We saw earlier that end systems (user PCs, PDAs, Web servers, mail servers, and
so on) connect into the Internet via a local ISP. The ISP can provide either wired or
wireless connectivity, using an array access technologies including DSL, cable,
FTTH, Wi-Fi, cellular, and WiMAX. Note the the local ISP does not have to be a
teleo or a cable company: it can be, for example, a university (providing Internet
acces~ to students, staff, and faculty) or a company (providing access for its
employees). But connecting end users and content providers into local ISPs is only
a small piece of solving the puzzle of connecting the hundreds of millions of end
systems and hundreds of thousands of networks that make up the Internet. The
Internet is a network of networks-understanding this phrase is the key to solving
this puzzle.
In the public Internet, access ISPs situated at the edge of the Internet are connected
to the rest of the Internet through a tiered hierarchy of ISPs, as shown in
Figure 1.15. Access ISPs are at the bottom of this hierarchy. At the very top of the
hierarchy is a relatively small number of so-called tier-lISPs. In many ways, a tier- I
ISP is the same as any network-it has links and routers and is connected to other
networks. In other ways, however, tier-liSPs are special. Their link speeds are often
622 Mbps or higher, with the larger tier-lISPs having links in the 2.5 to 10 Gbps
range; their routers must consequently be able to forward packets at extremely high
rates. Tier-lISPs are also characterized by being:
- Directly connected to each of the other tier-liSPs
- Connected to a large number of tier-2 ISPs and other customer networks
- International in coverage
Tier-lISPs are also known as Internet backbone networks. These include
Sprint, Verizon, MCI (previously UUNetlWoridCom), AT&T, NIT, Level3, Qwest,
and Cable & Wireless. Interestingly, no group officially sanctions tier-I status; as the
saying goes-if you have to ask if you are a member of a group, you're probably not.
A tier-2 ISP typically has regional or national coverage, amI (importantly) connects
to only a few of the tier-lISPs (see Figure I. I 5). Thus, in order to reach a
large portion of the global Internet, a tier-2 ISP needs to route traffic through one of the tier-lISPs to which it is connected. A tier-2 ISP is said to be a customer of the
tier-lISP to which it is connected, and the tier-lISP is said to be a provider to its
customer. Many large companies and institutions connect their enterprise's network
directly into a tier-lor tier-2 ISP, thus becoming a customer of that ISP. A provider
ISP charges its customer ISP a fee, which typically depends on the transmission rate
of the link connecting the two. A tier-2 network may also choose to connect directly
to other tier-2 networks, in which case traffic can flow between the two tier-2 networks
without having to pass through a tier-l network. Below the tier-2 ISPs are the
lower-tier ISPs, which connect to the larger Internet via one or more tier-2 ISPs. At
the bottom of the hierarchy are the access ISPs. Further complicating matters, some
tier-l providers are also tier-2 providers (that is, vertically integrated), selling Internet
access directly to end users and content providers, as well as to lower-tier ISPs.
When two ISPs are directly connected to each other at the same tier, they are said to
peer with each other. An interesting study [Subramanian 2002] seeks to define the
Internet's tiered structure more precisely by studying the Internet's topology in
terms of customer-provider and peer-peer relationships. For a readable discussion of
peering and customer-provided relationships, see [Van der Berg 2008].
Within an ISP's network, the points at which the ISP connects to other ISPs
(whether below, above, or at the same level in the hierarchy) are known as Points of
Presence (POPs). A POP is simply a group of one or more routers in the ISP's network
at which routers in other ISPs or in the networks belonging to the ISP's customers
can connect. A tier-l provider typically has many POPs scattered across
different geographicaf locations in its network, with multiple customer networks and
other ISPs connecting into each POP. For a customer network to connect to a
provider's POP, the customer typically leases a high-speed link from a third-party
telecommunications provider and directly connects one of its routers to a router at
the provider's POP. Furthermore, two ISPs may have multiple peering points, connecting
with each other at multiple pairs of POPs.
In summary, the topology of the Internet is complex, consisting of dozens of
tier-I and tier-2 ISPs and thousands of lower-tier ISPs. The ISPs are diverse in their
coverage, with some spanning multiple continents and oceans, and others limited
to narrow regions of the world. The lower-tier ISPs connect to the higher-tier
ISPs, and the higher-tier ISPs interconnect with one another. Users and content
providers are customers of lower-tier ISPs, and lower-tier lSPs are customers of
higher-tier ISPs.
zaterdag 13 november 2010
1.3.2 How Do Packet Make Their Way Through Packet-Switched Networks?
Earlier we said that a router takes a packet arriving on one of its attached communication
links and forwards that packet on to another of its attached communication
links. But how does the router determine the link onto which it should forward the
packet? This is actually done in different ways by different types of computer networks.
In this introductory chapter, we will describe one popular approach, namely,
the approach employed by the Internet.
In the Internet, each packet traversing the network contains the address of the
packet's destination in its header. As with postal addresses, this address has a hierarchical
structure. When a packet arrives at a router in the network, the router examines
a portion of the packet's destination address and forwards the packet to an
adjacent router. More specifically, each router has a forwarding table that maps
destination addresses (or portions of the destination addresses) to outbound links.
When a packet arrives at a router, the router examines the address and searches its
table using this destination address to find the appropriate outbound link. The router
then directs the packet to this outbound link.
We just learned that a router uses a packet's destination address to index a forwarding
table and detennine the appropriate outbound link. But this statement begs
yet another question: how do forwarding tables get set? Are they configured by hand
in each and every router, or does the Internet use a more automated procedure? This
issue will be studied in depth in Chapter 4. But to whet your appetite here, we'll note
now that the Internet has a number of special routing protocols that are used to automatically
set the forwarding tables. A routing protocol may, for example, determine
the shortest path from each router to each destination and use the shortest path
results to configure the forwarding tables in the routers.
The end-to-end routing process is analogous to a car driver who does not use
maps but instead prefers to ask for directions. For example, suppose Joe is driving
from Philadelphia to 156 Lakeside Drive in Orlando, Florida. Joe first drives to his
neighborhood gas station and asks how to get to 156 Lakeside Dri ve in Orlando,
Florida. The gas station attendant extracts the Florida portion of the address and tells
Joe that he needs to get onto the interstate highway 1-95 South, which has an
entrance just next to the gas station. He also tells Joe that once he enters Florida he
should ask someone else there. Joe then takes 1-95 South until he gets to Jacksonville,
Florida, at which point he asks another gas station attendant for directions.
The attendant extracts the Orlando portion of the address and tells Joe that he should
continue on 1-95 to Daytona Beach and then ask someone else. In Daytona Beach
another gas station attendant also extracts the Orlando portion of the address and
tells Joe that he should take 1-4 directly to Orlando. Joe takes 1-4 and gets off at the
Orlando exit. Joe goes to another gas station attendant, and this time the attendant
extracts the Lakeside Drive portion of the address and tells Joe the road he must follow
to get to Lakeside Drive. Once Joe reaches Lakeside Drive, he asks a kid on a bicycle how to get to his destination. The kid extracts the 156 portion of the address
and points to the house. Joe finally reaches his ultimate destination.
In the above analogy, the gas-station attendants and kids on bicycles are analogous
to routers. Their forwarding tables, which are in their brains, have been configured
by years of experience.
How would you actually like to see the end-to-end route that packets take in the
Internet? We now invite you to get your hands dirty by interacting with the Traceroute
program, by visiting the site http://www.traceroute.org. (For a discussion of
Traceroute, see Section 1.4.)
links and forwards that packet on to another of its attached communication
links. But how does the router determine the link onto which it should forward the
packet? This is actually done in different ways by different types of computer networks.
In this introductory chapter, we will describe one popular approach, namely,
the approach employed by the Internet.
In the Internet, each packet traversing the network contains the address of the
packet's destination in its header. As with postal addresses, this address has a hierarchical
structure. When a packet arrives at a router in the network, the router examines
a portion of the packet's destination address and forwards the packet to an
adjacent router. More specifically, each router has a forwarding table that maps
destination addresses (or portions of the destination addresses) to outbound links.
When a packet arrives at a router, the router examines the address and searches its
table using this destination address to find the appropriate outbound link. The router
then directs the packet to this outbound link.
We just learned that a router uses a packet's destination address to index a forwarding
table and detennine the appropriate outbound link. But this statement begs
yet another question: how do forwarding tables get set? Are they configured by hand
in each and every router, or does the Internet use a more automated procedure? This
issue will be studied in depth in Chapter 4. But to whet your appetite here, we'll note
now that the Internet has a number of special routing protocols that are used to automatically
set the forwarding tables. A routing protocol may, for example, determine
the shortest path from each router to each destination and use the shortest path
results to configure the forwarding tables in the routers.
The end-to-end routing process is analogous to a car driver who does not use
maps but instead prefers to ask for directions. For example, suppose Joe is driving
from Philadelphia to 156 Lakeside Drive in Orlando, Florida. Joe first drives to his
neighborhood gas station and asks how to get to 156 Lakeside Dri ve in Orlando,
Florida. The gas station attendant extracts the Florida portion of the address and tells
Joe that he needs to get onto the interstate highway 1-95 South, which has an
entrance just next to the gas station. He also tells Joe that once he enters Florida he
should ask someone else there. Joe then takes 1-95 South until he gets to Jacksonville,
Florida, at which point he asks another gas station attendant for directions.
The attendant extracts the Orlando portion of the address and tells Joe that he should
continue on 1-95 to Daytona Beach and then ask someone else. In Daytona Beach
another gas station attendant also extracts the Orlando portion of the address and
tells Joe that he should take 1-4 directly to Orlando. Joe takes 1-4 and gets off at the
Orlando exit. Joe goes to another gas station attendant, and this time the attendant
extracts the Lakeside Drive portion of the address and tells Joe the road he must follow
to get to Lakeside Drive. Once Joe reaches Lakeside Drive, he asks a kid on a bicycle how to get to his destination. The kid extracts the 156 portion of the address
and points to the house. Joe finally reaches his ultimate destination.
In the above analogy, the gas-station attendants and kids on bicycles are analogous
to routers. Their forwarding tables, which are in their brains, have been configured
by years of experience.
How would you actually like to see the end-to-end route that packets take in the
Internet? We now invite you to get your hands dirty by interacting with the Traceroute
program, by visiting the site http://www.traceroute.org. (For a discussion of
Traceroute, see Section 1.4.)
1.3.1 Circuit Switching and Packet Switching
There are two fundamental approaches to moving data through a network of links
and switches: circuit switching and packet switching. In circuit-switched networks,
the resources needed along a path (buffers, link transmission rate) to provide
for communication between the end systems are reserved for the duration of the
communication session between the end-systems. In packet-switched networks,
these resources are not reserved; a session 's messages use the resources on demand,
and as a consequence, may have to wait (that is, queue) for access to a communication
link. As a simple analogy, consider two restaurants, one that requires reservations
and another that neither requires reservations nor accepts them. For the
restaurant that requires reservations, we have to go through the hassle of calling
before we leave home. But when we arrive at the restaurant we can, in principle,
immediately communicate with the waiter and order our meal. For the restaurant
that does not require reservations, we don ' t need to bother to reserve a table. But
when we arrive at the restaurant, we may have to wait for a table before we can
communicate with the waiter.
The ubiquitous telephone networks are examples of circuit-switched networks.
Consider what happens when one person wants to send information (voice or facsimile)
to another over a telephone network. Before the sender can send the information,
the network must establish a connection between the sender and the
receiver. This is a bonafide connection for which the switches on the path between
the sender and receiver maintain connection state for that connection. In the jargon
of telephony, this connection is called a circuit. When the network establishes the
circuit, it also reserves a constant transmission rate in the network's links for the
duration of the connection. Since bandwidth has been reserved for this sender-toreceiver
connection, the sender can transfer the data to the receiver at the guaranteed
constant rate. Today's Internet is a quintessential packet-switched network. Consider what
happens when one host wants to send a packet to another host over the Internet. As
with circuit switching, the packet is transmitted over a series of communication
links. But with packet switching. the packet is sent into the network without reserving
any bandwidth whatsoever. If one of the links is congested because other packets
need to be transmitted over the link at the same time, then our packet will have
to wait in a buffer at the sending side of the transmission link, and suffer a dC1lay.
The internet makes its best effort to deliver packets in a timely manner, but ,it does
not make any guarantees.
Not all telecommunication networks can be neatly classified as pure circuitswitched
networks or pure packet-switched networks. Nevertheless, this fundamental
classification into packet- and circuit-switched networks is an excellent starting
point in understanding telecommunication network technology.
Circuit Switching
This book is about computer networks, the Internet, and packet switching, not about
telephone networks and circuit switching. Nevertheless, it is important to understand
why the Internet and other computer networks use packet switching rather
than the more traditional circuit-switching technology used in the telephone networks.
For this reason, we now give a brief overview of circuit switching.
Figure 1.12 illustrates a circuit-switched network. In this network, the four circuit
switches are interconnected by four links. Each of these links has n circuits, so
that each link can support n simultaneous connections. The hosts (for example, PCs
and workstations) are each directly connected to one of the switches. When two
hosts want to communicate, the network establishes a dedicated end-to-end connection
between the two hosts. (Conference calls between more than two devices
are, of course, also possible. But to keep things simple, let's suppose for now that
there are only two hosts for each connection.) Thus, in order for Host A to send messages
to Host B, the network must first reserve one circuit on each of two links.
Because each link has II circuits, for each link used by the end-to-end connection,
the connection gets a fraction lin of the link's bandwidth for the duration of the connection.
MulLiplexlllg in Circuit-Switched Networks
A circuit in a link is implemented with either frequency-division multiplexing
(FDM) or time-division multiplexing (TDM). With FDM, the frequency spectrum
of a link is divided up among the connections established across the link.
Specifically, the link dedicates a frequency band to each connection for the
duration of the connection. In telephone networks, this frequency band typically
has a width of 4 kHz (that is, 4,000 hertz or 4,000 cycles per second). The width of the band is called, not surprisingly, the bandwidth. FM radio stations also use FOM to share the frequency spectrum between 88 MHz and 108 MHz, with each station being allocated a specific frequency band. For a TOM link, time is divided into frames of fixed duration, and each frame is divided into a fixed number of time slots. When the network establishes a connecgist's
tion across a link, the network dedicates one time slot in every frame to this connecthat
tion. These slots are dedicated for the sole use of that connection, with one time slot available for lise (in every frame) to transmit the connect jon 's data. Figure 1.13 illustrates FOM and TDM for a specific network link suppOlting up to four circuits. For FDM, the frequency domain is segmented into four bands, each of bandwidth 4 kHz. For TOM, the time domain is segmented into frames, with four time slots in each frame; each circuit is assigned the same dedicated slot in the revolving TOM frames. For TOM, the transmission rate of a circuit is equal to the frame rate multiplied by the number of bits in a slot. For example, if the link transend-
mits 8,000 frames per second and each slot consists of 8 bits, then the transmission rate of a circuit is 64 kbps. Proponents of packet switching have always argued that circuit switching is wasteful because tbe dedicated circuits are idle during silent periods. For example, when one person in a telephone call stops talking, the idle network resources (fresion
quency bands or time slots in the links along the connection's route) cannot be used by other ongoing connections. As another example of how these resources can be underutilized, consider a radiologist who uses a circuit-switched network to
remotely access a series of x-rays. The radiologist sets up a connection, requests an
image, contemplates the image, and then requests a new image. Network resources
are allocated to the connection but are not used (i.e., are wasted) during the radiologist
's contemplation periods. Proponents of packet switching also enjoy pointing out
that establishing end-to-end circuits and reserving end-to-end bandwidth is complicated
and requires complex signaling software to coordinate the operation of the
switches along the end-to-end path.
Before we finish our discussion of circuit switching, let's work through a
numerical example that should shed further insight on the topic. Let us consider how
long it takes to send a file of 640,000 bits from Host A to Host B over a circuitswitched
network. Suppose that all links in the network use TDM with 24 slots and
have a bit rate of 1.536 Mbps. Also suppose that it takes 500 msec to establish an
end-to-end circuit before Host A can begin to transmit the file . How long does it take
to send the file? Each circuit has a transmission rate of (1.536 Mbps)/24 = 64 kbps,
so it takes (640,000 bits)/(64 kbps) =10 seconds to transmit the file. To this 10 seconds
we add the circuit establishment time, giving 10.5 seconds to send the file.
Note that the transmission time is independent of the number of links: The transmission
time would be 10 seconds if the end-to-end circuit passed through one link or a
hundred links. (The actual end-to-end delay also includes a propagation delay ; see
Section 1.4.)
Packet Switching
Distributed applications exchange messages in accomplishing their task. Messages
can contain anything the protocol designer wants. Messages may perform a control
function (for example, the "Hi" messages in our handshaking example) or can contain
data, such as an e-mail message, a JPEG image, or an MP3 audio file. In modern
computer networks, the source breaks long messages into smaller chunks of data
known as packets. Between source and destination, each of these packets travels
through communication links and packet switches (for which there are two predominant
types, routers and link-layer switches). Packets are transmitted over each
communication link at a rate equal to the filiI transmission rate of the link.
Most packet switches use store-and-forward transmission at the inputs to the
links. Store-and-forward transmission means that the switch must receive the entire
packet before it can begin to transmit the flfSt bit of the packet onto the outbound link.
Thus store-and-forward packet switches introduce a store-and-forward delay at the
input to each link along the packet's route. Consider how long it takes to send a packet
of L bits from one host to another host across a packet-switched network. Let's suppose
that there are Q links between the two hosts, each of rate R bps. Assume that this
is the only packet in the network. The packet must first be transmitted onto the first
link emanating from Host A; this takes UR seconds. It must then be transmitted on
each of the Q - I remaining links; that is, it must be stored and forwarded Q - I times,
each time with a store-and-forward delay ofLfR. Thus the total delay is QUR.
Each packet switch has multiple links attached to it. For each attached link, the
packet switch has an output buffer (also called an output queue), which stores
packets that the router is about to send into that link. The output buffers playa key
role in packet switching. If an arriving packet needs to be transmitted across a link
but finds the link busy with the transmission of another packet, the arriving packet
must wait in the output buffer. Thus, in addition to the store-and-forward delays,
packets suffer output buffer queuing delays. These delays are variable and depend
on the level of congestion in the network. Since the amollnt of buffer space is finite,
an arriving packet may find that the buffer is comple tely filled with other packets
waiting for transmission. In this case, packet loss will occur-either the arriving
packet or one of the already-queued packets will be dropped. Returning to our
restaurant analogy from earlier in this section, the queuing delay is analogoLls to the
amount of time you spend waiting at the restaurant's bar for a table to become free.
Packet loss is analogous to being told by the waiter that you must leave the premises
because there are already too many other people waiting at the bar for a table.
Figure 1.14 illustrates a simple packet-switched network. In this and subsequent
figures, packets are represented by three-dimensional slabs. The width of a slab represents
the number of bits in the packet. In this figure, all packets have the same
width and hence the same length. Suppose Hosts A and B are sending packets to
Host E. Hosts A and B first send their packets along 10 Mbps Ethernet links to the
first packet switch. The packet switch then directs these packets to the 1.5 Mbps link. If the arrival rate of packets to the switch exceeds the rate at which the switch
can forward packets across the 1.5 Mbps output link, congestion will occur as packets
queue in the link's output buffer before being transmitted onto the link. We ' ll
examine this queuing delay in more detail in Section 1.4.
Packet switching Versus. Circuit Switching: Statistical Multiplexing
Having described circuit switching and packet switching, let us compare the two.
Critics of packet switching have often argued that packet switching is not suitable
for real-time services (for example, telephone calls and video conference calls)
because of its variable and unpredictable end-to-end delays (due primarily to variable
and unpredictable queuing delays). Proponents of packet switching argue that
(I) it offers better sharing of bandwidth than circuit switching and (2) it is simpler,
more efficient, and less costly to implement than circuit switching. An interesting
discussion of packet switching versus circuit switching is [Molinero-Fernandez
2002]. Generally speaking, people who do not like to hassle with restaurant reservations
prefer packet switching to circuit switching.
Why is packet switching more efficient? Let's look at a simple example. Suppose
users share a I Mbps link. Also suppose that each user alternates between periods
of acti vity, when a user generates data at a constant rate of 100 kbps, and periods
of inactivity, when a user generates no data. Suppose further that a user is active
only 10 percent of the time (and is idly drinking coffee during the remaining 90 percent
of the time). With circuit switching, 100 kbps must be reserved for each user at
all times. For example, with circuit-switched TDM, if a one-second frame is divided into 10 time slots of 100 ms each, then each user would be allocated one time slot per frame. Thus, the circuit-switched link can support only 10 (= 1 MbpsllOO kbps) simultaneous
users. With packet switching, the probability that a specific user is active is 0.1 (that is, 10 percent). If there are 35 users, the probability that there are 11 or more simultaneously active users is approximately 0.0004. (Homework Problem P7 outlines how this probability is obtained.) When there are 10 or fewer simultanepacket?
ously active users (which happens with probability 0.9996), the aggregate arrival rate of data is less than or equal to 1 Mbps, the output rate of the link. Thus, when are 10 or fewer active users, users' packets flow through the link essentially
without delay, as is the case with circuit switching. When there are more than 10 simultaneously active users, then the aggregate arrival rate of packets exceeds the
output capacity of the link, and the output queue will begin to grow. (It continues to grow until the aggregate input rate falls back below I Mbps, at which point the queue will begin to diminish in length.) Because the probability of having more than 10 simultaneously active users is minuscule in this example, packet switching proWhen
vides essentially the same performance as circuit switching, but does so while allowing for more than three times the number of users. Let's now consider a second simple example. Suppose there are 10 users and that one user suddenly generates one thousand I,OOO-bit packets, while other users remain quiescent and do not generate packets. Under TDM circuit switching with 10 slots per frame and each slot consisting of 1,000 bits, the active user can only use its one time slot per frame to transmit data, while the remaining nine times slots in each frame remain idle. It will be to seconds before all of the active user's one million bits of data has been transmitted. In the case of packet switching, the active user can conmaticall
tinuously send its packets at the full link rate of 1 Mbps, since there are no other users generating packets that need to be multiplexed with the active user's packets. In this case, all of the active user's data will be transmitted within 1 second. The above examples illustrate two ways in which the performance of packet switching can be superior to that of circuit switching. They also highlight the crucial difference between the two forms of sharing a link's transmission rate among multineighbc
ple data streams. Circuit switching pre-allocates use of the transmission link regardFlorida.
less of demand, with allocated but unneeded link time going unused . Packet switching on the other hand allocates link use on demand. Link transmiss ion capacentranCf
ity will be shared on a packet-by-packet basis only among those users who have packets that need to be transmitted over the link. Such on-demand (rather than presonville
allocated) sharing ofresources is sometimes referred to as the statistical multiplexThe
ing of resources . Although packet switching and circuit switching are both prevalent in today 's telecommunication networks, the trend has certainly been in the direction of packet switching. Even many of today 's circuit-switched telephone networks are slowly migrating toward packet switching. In particular, telephone networks often use packet switching for the expensive overseas portion of a telephone call.
and switches: circuit switching and packet switching. In circuit-switched networks,
the resources needed along a path (buffers, link transmission rate) to provide
for communication between the end systems are reserved for the duration of the
communication session between the end-systems. In packet-switched networks,
these resources are not reserved; a session 's messages use the resources on demand,
and as a consequence, may have to wait (that is, queue) for access to a communication
link. As a simple analogy, consider two restaurants, one that requires reservations
and another that neither requires reservations nor accepts them. For the
restaurant that requires reservations, we have to go through the hassle of calling
before we leave home. But when we arrive at the restaurant we can, in principle,
immediately communicate with the waiter and order our meal. For the restaurant
that does not require reservations, we don ' t need to bother to reserve a table. But
when we arrive at the restaurant, we may have to wait for a table before we can
communicate with the waiter.
The ubiquitous telephone networks are examples of circuit-switched networks.
Consider what happens when one person wants to send information (voice or facsimile)
to another over a telephone network. Before the sender can send the information,
the network must establish a connection between the sender and the
receiver. This is a bonafide connection for which the switches on the path between
the sender and receiver maintain connection state for that connection. In the jargon
of telephony, this connection is called a circuit. When the network establishes the
circuit, it also reserves a constant transmission rate in the network's links for the
duration of the connection. Since bandwidth has been reserved for this sender-toreceiver
connection, the sender can transfer the data to the receiver at the guaranteed
constant rate. Today's Internet is a quintessential packet-switched network. Consider what
happens when one host wants to send a packet to another host over the Internet. As
with circuit switching, the packet is transmitted over a series of communication
links. But with packet switching. the packet is sent into the network without reserving
any bandwidth whatsoever. If one of the links is congested because other packets
need to be transmitted over the link at the same time, then our packet will have
to wait in a buffer at the sending side of the transmission link, and suffer a dC1lay.
The internet makes its best effort to deliver packets in a timely manner, but ,it does
not make any guarantees.
Not all telecommunication networks can be neatly classified as pure circuitswitched
networks or pure packet-switched networks. Nevertheless, this fundamental
classification into packet- and circuit-switched networks is an excellent starting
point in understanding telecommunication network technology.
Circuit Switching
This book is about computer networks, the Internet, and packet switching, not about
telephone networks and circuit switching. Nevertheless, it is important to understand
why the Internet and other computer networks use packet switching rather
than the more traditional circuit-switching technology used in the telephone networks.
For this reason, we now give a brief overview of circuit switching.
Figure 1.12 illustrates a circuit-switched network. In this network, the four circuit
switches are interconnected by four links. Each of these links has n circuits, so
that each link can support n simultaneous connections. The hosts (for example, PCs
and workstations) are each directly connected to one of the switches. When two
hosts want to communicate, the network establishes a dedicated end-to-end connection
between the two hosts. (Conference calls between more than two devices
are, of course, also possible. But to keep things simple, let's suppose for now that
there are only two hosts for each connection.) Thus, in order for Host A to send messages
to Host B, the network must first reserve one circuit on each of two links.
Because each link has II circuits, for each link used by the end-to-end connection,
the connection gets a fraction lin of the link's bandwidth for the duration of the connection.
MulLiplexlllg in Circuit-Switched Networks
A circuit in a link is implemented with either frequency-division multiplexing
(FDM) or time-division multiplexing (TDM). With FDM, the frequency spectrum
of a link is divided up among the connections established across the link.
Specifically, the link dedicates a frequency band to each connection for the
duration of the connection. In telephone networks, this frequency band typically
has a width of 4 kHz (that is, 4,000 hertz or 4,000 cycles per second). The width of the band is called, not surprisingly, the bandwidth. FM radio stations also use FOM to share the frequency spectrum between 88 MHz and 108 MHz, with each station being allocated a specific frequency band. For a TOM link, time is divided into frames of fixed duration, and each frame is divided into a fixed number of time slots. When the network establishes a connecgist's
tion across a link, the network dedicates one time slot in every frame to this connecthat
tion. These slots are dedicated for the sole use of that connection, with one time slot available for lise (in every frame) to transmit the connect jon 's data. Figure 1.13 illustrates FOM and TDM for a specific network link suppOlting up to four circuits. For FDM, the frequency domain is segmented into four bands, each of bandwidth 4 kHz. For TOM, the time domain is segmented into frames, with four time slots in each frame; each circuit is assigned the same dedicated slot in the revolving TOM frames. For TOM, the transmission rate of a circuit is equal to the frame rate multiplied by the number of bits in a slot. For example, if the link transend-
mits 8,000 frames per second and each slot consists of 8 bits, then the transmission rate of a circuit is 64 kbps. Proponents of packet switching have always argued that circuit switching is wasteful because tbe dedicated circuits are idle during silent periods. For example, when one person in a telephone call stops talking, the idle network resources (fresion
quency bands or time slots in the links along the connection's route) cannot be used by other ongoing connections. As another example of how these resources can be underutilized, consider a radiologist who uses a circuit-switched network to
remotely access a series of x-rays. The radiologist sets up a connection, requests an
image, contemplates the image, and then requests a new image. Network resources
are allocated to the connection but are not used (i.e., are wasted) during the radiologist
's contemplation periods. Proponents of packet switching also enjoy pointing out
that establishing end-to-end circuits and reserving end-to-end bandwidth is complicated
and requires complex signaling software to coordinate the operation of the
switches along the end-to-end path.
Before we finish our discussion of circuit switching, let's work through a
numerical example that should shed further insight on the topic. Let us consider how
long it takes to send a file of 640,000 bits from Host A to Host B over a circuitswitched
network. Suppose that all links in the network use TDM with 24 slots and
have a bit rate of 1.536 Mbps. Also suppose that it takes 500 msec to establish an
end-to-end circuit before Host A can begin to transmit the file . How long does it take
to send the file? Each circuit has a transmission rate of (1.536 Mbps)/24 = 64 kbps,
so it takes (640,000 bits)/(64 kbps) =10 seconds to transmit the file. To this 10 seconds
we add the circuit establishment time, giving 10.5 seconds to send the file.
Note that the transmission time is independent of the number of links: The transmission
time would be 10 seconds if the end-to-end circuit passed through one link or a
hundred links. (The actual end-to-end delay also includes a propagation delay ; see
Section 1.4.)
Packet Switching
Distributed applications exchange messages in accomplishing their task. Messages
can contain anything the protocol designer wants. Messages may perform a control
function (for example, the "Hi" messages in our handshaking example) or can contain
data, such as an e-mail message, a JPEG image, or an MP3 audio file. In modern
computer networks, the source breaks long messages into smaller chunks of data
known as packets. Between source and destination, each of these packets travels
through communication links and packet switches (for which there are two predominant
types, routers and link-layer switches). Packets are transmitted over each
communication link at a rate equal to the filiI transmission rate of the link.
Most packet switches use store-and-forward transmission at the inputs to the
links. Store-and-forward transmission means that the switch must receive the entire
packet before it can begin to transmit the flfSt bit of the packet onto the outbound link.
Thus store-and-forward packet switches introduce a store-and-forward delay at the
input to each link along the packet's route. Consider how long it takes to send a packet
of L bits from one host to another host across a packet-switched network. Let's suppose
that there are Q links between the two hosts, each of rate R bps. Assume that this
is the only packet in the network. The packet must first be transmitted onto the first
link emanating from Host A; this takes UR seconds. It must then be transmitted on
each of the Q - I remaining links; that is, it must be stored and forwarded Q - I times,
each time with a store-and-forward delay ofLfR. Thus the total delay is QUR.
Each packet switch has multiple links attached to it. For each attached link, the
packet switch has an output buffer (also called an output queue), which stores
packets that the router is about to send into that link. The output buffers playa key
role in packet switching. If an arriving packet needs to be transmitted across a link
but finds the link busy with the transmission of another packet, the arriving packet
must wait in the output buffer. Thus, in addition to the store-and-forward delays,
packets suffer output buffer queuing delays. These delays are variable and depend
on the level of congestion in the network. Since the amollnt of buffer space is finite,
an arriving packet may find that the buffer is comple tely filled with other packets
waiting for transmission. In this case, packet loss will occur-either the arriving
packet or one of the already-queued packets will be dropped. Returning to our
restaurant analogy from earlier in this section, the queuing delay is analogoLls to the
amount of time you spend waiting at the restaurant's bar for a table to become free.
Packet loss is analogous to being told by the waiter that you must leave the premises
because there are already too many other people waiting at the bar for a table.
Figure 1.14 illustrates a simple packet-switched network. In this and subsequent
figures, packets are represented by three-dimensional slabs. The width of a slab represents
the number of bits in the packet. In this figure, all packets have the same
width and hence the same length. Suppose Hosts A and B are sending packets to
Host E. Hosts A and B first send their packets along 10 Mbps Ethernet links to the
first packet switch. The packet switch then directs these packets to the 1.5 Mbps link. If the arrival rate of packets to the switch exceeds the rate at which the switch
can forward packets across the 1.5 Mbps output link, congestion will occur as packets
queue in the link's output buffer before being transmitted onto the link. We ' ll
examine this queuing delay in more detail in Section 1.4.
Packet switching Versus. Circuit Switching: Statistical Multiplexing
Having described circuit switching and packet switching, let us compare the two.
Critics of packet switching have often argued that packet switching is not suitable
for real-time services (for example, telephone calls and video conference calls)
because of its variable and unpredictable end-to-end delays (due primarily to variable
and unpredictable queuing delays). Proponents of packet switching argue that
(I) it offers better sharing of bandwidth than circuit switching and (2) it is simpler,
more efficient, and less costly to implement than circuit switching. An interesting
discussion of packet switching versus circuit switching is [Molinero-Fernandez
2002]. Generally speaking, people who do not like to hassle with restaurant reservations
prefer packet switching to circuit switching.
Why is packet switching more efficient? Let's look at a simple example. Suppose
users share a I Mbps link. Also suppose that each user alternates between periods
of acti vity, when a user generates data at a constant rate of 100 kbps, and periods
of inactivity, when a user generates no data. Suppose further that a user is active
only 10 percent of the time (and is idly drinking coffee during the remaining 90 percent
of the time). With circuit switching, 100 kbps must be reserved for each user at
all times. For example, with circuit-switched TDM, if a one-second frame is divided into 10 time slots of 100 ms each, then each user would be allocated one time slot per frame. Thus, the circuit-switched link can support only 10 (= 1 MbpsllOO kbps) simultaneous
users. With packet switching, the probability that a specific user is active is 0.1 (that is, 10 percent). If there are 35 users, the probability that there are 11 or more simultaneously active users is approximately 0.0004. (Homework Problem P7 outlines how this probability is obtained.) When there are 10 or fewer simultanepacket?
ously active users (which happens with probability 0.9996), the aggregate arrival rate of data is less than or equal to 1 Mbps, the output rate of the link. Thus, when are 10 or fewer active users, users' packets flow through the link essentially
without delay, as is the case with circuit switching. When there are more than 10 simultaneously active users, then the aggregate arrival rate of packets exceeds the
output capacity of the link, and the output queue will begin to grow. (It continues to grow until the aggregate input rate falls back below I Mbps, at which point the queue will begin to diminish in length.) Because the probability of having more than 10 simultaneously active users is minuscule in this example, packet switching proWhen
vides essentially the same performance as circuit switching, but does so while allowing for more than three times the number of users. Let's now consider a second simple example. Suppose there are 10 users and that one user suddenly generates one thousand I,OOO-bit packets, while other users remain quiescent and do not generate packets. Under TDM circuit switching with 10 slots per frame and each slot consisting of 1,000 bits, the active user can only use its one time slot per frame to transmit data, while the remaining nine times slots in each frame remain idle. It will be to seconds before all of the active user's one million bits of data has been transmitted. In the case of packet switching, the active user can conmaticall
tinuously send its packets at the full link rate of 1 Mbps, since there are no other users generating packets that need to be multiplexed with the active user's packets. In this case, all of the active user's data will be transmitted within 1 second. The above examples illustrate two ways in which the performance of packet switching can be superior to that of circuit switching. They also highlight the crucial difference between the two forms of sharing a link's transmission rate among multineighbc
ple data streams. Circuit switching pre-allocates use of the transmission link regardFlorida.
less of demand, with allocated but unneeded link time going unused . Packet switching on the other hand allocates link use on demand. Link transmiss ion capacentranCf
ity will be shared on a packet-by-packet basis only among those users who have packets that need to be transmitted over the link. Such on-demand (rather than presonville
allocated) sharing ofresources is sometimes referred to as the statistical multiplexThe
ing of resources . Although packet switching and circuit switching are both prevalent in today 's telecommunication networks, the trend has certainly been in the direction of packet switching. Even many of today 's circuit-switched telephone networks are slowly migrating toward packet switching. In particular, telephone networks often use packet switching for the expensive overseas portion of a telephone call.
1.3 The Network Core
Having examined the Internet's edge, let us now delve more deeply inside the network
core-the mesh of packet switches and links that interconnects the Internet's
end systems. Figure 1.11 highlights the network core with thick, shaded lines.
core-the mesh of packet switches and links that interconnects the Internet's
end systems. Figure 1.11 highlights the network core with thick, shaded lines.
1.2.3 Physical Media
In the previous subsection, we gave an overview of some of the most important network
access technologies in the Internet. As we described these technologies, we
also indicated the physical media used. For example, we said that HFC uses a combination
of flber cable and coaxial cable. We said rhat dial-up 56 kbps modems and
DSL use twisted-pair copper wire. And we said that mobile access networks use the radio spectrum. In this subsection we provide a brief overview of these and other transmission media that are commonly used in the Internet. In order to define what is meant by a physical medium, let us reflect on the brief life of a bit. Consider a bit traveling from one end system, through a series of links and routers, to another end system. This poor bit gets kicked around and transmitted many, many times! The source end system first transmits the bit, and shortly thereafter the first router in the series receives the bit; the first router then transmits the bit, and shortly thereafter the second router receives the bit; and so on. Thus our bit, when traveling from source to destination, passes through a series of transmitter-receiver pairs. For each transmitter-receiver pair, the bit is sent by propagating elec tromagnetic waves or optical pulses across a physical medium. The physical medium can take many shapes and forms and does not have to be of the same type for each transmitter-receiver pair along the path. Examples of physikbps
cal media include twisted-pair copper wire, coaxial cable, multi mode fiber-optic cable, terrestrial radio spectrum, and satellite radio spectrum. Physical media fall into two categories: guided media and unguided media. With guided media, the
waves are guided along a solid medium, such as a f iber-optic cable, a tw isted-pair
copper wire, or a coaxial cable. With unguided media, the waves propagate in the
atmosphere and in outer space, uch as in a wireless LAN or a digital satellite channe l. But before we get into the characteristics of the various media types, let us say a few words about their costs. The actual cost of the physical link (copper wire, fiber-optic cable, and so on) is often relatively minor compared with other networkhave
ing costs. In particular, the labor cost associated with the installation of the physical link can be orders of magn itude higher than the cost of the material. For this reason, builders install twisted pair, optical fiber, and coaxial cable in every room in a building. Even if only one medium is initiaJly used, there is a good chance that another medium could be used in the near future, and so money is saved by not havterns
ing to lay additional wires in the future.
Twisted-Pair Copper Wire
The least expensive and most commonly used guided transmission medium is lw isted-pair copper wire . For over a hundred years it has been used by telephone networks. In fact, more than 99 percent of the wired connections from the teleto
phone hand 'et to the local telephone switch use twisted-pair copper wire. Most of u have een twisted pair jn our homes and work environments. Twisted pair convery
sists of two insulated copper wires, each about I mm thick, arranged in a regular spiral pattern. The wires are twisted together to reduce the electrical interference from similar pairs close by. Typically, a number of pairs are bundled together in a cable by wrapping the pairs in a protective shield. A wire pair constitutes a single communication link. Unshielded twisted pair (UTP) is commonly used for computer networks within a building, that is , for LANs. Data rates for LANs
using twisted pair today range from 10 Mbps to I Gbps. The data rates that can
be achieved depend on the thickness of the wire and the distance between transmitter
and receiver.
When fiber-optic technology emerged in the 1980s, many people disparaged
twisted pair because of its relatively low bit rates. Some people even felt that fiberoptic
technology would completely replace twisted pair. But twisted pair did not
give up so easily. Modern twisted-pair technology, such as category 5 UTP, can
achieve data rates of 1 Gbps for distances up to a hundred meters. In the end, twisted
pair has emerged as the dominant solution for high-speed LAN networking.
As discussed earLier, twisted pair is also commonly used for residential Internet
access. We saw that dial-up modem technology enables access at rates of up to 56
kbps over twisted pair. We also saw that DSL (digital subscriber Line) technology
has ellabled residential users to access the Internet at rates in excess of 6 Mbps over
twisted pair (when users live close to the ISP's modem).
Coaxial Cable
Like twisted pair, coaxial cable consists of two copper conductors, but the two conductors
are concentric rather than parallel. With this construction and special insulation
and shielding, coaxial cable can have high bit rates. Coaxial cable is quite
common in cable television systems. As we saw earlier, cable television systems
have recently been coupled with cable modems to provide residential users with
Internet access at rates of I Mbps or higher. In cable television and cable Internet
access, the transmitter shifts the digital signal to a specific frequency band, and the
resulting analog signal is sent from the transmitter to one or more receivers. Coaxial
cable can be used as a guided shared medium. Specifically, a number of end systems
can be connected directly to the cable, with each of the end systems receiving
whatever is sent by the other end systems.
Fiber Optics
An optical fiber is a thin, flexible medium that conducts pulses of light, with each
pulse representing a bit. A single optical fiber can support tremendous bit rates, up
to tens or even hundreds of gigabits per second . They are immune to electromagnetic
interference, have very low signal attenuation up to 100 kilometers, and are
very hard to tap. These characteristics have made fiber optics the preferred longhaul
guided transmission media, particularly for overseas links. Many of the longdistance
telephone networks in the United States and elsewhere now use fiber optics
exclusively. Fiber optics is also prevalent in the backbone of the Internet. However,
the high cost of optical devices-such as transmitters, receivers, and switches-has
hindered their deployment for short-haul transport, such as in a LAN or into the home in a residential access network. The Optical Carrier (OC) standard link speeds range from 51 .8 Mbps to 39.8 Gbps; these specifications are often referred to as OC-n, where the link speed equals n x 51 .8 Mbps. Standards in use today include OC-l, OC-3, OC-12, OC-24, OC-48, OC-96, OC-I92, OC-768. [IEC Optical 2009; Goralski 200 I; Ramaswami 1998; and Mukherjee 1997] provide coverage of various aspects of optical networking.
Terrestrial Radio Channels
Radio channels carry signals in the electromagnetic spectrum. They are an attractive
medium because they require no physical wire to be in stalled, can penetrate walls, provide connectivity to a mobile user, and can potentially carry a signal for long distances. The characteristics of a radio channel depend significantly on the propagation environment and the distance over which a signal is to be carried.
Environmental considerations determine path loss and shadow fading (which
decrease the signal strength as the signal travels over a distance and around/through obstructing objects), multipath fading (due to signal retlection off
of interfering objects), and interference (due to other tran smissions and electroand
magnetic signals).
Terrestrial radio channels can be broadly classified into two groups: those that
operate in local areas, typically spanning from ten to a few hundred meters; and
those that operate in the wide area, spanning tens of kilometers. The wireless LAN
technologies described in Section 1.2.2 use local-area radio channels; the cellular
access technologies use wide-area radio channels. We' ll discuss radio channels in
detail in Chapter 6.
Satellite Radio Channels
A communication satellite links two or more Earth-based microwave transmitter/ receivers, known as ground stations. The satellite receives transmissions on one frewhen
quency band, regenerates the signal using a repeater (discussed below), and transmits the signal on another frequency. Two types of satellites are used in communications: geostationary satellites and low-earth orbiting (LEO) satellites. Geostationary satellites permanently remain above the same spot on Earth. This stationary presence is achieved by placing the satellite in orbit at 36,000 kilometers above Earth's surface. This huge distance from ground station through satellite back to ground station introduces a substantial signal propagation delay of 280 millisecthe
onds. Nevertheless, satellite links, which can operate at speeds of hundreds of Mbps, are often used in areas without access to DSL or cable-based Internet access. LEO satellites are placed much closer to Earth and do not remain permanently above one spot on Earth. They rotate around Earth (just as the Moon does) and may communicate with each other, as well as with ground stations. To provide continuous coverage to an area, many satellites need to be placed in orbit. There are currently
many low-altitude communication systems in development. Lloyd's satellite
constellations Web page [Wood 2009J provides and collects information on satellite
constellation systems for communications. LEO satellite technology may be used
for Internet access sometime in the future.
access technologies in the Internet. As we described these technologies, we
also indicated the physical media used. For example, we said that HFC uses a combination
of flber cable and coaxial cable. We said rhat dial-up 56 kbps modems and
DSL use twisted-pair copper wire. And we said that mobile access networks use the radio spectrum. In this subsection we provide a brief overview of these and other transmission media that are commonly used in the Internet. In order to define what is meant by a physical medium, let us reflect on the brief life of a bit. Consider a bit traveling from one end system, through a series of links and routers, to another end system. This poor bit gets kicked around and transmitted many, many times! The source end system first transmits the bit, and shortly thereafter the first router in the series receives the bit; the first router then transmits the bit, and shortly thereafter the second router receives the bit; and so on. Thus our bit, when traveling from source to destination, passes through a series of transmitter-receiver pairs. For each transmitter-receiver pair, the bit is sent by propagating elec tromagnetic waves or optical pulses across a physical medium. The physical medium can take many shapes and forms and does not have to be of the same type for each transmitter-receiver pair along the path. Examples of physikbps
cal media include twisted-pair copper wire, coaxial cable, multi mode fiber-optic cable, terrestrial radio spectrum, and satellite radio spectrum. Physical media fall into two categories: guided media and unguided media. With guided media, the
waves are guided along a solid medium, such as a f iber-optic cable, a tw isted-pair
copper wire, or a coaxial cable. With unguided media, the waves propagate in the
atmosphere and in outer space, uch as in a wireless LAN or a digital satellite channe l. But before we get into the characteristics of the various media types, let us say a few words about their costs. The actual cost of the physical link (copper wire, fiber-optic cable, and so on) is often relatively minor compared with other networkhave
ing costs. In particular, the labor cost associated with the installation of the physical link can be orders of magn itude higher than the cost of the material. For this reason, builders install twisted pair, optical fiber, and coaxial cable in every room in a building. Even if only one medium is initiaJly used, there is a good chance that another medium could be used in the near future, and so money is saved by not havterns
ing to lay additional wires in the future.
Twisted-Pair Copper Wire
The least expensive and most commonly used guided transmission medium is lw isted-pair copper wire . For over a hundred years it has been used by telephone networks. In fact, more than 99 percent of the wired connections from the teleto
phone hand 'et to the local telephone switch use twisted-pair copper wire. Most of u have een twisted pair jn our homes and work environments. Twisted pair convery
sists of two insulated copper wires, each about I mm thick, arranged in a regular spiral pattern. The wires are twisted together to reduce the electrical interference from similar pairs close by. Typically, a number of pairs are bundled together in a cable by wrapping the pairs in a protective shield. A wire pair constitutes a single communication link. Unshielded twisted pair (UTP) is commonly used for computer networks within a building, that is , for LANs. Data rates for LANs
using twisted pair today range from 10 Mbps to I Gbps. The data rates that can
be achieved depend on the thickness of the wire and the distance between transmitter
and receiver.
When fiber-optic technology emerged in the 1980s, many people disparaged
twisted pair because of its relatively low bit rates. Some people even felt that fiberoptic
technology would completely replace twisted pair. But twisted pair did not
give up so easily. Modern twisted-pair technology, such as category 5 UTP, can
achieve data rates of 1 Gbps for distances up to a hundred meters. In the end, twisted
pair has emerged as the dominant solution for high-speed LAN networking.
As discussed earLier, twisted pair is also commonly used for residential Internet
access. We saw that dial-up modem technology enables access at rates of up to 56
kbps over twisted pair. We also saw that DSL (digital subscriber Line) technology
has ellabled residential users to access the Internet at rates in excess of 6 Mbps over
twisted pair (when users live close to the ISP's modem).
Coaxial Cable
Like twisted pair, coaxial cable consists of two copper conductors, but the two conductors
are concentric rather than parallel. With this construction and special insulation
and shielding, coaxial cable can have high bit rates. Coaxial cable is quite
common in cable television systems. As we saw earlier, cable television systems
have recently been coupled with cable modems to provide residential users with
Internet access at rates of I Mbps or higher. In cable television and cable Internet
access, the transmitter shifts the digital signal to a specific frequency band, and the
resulting analog signal is sent from the transmitter to one or more receivers. Coaxial
cable can be used as a guided shared medium. Specifically, a number of end systems
can be connected directly to the cable, with each of the end systems receiving
whatever is sent by the other end systems.
Fiber Optics
An optical fiber is a thin, flexible medium that conducts pulses of light, with each
pulse representing a bit. A single optical fiber can support tremendous bit rates, up
to tens or even hundreds of gigabits per second . They are immune to electromagnetic
interference, have very low signal attenuation up to 100 kilometers, and are
very hard to tap. These characteristics have made fiber optics the preferred longhaul
guided transmission media, particularly for overseas links. Many of the longdistance
telephone networks in the United States and elsewhere now use fiber optics
exclusively. Fiber optics is also prevalent in the backbone of the Internet. However,
the high cost of optical devices-such as transmitters, receivers, and switches-has
hindered their deployment for short-haul transport, such as in a LAN or into the home in a residential access network. The Optical Carrier (OC) standard link speeds range from 51 .8 Mbps to 39.8 Gbps; these specifications are often referred to as OC-n, where the link speed equals n x 51 .8 Mbps. Standards in use today include OC-l, OC-3, OC-12, OC-24, OC-48, OC-96, OC-I92, OC-768. [IEC Optical 2009; Goralski 200 I; Ramaswami 1998; and Mukherjee 1997] provide coverage of various aspects of optical networking.
Terrestrial Radio Channels
Radio channels carry signals in the electromagnetic spectrum. They are an attractive
medium because they require no physical wire to be in stalled, can penetrate walls, provide connectivity to a mobile user, and can potentially carry a signal for long distances. The characteristics of a radio channel depend significantly on the propagation environment and the distance over which a signal is to be carried.
Environmental considerations determine path loss and shadow fading (which
decrease the signal strength as the signal travels over a distance and around/through obstructing objects), multipath fading (due to signal retlection off
of interfering objects), and interference (due to other tran smissions and electroand
magnetic signals).
Terrestrial radio channels can be broadly classified into two groups: those that
operate in local areas, typically spanning from ten to a few hundred meters; and
those that operate in the wide area, spanning tens of kilometers. The wireless LAN
technologies described in Section 1.2.2 use local-area radio channels; the cellular
access technologies use wide-area radio channels. We' ll discuss radio channels in
detail in Chapter 6.
Satellite Radio Channels
A communication satellite links two or more Earth-based microwave transmitter/ receivers, known as ground stations. The satellite receives transmissions on one frewhen
quency band, regenerates the signal using a repeater (discussed below), and transmits the signal on another frequency. Two types of satellites are used in communications: geostationary satellites and low-earth orbiting (LEO) satellites. Geostationary satellites permanently remain above the same spot on Earth. This stationary presence is achieved by placing the satellite in orbit at 36,000 kilometers above Earth's surface. This huge distance from ground station through satellite back to ground station introduces a substantial signal propagation delay of 280 millisecthe
onds. Nevertheless, satellite links, which can operate at speeds of hundreds of Mbps, are often used in areas without access to DSL or cable-based Internet access. LEO satellites are placed much closer to Earth and do not remain permanently above one spot on Earth. They rotate around Earth (just as the Moon does) and may communicate with each other, as well as with ground stations. To provide continuous coverage to an area, many satellites need to be placed in orbit. There are currently
many low-altitude communication systems in development. Lloyd's satellite
constellations Web page [Wood 2009J provides and collects information on satellite
constellation systems for communications. LEO satellite technology may be used
for Internet access sometime in the future.
1.2.2 Access Networks
Having considered the applications and end systems at the "edge of the network,"
let's next consider access networks-the physical links that connect an end system
to the first router (also known as the "edge router") on a path from the end system to
any other distant end system. Figure 1.4 shows several types of access links from
end system to edge router; the access links are highlighted in thick, shaded lines.
This section surveys many of the most common access network technologies,
roughly from low speed to high speed.
We'll soon see that many of the access technologies employ, to varying degrees,
portions of the traditional local wired telephone infrastructure. The local wired telephone
infrastructure is provided by a local telephone provider, which we will simply
refer to as the local telco. Examples of local telcos include Verizon in the United States and France Telecom in France. Each residence (household and apartment) has a
direct, twisted-pair cooper link to a nearby teleo switch, which is housed in a building
called the central, office (CO) in telephony jargon. (We will discuss twisted-pair
cooper wire later in this section.) A local teleo will typically own hundreds of COs,
and will link each of its customers to its nearest CO.
Dial-Up
Back in the 1990s, almost all residential users accessed the Internet over ordinary
analog telephone lines using a dial-up modem. Today, many users in underdeveloped
countries and in rural areas in developed countries (where broadband access is
unavailable) still access the Internet via dial-up. In fact, it is estimated that 10% of
residential users in the United States used dial-up in 2008 [Pew 20081.
The term "dial-up" is employed because the user's software actually dials an
ISP's phone number and makes a traditional phone connection with the ISP (e.g.,
with AOL). As shown in Figurc 1.5, the PC is attached to a dial-up modem, which is
in turn attached to the home's analog phone line. This analog phone line if; made of
twisted-pair copper wire and is the same telephone line used to make ordinary phone
calls. The home modem converts the digital output of the PC into an analog format
appropriate for transmission over the analog phone line. At the other end of the connection
, a modem in the ISP converts the analog signal back into digital form fOl"
input to the ISP's router.
Dial-up Internet access has two major drawbacks. First and foremost, it is
excruciatingly slow, providing a maximum rate of 56 kbps. At a 56 kbps, it takes
approximately eight minutes to download a single three-minute MP3 song and several
days to download a I Gbyte movie! Second, dial-up modem access ties up a
user's ordinary phone line-while one family member uses a dial-up modem to surf the Web, other family members cannot receive and make ordinary phone calls over
the phone line.
DSl
Today the two most prevalent types of broadband residential access are digital subscriber
line (DSL) and cable. In most developed countries today, more than 50% of
the households have broadband access, with South Korea, Iceland, Netherlands,
Denmark, and Switzerland leading the way with more
households as of 2008 [ITlF 20081. In the United States, DSL and cable have about
the same market share for broadband access [Pew 20081. Outside the United States
and Canada, DSL dominates, particularly in Europe where more than 90% of the
broadband connections are DSL in many countries.
A residence typically obtains DSL Internet access from the same company that
provides it wired local phone access (i.e., the local teleo). Thus, when DSL is used,
a cu.'tomer's teleo is al so its ISP. As shown in Figure l.6, each customer's DSL
modem nses the existing telephone line (twisted-pair copper wire) to exchange data
with a digital subscriber line access multiplexer (DSLAM), typicalJy located in the
telco's CO. The telephone line carries simultaneously both data and telephone signals,
which are encoded at different frequencies:
• A high-speed downstream channel, in the 50 kHz to I MHz band
• A medium-speed upstream channel, in the 4 kHz to 50 kHz band
• An ordinary two-way telephone channel, in the 0 to 4 kHz band
This approach makes the single DSL Iink appear as if there were three separate
links, so that a telephone call and an Internet connection can share the DSL link at
the same time. (We'll describe this technique of frequency-division multiplexing in
Section 1.3.1). On the customer side, for the signals arriving to the home, a splitter
separates the data and telephone signals and forwards the data signal to the DSL
modem. On the teleo side, in the CO, the DSLAM separates the data and phone signals
and sends the data into the Internet. Hundreds or even thousands of households
connect to a single DSLAM [Cha 2009, Dischinger 2007J.
DSL has two major advantages over dial-up Internet access. First, it can transmit
and receive data at much higher rates. Typically, DSL customer will have a transmiss
ion rate in the I to 2 Mbps range for downstream (CO to residence) and in the
128 kbps to I Mbps range for upstream. Because the downstream and upstream rates
are different, the access is said to be asymmetric. The second major advantage is that
lIsers can simultaneously talk on the phone and access the Internet. Unlike dial-up,
users do not dial an ISP phone number to get Internet access; instead, they have an
"always-on" permanent connection to the ISP's DSLAM (and hence to the Internet).
The actual downstream and upstream transmission rate available to the residence
is a function of the distance between the home and the CO, the gauge of the twistedpair
line and the degree of electrical interference. Engineers have expressly designed
DSL for short distances between the home and the CO, allowing for substantially
higher transmission rates than dial-up access. To boost the data rates. DSL rel ies on advanced signal processing and error correction algorithms, which can lead to high
packet delays. However, if the residence is not located within 5 to 10 miles of the CO, DSL signal-processing technology is no longer effective, and the resid ence must
resort to an alternative form of Internet access.
There are also a variety of higher-speed DSL technologies enjoying penetration
in a handful of countries today. For example, very-high speed DSL (VDSL), with
highest penetration today in South Korea and Japan. provides impress ive rates of 12
to 55 Mbps for downstream and 1.6 to 20 Mbps for upstream [DSL 2009].
Cable
Many resid ences in the North America and elsewhere receive hundreds of broadcast
televi sion channels over coaxial cable netwurks. (We will discuss coaxial cable later
in this section.) In a traditional cable television system, a cable head end broadcas ts
television channels through a distribution network of coaxial cable and amplifiers to
residences.
While DSL and dial-LIp make use of the teleo 's existing local telephone infrastructure,
cable Internet access makes use the cable television company's existing cable television
infrastructure. A residence obtains cable Internet access from the same
company that provides it cable television. As illustrated in Figure 1.7, fiber optics connect
the cable head cnLi to neighborhood-level junctions, from which traditional coaxial
cable is then used to reach individual houses and apartments. Each neighborhood
junction typically supports 500 to 5,000 homes. Because both fiber and coaxial cable
arc employed in this system, it is often rcfen-ed to as hybrid fiber coax (HFC).
Cable Intemet access requires special modems, called cable modems. As with
a DSL modem, the cable modem is typically an external device and connects to the
home PC through an Ethernet port. (We will discuss Ethernet in great detail in Chapter
5.) Cable modems divide the HFC network into two channels, a downstream and
an upstream channel. As with DSL, access is typically asymmetric, with the downstream
channel typically allocated at a higher transmission rate than the upstream
channel.
One important characteristic of cable Internet access is that it is a shared broadcast
medium. In particular, every packet sent by the head end travels downstream on
every link to every home; and every packet sent by a home travels on the upstream
channel to the head end. For this reason, if several users are simultaneously downloading
a video file on the downstream channel, the actual rate at which each user
receives its video file will be significantly lower than the aggregate cable downstream
rate. On the other hand, if there are only a few active users and they are all
Web surfing, then each of the users may actually receive Web pages at the fuJI cable
downstream rate, because the users will rarely request a Web page at exactly the
same time. Because the upstream channel is also shared, a distributed multipleaccess
protocol is needed to coordinate transmissions and avoid collisions. (We'll
discuss this collision issue in some detail when we discuss Ethernet in Chapter 5.)
Advocates of DSL are quick to point out that DSL is a point-to-point connection
between the home and ISP, and therefore, the entire transmission capacity of
the DSL link between the home and the ISP is dedicated rather than shared. Cable
advocates, however, argue that a reasonably dimensioned HFC network provides higher transmission rates than DSL. The battle between DSL and HFC for highspeed
residential access is raging, particularly in North America. In rural areas,
where neither DSL nor HFC is available, a satellite link can be used to connect a residence
to the Internet at speeds of more than I Mbps; StarBand and HughesNet are
two such satellite access providers.
Fiher-To-The-llome (fTTH)
Fiber optics (to be discussed in Section 1.2.3) can offer significantly higher transmission
rates than twisted-pair copper wire or coaxial cable. Some local telcos (in
many different countries), having recently laid optical fiber from their COs to
homes, now provide high-speed Internet access as well as traditional phone and television
services over the optical fibers. In the United States, Verizon has been particularly
aggressive with FITH with its FIOS service [Verizon FIOS 2009].
There are several competing technologies for optical distribution from the
CO to the homes. The simplest optical distribution network is called direct
fiber, for which there is one fiber Jeaving the CO for each home. Such distribution
can provide high bandwidth, since each customer gets its own dedicated
fiber all the way to the central office. More commonly, each fiber leaving the
central office is actually shared by many homes; it is not until the fiber gets relatively
close to the homes that it is split into individual customer-specific fibers.
There are two competing optical-distribution network architectures that perform
this splitting: active optical networks (AONs) and passive optical networ.ks
(PONs). AON is essentially switched Ethernet, which is discussed in Chapter 5.
Here we briefly discuss PON, which is used in Verizon 's FIOS service. Figure
1.8 shows FTTH using the PON distribution architecture . Each home has an
optical network terminator (ONT), which is connected by dedicated optical
fibter to a neighborhood splitter. The splitter combines a number of homes (typically
less than 100) onto a single , shared optical fiber, which connects to an
optical line terminator (OLT) in the telco's CO. The OLT, providing conversion
between optical and electrical signals, connects to the Internet via a telco rOllter.
In the home, users connect a home router (typically a wireless router) to the
ONT and access the Internet via this home router. In the PON architecture, all
packets sent from OLT to the splitter are replicated at the splitter (similar to a
cable head end) .
FTTH can potentially provide Internet access rates in the gigabits per second
range. However, most FTTH ISPs provide different rate offerings, with the higher
rates naturally costing more money. Most FTTH customers today enjoy download
rates in the 10 to 20 Mbps range and upload rates in the 2 to IO Mbps range. In addition
to Internet access, the optical fibers carry broadcast television services and traditional
phone service.
Ethernet
On corporate and university campuses, a local area network (LAN) is typically used
to connect an end system to the edge router. Although, there are many types of LAN
technologies, Ethernet is by far the most prevalent access technology in corporate
and university networks. As shown in Figure 1.9, Ethernet users use twisted-pair
copper wire to connect to an Ethernet switch, a technology discussed in detail in
Chapter S. With Ethernet access, llsers typically have 100 Mbps access, whereas
servers may can have I Gbps or even 10 Gbps access.
Wifi
Increasingly, people access the Internet wirelessly, either through a laptop computer
or from a mobile handheld device, such as an iPhone, Blackberry, or Google phone
(see earlier sidebar on A Dizzying Array of Internet End Systems). Today, there are
two common types of wireless Internet access. In a wireless LAN, wireless users
transmit/receive packets to/from an access point that in turn is connected to the
wired Internet. A wireless LAN user must typically be within a few tens of meters
of the access point. In wide-area wireless access networks, packets are transmitted
to a base station over the same wireless infrastructure used for ceUular tC'lephony.
In this case, the base station is managed by the cellular network provider and a user
must typically be within a few tens of kilometers of the base station.
Wireless LAN access based on IEEE 802. 11 technology, that is WiFi, is now
just about everywhere- universi ties, business offices, cafes, airports, homes, and
even in airplanes. Most universities have installed IEEE 802.11 base stations
across their entire campus, allowing students to send and receive e-mail or surf
the Web from anywhere on campus. In many cities, one can stand on a street corner
and be within range of ten or twenty base stations (for a browseable global
map of 802.11 base stations that have been discovered and logged on a Web site
by people who take great enjoyment in doing such things, see [wigle. net 2009J).
As discussed in detail in Chapter 6, 802. I I today provides a shared tran smission
rate of up to 54 Mbps.
Many homes combine broadband residential access (that is, cable modems or
DSL) with inexpensive wireless LAN technology to create powerful home networks.
Figure 1.10 shows a schematic of a typical home network. This home
network consists of a roaming laptop as well as a wired PC; a base station (the wireless
access point), which communicates with the wireless PC; a cable modem, providing
broadband access to the Internet; and a router, which interconnects the base
station and the stationary PC with the cable modem. This network allows household
members to have broadband access to the Internet with one member roaming from
the kitchen to the backyard to the bedrooms.
Wide-Area Wireless Access
When you access the Internet through wireless LAN technology, you typically need
to be within a few tens of meters of the access point. This is feasible for home
access, coffee shop access, and more generally, access within and around a building.
But what if you are on the beach. on a bus, or in your car, and you need Internet
access? For such wide-area access, roaming Internet users make use of the cellular
phone infrastructure, accessing base stations that are up to tens of kilometers away.
Telecommunications companies have made enormous investments in so-called
third generation (3G) wireless, which provides packet-switched wide-area wireless Internet access at speeds in excess of ] Mbps . Today millions or users are using
these networks to read and send email, surf the Web, and download music while on
the run.
WiMAX
As always, there is a potential "killer" technology waiting to dethrone these standards.
WiMAX [Intel WiMAX 2009, WiMAX Forum 2009], also known as IEEE
802.16, is a long-distance cousin of the 802.11 WiFi protocol discus sed above .
WiMAX operates independently of the cellular network and promises speeds of 5 to
10 Mbps or higher over distances of tens of kilometers. Sprint-Nextel has committed
billions of dollars towards deploying WiMAX in 2007 and beyond. We'll cover
WiFi, Wi MAX, and 3G in detail in Chapter 6.
let's next consider access networks-the physical links that connect an end system
to the first router (also known as the "edge router") on a path from the end system to
any other distant end system. Figure 1.4 shows several types of access links from
end system to edge router; the access links are highlighted in thick, shaded lines.
This section surveys many of the most common access network technologies,
roughly from low speed to high speed.
We'll soon see that many of the access technologies employ, to varying degrees,
portions of the traditional local wired telephone infrastructure. The local wired telephone
infrastructure is provided by a local telephone provider, which we will simply
refer to as the local telco. Examples of local telcos include Verizon in the United States and France Telecom in France. Each residence (household and apartment) has a
direct, twisted-pair cooper link to a nearby teleo switch, which is housed in a building
called the central, office (CO) in telephony jargon. (We will discuss twisted-pair
cooper wire later in this section.) A local teleo will typically own hundreds of COs,
and will link each of its customers to its nearest CO.
Dial-Up
Back in the 1990s, almost all residential users accessed the Internet over ordinary
analog telephone lines using a dial-up modem. Today, many users in underdeveloped
countries and in rural areas in developed countries (where broadband access is
unavailable) still access the Internet via dial-up. In fact, it is estimated that 10% of
residential users in the United States used dial-up in 2008 [Pew 20081.
The term "dial-up" is employed because the user's software actually dials an
ISP's phone number and makes a traditional phone connection with the ISP (e.g.,
with AOL). As shown in Figurc 1.5, the PC is attached to a dial-up modem, which is
in turn attached to the home's analog phone line. This analog phone line if; made of
twisted-pair copper wire and is the same telephone line used to make ordinary phone
calls. The home modem converts the digital output of the PC into an analog format
appropriate for transmission over the analog phone line. At the other end of the connection
, a modem in the ISP converts the analog signal back into digital form fOl"
input to the ISP's router.
Dial-up Internet access has two major drawbacks. First and foremost, it is
excruciatingly slow, providing a maximum rate of 56 kbps. At a 56 kbps, it takes
approximately eight minutes to download a single three-minute MP3 song and several
days to download a I Gbyte movie! Second, dial-up modem access ties up a
user's ordinary phone line-while one family member uses a dial-up modem to surf the Web, other family members cannot receive and make ordinary phone calls over
the phone line.
DSl
Today the two most prevalent types of broadband residential access are digital subscriber
line (DSL) and cable. In most developed countries today, more than 50% of
the households have broadband access, with South Korea, Iceland, Netherlands,
Denmark, and Switzerland leading the way with more
households as of 2008 [ITlF 20081. In the United States, DSL and cable have about
the same market share for broadband access [Pew 20081. Outside the United States
and Canada, DSL dominates, particularly in Europe where more than 90% of the
broadband connections are DSL in many countries.
A residence typically obtains DSL Internet access from the same company that
provides it wired local phone access (i.e., the local teleo). Thus, when DSL is used,
a cu.'tomer's teleo is al so its ISP. As shown in Figure l.6, each customer's DSL
modem nses the existing telephone line (twisted-pair copper wire) to exchange data
with a digital subscriber line access multiplexer (DSLAM), typicalJy located in the
telco's CO. The telephone line carries simultaneously both data and telephone signals,
which are encoded at different frequencies:
• A high-speed downstream channel, in the 50 kHz to I MHz band
• A medium-speed upstream channel, in the 4 kHz to 50 kHz band
• An ordinary two-way telephone channel, in the 0 to 4 kHz band
This approach makes the single DSL Iink appear as if there were three separate
links, so that a telephone call and an Internet connection can share the DSL link at
the same time. (We'll describe this technique of frequency-division multiplexing in
Section 1.3.1). On the customer side, for the signals arriving to the home, a splitter
separates the data and telephone signals and forwards the data signal to the DSL
modem. On the teleo side, in the CO, the DSLAM separates the data and phone signals
and sends the data into the Internet. Hundreds or even thousands of households
connect to a single DSLAM [Cha 2009, Dischinger 2007J.
DSL has two major advantages over dial-up Internet access. First, it can transmit
and receive data at much higher rates. Typically, DSL customer will have a transmiss
ion rate in the I to 2 Mbps range for downstream (CO to residence) and in the
128 kbps to I Mbps range for upstream. Because the downstream and upstream rates
are different, the access is said to be asymmetric. The second major advantage is that
lIsers can simultaneously talk on the phone and access the Internet. Unlike dial-up,
users do not dial an ISP phone number to get Internet access; instead, they have an
"always-on" permanent connection to the ISP's DSLAM (and hence to the Internet).
The actual downstream and upstream transmission rate available to the residence
is a function of the distance between the home and the CO, the gauge of the twistedpair
line and the degree of electrical interference. Engineers have expressly designed
DSL for short distances between the home and the CO, allowing for substantially
higher transmission rates than dial-up access. To boost the data rates. DSL rel ies on advanced signal processing and error correction algorithms, which can lead to high
packet delays. However, if the residence is not located within 5 to 10 miles of the CO, DSL signal-processing technology is no longer effective, and the resid ence must
resort to an alternative form of Internet access.
There are also a variety of higher-speed DSL technologies enjoying penetration
in a handful of countries today. For example, very-high speed DSL (VDSL), with
highest penetration today in South Korea and Japan. provides impress ive rates of 12
to 55 Mbps for downstream and 1.6 to 20 Mbps for upstream [DSL 2009].
Cable
Many resid ences in the North America and elsewhere receive hundreds of broadcast
televi sion channels over coaxial cable netwurks. (We will discuss coaxial cable later
in this section.) In a traditional cable television system, a cable head end broadcas ts
television channels through a distribution network of coaxial cable and amplifiers to
residences.
While DSL and dial-LIp make use of the teleo 's existing local telephone infrastructure,
cable Internet access makes use the cable television company's existing cable television
infrastructure. A residence obtains cable Internet access from the same
company that provides it cable television. As illustrated in Figure 1.7, fiber optics connect
the cable head cnLi to neighborhood-level junctions, from which traditional coaxial
cable is then used to reach individual houses and apartments. Each neighborhood
junction typically supports 500 to 5,000 homes. Because both fiber and coaxial cable
arc employed in this system, it is often rcfen-ed to as hybrid fiber coax (HFC).
Cable Intemet access requires special modems, called cable modems. As with
a DSL modem, the cable modem is typically an external device and connects to the
home PC through an Ethernet port. (We will discuss Ethernet in great detail in Chapter
5.) Cable modems divide the HFC network into two channels, a downstream and
an upstream channel. As with DSL, access is typically asymmetric, with the downstream
channel typically allocated at a higher transmission rate than the upstream
channel.
One important characteristic of cable Internet access is that it is a shared broadcast
medium. In particular, every packet sent by the head end travels downstream on
every link to every home; and every packet sent by a home travels on the upstream
channel to the head end. For this reason, if several users are simultaneously downloading
a video file on the downstream channel, the actual rate at which each user
receives its video file will be significantly lower than the aggregate cable downstream
rate. On the other hand, if there are only a few active users and they are all
Web surfing, then each of the users may actually receive Web pages at the fuJI cable
downstream rate, because the users will rarely request a Web page at exactly the
same time. Because the upstream channel is also shared, a distributed multipleaccess
protocol is needed to coordinate transmissions and avoid collisions. (We'll
discuss this collision issue in some detail when we discuss Ethernet in Chapter 5.)
Advocates of DSL are quick to point out that DSL is a point-to-point connection
between the home and ISP, and therefore, the entire transmission capacity of
the DSL link between the home and the ISP is dedicated rather than shared. Cable
advocates, however, argue that a reasonably dimensioned HFC network provides higher transmission rates than DSL. The battle between DSL and HFC for highspeed
residential access is raging, particularly in North America. In rural areas,
where neither DSL nor HFC is available, a satellite link can be used to connect a residence
to the Internet at speeds of more than I Mbps; StarBand and HughesNet are
two such satellite access providers.
Fiher-To-The-llome (fTTH)
Fiber optics (to be discussed in Section 1.2.3) can offer significantly higher transmission
rates than twisted-pair copper wire or coaxial cable. Some local telcos (in
many different countries), having recently laid optical fiber from their COs to
homes, now provide high-speed Internet access as well as traditional phone and television
services over the optical fibers. In the United States, Verizon has been particularly
aggressive with FITH with its FIOS service [Verizon FIOS 2009].
There are several competing technologies for optical distribution from the
CO to the homes. The simplest optical distribution network is called direct
fiber, for which there is one fiber Jeaving the CO for each home. Such distribution
can provide high bandwidth, since each customer gets its own dedicated
fiber all the way to the central office. More commonly, each fiber leaving the
central office is actually shared by many homes; it is not until the fiber gets relatively
close to the homes that it is split into individual customer-specific fibers.
There are two competing optical-distribution network architectures that perform
this splitting: active optical networks (AONs) and passive optical networ.ks
(PONs). AON is essentially switched Ethernet, which is discussed in Chapter 5.
Here we briefly discuss PON, which is used in Verizon 's FIOS service. Figure
1.8 shows FTTH using the PON distribution architecture . Each home has an
optical network terminator (ONT), which is connected by dedicated optical
fibter to a neighborhood splitter. The splitter combines a number of homes (typically
less than 100) onto a single , shared optical fiber, which connects to an
optical line terminator (OLT) in the telco's CO. The OLT, providing conversion
between optical and electrical signals, connects to the Internet via a telco rOllter.
In the home, users connect a home router (typically a wireless router) to the
ONT and access the Internet via this home router. In the PON architecture, all
packets sent from OLT to the splitter are replicated at the splitter (similar to a
cable head end) .
FTTH can potentially provide Internet access rates in the gigabits per second
range. However, most FTTH ISPs provide different rate offerings, with the higher
rates naturally costing more money. Most FTTH customers today enjoy download
rates in the 10 to 20 Mbps range and upload rates in the 2 to IO Mbps range. In addition
to Internet access, the optical fibers carry broadcast television services and traditional
phone service.
Ethernet
On corporate and university campuses, a local area network (LAN) is typically used
to connect an end system to the edge router. Although, there are many types of LAN
technologies, Ethernet is by far the most prevalent access technology in corporate
and university networks. As shown in Figure 1.9, Ethernet users use twisted-pair
copper wire to connect to an Ethernet switch, a technology discussed in detail in
Chapter S. With Ethernet access, llsers typically have 100 Mbps access, whereas
servers may can have I Gbps or even 10 Gbps access.
Wifi
Increasingly, people access the Internet wirelessly, either through a laptop computer
or from a mobile handheld device, such as an iPhone, Blackberry, or Google phone
(see earlier sidebar on A Dizzying Array of Internet End Systems). Today, there are
two common types of wireless Internet access. In a wireless LAN, wireless users
transmit/receive packets to/from an access point that in turn is connected to the
wired Internet. A wireless LAN user must typically be within a few tens of meters
of the access point. In wide-area wireless access networks, packets are transmitted
to a base station over the same wireless infrastructure used for ceUular tC'lephony.
In this case, the base station is managed by the cellular network provider and a user
must typically be within a few tens of kilometers of the base station.
Wireless LAN access based on IEEE 802. 11 technology, that is WiFi, is now
just about everywhere- universi ties, business offices, cafes, airports, homes, and
even in airplanes. Most universities have installed IEEE 802.11 base stations
across their entire campus, allowing students to send and receive e-mail or surf
the Web from anywhere on campus. In many cities, one can stand on a street corner
and be within range of ten or twenty base stations (for a browseable global
map of 802.11 base stations that have been discovered and logged on a Web site
by people who take great enjoyment in doing such things, see [wigle. net 2009J).
As discussed in detail in Chapter 6, 802. I I today provides a shared tran smission
rate of up to 54 Mbps.
Many homes combine broadband residential access (that is, cable modems or
DSL) with inexpensive wireless LAN technology to create powerful home networks.
Figure 1.10 shows a schematic of a typical home network. This home
network consists of a roaming laptop as well as a wired PC; a base station (the wireless
access point), which communicates with the wireless PC; a cable modem, providing
broadband access to the Internet; and a router, which interconnects the base
station and the stationary PC with the cable modem. This network allows household
members to have broadband access to the Internet with one member roaming from
the kitchen to the backyard to the bedrooms.
Wide-Area Wireless Access
When you access the Internet through wireless LAN technology, you typically need
to be within a few tens of meters of the access point. This is feasible for home
access, coffee shop access, and more generally, access within and around a building.
But what if you are on the beach. on a bus, or in your car, and you need Internet
access? For such wide-area access, roaming Internet users make use of the cellular
phone infrastructure, accessing base stations that are up to tens of kilometers away.
Telecommunications companies have made enormous investments in so-called
third generation (3G) wireless, which provides packet-switched wide-area wireless Internet access at speeds in excess of ] Mbps . Today millions or users are using
these networks to read and send email, surf the Web, and download music while on
the run.
WiMAX
As always, there is a potential "killer" technology waiting to dethrone these standards.
WiMAX [Intel WiMAX 2009, WiMAX Forum 2009], also known as IEEE
802.16, is a long-distance cousin of the 802.11 WiFi protocol discus sed above .
WiMAX operates independently of the cellular network and promises speeds of 5 to
10 Mbps or higher over distances of tens of kilometers. Sprint-Nextel has committed
billions of dollars towards deploying WiMAX in 2007 and beyond. We'll cover
WiFi, Wi MAX, and 3G in detail in Chapter 6.
1.2.1 Client and Server Programs
In the context of networking software, there is another definition of a client and
server, a definition that we'll refer to throughout this book. A client program is a
program running on one end system that requests and recei ves a service from a
server program running on another end system. The Web, e-mail, file transfer,
remote login, newsgroups, and many other popular applications adopt the clientserver
model. Since a client program typically runs on one computer and the server
program runs on another computer, client-server Internet applications are, by definition,
distributed applications. The client program and the server program
interact by sending each other messages over the Internet. At this level of abstraction,
the routers, links, and other nuts and bolts of the Internet serve collectively as
a black box that transfers messages between the distributed, communicating
components of an Internet application . This is the level of abstraction depicted in
Figure 1.3.
Not all Internet applications today consist of pure client programs interacting
with pure server programs. Increasingly, many applications are peer-to-peer (P2P)
applications, in which end systems interact and run programs that perform both
client and server functions. For example, in P2P file- sharing applications (such as
BitTorrent and eMuIe), the program in the user's end system acts as a client when it
requests a file from another peer; and the program acts as a server when it sends a
file to another peer. In Internet telephony, the two communicating parties interact as
peers-the communication session is symmetric, with both parties sending and
receiving data. We'll compare and contrast client-server and P2P architectures in
detail in Chapter 2.
server, a definition that we'll refer to throughout this book. A client program is a
program running on one end system that requests and recei ves a service from a
server program running on another end system. The Web, e-mail, file transfer,
remote login, newsgroups, and many other popular applications adopt the clientserver
model. Since a client program typically runs on one computer and the server
program runs on another computer, client-server Internet applications are, by definition,
distributed applications. The client program and the server program
interact by sending each other messages over the Internet. At this level of abstraction,
the routers, links, and other nuts and bolts of the Internet serve collectively as
a black box that transfers messages between the distributed, communicating
components of an Internet application . This is the level of abstraction depicted in
Figure 1.3.
Not all Internet applications today consist of pure client programs interacting
with pure server programs. Increasingly, many applications are peer-to-peer (P2P)
applications, in which end systems interact and run programs that perform both
client and server functions. For example, in P2P file- sharing applications (such as
BitTorrent and eMuIe), the program in the user's end system acts as a client when it
requests a file from another peer; and the program acts as a server when it sends a
file to another peer. In Internet telephony, the two communicating parties interact as
peers-the communication session is symmetric, with both parties sending and
receiving data. We'll compare and contrast client-server and P2P architectures in
detail in Chapter 2.
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