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.