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.