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 fibre cable and coaxial cable. We said that DSL and Ethernet use copper wire. And we said that mobile access networks use the radio spectrum.
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 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 therefore 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 electromagnetic 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 physical media include twisted-pair copper wire, coaxial cable, multimode fibre-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 social medium, such as fibre-optic cable, a twisted-pair copper wire, or a coaxial cable. With unguided media, the waves propagate in the atmosphere and in outer space, such as in a wireless LAN or a digital satellite channel.
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, fibre-optic cable, and so on) is often relatively minor compared with another networking costs. In particular, the labor cost associated with the installation of the physical link can of orders of magnitude higher than the cost of the material. For this reason, many builders install twisted pair , optical fibre, and coaxial cable in every room in a building. Even if only one medium is initially used, there is a good change that another medium could be used in the near future, and so money is saved by not having to lay additional wires in the future.
Twisted-Pair Copper Wire
The least expensive and most commonly used guided transmission medium is twisted-pair copper wire. For over a hundred years it has been used by telephone networks. In fact, more than 99 percent of the wired connected from telephone handset to the local telephone switch use twisted-pair copper wire. Most of us have seen twisted pair in our homes and work environments. Twisted pair consists of two insulated coper wires, each about 1 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 10 Gbps. The data rates that can be achieved depend on the thickness of the wire and the distance between transmitter and receiver.
When fibre-optic technology emerged in 1980s, many people disparaged twisted pair because of its relatively low bit rates. Some people even felt that fibre optic technology would completely replace twisted pair. But twisted pair did not give up easily. Modern twisted-pair technology , such as category 6a cable, can achieve data rates of 10 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 rates of up to 56kbps over twisted pair. We also saw that DSL (digital subscriber line) technology has enabled residential users to access internet at tens of Mbps over twisted pair (when users live close to the ISP’s modem).
Like twisted pair, coaxial cable consists of two copper conductors but the two conductor are concentric rather than parallel. With this construction and special insulation and shielding, coaxial cable can achieve high data transmission 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 tens of Mbps. 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.
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 upto 100 kilometers, and are very hard to tap. These characteristics have made fiber optics the preferred long-haul guided transmission media, particularly for overseas links. Many of the long-distance 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 nx51.8 Mbps. Standards in use today include OC-1, OC-3, OC-12, OC-24, OC-48. OC-96, OC-192, OC-768
Terrestrial Radio Channels
Radio channels carry signals in the electromagnetic spectrum. They are an attractive medium because they require no physical wire to be installed, 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 carries. 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 reflection off of interfering objects), and interference (due to other transmissions and electromagnetic signals.)
Terrestrial radio channels can be broadly classified unto three groups: those that operate over very short distance (e.g. with one or two meters); those that operate in local areas, typically spanning from 10 to a few hundred meters; and those that operate in the wide area, spanning tens of kilometres. Personal devices such as wireless handsets, keyboards, and medical devices operate over short distances; the wireless LAN technologies use local-area radio channels; the cellular access technologies use wide-area radio channels.
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 frequency band, regenerates the signal using a repeater, and transmits the signal on another frequency. Two types of satellites are used in communications: geostationary satellites and low-earth orbiting (LEO) satellites.
LEO satellites are places 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 provides and collects information on satellite constellation systems for communications. LEO satellite technology may be used for internet access sometime in the future.