The second transmission medium to be introduced was coaxial cable (often called coax), which began being deployed in telephony networks around the mid-1920s. Figure 2.2 shows the components of coax. In the center of a coaxial cable is a copper wire that acts as the conductor, where the information travels. The copper wire in coax is thicker than that in twisted-pair, and it is also unaffected by surrounding wires that contribute to EMI, so it can provide a higher transmission rate than twisted-pair. The center conductor is surrounded by plastic insulation, which helps filter out extraneous interference. The insulation is covered by the return path, which is usually braided-copper shielding or aluminum foil–type covering. Outer jackets form a protective covering for coax; the number and type of outer jackets depend on the intended use of the cable (e.g., whether the cable is meant to be strung in the air or underground, whether rodent protection is required).
Figure 2.2 Coaxial cable
Characteristics of Coaxial Cable
Coax affords a great deal more frequency spectrum than does twisted-pair. Traditional coaxial cable television networks generally support 370MHz. Newer deployments, such as hybrid fiber coax (HFC) architectures, support 750MHz or 1,000MHz systems. (HFC is discussed in detail in Chapter 12.) Therefore, coax provides from 370 to 1,000 times more capacity than single twisted-pair. With this additional capacity, you can carve out individual channels, which makes coax a broadband facility. Multiplexing techniques can be applied to coax to derive multiple channels. Coax offers slightly better performance than twisted-pair because the metallic shielding protects the center conductor from outside interferences; hence the performance of coax is on the order of 10–9 (i.e., 1 in 1 billion) bps received in error. Amplifiers need to be spaced every 1.5 miles (2.5 km), which is another improvement over twisted-pair, but it still means a substantial number of amplifiers must be deployed throughout the network.
Cable TV operators, like the telephony network providers, have been prevalent users of coax, but in the past decade, they have been reengineering their backbone networks so that they are fiber based, thereby eliminating many amplifiers and subsequently improving performance. Remember from Chapter 1, "Telecommunications Technology Fundamentals," that amplifiers accumulate noise over a distance. In a large franchise area for a cable TV operator, toward the outer fringes, greater noise accumulates and a lower service level is provided there than to some of the users upstream. By reengineering their backbones to be fiber based, cable providers can also limit how many amplifiers have to be deployed, along with reaping other advantages, such as high bandwidth, immunity to electrical interference, and improved security.
One problem with coax has to do with the deployment architecture. Coaxial cable and HFC architectures are deployed in bus topologies. In a bus topology, the bandwidth is shared, which means congestion levels increase as more users in the neighborhood avail themselves of these features and services. A bus topology also presents security risks. It's sort of like going back to the party line in telephony. You do not have your own dedicated twisted-pair that's yours and only yours. Instead, several channels devoted to voice telephony are shared by everyone in the neighborhood, which makes encryption important. Also, there are some problems with noise in bus topologies. The points where the coax connects into set-top boxes or cable-ready TV sets tend to collect noise, so the cable tends to pick up extraneous noise from vacuum cleaners or hair dryers or passing motorcycles. Thus, if every household on the network is running a hair dryer at 6:30 AM, the upstream paths are subjected to this noise, resulting in some performance degradation. (Bus topologies are discussed in more detail in Chapter 6, "Local Area Networking.")
Applications of Coaxial Cable
In the mid-1920s, coax was applied to telephony networks as interoffice trunks. Rather than having to add more copper cable bundles with 1,500 or 3,000 pairs of copper wires in them, it was possible to replace those big cables (which are very difficult to install cost-effectively) with a much smaller coaxial cable.
The next major use of coax in telecommunications occurred in the 1950s, when it was deployed as submarine cable to carry international traffic. It was then introduced into the data-processing realm in the mid- to late 1960s. Early computer architectures required coax as the media type from the terminal to the host. LANs were predominantly based on coax from 1980 to about 1987.
Coax has been used in cable TV and in the local loop, in the form of HFC architectures. HFC brings fiber as close as possible to the neighborhood; then on a neighborhood node, it terminates that fiber, and from that node it fans the coax out to the home service by that particular node. (This is described in detail in Chapter 12.)
Advantages and Disadvantages of Coaxial Cable
The advantages of coax include the following:
- Broadband system—Coax has a sufficient frequency range to support multiple channels, which allows for much greater throughput.
- Greater channel capacity—Each of the multiple channels offers substantial capacity. The capacity depends on where you are in the world. In the North American system, each channel in the cable TV system is 6MHz wide, according to the National Television Systems Committee (NTSC) standard. In Europe, with the Phase Alternate Line (PAL) standard, the channels are 8MHz wide. Within one of these channels, you can provision high-speed Internet access—that's how cable modems operate. But that one channel is now being shared by everyone using that coax from that neighborhood node, which can range from 200 to 2,000 homes.
- Greater bandwidth—Compared to twisted-pair, coax provides greater bandwidth systemwide, and it also offers greater bandwidth for each channel. Because it has greater bandwidth per channel, it supports a mixed range of services. Voice, data, and even video and multimedia can benefit from the enhanced capacity.
- Lower error rates—Because the inner conductor is in a Faraday shield, noise immunity is improved, and coax has lower error rates and therefore slightly better performance than twisted-pair. The error rate is generally 10–9 (i.e., 1 in 1 billion) bps.
- Greater spacing between amplifiers—Coax's cable shielding reduces noise and crosstalk, which means amplifiers can be spaced farther apart than with twisted-pair.
The main disadvantages of coax are as follows:
- Problems with the deployment architecture—The bus topology in which coax is deployed is susceptible to congestion, noise, and security risks.
- Bidirectional upgrade required—In countries that have a history of cable TV, the cable systems were designed for broadcasting, not for interactive communications. Before they can offer to the subscriber any form of two-way services, those networks have to be upgraded to bidirectional systems.
- Great noise—The return path has some noise problems, and the end equipment requires added intelligence to take care of error control.
- High installation costs—Installation costs in the local environment are high.
- Susceptible to damage from lightning strikes—Coax may be damaged by lightning strikes. People who live in an area with a lot of lightning strikes must be wary because if that lightning is conducted by a coax, it could very well fry the equipment at the end of it.