9.1 Transmission Media
In a geographically close environment, computers can be networked with private cables in a number of configurations. For geographically distant systems, the public switched telephone network (PSTN) can be used. Users connect to the PSTN with modems that convert logical bits into audible sounds. People can hear at frequencies up to about 20 KHz, but only speak at frequencies up to about 4 KHz, which is approximately the bandwidth that traditional telephony will pass on a voice-grade line. An analog signal (such as voice) that is approximated with a digital signal needs to be sampled at least twice per cycle (to capture the high and low values), and so a sampling rate of 8 KHz is needed to digitize a voice-grade line. At 8 bits per sample, that gives a bit rate of 8 bits/cycle ´ 8 KHz = 64 Kbits/sec which is what is available on an ordinary telephone line in North America. One sample out of every 8 is used by the telephone company to administer the line, and so the maximum bit rate possible on a voice-grade line is 56 Kbits/sec.
A transmitted binary sequence is converted into high/low values, but the wave- form gets attenuated and distorted, more so at high frequencies and long distances. Figure 9-4 illustrates the sampling problem. The binary pattern
01011001 is represented by an ideal wave, which is only approximated by a transmitted wave. The ideal wave contains discontinuities, which are difficult to produce with a real wave. In terms of analysis, we can think of the ideal wave as being approximated with a superposition of sinusoidal waves, with sharper edges achieved at higher frequencies.
Unfortunately, high frequencies are attenuated more greatly than low frequencies in most media, and different frequencies propagate at different rates, which leads to distortions of the wave as it propagates. The degree of distortion varies with the transmission medium, several of which are described here.
9.2.1 TWO-WIRE OPEN LINES
In one of the simplest scenarios, a pair of wires, open to free space, carries a signal and a return (the “ground”). The two-wire open line configuration is shown in Figure 9-5a. The lines emit electromagnetic radiation, and they also pick up
noise, not necessarily the same amount of noise for each line, which distorts the difference signal. The lines are also vulnerable to “capacitive coupling” which means they pick up unwanted signals from neighboring wires. The speed and distance for reliable transmission is limited to about 19.2 Kbps and 50 m.
9.2.2 TWISTED-PAIR LINES
If we twist the pair of lines in the two-wire open line configuration, then any spurious external noise that is introduced to the line affects both the signal and ground (reference) wires in the same way. Figure 9-5b shows the twisted-wire configuration. The difference signal is thus unaffected, and reliable transmission can go up to 1 Mbps over 100 m.
9.2.3 COAXIAL CABLE
For higher speeds (10 Mbps) and longer distances (hundreds of meters), the sig- nal wire is placed inside of the reference conductor (coaxially) with an insulator between the two, as shown in Figure 9-5c. (The braiding of the outer conductor makes the cable more flexible.) The idea is that the center conductor is effectively shielded from external interference, and is also shielded from losses from electro- magnetic radiation.
9.2.4 OPTICAL FIBER
Optical communication is immune to electromagnetic interference and crosstalk, and supports a much wider bandwidth. There is a need for optoelectronic conversions on each end, which is commercially available up to a few Gbps (using laser diodes.) Optical fiber consists of the optical core, optical clad- ding, and a plastic coating as shown in Figure 9-5d.
A light emitting diode (LED) is a less expensive light source than a laser diode, but it emits light at various angles, and so a multimode stepped index fiber is used that reflects light less than the critical angle back into the core. Because the path lengths differ, the received pulse is wider, and only modest bit rates can be supported. The LEDs are inexpensive, however, and bending tolerances are a less significant issue than for laser diodes.
With a multimode graded index fiber, light is refracted more greatly as it moves away from the core, which narrows the pulse and reduces losses.
Single mode (monomode) fiber reduces the core diameter to a single wavelength so that light travels along a single dispersionless path. Laser diodes are commonly used as sources for single mode fiber, and can operate up to several Gbps over tens of kilometers.
9.2.5 SATELLITES
Manmade satellites that are launched into orbit around the Earth are used for communication when a broad area of coverage is needed at a lower cost than a wireline network (including optical fibers). Natural satellites, inside and outside of Earth orbits (such as the Moon and asteroids), can also be used for communication, but are not generally in use for such purposes.
In satellite communication, a collimated microwave beam is transmitted from the ground to a satellite, where a transponder that covers a certain band of frequencies retransmits the signal to an area of coverage on the Earth. The satellite configuration is shown in Figure 9-5e.
A typical satellite has several transponders at 500 MHz per channel. A small area of coverage means that the transmitted signal is stronger and the receiving dishes can be smaller. This is typical for direct broadcast satellite (DBS) television, in which very small receiving dishes are used. The DBS satellites orbit the Earth at a low orbit, approximately 700 Km, and so a smaller collecting area is needed than for satellites that are placed in geosynchronous orbit (23,000 miles above the surface of the Earth), where the Earth’s attractive gravitational force and the repel- ling centrifugal force are balanced, so that the satellite appears stationary over the ground when the orbit collocates with the Equator. This is why large satellite dishes are aimed in the direction of the Equator.
For two-way satellite network communication, the delay between the end-user and the satellite needs to be tolerable. The uplink to the satellite is generally slower than the downlink. This matches the typical mode of operation for an end-user on the Internet, since less than 10% of the network traffic goes from the end-user to the Internet, and over 90% of the network traffic goes from the Internet to the end-user. The speed of communication is limited by c (the speed of light in a vacuum) which is approximately 1 ns per foot, or 5 µs per mile (5280 feet per mile). Over a distance of 23,000 miles, the free space delay is more than 100 ms to the satellite and another 100 ms back to the Earth, plus a processing delay. This is more than the acceptable average delay of 100 ms needed for a keystroke response. Low Earth orbit (LEO) is only 700 Km, and introduces a much smaller delay, on the order of spanning the distance of a few states in the United States, and is therefore better suited for interactive networking.
9.2.6 TERRESTRIAL MICROWAVE
Ground based line-of-sight links are effective up to 50 Km, particularly for crossing difficult terrain, although they are prone to atmospheric disturbances, flocks of geese, etc.
9.2.7 RADIO
In cellular radio communication, a radio base station is placed in the middle of a cell, which is generally less than 20 Km in diameter. A restricted band of frequencies is used within a cell, for communication between roaming cellular devices and the base station. Neighboring cells use a different band of frequencies, so that there is no confusion at the cell boundary where a handoff is made as a roaming end-user transits from one cell to another, which normally involves a frequency change.
Total available bandwidth in a cell is small, on the order of 2 MB/s, which is sub- divided over the number of channels in use. In congested areas, cell sizes are smaller than in less densely populated areas, sometimes extending no farther than a single building.