This chapter discusses circuits, the actual lines and methods used to establish communications. The objectives important to this chapter are:
Concepts:The chapter begins with more vocabulary: a circuit is defined as "the path over which communications take place", on page 314. A line is compared to a circuit, and we are told the words mean about the same thing. A circuit is usually called a line if it is a physical path, like a cable. A link is a part of a circuit, for example the cable from computer at work to its network jack. It may take several links to make a circuit. A channel is a circuit that flows one direction, like a television channel. (That is why it is called a channel.) A node can be several things: a computer (or other device) in a network, a connector in a circuit, or an end of a circuit.
Circuits are usually divided into two types: point-to-point an multipoint (also called multidrop). A point-to-point circuit connects two devices, like a cable that connects a printer directly to a personal computer. A multipoint circuit connects several devices together that can communicate, like the telephone system.
A distinction is made on page 315 between two-wire and four-wire circuits. It takes two wires, in most cases, to create a flow of electricity. Two-wire circuits are usually half-duplex. Four wires imply two possible circuits, so a four-wire circuit is often full duplex. Four-wire circuits cost more, so they are less common.
Analog and digital circuits were discussed in the last chapter. A better illustration of why we prefer digital circuits is on page 317. Note the action of the amplifier in the analog circuit. It can make a weak signal stronger, but it does not correct the distortion of the signal. In the illustration of the digital circuit, however, the repeater is capable of interpreting the distorted signal and make a best guess about the original signal. The repeater then recreates the original signal (as best it can) and sends it on.
On page 318, you see an illustration of ISDN circuits. In some markets, these are available to commercial and residential customers. This is actually a bundling of several circuits that the customer may use all or part of, depending on what they lease from the provider. The line speeds are measured in kilobits per second. The ISDN Primary Access service is comparable to a T-1 line.
At Baker, we have a T-1 line to the Internet. T-carrier service is discussed on page 320. Note that the line speeds are measured in megabits per second. A T-1 line runs at 1.544 Mbps. The average business customer may not need this much capacity, so Fractional T-1 service was devised. This is essentially a shared T-1 line or shared ADSL line, where the customer pays for capacity in increments of 64 Kbps.
A more likely product for home and business use is ADSL service, described on page 322. This service can beat T-1 speed, reaching 6 Mbps one direction and 640 Kbps going the other way. When you consider that most of your time surfing the Internet is spent downloading, it makes sense to put most of the bandwidth on that line.
The most common medium used for circuits is described on page 323. Wire is used in most circuits, in one form or another. The most common form for voice or data is
UTP, Unshielded Twisted Pair. It is called unshielded because
it does not have a foil sheath inside the insulator, such as you find
in coaxial cable. A UTP cable often has four pairs of wires
in it. Each pair forms a circuit. The two wires in a pair are twisted
together to minimize crosstalk, the leakage of signal from one
pair of wires to another pair. Cable of this type is classified as Category
1, 2, 3, 4 or 5.
Coaxial cable is described on page 326. Most people are familiar with "coax" from cable television. It has a central wire, surrounded by an insulating foam, which is surrounded by a conducting sheath and finally an insulating outer sheath. Coax has a high capacity, up but is not commonly used in networks for capacity over 10 Mbps.
Fiber optic, or optical fiber cable uses light instead of electrical pulses. It is a glass or plastic cable, called a core. The core is surrounded by a reflective layer called cladding, which acts to reflect light back toward the center of the core. The light carried on fiber optic cable can be from an LED or a laser, and it flows only one direction, so two cables are needed for a circuit. Two kinds of fiber optic cable are described on page 328: single mode and multimode. Multimode cable can carry more simultaneous signals, but single mode cable can be run farther between repeaters. The actual core used is often thinner than a human hair, so many cables can be bundled together and run in the same space as one of the other types of cables.
Some data circuits are not cabled. On page 334, the text describes microwave radio as the transmission medium. This medium is only useful for line of sight transmission (the sender and receiver have to be able to see each other), but it has the advantage of working where you cannot run a cable, such as over water or through a city.
An extension of this idea is the use of communication satellites, first proposed by the writer Arthur C. Clarke. A satellite that orbits the Earth at an altitude of 22,300 miles is in geosynchronous orbit. It seems to stay in the same place in the sky all the time, when seen from the ground. This means that any two points on the ground that can see the same satellite can use it to communicate. On page 337, the author discusses propagation delay. This is the time it takes for a signal to go from one end of a circuit to the other. In an electrical cable, the time is usually the distance traveled divided by the speed of the signal (about 80% of the speed of light in a vacuum. Mr. Rowe's calculation on page 337 is actually a bit off.) For the most part, a signal to a satellite travels through the vacuum of space, so it does not slow down much. It does, however, have to travel about 44,600 miles, so his calculation of about a quarter of a second is valid.
On page 340, a chart shows you the five media types discussed, and compares typical values of their characteristics. You should review this chart.
Circuits must be owned by someone. Private circuits are owned by one company, and used only by them.
Leased circuits are used by customers who lease time on the circuits from the owners. Lease customers expect that leased lines will not be used by others, that the line will always be available. A leased line is also usually guaranteed to have a certain capacity, and a certain quality.
Switched (dial-up circuits) are what you get when you make an ordinary phone call: the next available free circuit is assigned to your call. Sometimes a circuit is not available, or the line may have more static than the customer may be able to tolerate, which can make the leased line attractive to heavy users. Switched circuits are also less secure, more open to eavesdropping and hacking than leased lines. Only allowing dial-up connections from certain known lines (a list of approved phone numbers, for example) can make this system more secure.
The idea of multiplexing is covered next. A multiplexer can work several different ways. We have already discussed Frequency Division Multiplexing (FDM). This is simply putting separate signals into different frequency ranges, so that we can put more than one set of signals on a medium at the same time.
Time Division Multiplexing (TDM) is another method. In the illustration on page 346, four terminals are going to send signals across one medium. The system will send each of their signals for a certain amount of time, in rotation, so that each signal is sent for a while, each station gets a turn to send, and all the signals get across the medium a little at a time. The actual amount of time given to each station varies with the implementation.
Statistical Time Division Multiplexing (STDM or Stat-TDM) is like TDM, but it takes advantage of the fact that every station does not always have anything to send. The traffic in the system is analyzed, and heavy users get more turns, while light users get fewer turns.
A concentrator, in terms of circuits, is a device that takes several low capacity lines and combines their signals for transmission across a high capacity line. They are split up again at the other end.
An inverse concentrator does the opposite: if no high capacity line is available, an inverse concentrator can split a signal across several low capacity lines. The original signal must be reassembled at the receiving end.
Several common circuit errors are discussed:
Errors in transmission still happen, so it is necessary to detect them. When an error in data transmission happens, it is usually simple to ask for a retransmission. Your system probably does this several times when downloading a file from the Internet without ever telling you that it happened. Error Detection Methods: