Computer Networks: The Physical Layer

2.4 The Telephone System

Highlights

2.4.1 Structure of the Telephone System

Digital signaling is superior to analog in many ways: it has easily determined bit values (+5 volts and -5 volts) which lose no value or information when amplified, and amplification does not compound errors in digital signaling.

2.4.2 The Politics of Telephones

Before 1984, AT&T owned all of the regional telephone companies as well as the long distance lines. In 1984, AT&T was forced to split up into smaller subgroups of the regional BOCs and the long distance company it is today. But most of the telecommunications infrastructure we have today is due to AT&T.

2.4.3 The Local Loop

The local loop will not likely convert to digital for a while, so the present analog services will have to suffice for now when computers want to talk over the telephone system. Modems convert computers' digital data to an analog form so that it can travel over the wires. In order to maximize the data transmission rates, strategies have been developed to allow more than one bit to be sent per signal change, such as Quadrature Amplitude Modulation (QAM).

Computers have been talking to each other since the earliest part of computing history. The first standard for communication between computers (over a modem) was RS-232-C. A new standard, called RS-449, is replacing it.

For the future of communications, the current copper system will not be sufficient. There are conflicting camps about what should be done to put our communications infrastructure closer to the needs of the future. Some want Fiber to the Home (FTTH), but that is unrealistic at this point in time because of the cost of ripping out the existing wire and replacing it with fiber. A more realistic solution is to run Fiber to the Curb (FTTC), making the copper wires connecting the house to the fiber cable relatively short and capable of higher speeds.

2.4.4 Trunks and Multiplexing

Multiplexing is a necessity for the higher bandwidth lines so that their capabilities are fully utilized. The methods of apportioning bandwidth vary: certain frequencies can be reserved (FDM), certain time slots can be reserved (TDM), or (on fiber) certain wavelengths can be reserved (WDM) for use by one system.

There are different ways of multiplexing different data input streams onto one data output stream. T1 uses a TDM scheme to multiplex analog signals. SONET is a way to multiplex digital channels. The data rate for ATM is designed to fit well for running over SONET at 155.52 Mbps.

2.4.5 Switching

When information has to go over a switch in the communications system, there are several choices of how to switch the information. A circuit could be set up, causing no delay between switches, but causing setup time. Message switching could be used; it involves sending an entire message from one switch to the next before forwarding is possible. And then there is packet switching, in which a message is cut into several smaller fixed-sized packets, thus reducing the wait time at each switch when compared to message switching.

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