Because of the role played by phone companies in data transmission, analog transmission has dominated communications for quite some time. Today, everyone agrees that ber is the way to go. As much as we prefer ber, analog communications will be with us for a long time. Consider the phone system. It is characterized by:
1. Low bandwidth: It carries a bandwidth of about 3 kHz. That is, the system only allows signals between 0-3kHz to pass through | all higher frequencies are chopped o . The 0-3kHz spectrum covers the most important frequencies of human voice, which is precisely what the phone system has been designed to carry.
2. High error rate: Relative to LANs, the error rate is roughly 11 orders of magnitude
higher! Errors don’t matter as much to analog communication, especially when voice
is involved. For digital communications, of course, a 1-bit error can have devastating
The phone system is organized into a hierarchy:
Local phones are connected to a central oce over a 2-wire circuit. That is, only two
wires are needed to connection your phone to the phone system.
The 2-wire path is called the local loop, and is no longer than 10 km in length.
An area code and the rst three digits of a telephone number uniquely identify or
address a central oce.
Calls between neighbors connected to the same central oce go only through the
Each call ties up a circuit in the central oce. Typically only a small fraction of the
circuits can be used simultaneously. That is, the phone company plans for expected
use rather than worst case use.
Central oces connect to toll oces over toll connecting trunks, which typically have
higher bandwidths then the local loop. In fact, higher bandwidth trunks carry
multiple voice grade” lines over a single physical channel.
Toll oces are then connected in various ways.
Fiber is increasingly connecting toll oces and central oces, but not the local loop.
Why? It’s not cost eective to replace wiring at the local loop level. For trunks,
however, many phone customers essentially share the lines, making the costs
CS 513 2 week2-physcont.tex
Digital Data/Analog Signals
Must convert digital data to analog signal. One such device is a modem to translate
between bit-serial and modulated carrier signals.
To send digital data using analog technology, the sender generates a carrier signal at some
continuous tone (e.g. 1-2 kHz in phone circuits) that looks like a sine wave. The following
techniques are used to encode digital data into analog signals (Fig 2-18)
Resulting bandwidth is centered on the carrier frequency.
amplitude-shift modulation (keying): vary the amplitude (e.g. voltage) of the signal.
Used to transmit digital data over optical ber.
frequency-shift modulation: two (or more tones) are used, which are near the carrier
frequency. Used in a full-duplex modem (signals in both directions).
phase-shift modulation: systematically shift the carrier wave at uniformly spaced