Various asynchronous standards define the interface or signaling that goes on between the DTE and the
DCE, and between the two DCEs. The connection between two modems is an ordinary telephone line.
Because the connection is a dial-up connection, it is also called a switched line. For this reason, the
telephone lines are known as a public switched telephone network (PSTN). MCI, Sprint, and AT&T are all
PSTNs.
Sometimes the telephone lines are simply referred to as the plain old telephone system (POTS). An
ordinary phone line has a physical limitation in transmitting data. The bandwidth of a an ordinary phone
line lies within the range of approximately 300 to 3,000 Hz—a range that covers the most frequently
used spoken tones, but that is far less than the full range of human hearing. PSTN bandwidth is
comparable to less than half the playable notes on a standard piano.
This bandwidth limitation for telephone lines means that the maximum of 2,400 distinct tones per
second can be transmitted on the line. This data transfer rate is measured as the baud rate. A baud is a
single change of state in the analog signal per second. This change may be in frequency, amplitude, or
phase of the carrier signal. Therefore, 2400 baud is the maximum throughput a PSTN will allow without
the aid of some modulation scheme to increase the bps data throughput for the same maximum baud
rate.
Sometimes a simple telephone line is a very fast physical link that is split at each end into many slower
lines, each of which operates as an individual phone line. For example, a statistical multiplexer is one
piece of equipment that splits a high-speed (T1) trunk line into many slower (9600 bps) lines so that
each of the slower lines appears to have a dedicated modem attached to it.
Another variation on this same principle is the implementation of a hunt group. Using a hunt group over
a high bandwidth line, one phone number is routed to a series of 5, 10, or more different inbound
extensions, each of which has an individual modem. The hunt group finds the first available modem from
the bank of modems and connects the caller to that modem. This process continues for each successive
caller until all available modems are in use. All subsequent callers get a busy signal until one of the
modems is free to take a call.
Modems take digital input from terminals or computers and map the discreet bits onto an analog sine
wave (the carrier wave). The carrier and the modulated signal are transmitted to the remote modem,
which strips off the signal riding the carrier, converts it back into a digital bit stream, and then sends the
re-digitized data stream to the remote computer or terminal. The process is called modulation-
demodulation, from which the modem takes its name.
There are five or six major schemes called modulation standards for mapping or unmapping the digital
signal onto or from the carrier frequency. A discussion of these modulation standards is beyond the
scope of this paper; however, the modulation standard attributes discussed in the next section are
important to understand.
The frequency of the carrier sine wave, how many carrier waves there actually are, the physical
modulation technique, and the speed at which the two modems can modulate or demodulate the digital
signal are all attributes of the modulation standard. For two modems to communicate, they must be able
to first negotiate to a common modulation standard.
Notice that the modulation technique is not equivalent to the modulation standard. The following is a list
of some of the most commonly used modulation techniques:
l Frequency shift keying (FSK): This technique is built on the principle of frequency modulation
(FM) of the carrier signal.
l Phase shift keying (PSK): This technique is built on the method of phase modulation (PM) of the
carrier signal.
l Quadrature amplitude modulation (QAM): This technique involves changing the amplitude and
frequency of the carrier wave at the same time.
l Trellis coding modulation (TC or TCM): This is a variant on QAM that includes a method for
forward error correction; that is, error correcting information is already carried with the data,
Modulation Standard Attributes