Hardware Reference
In-Depth Information
by 45, 135, 225, or 315 degrees, to allow 2 bits per time interval, called
dibit
phase
encoding. For example, a phase shift of 45 degrees could represent 00, a phase
shift of 135 degrees could represent 01, and so on. Schemes for transmitting 3 or
more bits per time interval also exist. The number of time intervals (i.e., the num-
ber of potential signal changes per second) is the
baud
rate. With 2 or more bits
per interval, the bit rate will exceed the baud rate. Many people confuse these two
terms. Again: the baud rate is the number of times the signal changes per second
whereas the bit rate is the number of bits transmitted per second. The bit rate is
generally a multiple of the baud rate, but theoretically it can be lower.
If the data to be transmitted consist of a series of 8-bit characters, it would be
desirable to have a connection capable of transmitting 8 bits simultaneously—that
is, eight pairs of wires. Because voice-grade telephone lines provide only one
channel, the bits must be sent serially, one after another (or in groups of two if dibit
encoding is being used). The device that accepts characters from a computer in the
form of two-level signals, 1 bit at a time, and transmits the bits in groups of 1 or 2,
in amplitude-, frequency-, or phase-modulated form, is the modem. To mark the
start and end of each character, an 8-bit character is normally sent preceded by a
start bit and followed by a stop bit, making 10 bits in all.
The transmitting modem sends the individual bits within one character at regu-
larly spaced time intervals. For example, 9600 baud implies one signal change
every 104
μ
sec. A second modem at the receiving end is used to convert a modu-
lated carrier to a binary number. Because the bits arrive at the receiver at regularly
spaced intervals, once the receiving modem has determined the start of the charac-
ter, its clock tells it when to sample the line to read the incoming bits.
Modern modems run at 56 kbps, usually at much lower baud rates. They use a
combination of techniques to send multiple bits per baud, modulating the ampli-
tude, frequency, and phase. All are
full-duplex
, meaning they can transmit in both
directions at the same time (on different frequencies). Modems or transmission
lines that can transmit only in one direction at a time (like a single-track railroad
that can handle northbound trains or southbound trains but not at the same time)
are called
half-duplex
. Lines that can transmit in only one direction are
simplex
.
Digital Subscriber Lines
When the telephone industry finally got to 56 kbps, it patted itself on the back
for a job well done. Meanwhile, the cable TV industry was offering speeds up to
10 Mbps on shared cables, and satellite companies were planning to offer upward
of 50 Mbps. As Internet access became an increasingly important part of their bus-
iness, the
telcos
(
telephone companies
) began to realize they needed a more com-
petitive product than dialup lines. Their answer was to start offering a new digital
Internet access service. Services with more bandwidth than standard telephone
service are sometimes called
broadband
, although the term really is more of a
marketing concept than anything else. From a very narrow technical perspective,
