Bits, Bauds and Hz


art@acc.arpa
12 Jan 86 22:28:00 PST


This is not really about tcp/ip, but the excerpt came off this newgroup
and someone may find it interesting since most protocols usually get
down to a real comm channel.

> The broadband communications use one 6MHz channel,
> and can handle 10Mbits/sec. (Yes, it is possible to get more bits in
> a channel than its bandwidth. This has always seemed to me to violate
> some basic principle, but sophisticated communications technology can
> get more bits/sec than Hz.)

Remember that these types of communication channels are really analog
circuits. Whereas digital channels have basically only two variables
(binary state and bit duration), analog channels have such variables
as frequency, amplitude, phase change. Analog channels use these
variables to carry more information in a given unit of time.

9600 4-wire modems still have to communicate over phone circuits which
have a useful bandwidth somewhat over 3kHz. This is accomplished by
sending 4 bits at a time, 2400 groups/second. The four bits are
encoded as one of eight phase shifts (3 bits) and one of two amplitudes
(4th bit). The time unit of a particular modulation state is named
after Baudot and is called a "Baud", and the rate at which the modulation
state changes is the "Baud Rate". Thus, most 9600 bit/second modems
are not really "9600 Baud" modems. But, most 300/1200 bit/second
modems use Frequency-Shift-Keying (FSK) where the binary state of the
data directly controls which of two frequencies is being transmitted.
In this case the bit/second rate is the same as the baud rate.

Most broadband systems are based on CATV technology developed for
video distribution. These systems use a range of frequencies broken
into 6mHz bands. By modulating the carrier frequency within a 6 mHz
band, each band can carry separate information. The 802.4 token bus
protocol used in GM's MAP specifications uses two pairs of adjacent
6mHz channels, one pair for transmitting and the other for receiving.
The channel is modulated with a technique called Duobinary AM-PSK
which uses both amplitude and phase to convey information at a
10 mHz rate.

Unless laboratory developments change things, fiber optics will be
used digitally. A laser will generate monochromatic light which is
either on or off. But Bell Labs has demonstrated that the light can
be switched (and detected) many billions of times per second over
useful distances. (24 telephone voice channels only use 1.544 Mbit/sec)

                                        <Art@ACC.ARPA>

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