Modem Terminology Overview
Modem Terminology Overview - Page
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SPOT 14.4
SPOT 14.4 User's Guide
User's Guide
38GDE00001SCF - Issue 4 (11/96)
38GDE00001SCF - Issue 4 (11/96)
Speed
Speed
Transmission speed is measured in bits per second
(bps).
This is a
measure of the number of data bits that are transmitted/received per
second and, depending on the selected mode, the speed can vary from
75 to 14400 bps. The speed at which the characters are transmitted
(CPS) depends on the word length (7 or 8 bits) plus the parity and
start/stop bit configuration. For example if you are using an 8 bit ASCII
character with one stop bit and a parity bit (10 bits in all), then at a
transmission speed of 300 bps you will transfer 30 characters per
second.
Baud
Baud
The baud rate measures the number of times per second a signal on the
telephone line changes. The term baud is often confused with bits per
second (bps). But for speeds above 600bps, modems transmit more than
one bit at a time by using different tones for different groups of bits. For
example, when the modem transmits at V.22bis (2400bps), the baud rate
is 600 with four data bits being transmitted for each baud.
Data Compression
Data Compression
The rate at which signals can be passed along copper telephone wires is
limited by the laws of physics. This leaves data compression as the only
route to improve modem speeds (until we all have fibre-optic phone
lines). The drawback on compression is that the receiving PC or modem
must understand the method being used in order to decompress the data.
Thus several standard for automatic data compression and
decompression exist.
– MNP 5
MNP 5:: Can produce compression ratios of up to 2 to 1.
– V.42bis
V.42bis:: Can produce compression ratios of up 3.5 to 1.
The drawback of data compression on modem links is that except for
plain text file, many PC files are already encoded and compressed in
some way. The effect of compressing these files further is not so great.
