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)
Stop & Start bits
Stop & Start bits
We have already said that modems transmit data as either a 7 or 8 bit
byte. The modem can also add other bits to enable the receiving modem
to identify the beginning and end of each byte. Without these Stop and
Start bits, the data pulses could run into one another and could lead to
errors. When configuring your modem, you need to select the number of
start/stop bits according to the transmission mode in use. Start bits are
somewhat archaic and not generally used with modem speeds greater
than 100 bps.
Error Detection
Error Detection
Modems use several methods of error correction.
Parity Checking
Parity Checking
A simple form of error detection used with modems is called parity
checking. It is achieved by automatically adding a parity bit to each
transmitted byte.
A parity bit is simply an extra bit that follows the character bits. Its value
makes the number of 1 bits add up to either an odd or even number
depending on whether
odd
or
even
parity is the selected protocol.
For example, we have already seen that the letter K has the ASCII bit
pattern 01001011. This has an even number of 1's (high bits). If the
transmit protocol demands even parity, then the parity bit will be 0.
However, if the protocol calls for odd parity, then the parity bit will be 1,
making the total number of 1's an odd number.
When the data is transmitted, if the parity bit received does not agree with
other bits, then the receiving computer signals that an error may have
occurred during transmission.
MNP Error Correction
MNP Error Correction
Another method of error detection widely supported is known as MNP
(Microcom Networking Protocol)
. This protocol doesn't use start and stop
bits, but instead groups the data bytes together and sends them as
packets complete with a check field. When the modem receives the
packet it checks that the check field is correct, and requests that the
packet is retransmitted if an error is detected. SPOT supports MNP 2, 3,
4 and 5 (MNP 5 adds also adds data compression).
