Information Technology Reference
In-Depth Information
0
B
0
B
1
B
2
B
3
B
4
B
5
B
6
P
S
1
S
2
1
ASCII character
Start
bit
Parity
Stop
bit(s)
RS-232 character
RS-232 character
RS-232 character
Start
bit
Stop
bit(s)
Figure 15.2
RS-232 frame format
For asynchronous transmission the start and stop bits are added in addition to the seven
ASCII character bits and the parity. Thus a total of 10 bits are required to transmit a single
character. With 2 stop bits, a total of 11 bits are required. If 10 characters are sent every sec-
ond and if 11 bits are used for each character, then the transmission rate is 110 bits per sec-
ond (bps). The fastest modem thus has a character transmission rate of 2880 characters per
second.
In addition to the bit rate, another term used to describe the transmission speed is the
baud rate. The bit rate refers to the actual rate at which bits are transmitted, whereas the baud
rate is the rate at which signalling elements, used to represent bits, are transmitted. As one
signalling element encodes 1 bit, the two rates are then identical. Only in modems does the
bit rate differ from the baud rate.
15.3 Modem standards
The CCITT (now known as the ITU) has defined standards which relate to RS-232 and mo-
dem communications. Each uses a V number to define their type. Modems tend to state all
the standards they comply with. An example FAX/modem has the following compatibility:
•
V.32bis
(14.4 Kbps).
V.32
(9.6 Kbps).
•
V.22bis
(2.4 Kbps).
V.22
(1.2 Kbps).
•
Bell 212A (1.2 Kbps).
Bell 103 (300 bps).
•
V.17
(14.4 bps FAX).
V.29
(9.6 Kbps FAX).
•
V.27ter
(4.8 Kbps FAX).
V.21
(300 bps FAX - secondary channel).
•
V.42bis
(data compression).
V.42
(error correction).
•
MNP5
(data compression).
MNP2-4 (error correction).
A 28.8 Kbps modem also supports the V.34 standard.
