Technical data

70 Theoretical and general applications www.westermo.com
Connection
When a modem connection is established handshaking occurs where the data transfer
rate and level of error correction are negotiated. The specification
below shows the connection times between two modems for different
protocol settings.
This measurement illustrates that the fastest data rate is not always the
most effective. The connection time is the key factor when you need
to call several devices and only transfer a small amount of data.
Telecom modem language
In order to configure a connection, a terminal or a computer with communication soft-
ware that uses the computer’s serial port is required to communicate with the
modem. Instructions are required to control the telecom modem. Hayes Microcom-
puter Products developed a command set that has become a standard, these are
called Hayes
®
-commands. This is a set of commands for telecom modems that can
either be sent manually from a computer, via the keyboard, or automatically from a
connected device to provide different settings as required.
Error correction and compression
Most telecom modems transmit synchronously between each other, even when com-
munication between the computer and the serial port are asynchronous thus provid-
ing simple data compression. In order to monitor reliability data can be divided into
blocks, where each block is assigned a checksum. If the data transfer is disrupted and
the checksum does not correspond the receiver requests the block to be resent. This
is known as ARQ (Automatic Repeat reQuest) and the most common method for
this is, according to ITU-T, V.42 error correction which is supported by MNP
(Microcom Networking Protocol) and LAPM (Link Access Procedure for Modems).
Searching and file transfer
Using a telecom modem you can connect to other computers, directly or indirectly via
a network. In a short time, the Internet has expanded into the largest global network
with up 250 million users. Based on the Internet’s TCP/IP-protocol, electronic mail, dis-
cussion groups, World Wide Web (databases, information and marketing), file down-
loading and uploading, telephony, video conferencing, chatting, etc are all available.
However, there are also other networks and services that are accessed via modems,
for example, MEMO, Lotus Notes, Compuserve, etc. The telecom modem also makes
homeworking possible as well as to allow to company’s computers to be connected
via mobile GSM.
Protocol Connection
time
V.32 bis 16 sec
error corrected
V.32 bis 13 sec
V.22 bis 12 sec
error corrected
V.22 bis 7 sec
V.23 6 sec
V.21 7 sec
ARQ and MNP
MNP Level 1:
asynchronous protocol,
half duplex
MNP Level 2:
asynchronous protocol, full
duplex. Data divided into
blocks. Actual data transmis-
sion speed somewhat lower
than normal.
MNP Level 3:
synchronous protocol, full
duplex. Data in blocks. 10%
higher speed with error-free
transmissions.
B A C K