Technical data

66 Theoretical and general applications www.westermo.com
Remote Connections
PSTN Dial-up lines
Data communication over the telephone network
Remote communication is an important supplement to local data communication.
That is to say, the possibility to connect to remote data sources to search for informa-
tion about for example markets, prices quoted on the stock exchange or public regis-
ters. The number of data sources has increased significantly and they are often linked
via global networks. Despite connecting to a data source in one country you can quite
easily end up in an international finance data source in New York. There are many rea-
sons to establish remote data communication, among others to connect with your
workplace and company computer via the telephone network while out in the field.
Today, a computer, modem, GSM telephone and fax are often all combined into a sin-
gle portable computer.
Dial-up connection
The principle of remote communication via the telephone network is based on calling
the recipient’s modem, which answers and then both modems establish a carrier, over
the telephone line. The carrier is a signal that a modem listens for. Once the modems
can hear each other’s carrier they lock-on or synchronize with this. Transfer rates over
the telephone network have increased and nowadays 2400– 56000 bit/s are common-
place. It is not just the modem that limits transfer rates but also the telephone line.
Distance, the number of exchanges and relays significantly affect the quality of the line.
Most high speed modems have the capability to automatically retrain to maintain good
transmission quality. Within telemodem communication it is vital to conform to stan-
dards, as the transmitter and receiver are often from different manufacturers. In the
table on page 69 the bitrates associated with specific standards are presented.
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