Within the premises or local area environment the short haul modem is a
convenient device for configuring a reliable communications link. When
the distance between communicating data equipment gets beyond 100 feet
signals need 'to be boosted' or they will not be received and decoded
reliably. Using a pair of short haul modems in the link, one for
transmitting and one for receiving in each direction, boosts the signals
and gives the reliability.
However, a number of items always seem to come up when using a pair of
short haul modems to deal with this rather straightforward problem.
First, there is the matter of powering the modems. A modem needs
electrical power to work. Many times the network installer can look
around for an electrical outlet and find that either one is not
available or is quite far from the device and currently in use.
Secondly, there is the 'headache' of getting the data communications
link up and running. Something always seems to happen, and the installer
always has to look at some signals going from one end of the link to
another. The installer has to look at the status of the link and maybe
even after the link is up the user has to check status from time to time
to assure himself/herself that everything is OK.
All of these issues must be dealt with in the context of the modem
supporting the data rate adequate for the communicating devices and over
the communication distance of interest.
These can be conveniently handled by the Model 227 short haul modem. Its
employment in a typical application is provided by the illustration
Here we have two communicating data devices a PC and a terminal server
associated with an Ethernet Local Area Network (LAN). The PC user wants
to be part of the LAN, but is located some considerable distance away
from it - measured by Ethernet LAN rules - say a few miles. The PC users
want access to the LAN and will have to transmit not at 'full LAN'
rates, but say at significant rates of up to 115.2 KBPS. This will be
asynchronous, which is in the nature of a user sitting at a keyboard,
and full duplex.
As you see this is accomplished by having two twisted pair cables. Each
cable is dedicated to a specific direction of the communication, either
badge reader-to-host computer or vice versa. While you can not see it in
the illustration there is no available, that is unused, electrical
outlet either near the PC or the terminal server.
The Model 227 shown in the illustration fits the needs of this
application quite well. It is a full duplex, short haul modem where full
duplex is achieved by using two separate cable pairs. It transmits data
in an asynchronous mode which is just what this PC originating
application needs. It transmits at data rates up to 115.2 KBPS for links
of about 1 km on 26 AWG cable. It can provide reliable transmission for
up to about 9 km if a lower data rate of 2.4 KBPS is OK.
The modem also operates by powering itself from the interface to which
it is attached. It does not need an electrical outlet. Finally, the
Model 227 has a convenient LCD display for showing link status. This
effectively integrates a breakout box right into the modem. It allows
the network manager to view the status of the data lines and the control
pins. It is ideal for helping bring the link up and for monitoring its