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 question of whether you want asynchronous or synchronous communications. Asynchronous transmission is matched to the situation where the communicating devices generate data sporadically - seemingly at random. Synchronous transmission is matched to the situation where the data is being generated at high speed and continuously and you need maximum utilization of the communications link in order to realize the throughput objectives. There are applications where some times the need is for asynchronous communications and at other times for synchronous communications. For example, it may be that during the day a point of sale terminal communicating with a store's inventory program in a 'query-response' manner may be adequately served by asynchronous communications. At the end of day these same terminals may need to send 'long' records summarizing sales details to an accounting application. This type of situation may better be served by synchronous communications.

Secondly, 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.

Thirdly, there is the issue of whether the modem can support full duplex transmission or just half duplex transmission.

Finally, there is the question of what data transmission rate can be supported and over what distance.

The Model 92 can handle all of these issues. Its employment in a typical application is shown in the illustration above.

Here we have a Computer Workstation on the bottom communicating with a Network Controller on the top. The distance between these units is less than 1 mile and the required transmission rate is 19.2 KBPS over 26 AWG cable. Throughput demands of the Controller's network demand that the connection be both synchronous and full duplex. There are 2 cable pairs present to effect the full duplex transmission. There are no available wall outlets in this application for powering the modem.

The Model 92 provides the, full duplex, synchronous transmission over the 2 cable pairs. As it derives power from its interface signals the absence of electric wall outlets does not preclude its operation. It can support the desired transmission rate over the distance required.

The application described above is synchronous. However, it may be that at some point in time the Network Controller no longer demands this for its throughput objective or the Network Controller itself is replaced by a simpler PC. In such a situation the transmission can be changed to asynchronous without having to replace the modems. The Model 92 supports both synchronous and asynchronous transmission. Or it may be that the network architect who is tasked with dealing with this application must also deal with another application where only asynchronous transmission, at these moderate distances and rates, is required. By purchasing a larger quantity of Model 92's the network architect can satisfy both applications and get greater discounts.