Model 2431/431 Application Notes
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 issue of transmission speed and distance. The
network architect always wants to get the highest speed over the
longest
distance and, of course, in the most economic way. One may also
have to meet different distance requirements in different configurations
and may want the flexibility to trade speed for distance. That
is,
in putting up many links the architect may have to accommodate
a particular long distance between two data devices and as a consequence
would be willing to live with a lower data rate on this particular
link. He may want to be able to choose from a menu of 'speed-distance'
possibilities.
Secondly, there is the issue of cabling. In many premise cabling
environments, cabling to form the data communications link connecting
the modems
is not always readily available. Additional pairs of cable are
'tough to get.' This presents a particular problem when full duplex
transmission
is needed. Being able to realize it by dedicating a single pair
of cables to each particular direction may be 'out of the question.'
The network engineer may have to look hard to achieve full duplex
transmission by using just a single cable pair to implement the
link.
Thirdly, 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.
Fourthly, there is the question of flow control. It may be that
one of the communicating data devices is ready to transmit, but
the other is not yet ready to receive. It may still be carrying
out a prior task. Flow control procedures assure that communications
only proceeds when both devices are ready. Often these can be carried
out by software. But, this may not be always. In the absence of
software flow control there is frequently a demand for a hardware
flow control feature - a hardware 'handshake' between both ends
of a link. This often is as simple as once end of the link raising
the RTS (Request To Send) flag, sending it to the other end of the
link which responds with the CTS (Clear To Send).
Finally, there is the issue of the interface on which the modem
obtains/sends out the data to data devices. In many cases this is
RS-232, but it can be others. For example, V.35 is often found when
the data equipment is dealing with high rates.
All of these issues can be effectively dealt with by the Model 2431
and Model 431. Its employment in a typical application is provided
by the illustration above. This application refers to an Internet
Service Provider (ISP) and one of its customers. The right hand
side of the illustration is the customer's site. Here we have a
mainframe computer and possibly other computers, PCs not shown,
which may all be part of some premises data communications network
at the ISP customer site. Occasionally, the users of the customer's
computers want Internet access. To do this they make their connections
through the customer Router that is indicated. The router needs
to have a data communications link to the ISP. The left hand side
of the illustration is the ISP site. Within the ISP site there is
often an Ethernet LAN. This LAN takes in data from all of the ISP's
customers and after various levels of processing sends it to the
Internet. This is not shown in the illustration. What is shown in
the illustration is the Ethernet Server associated with this LAN.
This takes in and sends out data from/to the various ISP customers.
It does this through a bridge or router. A bridge is shown in the
illustration. This bridge must be linked to the ISP customer.
So, looking at both sides of the illustration, we see that there
has to be a link between the ISP customer and the ISP.
Because of the throughput demanded by an ISP's customers, this link
should be full duplex and at as high rate as possible. This full
duplex transmission should be accommodated on a single cable pair
so that cabling cost can be kept as low as possible.
Because some customers of the ISP may be much further in distance
from it than others there should be the possibility of trading transmission
rate for distance. That is, connecting to far away customers but
at lower rates.
With long files being sent back and forth synchronous transmission
is really needed.
Routers and bridges associated with the LANs shown in this application
often employ the V.35 interface since this is well matched to the
speeds being present.
Finally, because in periods of intense customer demand the ISP may
not be able to serve everybody simultaneously, there should be control
signals embedded in the communication.
All of these needs can be met by employing the Model 2431 and Model
431 for this type of application. This is exactly what is shown.
|
Model 2431
Model 431
|
Telebyte Home
Bomara Homepage