Within the premises data communication environment economies can be gained by sharing resources. Such resource sharing may take a number of different forms. The communication capacity, that is the actual cable, can be shared by a number of different data sources or their applications. Communication and peripheral equipment can also be shared.
Let’s consider the problem of capacity sharing first. If you were to take a careful look at the actual cable connecting a single pair of communicating data equipments you would find that its
signaling capacity, the rate at which it can transfer data, probably far exceeds the needs of the particular data processing application being run. To the point, of course the cable can be used to connect a single data equipment type to another isolated data equipment type. However, in general, the data transferred in a single application is far less than the actual capability of the wire. You can then conclude that the single cable need not be dedicated just to a single pair of communicating data units. Rather, it can be shared among a number of different users.
You may ask how may this sharing occur and the resultant cable cost reduction be achieved? It is really quite simple. Consider the top illustration, Figure 12. Here is a group of different terminals. Also shown is a multi-user computer to which the terminals must be connected. the computer may be somewhere else on the premises perhaps in the basement of the office building.
When the terminals are scattered throughout the building the most straightforward approach to connecting them to the computer is also probably (close to) the most economical. This is shown in the middle illustration of Figure 12. Here, each terminal has its own dedicated pair of wires connecting it to the computer. Short haul modems are used for signal boosting if required.
When the group of terminals is not scattered but clustered in the same office or floor there is a more economical alternative. A single cable pair has the capability to effect the transfer of the data from all the terminals. Consequently, the cluster of terminals can share the single cable bringing about a savings in cabling and equipment costs. A device called a
short haul multiplexer is required to bring about the efficient and reliable sharing of the single cable. The bottom illustration of Figure 12 shows this. The cabling cost here is one-sixth that of the the situation shown in Figure 12B with the terminals at the same locations. Of course, one has to compare the cost of the short haul multiplexer with its significant intelligence with that of the distinct short haul modem pairs to determine the actual cost savings.
Figure 12A:Terminal Cluster Isolated from Multi-user Computer
Note:All Terminals Use Short Haul Modems to
Support Cable Lengths in Excess of 50 Feet
Figure 12B:Terminals in cluster each connected by dedicated cables to multi-user computer
Figure 12C:Terminals sharing a single cable to multi-user computer by multiplexing
Different short haul multiplexers manufactured by Telebyte are shown in Figure 13.
Figure 13: Telebyte Short Haul Multiplexers
Short haul multiplexers operate in a number of different ways. The most common way is to share time on the cable between the different terminals. This is usually referred to as time division multiplexing (TDM). More advanced forms of short haul multiplexers, such as the Telebyte Model 570 Quick Mux, have the ability to incorporate bi-directional control signals for each terminal. Short haul multiplexers generally incorporate specialized short haul modems necessary to effect the signaling on the line.
When considering short haul multiplexing as an economic measure in your premises data communication environment you should be aware of a number of different issues. These are brought to your attention now.
To begin with there is the number of ports that the short haul multiplexers can support. This limits the number of terminals that can be plugged into it and can share the cable.
Be aware of the port speed. This limits the maximum data that can be transferred from a data equipment unit into the multiplexer.
Pay attention to the aggregate speed on the output side of the multiplexer driving the line. You obviously need transmission media which can support the aggregate speed.
Be concerned with the possible problem of lightning protection. Often the situation arises where the cluster of terminals is located in one building and the multi-user computer is in another building. The interconnecting line must be protected. Even if buried it is subject to lightning strikes and appropriate lightning protection must be present.
Within short haul multiplexers there are a number of different types. Some are stand-alone, others are integrated as cards within the computer themselves.
Finally, be concerned with the transmission media employed. Besides short haul multiplexers for twisted pair cable there are ones which interface fiber optic cable. Telebyte’s
Model 575 short haul multiplexer mated with Model 275 adapter is one such example.
5.2 PORT SHARING
Port sharing devices allow a number of different DTE’s to share the same peripheral such as a printer, plotter, or a high speed modem. This sharing can be accomplished via a simple mechanical switch or sophisticated electronic devices. With simple devices there is an external switch, which physically must be thrown to allow one data equipment unit to seize control of the peripheral. Somewhat more complicated than the mechanical port sharing devices are equipments which are active and allow terminals or computers to share expensive peripherals. Such devices generally go under the name of modem sharing units, peripheral sharing units or modem allocating units. These allow computers or terminals to share the peripherals on a contention basis. The sharing unit resolves the contention. Usually, these devices operate by polling the connecting terminals, seeing which has raised an RTS flag thus requesting use of the peripheral. The sharing unit then grants the requesting terminal access to the peripheral terminal while keeping other terminals waiting for its use.
Figure 14: Telebyte's Port Sharing Devices, Models 335 and 576
When considering such sharing units there are a number of facts which you must know. To begin with know the number of ports, that is the number of terminals which can share the peripheral and the signal lines supported. You must know the speed which data can be transferred through the device to peripheral. You would like to know the dwell time and polling cycle time. In other words know how long it will take for the sharing device to connect you through to the peripheral beginning with the first service request if nobody else is demanding connection. You would like to know if received data is routed only to the selecting terminal or broadcast to all connecting devices. Finally, you would like to know if the sharing unit has internal buffering so that it can save data if the connecting terminal or peripheral is not ready to receive it and control signals cannot effectively throttle the flow.