Carrying out data communications using fiber optic cables in the premises environment presents several ready advantages. First, there is tremendous bandwidth potential. Consequently, applications that require very high data transmission rates can be easily accommodated. Secondly, there is the protection that fiber optic transmission provides against the variety of deleterious effects which plague transmission over copper cable. These include the resistance that fiber optic transmission has to Electromagnetic Interference (EMI), lightning induced current surges and ground loops. Finally, there is the protection that fiber optic transmission has with respect to 'tapping.' It is much more secure with no effective radiation of the communication occurring out of the cable.
Against these advantages there is the issue of economics - basically, the cost of installing fiber optic cable for the premises environment. Costs have come down markedly in the last decade. However, they are still an issue with which the network architect should be concerned.
One approach to dealing with the cost of installing fiber optic cable is to multiplex a number of different communication transmissions on a given cable - that is, use it to configure more than one link simultaneously. It can then serve more than one user application simultaneously. This brings cable cost per link transmission down. The tremendous bandwidth associated with fiber optic cables is well suited for mutliplexing.
There are two basic approaches to multiplexing different communications links on a single fiber optic cable: Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM).
With TDM, different communication links share the same fiber optic cable on the basis of time. There is a continuous sequence of time slots. The duration of the time slots depends upon engineering factors that we need not address here. Each specific link is assigned specific time slots during which it is allowed to send its data from one end to the other and during which no other link is allowed to send data.
TDM can be engineered to accommodate different link types - that is links carrying different types of traffic and at different transmission rate. TDM can also be engineered to have different time slot assignment strategies. Slots can be permanently assigned or assigned upon demand. The time duration can even be made to vary depending upon the characteristics of the link being configured. However, there is strong anecdotal evidence that, in the premises environment, TDM works best when it is dealing with links all of the same type and all given permanently assigned time slots of equal duration. This occurs, for example, when each and every link is being used to handle data links at speeds of at most 32 KBPS coming from the same type of data device.
With WDM different communication links share the same fiber on the basis of wavelength. Information associated with each link first goes through a modulation process. The result is the generation of light modulated by the information. The resulting information associated with each link can be placed on light at a different wavelength - that is, each link is associated with a different optical wavelength. The light from all of the links can easily be coupled into a single fiber optic cable and then transmitted together down the optical fiber. At the receiving end the different links can then be separated on the basis of wavelength using prisms - the demultiplexing operation. The resulting received information is then directed to the appropriate data device destination.
WDM has some significant advantages over TDM and some drawbacks relative to TDM for the premises environment. Compared to TDM it is much easier to mix different traffic types with this approach. The input channels to the multiplexing operation do not have to appear homogeneous from a traffic characteristic point of view. On the other hand, for the premises environment, WDM can usually only accommodate two input channels for the multiplexing operation. Furthermore, isolation between the channels may not be that great. There may be interference - crosstalk. However, there may be improvements coming down the road on the issue also.
The Model 273 presents a convenient TDM approach to multiplexing four different channels on fiber optic cable in the premises environment. It not only multiplexes data from four channels for transmission on a common fiber optic cable, it also multiplexes control signals from the channels.
The illustration above shows a typical application where the Model 273 would prove attractive. Here we have a manufacturing environment with all of the harsh interference conditions that this implies. On the right hand side we have a cluster of four data terminals. They are all communicating with a host computer which is not shown, but which is serving the factory. The terminals are doing this by sending data and control signals to a Front End Processor, which handles all of the data communications tasks for the host computer. Each terminal is not only sending and receiving data from the host computer. It is also sending a bi-directional control signal; e.g., sending RTS/receiving CTS. Why? Because the host computer may be busy with a variety of tasks and may not be ready to receive and process data when a particular terminal is ready to send data.
Because of the harsh interference conditions transmission of data over fiber optic cable stands out immediately as a possibility. To reduce cabling costs from the host computer to the four terminals, multiplexing stands out immediately as a desirable way to implement the network. Each terminal-to-host connection has similar traffic, and there are four connections desired. Consequently, TDM rather than WDM would appear to be the desired multiplexing technique. Since bi-directional control signals are needed for each connection, the Model 273 as shown in the illustration presents itself as the obvious choice for the multiplexer. It can meet all of these requirements.