Sharing the transmission medium
You are the network manager of a company. You have a Source-User link requirement given to you. In response you install a premises fiber optic data link. However, the bandwidth requirements of the specific source users to adapt to the source for bandwidth speed requirements, is much, much less than is available from the fiber optic data link. The tremendous bandwidth of the installed optical fiber cable is being wasted. On the surface, this is not an economically efficient installation.
You want to prove that the install link to the controller of the company, the person who comment on your budget. The controller don’t understand the attenuation benefits of fiber optic cable. The controller doesn’t understand the interference benefits of fiber optic cable. The controller hates waste. He just wanted to see most of the bandwidth of the fiber optic cable used waste nothing. There is a solution to this problem. Don’t just put the huge bandwidth of optical fiber cable, a single, specific, the source user communication needs. Instead, allow it to be shared by a multiplicity of
Source-User requirements. It allows it to carve a multiplicity of fiber optic data links out of the same fiber optic cable.
The technique used to bring about this sharing of the fiber optic cable among a multiplicity of Source-User transmission requirements is called multiplexing. It is not particular to the fiber optic cable. It happens in any transmission medium, such as wire, microwave, etc., the available bandwidth far surpasses any individual Source-User requirement. However, multiplexing is particularly attractive is a fiber optic cable transmission medium.
Conceptually, multiplexing as shown in figure 1. This figure shows the “N” Source-User pairs indexed as 1, 2, … There is a multiplexer provided at each end of the fiber optic cable. The multiplexer on the left takes the data provided by each of the source. It combines these data steams together and sends the resultant steam out on the fiber optic cable. Such a single source to generate data flow sharing the fiber optic cable. The multiplexer on the left performs what is called a multiplexing or combining function. The right of the multiplexer takes the combined stream put out by the fiber optic cable. It separates the combined stream into the individual Source streams composing it. It directs each of these component steams to the corresponding User. The multiplexer on the right performs what is called a demultiplexing function.
A few things should be noted about this illustration shown in Figure 1.
Figure 1: Conceptual view of Multiplexing. A single fiber optic cable is “carved” into a multiplicity of fiber optic data links.
First, the Transmitter and Receiver are still present even though they are not shown. Device is considered to be part of the fiber optic multiplexer is on the left and the receiver are considered to be part of the multiplexer is on the right.
Secondly, the Sources and Users are display close to the multiplexer. Multiplexing to understand this is a common situation. Connectioin from the source to the multiplexer and multiplexer to User is called a tail circuit. If the tail circuit is too long a separate data link may be needed just to bring data from the Source to the multiplexer or from the multiplexer to the User. The cost of this separate data link may counter any savings effected by multiplexing.
Thirdly, the link between the multiplexer, the link in this case realized by the fiber optic cable, is termed the composite link. This is the link, the transportation is by all of the source of the river alone.
Finally, sepatate users are shown in figure 1. However, it may be only user provide separate ports and source of all the common user communication. There may be a change in this. The source user does not need all of the same type. They may be completely different types of data devices serve different applications and different speed requirements.
Within the context of premise data communications a typical situation where the need for multiplexing arises is illustrated in figure 2. This shows a cluster of terminals. In this case there are six terminals. All of these terminals are fairly close to one another. All are at a distance from and want to communicate with a multi-user computer. This can be a multi-use PC or a mini-computer. This situation may arise when all of the terminals are co-located on the same floor of an office building and the multi-user computer is in a computer room on another floor of the building.
The communication connection of each of these terminals could be effected by the approach illustrated in figure 3. Here each of the terminals are connected to a dedicated port at the computer by a separated cable. The cable could be a twisted pair cable or a fiber optic cable. Of course, the six cables are required and the bandwidth of each cable may far exceed the terminal to computer speed requirements.
Figure 2: Terminal cluster isolated from multi-user computer
Figure 3: Terminals in cluster. Each connected by dedicated cables to multi-user computer
Figure 4: Terminals sharing a single cable to multi-user computer by multiplexing
A more economically efficient way of communication connection as shown in figure 4. Here each of the six terminals are connected to a multiplexer. The data streams from these terminals are collected by the multiplexer. The streams are combined and then sent on a single cable to another multiplexer located near the multi-user computer. This second multiplexer separates out the individual terminal data streams and provides each to its dedicated port. The connection going from the computer to the terminals is similarly handled. The six cables shown in figure 3 has been replaced by the single composite link cable shown in figure 4. Cable cost has dropped significantly. Of course, this comes at the cost of two multiplexers. Yet, if the terminals are in a cluster the tradeoff is in the direction of a net decrease in cost.
There are two techniques for carrying out multiplexing on fiber optic cable in the premise environment. These two techniques are Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM). These techniques are described in the sequel. Examples of specific products to
implement these technologies are introduced. These products are readily available from FiberStore.
Time Division Multiplexing (TDM) with Fiber Optic Cable
With TDM a multiplicity of communication links, each for a given Source – User pair, share the same fiber optic cable on the basis of time. The multiplexer set up a continuous sequence of time using the clock. The duration of the period depends on a number of different engineering design factors; most notably the needed transmission speeds for the different links. Each communication link assigned a specific time period, TDM channel, during this period it is allowed to send its data from the source to the client. At this time there is no other link allows sending data. The multiplexer in source side receiving data from the source is connected to it. Then load the data from each source to the TDM channel accordingly. In multiplexer uninstall from each channel at the end of the user data and sends it to the corresponding user.
Wavelength Division Multiplexing (WDM) With Fiber Optic Cable
With WDM a multiplicity of communication links, each for a given Source-User pair, share the same fiber optic cable on the basis of wavelength. The data steam from each source is assigned an optical wavelength. The multiplexer has within it the modulation and transmission processing circuitry. The multiplexer modulates each data stream from each source. After the modulation process the resulting optical signal generated for each source data stream is placed on its assigned wavelength. The multiplexer then couples the totality of optical signals generated for all source data streams into the fiber optic cable. These different wavelength optical signals propagate simultaneously. This is in contrast to TDM.
The fiber optic cable is thereby carved into a multiplicity of data links – each data link corresponding to a different one of these optical wavelengths assigned to the Sources. At the user end the multiplexer receives these simultaneous optical signals. It separates these signals out according to their different wavelengths by using prisms. This constitutes the demultiplexing operation. The separated signal corresponding to the different source user data streams. These are further demodulation. The resulting data separated data streams are then provided to the respective users.