Time Division Multiplexing (TDM)

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TDM is method of sharing a communication channel in which the total time available in the channel is divided between several users and each user of the channel is allotted a time slice during which he may transmit a message. The capacity is fully utilized by interleaving a number of data streams belonging to different users into one data stream.

FDM suffers from the problem that channels are permanently to be assigned. Although TDM is more efficient than FDM, in that it does not require guard bands and it operates directly in digital form but both are left behind by the advantages of Statistical Time Division Multiplexing (STATDM) which takes advantage of the statistics in data transfer in several sophisticated ways. This permits the efficiency of channel use to increase by ten-fold and in some cases even more.

           

            TDM is fast because it does not do error checking. Data are transparent to it. It just provides the slots for the data. TDM works well enough for applications that need continuous slots on the channel, such as voice and video.

                                                

            Consider sending of three messages of varying length as shown on figure 1. The main drawback is that message C must wait until messages A and B are sent before it can be transmitted.

                                                

            In Figure 2, the three messages are to be sent to three different places. The three stations should receive the messages just at the same time. To solve such a problem, all the three messages need to be reformed into smaller parts called packets. These packets are of equal length, as seen in Figure 3.

            

                        

            The packets forming messages A, B and C are interleaved and assigned time slots as seen in the lower diagram of this figure. A header (shaded area), containing the address and packet number information, precedes each packet. The interleaved packets are transmitted and received by the receiving station. The appropriate packets (determined by destination address in the header) are extracted by each station as they are received and reassembled (by packet number, included in the header) into their original message form. This is the full operation of Time Division Multiplexing. The two basic forms of TDM are:

                   

                   1.      Synchronous Time Division Multiplexing (STDM)

                   2.      Asynchronous Time Division Multiplexing (ASTDM) or Statistical TDM (STATDM)

Synchronous Time Division Multiplexing (STDM)

Synchronous TDM assigns time slots of equal length to all packets regardless whether or not anything is to be sent by each station with an assigned time slot. For example, if the message A is not having any message to sent, then its allotted time would still be allocated. Thus, A would still be allotted time but time slots for message A would not contain information. STDM systems are comparatively easy to implement once the software allocates the time slots.

Asynchronous Time Division Multiplexing (ASTDM)

ASTDM or STATDM does not make a fixed assignment of time slots so that any port which is idle does not receive a (full) slot. In order to identify which slot corresponds to which data stream, it is necessary to append address and control symbols to each slot that is used. This “overhead” is small and is more than compensated for by the increased efficiency.

            

            These systems are more complex but they allow the means of reassigning time slots that are not in use. STATDM networks assign time slots only when they are to be used and delete them when they are idle. The total time used for a STATDM frame varies with the amount of traffic currently being handled. STATDM systems are most suitable for high-density, high traffic applications. The continuous messages are assigned time slots and interleaved as each channel on the send side becomes active and requires communications with another channel. If a channel does not find any traffic, its time slots are deleted and reassigned to an active channel. In this way the interconnecting media achieves a higher efficiency that with STDM systems.

           

             Figure 4 illustrates the comparison of FDM, TDM and Statistical TDM with fixed frame and variable frame methods. In the variable-frame method of STDM, the size of the slots and the frame are not fixed but depend on the data itself. Statistical TDM are also built either for asynchronous or synchronous data or both.

            

           TDM and STATDM require a modem in order to interface with the voice line, but this may be builtin device. All modern STATDM systems have at least one or more microprocessors with programmed and programmable functions of great diversity available. They are thus named as “Smart” or “Intelligent MUXs or multiplexers.”

Multichannel TDM

Many channels of communication on a single line are managed by a broadband system. In this method, each channel occupies a portion of that bandwidth. This would require the bandwidth of the system to be larger so as to contain all the channels. Multichannel use of TDM system relies on sharing transmission time periods rather than a system’s bandwidth. [See Figure 5]