Time-division multiplexing (TDM) is a digital process that can be applied when the data rate capacity of the transmission medium is greater than the data rate required by the sending and receiving devices. In such a case, multiple transmissions can occupy a single link by subdividing them and interleaving the portions.
Figure 7.3-1 gives a conceptual view of TDM. Note that the same link is used as in the FDM; here, however, the link is shown sectioned by time rather than frequency.
In the TDM figure, portions of signals of channels 1, 2 and 3 occupy the link sequentially. As an analogy, imagine a ski lift that serves several runs. Each run has its own line and the skiers in each line take turns getting on the lift. As each chair reaches the top of the mountain, the skier riding it gets off and skis down the run for which he or she waited in line.
It should be noted that TDM is able to multiplex both analog and digital signals. In practice, however, it is used to multiplex digital signals only.
As we know that TDM can be implemented in two ways: synchronous TDM and asynchronous TDM.
7.3.1 Synchronous TDM
In time-division multiplexing, the term synchronous has a different meaning from that used in other areas of telecommunications. Here synchronous means that the multiplexer allocates exactly the same time slot to each device at all times, whether or not a device has anything to transmit. Time slot A, for example, is assigned to device A alone and cannot be used by any other device. Each time its allocated time slot comes up, a device has the opportunity to send a portion of its data. If a device is unable to transmit or does not have data to send, its time slot remains empty.
Frames Time slots are grouped into frames. A frame consists of one complete cycle of time slots, including one or more slots dedicated to each sending device, plus framing bits (see Figure 7.3-5). In a system with n input lines, each frame has at least n slots, with each slot allocated to carry data from a specific input line. If all the input devices sharing a link are transmitting at the same data rate, each device has one time slot per frame. However, it is possible to accommodate varying data rates. A transmission with two slots per frame will arrive twice as quickly as one with one slot per frame. The time slots dedicated to a given device occupy the same location in each frame and constitute that device's channel. In Figure 7.3-2, we show five input lines multiplexed onto a single path using synchronous TDM. In this example, all of the inputs have the same data rate, so the number of time slots in each frame is equal to the number of input lines.
Interleaving Synchronous TDM can be compared to a very fast rotating switch. As the switch opens in front of a device, that device has the opportunity to send a specified amount (x bits) of data onto the path. The switch moves from device to device at a constant rate and in a fixed order. This process is called interleaving.
Interleaving can be done by bit, by byte, or by any other data unit. In other words, the multiplexer can take one byte from each device, then another byte from each device, and so on. In a given system, the interleaving units will always be of the same size.
Figure 7.3-3 shows interleaving and frame building. In the example, we interleave the various transmissions by character (equal to one byte each), but the concept is the same for data units of any length. As you can see, each device is sending a different message. The multiplexer interleaves the different messages and forms them into frames before putting them onto the link.
At the receiver, the demultiplexer decomposes each frame by discarding the framing bits and extracting each character in turn. As a character is removed from a frame, it is passed to the appropriate receiving device (see Figure 7.3-4).
Figure 7.3-3 and Figure 7.3-4 also point out the major weakness of synchronous TDM. By assigning each time slot to a specific input line, we end up with empty slots whenever not all the lines are active. In Figure 7.3-3, only the first three frames are completely filled. The last three frames have a collective six empty slots. Having 6 empty slots out of 24 means that a quarter of the capacity of the link is being wasted.
Framing Bits Because the time slot order in a synchronous TDM system does not vary from frame to frame, very little overhead information needs to be included in each frame. The order of receipt tells the demultiplexer where to direct each time slot, so no addressing is necessary; Various factors, however, can cause timing inconsistencies. For this reason, one or more synchronization bits are usually added to the beginning of each frame. These bits, called framing bits, follow a pattern, frame to frame, that allows the demultiplexer to synchronize with the incoming stream so that it can separate the time slots accurately. In some cases, this synchronization information consists of one bit per frame, alternating between 0 and 1 (01010101010), as shown in Figure 7.3-5.
Synchronous TDM Example Imagine that we have four input sources on a synchronous TDM link, where transmissions are interleaved by character. If each source is creating 250 characters per second, and each frame is carrying 1 character from each source, the transmission path must be able to carry 250 frames per second (see Figure 7.3-6).
Fig. 7.3-5 Frame bits
Fig. 7.3-5 Frame bits
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