TDM-Time Division Multiplexing Technique.
What is TDM?
Time-division multiplexing (TDM) is a method of putting multiple data streams in a single signal by separating the signal into many segments, each having a very short duration. Each individual data stream is reassembled at the receiving end based on the timing.
1. TDM is the digital multiplexing technique.
2. In TDM, the channel/link is not divided on the basis of frequency but on the basis of time.
3. Total time available in the channel is divided between several users.
4. Each user is allotted a particular a time interval called time slot or time slice during which the data is transmitted by that user.
5. Thus each sending device takes control of entire bandwidth of the channel for fixed amount of time.
6. In TDM the data rate capacity of the transmission medium should be greater than the data rate required by sending or receiving devices.
7. In TDM all the signals to be transmitted are not transmitted simultaneously. Instead, they are transmitted one-by-one.
8. Thus each signal will be transmitted for a very short time. One cycle or frame is said to be complete when all the signals are transmitted once on the transmission channel.
9. The TDM system can be used to multiplex analog or digital signals, however it is more suitable for the digital signal multiplexing.
10. The TDM signal in the form of frames is transmitted on the common communication medium.
Types of TDM:
1. Synchronous TDM
2. Asynchronous TDM
Multiplexing Process in STDM:
1. In STDM every device is given the opportunity to transmit a specific amount of data onto the link.
2. Each device gets its turn in fixed order and for fixed amount of time. This process is known as interleaving.
3. We can say that the operation of STDM is similar to that of a fast interleaved switch. The switch opens in front of a device; the device gets a chance to place the data onto the link.
4. Such an interleaving may be done on the basis of a hit, a byte or by any other data unit.
5. In STDM, the interleaved units are of same size i.e. if one device sends a byte, other will also send a byte and so on.
6. As shown in the fig. interleaving is done by a character (one byte). Each frame consists of four slots as there are four input devices. The slots of some devices go empty if they do not have any data to send.
7. At the receiver, demultiplexer decomposes each frame by extracting each character in turn. As a character is removed from frame, it is passed to the appropriate receiving device.
Advantages of TDM:
1. Full available channel bandwidth can be utilized for each channel.
2. lnter modulation distortion is absent.
3. TDM circuitry is not very complex.
4. The problem of crosstalk is not severe.
Disadvantages of TDM:
1. Synchronization is essential for proper operation.
2. Due to slow narrow band fading, all the TDM channels may get wiped out.
TDM Time Division Multiplexing Circuit Diagram: