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Time division multiplex

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Time Division  Multiplexing (TDM)

Time division multiplexing (TDM) is a communications process that transmits two or more streaming digital signals over a common channel. In TDM, incoming signals are divided into equal fixed-length time slots. After multiplexing, these signals are transmitted over a shared medium and reassembled into their original format after de-multiplexing. Time slot selection is directly proportional to overall system efficiency.TDM was initially developed in 1870 for large system telegraphy implementation. Packet switching networks use TDM for telecommunication links, i.e., packets are divided into fixed lengths and assigned fixed time slots for transmission. Each divided signal and packet, which must be transmitted within assigned time slots, are reassembled into a complete signal at the destination. TDM is comprised of two major categories: TDM and synchronous time division multiplexing (sync TDM). TDM is used for long-distance communication links and bears heavy data traffic loads from end users. Sync TDM is used for high-speed transmission. During each time slot a TDM frame (or data packet) is created as a sample of the signal of a given sub-channel; the frame also consists of a synchronization channel and sometimes an error correction channel. After the first sample of the given sub-channel (along with its associated and newly created error correction and synchronization channels) are taken, the process is repeated for a second sample when a second frame is created, then repeated for a third frame, etc.; and the frames are interleaved one after the other. When the time slot has expired, the process is repeated for the next sub-channel. Examples of utilizing TDM include digitally transmitting several telephone conversations over the same four-wire copper cable or fiber optical cable in a TDM telephone network; these systems may be pulse code modulation (PCM) or plesiochronous digital hierarchy (PDH) systems. Another example involves sampling left and right stereo signals using resource interchange file format (RIFF), also referred to as waveform audio file format (WAV), audio standard interleaves. Also synchronous Digital Hierarchy (SDH) and synchronous optical networking (SONET) network transmission standards have incorporated TDM; and these have surpassed PDH. TDM can also be used within time division multiple access (TDMA) where stations sharing the same frequency channel can communicate with one another. GSM utilizes both TDM and TDMA. Time division multiplexing (TDM) is also known as a digital circuit switched.


Time-division  multiplexing (TDM)

– each channel occupies the entire bandwidth of the link for a very short period of time

– a channel is made up of a sequence of time slots

– Synchronous TDM

• time slot are assigned to each channel in a regular sequence

• Example: multiplexing digitalized voice signals and data streams

Transmitter

 

A number of signals are to be multiplexed onto the same transmission medium. The incoming data from each source are briefly buffered. Each buffer is typically one bit or one character in length. The buffers are scanned sequentially to form a composite digital data stream .The scan operation is sufficiently rapid so that each buffer is emptied before more data can arrive .The composite digital signal stream may be transmitted directly or passed through a modem so that an analog signal is transmitted. The data are organized into frames. Each frame contains a cycle of time slots. In each frame, one or more slots is dedicated to each data source. The sequence of slot dedicated to one source, from frame to frame, is called a channel Length of slot .The slot length equals the transmitter buffer length, typically a bit or a character. The characteristic-interleaving technique is used with asynchronous sources. Each time slot contains one character of data .The start and stop bits of each character are eliminated before transmission and reinserted by the receiver.

Receiver

 The interleaved data are de multiplexed and routed to the appropriate destination buffer Synchronous TDM is called synchronous not because synchronous transmission is used but because the time slots are pre assigned to sources and fixed. The time slots for each source are transmitted whether or not the source has data to send capacity is wasted to simplify the hardware implementation It is possible for a synchronous TD device to handle sources of different data rates. For example, the slowest input device could be assigned one slot per cycle, while faster devices are assigned multiple

   Source 1: Analog, 2 kHz bandwidth

– Source 2: Analog, 4 kHz bandwidth

– Source 3: Analog, 2 kHz bandwidth

– Source 4-11: Digital, 7200 bps synchronous

The analog sources are converted to digital using PCM Sampling rate = 2 x bandwidth .The samples are quantized using a 4 bit A/D For the digital sources, pulse stuffing is used to raise each source to a rate of 8 kbps .A frame can consist of multiple cycles of 32 bits, each containing 16 PCM bits and two bits from each of the eight digital source

Time division multiplex

Time division multiplexing is a technique used to transmit a signal over a single communication channel by dividing the time frame into slots – one slot for each message signal. Time-division multiplexing is primarily applied to digital signals as well as analog signals,wherein several low speed channels are multiplexed into high-speed channels for transmission. Based on the time, each low-speed channel is allocated to a specific position, where it works in synchronized mode. At both the ends, i.e.,the multiplexer and demultiplexer are timely synchronized and simultaneously switched to the next channel.

Time division multiplexing is classifieds into four types:

  • Synchronous time-division multiplexing
  • Asynchronous time-division multiplexing
  • Interleaving time-division multiplexing
  • Statistical time-division multiplexing


synchronous Time-Division  Multiplexing

Synchronous time division multiplexing can be used for both analog and digital signals. In synchronous TDM, the connection of input is connected to a frame. If there are ‘n’ connections, then a frame is divided into ‘n’ time slots – and, for each unit, one slot is allocated – one for each input line. In this synchronous TDM sampling, the rate is same for all the signals, and this sampling requires a common clock signal at both the sender and receiver end. In synchronous TDM, the multiplexer allocates the same slot to each device at all times.


Asynchronous Time-Division  Multiplexing

In asynchronous time-division multiplexing, the sampling rate is different for different signals, and it doesn’t require a common clock. If the devices have nothing to transmit, then their time slot is allocated to another device. Designing of a commutator or de-commutator is difficult and the bandwidth is less for time-division multiplexing. This type of time-division multiplexing is used in asynchronous transfer mode networks.



Interleaving

Time-division multiplexing can be visualized as two fast rotating switches on the multiplexing and demultiplexing side. At the same speed these switches rotate and synchronize, but in opposite directions. When the switch opens at the multiplexer side in front of a connection, it has the opportunity to send a unit into the path. In the same way, when the switch opens on the demultiplexer side in front of a connection that has the opportunity to receive a unit from the path. This process is called interleaving.


Statistical Time-Division Multiplexing

Statistical time-division multiplexing is used to transmit several types of data concurrently across a single transmission cable. This is often used for managing data being transmitted via LAN or WAN. The data is simultaneously transmitted from the input devices that are connected to the network including printers, fax machines, and computers.This type of multiplexing is also used in telephone switch board settings to manage the calls. Statistical TDM is similar to dynamic bandwidth allocation,an in this type of time-division multiplexing, a communication channel is divided into an arbitrary number of data streams. These are the different types of multiplexing techniques used in communication system for efficient transferring and receiving of the data. 


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