Time Division Multiplexing (TDM ) is an analog or digital multiplexing technology in which several signals or bitstreams are transmitted simultaneously as subchannels in one communication channel.
Data transmission in such a channel is divided into time intervals ( time slots ) of a fixed length, separate for each channel. For example: a data block or subchannel 1 is transmitted during time slot 1, subchannel 2 to time slot 2, etc. One TDM frame consists of one time slot allocated to one particular subchannel. After the transmission of the frame of the last of the subchannels, the frame of the first subchannel, etc., is transmitted in order.
There was a version that in the future TDM may give way to ATM (although this technology has been introduced extremely slowly, and today the development of ATM is completely stopped); seems more realistic [to whom? ] that TDM may give way to IP ; but today, TDM is the predominant local access technology.
Content
- 1 TDM and packet data
- 2 Transmission using TDM
- 3 Usage Examples
- 4 Synchronous TDM (Sync TDM)
- 5 See also
- 6 notes
- 7 References
TDM and Packet Data
In its primary form, TDM is used for communication circuits using a constant number of channels and constant bandwidth in each channel.
The main difference between time division multiplexing and statistical multiplexing, such as packet multiplexing, is that the time slots in it follow in a given, periodically repeating order, in contrast to batch processing (as packets arrive). Statistical multiplexing is similar, but should not be construed as time division multiplexing.
In dynamic TDMAccess, the scheduling algorithm dynamically reserves a variable number of time slots to organize a dynamic change in throughput based on the traffic requirements of each data stream. Dynamic TDMA is used in:
- IEEE 802.11 ;
- IEEE 802.16a .
TDM Transmission
In networks with switched communication channels, such as, for example, city public telephone networks, there is a need to transmit simultaneously many calls of different subscribers in one transmission medium. To implement this task, you can use TDM. The standard voice signal ( DS0 ) uses 64 kbps. TDM takes voice frames and multiplexes them into TDM frames, which are transmitted with higher bandwidth. Thus, if a TDM frame contains n voice frames, then the throughput will be n * 64 kbit / s.
Each voice time slot in a TDM frame is called a channel. In European systems, the TDM frame consists of 30 digital voice channels, in the American standard there are 22 of them (see E1 and T1 ). Both of these standards include bit timeslots for signaling (see: OKS-7 ) and synchronization bits.
Multiplexing of more than 30 and 22 digital voice channels is called higher order multiplexing, which can be achieved by multiplexing standard TDM frames. For example, a European 120-channel TDM frame is formed by multiplexing four 30-channel TDM frames. Each higher-order multiplexing combines 4 frames of the previous order created by n × 64 kbit / s multiplexing, where n = 120, 480, 1920, etc.
Use
- In PDH , also known as PCM systems, for the digital transfer of multiple telephone calls over a single copper cable with four wires ( T1 or E1 lines) or over a fiber optic line;
- In SDH and Synchronous Optical Networks ( SONET );
- RIFF ( WAV ), a sound standard, alternating left and right stereo channels from one source;
- In stereoscopic glasses, when divided into left and right channels.
TDM can be expanded in the future for use in time division multiple access ( TDMA ) networks, where several stations, with the same physical communication medium, can communicate together using the same frequency channel (for example, GSM networks).
Synchronous TDM (Sync TDM)
There are three types of Sync TDM: T1 , SONET / SDH , and ISDN .
See also
- TDMA
- Fdma
- CDMA