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Chapter 3 • Time Division Multiplexing • The concept of Time Division Multiplexing • TDM Examples • Frame Synchronization • TDM Hierarchy • Packet Transmission Huseyin Bilgekul Eeng360 Communication Systems I Department of Electrical and Electronic Engineering Eastern Mediterranean University
Time Division Multiplexing Definition: Time Division Multiplexing (TDM) is the time interleaving of samples from several sources so that the information from these sources can be transmitted serially over a single communication channel. At the Transmitter • Simultaneous transmission of several signals on a time-sharing basis. • Each signal occupies its own distinct time slot, using all frequencies, for the duration of the transmission. • Slots may be permanently assigned on demand. At the Receiver • Decommutator (sampler) has to be synchronized with the incoming waveform Frame Synchronization • Low pass filter • ISI – poor channel filtering • Feedthrough of one channel's signal into another channel -- Crosstalk Applications of TDM: Digital Telephony, Data communications, Satellite Access, Cellular radio.
Time Division Multiplexing Conceptual diagram of multiplexing-demultiplexing. PAM TDM System
Block diagram of TDM system. PAM TDM System A Typical Framing Structure for TDM
Time Division Multiplexing Frame structure of a certain TDM signal Composite Signal Format
Time Division Multiplexing Pulse width of TDM PAM: Pulse width of TDM PCM:
Pulse Stuffing in TDM • Stuff bits, which are dummy bits are inserted in the TDM output data when the different inputs are not completeley synchronized or the different input rates are not related by a ratinal number.
Pulse Stuffing in TDM • Stuff bits, which are dummy bits are inserted in the TDM output data when the different inputs are not completeley synchronized or the different input rates are not related by a ratinal number. Multiplexing of two data streams with bit stuffing
TDM Example • Source 1: 2 kHz bandwidth. • Source 2: 4 kHz bandwidth. • Source 3: 2 kHz bandwidth. • Source 4-11: Digital 7200 bits/sec. 16 ksam/s 64 kb/s 8x7.2=57.6 kb/sUse stuff bits to complete 7.2 to 8 kb/s.Now 8 and 64 rates are complete multıples 128 kb/s
Frame Synchronization • To sort and direct the received multiplexed data to the appropriate output channel • Two ways to provide frame sync to the demultiplexer circuit - Over a separate channel - Deriving from the TDM signal itself • Frame sync (unique k-bits) +Information words of an N-channel TDM system
Packet Transmission System • TDM is Synchronous Transfer Mode (STM) technology - Data source is assigned a specific time slot – fixed data rate - More efficient when sources have a fixed data rate - Inefficient to accommodate bursty data source Solution? • Packet Transmission System - Partitions source data into data packets (destination address, header) - Efficiently assigns network resources when the sources have bursty data - Examples : Internet TCP/IP technology and the Asynchronous Transfer Mode (ATM) technology.
Summary • How information in analog waveforms can be represented by digital signaling • How to compute the spectra for line codes • How filtering of the digital signal, due to the communication channel affects our ability to recover the digital information at the receiver [ISI] • How we can merge information from several sources into one digital signal by using time division multiplexing (TDM)