Synchronization By: Loren K. Schwappach Prepared for: Dr. Santiago CTU, EE463, Communications 1 October 2010
First some quick notes about TDM:• As the number of independent message sources increases the intervals allotted to each source by the commutator is reduced, all sources must be accommodated into a time interval equal to the sampling interval.• Since pulses too short can impair transmit it is necessary in practice to reduce the number. In a North American Public Switched Telephone Network (PSTN) T1 System, TDM is used to transmit 24 Channels.
Synchronization:• In a PCM system with TDM it is essential that timing operations at the receiver (exempting time lost in transmission and regenerative repeating) closely follow the operations of the transmitter, this amounts to requiring a local clock at the receive operating in synchronization with the transmitter.• Used in Time Division Multiplexing to lock the commutator and decommutator and keep the transmitter and receiver in phase.
Synchronization:• One method of synchronization is to set aside a code word at the end of a frame (transmitted every other pulse) and send it via the Commutator. Thus a clock is created at half the frame rate and can be used by the receiver to establish synchronization.• Re-synchronization is the procedure of examining code elements one by one by setting the clock back by one code element until the synchronizing pulse is detected, this process often includes the use of buffers.
Low-pass Low-pass (reconstruction)Message (anti-aliasing) TDM system block diagram filters filters Message Inputs By Loren K. Schwappach Outputs 1 LPF LPF 1 Synchronized Pulse Communication Pulse 2 LPF Modulator Channel Demodulator LPF 2 Commutator DeCommutator N LPF LPF N Clock pulses Clock pulses Anti-aliasing filter: Removes non-essential frequencies. Commutator: Uses electronic switching to (1) take a narrow sample of each of the N input messages at a rate fs (slightly higher than 2W), (2) sequentially interleave these N samples inside the sampling interval. Pulse Modulator/Demodulator: Transforms the multiplexed signal into a suitable form. Often requires equalization to ensure satisfactory operation. Decommutator: Performs opposite operation of commutator. Operates in synchronization. Example of Synchronization: Sync Sync Sync Sync Sync Sync M1 Element M2 Element MN Element M1 Element M2 Element MN Element 1 0 1 0 1 0