Asynchronous Time Division

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Asynchronous Time Division ( ATD ) or Asynchronous Time Division Multiplexing ( ATDM ), in German asynchronous time division multiplex (also statistical time division multiplex ), is a dynamic multiplex process in which the data blocks of the users are not transmitted in a fixed cycle, but alternately and as required.

A data stream arriving at the multiplexer is divided into packets, which are then sent in time segments of fixed or variable length ( time slots or time slices ). The packets of the different users are sent alternately. For each user it looks as if he has the channel to himself, albeit - depending on the load - at possibly slower speed.

In contrast to the synchronous time division multiplex (STD), in which each user is served alternately, but always receives a time slot of the same size while he can transmit data, with the ATD the time slices are not evenly, but distributed among the users as required. This is done by additionally sending channel information in order to clearly identify the transmitter. This makes better use of the channel when some participants are not transmitting, which enables a variable transmission rate. The disadvantage is a greater effort when restoring (demultiplexing) the data and a greater protocol effort, since this can reduce the effective bandwidth in the case of full utilization in contrast to synchronous time division multiplexing. The STD method is simpler, but less effective than the ATD, since with the STD the time slot is reserved even if there is no data to be transmitted.

Comparison of ATDM and STDM

  • ATDM makes better use of the transmission capacity of the line than STDM.
  • ATDM works worse under heavy loads, as collisions can hinder transmission when the same time slot is accessed at the same time.
  • STDM can guarantee transmission capacity to every user, which ATDM cannot.
  • STDM divides the available transmission capacity under full load exactly evenly among the users. With users, each user receives the transmission capacity.

All broadband high-speed networks use STDM.