Synchronous transfer mode

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Unlike interactive data applications, the traffic generated by voice and video sources requires a constant delivery rate to ensure faithful reproduction of the signal at the receiving end. Statistical multiplexing is unacceptable for these applications because of the variable delay a statistical multiplexer can introduce. Round-robin TDM allocates a time slice, or time slot, to each user on the access link at fixed time intervals. A time slot is dedicated to a user and will remain idle if the user has no information to send. This reservation of time slots provides a constant delivery of information to the receiver.

In digital systems, a single time slot represents a 64 kbps channel. For applications requiring more bandwidth than 64 kbps, such as video, multiple time slots can be allocated to a single user application. The capacity of the transmission system is determined by the number of users and the size of the time slots required by each user. This transmission scheme is sometimes called synchronous transfer mode (STM).


The visual shows STM multiplexing. Four users running different applications are attached to a single STM multiplexer. Because bandwidth is dedicated to each user, whether or not it is used, the multiplexer output must be at a rate that is the sum of the four input lines. If each input line operates at 64 kbps, for example, then the output line must operate at a speed of at least 256 kbps. The synchronous nature of this form of communication is that time implies ownership; therefore, the time at which an item of data arrives actually identifies the owner of the data.

In the visual, the four inputs represent (from top to bottom) a voice connection (V), a packet data connection (P1), an idle connection, and another packet data connection (P2). Assuming that the STM multiplexer takes one octet of information from each input 8000 times per second (or once every 125 µs), then the output link will require sufficient bandwidth to handle four octets every 125 µs. Again, note the amount of wasted bandwidth due to unused time slots.

This type of multiplexing scheme is very common in isochronous environments. Isochronous applications are those where precise timing on an end-to-end basis is critical, such as voice and circuit emulation services. In these environments, the two end-user devices must send and receive samples at constant intervals so that the two devices appear to use the (nearly) exact same clock rate.