Multiplexing Case

Multiplexing

Asynchronous Mode

Statistical time-division multiplexing (stat TDM)

Statistical TDM (STDM) is an advanced version of TDM in which both the address of the terminal and the data itself are transmitted together for better routing. Using STDM allows bandwidth to be split over one line. Many college and corporate campuses use this type of TDM to distribute bandwidth.

If there is one 10-MBit line coming into the building, STDM can be used to provide 178 terminals with a dedicated 56k connection (178 * 56k = 9.96Mb). A more common use however is to only grant the bandwidth when that much is needed. STDM does not reserve a time slot for each terminal, rather it assigns a slot when the terminal is requiring data to be sent or received.

This is also called asynchronous time-division multiplexing (ATDM), in an alternative nomenclature in which STDM designates "synchronous time-division multiplexing", the older method that uses fixed time slots.

Synchronous Mode

Synchronous digital hierarchy (SDH)

Plesiochronous digital hierarchy (PDH) was developed as a standard for multiplexing higher order frames. PDH created larger numbers of channels by multiplexing the standard Europeans 30 channel TDM frames. This solution worked for a while; however PDH suffered from several inherent drawbacks which ultimately resulted in the development of the Synchronous Digital Hierarchy (SDH). The requirements which drove the development of SDH were these:

* Be synchronous - All clocks in the system must align with a reference clock.

* Be service-oriented - SDH must route traffic from End Exchange to End Exchange without worrying about exchanges in between, where the bandwidth can be reserved at a fixed level for a fixed period of time.

* Allow frames of any size to be removed or inserted into an SDH frame of any size.

* Easily manageable with the capability of transferring management data across links.

* Provide high levels of recovery from faults.

* Provide high data rates by multiplexing any size frame, limited only by technology.

* Give reduced bit rate errors.

SDH has become the primary transmission protocol in most PSTN networks. It was developed to allow streams 1.544 Mbit/s and above to be multiplexed, in order to create larger SDH frames known as Synchronous Transport Modules (STM). The STM-1 frame consists of smaller streams that are multiplexed to create a 155.52 Mbit/s frame. SDH can also multiplex packet based frames e.g. Ethernet, PPP and ATM.

While SDH is