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DTM (Dynamic Synchronous Transfer Mode)

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DTM is a form of circuit switching for fiber-optic networks that employs TDM (time division multiplexing) in a new way that dynamically reallocates available bandwidth to users that need it. DTM was designed to remove the bottleneck at fiber network access points. These bottlenecks are typically caused by the need to process and buffer large amounts packet-based data. DTM seeks to limit complexity and use transmission capacity more efficiently. In particular, DTM can fully support high-bit-rate, real-time traffic, and multicasting. When used as a link layer for IP networks, it can support strict QoS.

Along with Gigabit Ethernet and Cisco DPT (Dynamic Packet Transport), DTM is considered one of the technologies to use in new fiber-access metropolitan area networks.

Circuit switching has always been more reliable than packet switching, and provides a nonblocking data transmission system (it's predictable because you get all the bandwidth you paid for). At the same time, packet switching has many benefits, including the ability to use bandwidth efficiently by multiplexing the transmissions of many users over mesh-topology links. The global Internet is testament to the advantages of packet switching. But as network capacity has improved due to fiber-optic cable, and as bandwidth requirements have increased for real-time traffic like voice and video, the need for circuits is real.

As mentioned, DTM is a circuit-switching scheme that uses TDM. Thus, DTM can guarantee bandwidth to users of the system. DTM uses SONET/SDH framing schemes, but extends the scheme with a dynamic reallocation mechanism that can redistribute bandwidth not being used by one user to another user who needs it. DTM basically allocates, on demand, any available bandwidth to other users. New channels can be set up at very high speed (less than a millisecond).

Users can be allocated bandwidth according to several schemes. The best scheme is guaranteed bandwidth, which allocates a certain number of time slots to a user that will guarantee the bandwidth the user needs. The on-demand bandwidth scheme gives users bandwidth when they ask for it, at an extra cost. Finally, the on-demand bandwidth with best effort is a scheme that gives users bandwidth when requested, but only when it is available.

While DTM is primarily circuit oriented, it differs in several ways. First, DTM channels are simplex to achieve high bandwidth. Interactive sessions between two hosts will require two channels. DTM supports multirate bandwidth allocation, from 512 Kbits/sec up to full link capacity. DTM also supports multicast so that any one channel can be connected to any number of receivers. DTM is fault tolerant. It supports identical nodes (master and slave) and redundant dual-fiber connections between adjacent dual nodes. The switching nodes are used in parallel, but provide immediate failover in case one fails.

More information about DTM is available at the Web sites listed on the related entries page. Dynarc and Net Insight are the two primary vendors advocating DTM. Net Insight developed an 8-port single-chip DTM switch that other vendors are using to build DTM equipment.

Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright conventions.