Dynamic synchronous transfer mode
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|Internet protocol suite|
Dynamic synchronous transfer mode (DTM) is an optical networking technology standardized by the European Telecommunications Standards Institute (ETSI) in 2001 beginning with specification ETSI ES 201 803-1. DTM is a time division multiplexing and a circuit-switching network technology that combines switching and transport. It is designed to provide a guaranteed quality of service (QoS) for streaming video services, but can be used for packet-based services as well. It is marketed for professional media networks, mobile TV networks, digital terrestrial television (DTT) networks, in content delivery networks and in consumer oriented networks, such as "triple play" networks.
In DTM, capacity is allocated to a channel by assigning a number of time slots to it. It is basically a time-division multiplexing (TDM) system. What sets it apart from other TDM systems is the capability to assign any number of time slots to a channel, and vary this number of slots as traffic demands. The basic argument for this technique is that it provides a guaranteed QoS for a service since resources are physically allocated to the channel and traffic from other channels will have no impact on this channel.
Time slots belong to a "DTM frame". The frame is 125 µs long and contains a number of 64-bit time slots. Thus the number of time slots per frame depends on the link bit-rate. A number of these time slots are associated to form a channel. The simplest channel consists of 1 time slot that is repeated each 125 µs. The capacity of this one slot channel is then 64 bits / 125 µs = 512 kbit/s. A channel consisting of N time slots thus have a capacity of N x 512 kbit/s. Thus 512 kbit/s is the "granularity" of bandwidth allocation for a service.
DTM specifies that channels may be switched, which sets it apart from common transmission techniques such as Synchronous Digital Hierarchy (SDH) or Synchronous optical networking (SONET). A DTM channel is automatically provisioned end-to-end over a general topology network using control signalling. DTM is thus a circuit switched system. The switches are generally time-space switches with the guaranteed QoS property, since resources are physically allocated per channel also in the switch. This is opposed to packet or cell based switches, in which the packets and cells are competing for resources and as a result of this competition may have packets or cells delayed or discarded. For packet and cell switches this shared resource allocation mechanism imposes a limit to how high the utilization of a network can be before the QoS get un-acceptably low. In DTM network there is no such shared resource allocation, implying that a network theoretically can be loaded to 100% and still have guaranteed QoS for its services. Real utilization becomes thus more a question of adapting the network topology and link capacities to the actual traffic matrix than to accommodating for QoS.
Why not packet/cell switching?
||This section possibly contains original research. (June 2012)|
Packet and cell networks were designed to perform statistical multiplexing. This means that when different packet streams in a switch or router arrive to a common outgoing link they are buffered until there are resources free on this link. This makes it possible to utilize the outgoing link to a high degree at the cost of varying delays. This fits best effort traffic; however streaming media has specific QoS requirements. Streaming traffic is not particularly statistical in its nature and is better served by a fixed resource allocated channel end-to-end.
DTM technology is typically applied to video or audio services. This does not exclude IP traffic which can gain from being served by a guaranteed QoS transport, especially if the IP traffic contains video/audio. IP and Ethernet technologies were also adapted for streaming media (see for example professional video over IP).
Alternately, applying Multiprotocol Label Switching (MPLS) technology to the carriage network can provide the reliability and determinism required by video and other streaming media. This, used in conjunction with forward error correction techniques can provide an appropriate transport mechanism with features similar to that of ATM, as ATM served as the basis of design for MPLS. However, like ATM and unlike DTM, MPLS-enabled networks still suffer from greater variances in frame jitter and packet loss under heavy congestion conditions.
Progress in carrier Ethernet technologies such as Provider Backbone Bridge Traffic Engineering made Ethernet suitable for voice and video transmission with properties such as hard QoS, deterministic forwarding and sub 50ms protection switching.
The research team was split into two spin-off companies, reflecting two different approaches to use the technology. One of these companies remains active in the field and delivers commercial products based on the DTM technology.
- ETSI ES 201 803-1 V1.1.1 Dynamic synchronous Transfer Mode (DTM); Part 1: System description. March 2001.
- Matthew N.O.Sadiku (2003). "6: Dynamic synchronous transfer mode". In Mohammad Ilyas, Hussein T. Mouftah. The handbook of optical communication networks. CRC Press. pp. 99–106. ISBN 978-0-8493-1333-2.
- C. Bohm; M. Hidell; P. Lindgren; L. Ramfelt.; Peter Sjödin (February 1996). "Fast circuit switching for the next generation of high performance networks". IEEE Journal on Selected Areas in Communications 14 (2): 298–305. doi:10.1109/49.481937.
- Lars Kahn (15 June 1998). "DTM: a networking technique for IP traffic over optical fibres". Net Insight AB.
- P. Lindgren, C. Bohm, S. Jörbeck and J. Antell (20 August 2002). "Towards a Media Extranet". Net Insight AB.