Dynamic synchronous Transfer Mode
Encyclopedia
Dynamic synchronous transfer mode (DTM) is an optical networking technology standardized by the European Telecommunications Standards Institute
(ETSI) in 2001 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 was marketed for professional media networks, mobile TV networks, digital terrestrial television (DTT) networks, in content delivery network
s and in consumer oriented networks, such as "triple play
" networks.
(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.
(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.
DTM technology was 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.
(KTH) in Sweden
.
It was published in February 1996.
European Telecommunications Standards Institute
The European Telecommunications Standards Institute is an independent, non-profit, standardization organization in the telecommunications industry in Europe, with worldwide projection...
(ETSI) in 2001 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
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...
(QoS) for streaming video services, but can be used for packet-based services as well. It was marketed for professional media networks, mobile TV networks, digital terrestrial television (DTT) networks, in content delivery network
Content Delivery Network
A content delivery network or content distribution network is a system of computers containing copies of data placed at various nodes of a network....
s and in consumer oriented networks, such as "triple play
Triple play (telecommunications)
In telecommunications, triple play service is a marketing term for the provisioning of two bandwidth-intensive services, high-speed Internet access and television, and a less bandwidth-demanding service, telephone, over a single broadband connection. Triple play focuses on a combined business...
" networks.
Basic principle
In DTM, capacity is allocated to a channel by assigning a number of time slots to it. It is basically a time-division multiplexingTime-division multiplexing
Time-division multiplexing is a type of digital multiplexing in which two or more bit streams or signals are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent...
(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.
Switching
DTM specfies that channels may be switched, which sets it apart from common transmission techniques such as Synchronous Digital Hierarchy (SDH) or Synchronous optical networkingSynchronous optical networking
Synchronous Optical Networking and Synchronous Digital Hierarchy are standardized multiplexing protocols that transfer multiple digital bit streams over optical fiber using lasers or highly coherent light from light-emitting diodes . At low transmission rates data can also be transferred via an...
(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?
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 was 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
Professional video over IP
Professional video over IP systems use some existing standard video codec to reduce the program material to a bitstream , and then to use an Internet Protocol network to carry that bitstream encapsulated in a stream of IP packets...
).
Alternately, applying Multiprotocol Label Switching
Multiprotocol Label Switching
Multiprotocol Label Switching is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links between...
(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
Forward error correction
In telecommunication, information theory, and coding theory, forward error correction or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels....
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
Carrier Ethernet
Carrier Ethernet is a marketing term for extensions to Ethernet to enable telecommunications network providers to provide Ethernet services to customers and to utilize Ethernet technology in their networks....
technologies such as Provider Backbone Bridge Traffic Engineering
Provider Backbone Bridge Traffic Engineering
Provider Backbone Bridge Traffic Engineering is an approved telecommunications networking standard, IEEE 802.1Qay-2009. PBB-TE adapts Ethernet technology to carrier class transport networks...
made Ethernet suitable for voice and video transmission with properties such as hard QoS, deterministic forwarding and sub 50ms protection switching.
History
The DTM architecture was conceived in 1985 and developed at the Royal Institute of TechnologyRoyal Institute of Technology
The Royal Institute of Technology is a university in Stockholm, Sweden. KTH was founded in 1827 as Sweden's first polytechnic and is one of Scandinavia's largest institutions of higher education in technology. KTH accounts for one-third of Sweden’s technical research and engineering education...
(KTH) in Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
.
It was published in February 1996.