Dynamic synchronous Transfer Mode
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Dynamic synchronous Transfer Mode , or DTM for short, is a networking technology, standardized by ETSI. It is designed to provide a guaranteed Quality of Service (QoS) for the emerging class of e.g. streaming video services, but can be used for packet based services as well. It has its most use in Professional Media Networks, MobileTV networks, Digital Terestrial Television (DTT) networks, in Content Distribution Networks and in consumer oriented networks, such as "Triple Play" networks.
It is a combined switching and transport technology for optical fiber based networks, aimed to provide a "QoS" or bandwidth management layer in-between the transmission layer and the upper IP/Service layers.
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[edit] Introduction
[edit] Basic principle
In DTM, capacity is allocated to a channel by assigning a number of time slots to it. I.e. 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 also varying this number of slots as user 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.
The time slots belongs to a "DTM frame" that is structured as follows. 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.
[edit] Switching
The DTM standard also specfies that DTM channels may be switched, which sets it apart from ordinary transmission techniques, such as SDH or SONET. A DTM channel is thus 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 that also has the guaranteed QoS property, since resources are physically allocated per channel also in the switch. This as opposed to packet or cell based routers/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.
[edit] Why not packet/cell switching?
Packet/cell networks are good (actually were designed) to perform statistical multiplexing. This means that when different packet streams in a switch or router arrives 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 is a great property for best effort traffic, for which no QoS level is specified. However with the increasing demand for streaming media with tight QoS requirements, this paradigm falls short. Streaming traffic is not particularly statistical in its nature and is better served by a fixed resource allocated channel end-to-end.
It is to this kind of streaming media services the DTM technology is typically applied. i.e for 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. Indeed there are under-going efforts for adapting IP and Ethernet technologies for streaming media (see for example Professional video over IP). These efforts aims in much to emulate the basic principles for DTM with rather complicated mechanisms.
DTM is specified in ETSI recommendations ETSI ES 201 803.
