For the purposes of the present document, the following terms and definitions apply:
Access token:- right to use a slot for transmitting data on a physical interface for a certain number of bypass hops
Address, DTM address:- 64 bit numerical value that uniquely identifies a node in a DTM network
Allocation domain:- same as a bypass chain where, if the topology is point-to-point or bus, the last node is not counted as member of the AD.
Bypass Chain (BC):- series of concatenated physical links, where data can be transported end-to-end using bypass switching.
Bypass switching:- space switching of slots from a receiver to a transmitter on the same physical interface on a per slot basis.The DTM Resource Management Protocol (DRMP) handles control of transmission resources over bypass chains.
Resource Management handles the right to use a time slot for transmission and the right to administer usage rights. The latter function is referred to as slot ownership. A physical interface owns a set of time slots on the bypass chain. The owner of a time slot has the right to use it for transmission, but can also lend the right to another physical interface if it is in greater need. Ownership can either be statically configured on a bypass chain, or dynamic. In the latter case, the Dynamic Ownership part of DRMP, distributes ownership along the physical interfaces on the bypass chain according to a configured policy. Typically the physical interfaces are configured to own an equal share that exhausts the capacity of the bypass chain.The right to transmit on a time slot can be limited spatially, so as to cover only as many physical links as is needed on a bypass chain for the intended transmission. On the next physical page link downstream of the receiver, and the next physical page link upstream of the transmitter, the same time slot can be used again for other data transmissions. This spatial reuse of time slots is referred to as slot scoping.
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DTM (Dynamic Synchronous Transfer Mode)
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 page 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 page 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.
DEFINITION AND OVERVIEW
Definition
Dynamic synchronous transfer mode (DTM) is an exciting
networking technology. The idea behind it is to provide high-speed
networking with top-quality transmissions and the ability to adapt the
bandwidth to traffic variations quickly. DTM is designed to be used in
integrated service networks for both distribution and one-to-one
communication. It can be used directly for application-to-application
communication or as a carrier for higher-layer protocols such as
Internet protocol (IP).
DTM, Dynamic synchronous Transfer Mode, is a broadband
network architecture based on circuit switching augmented with
dynamic reallocation of time slots. DTM provides a service based on
multicast, multirate channels with short set-up delay. DTM supports
applications with real-time QoS requirements as well as applications
characterized by bursty, asynchronous traffic
Overview
This tutorial explores the development of DTM in light of the
demand for network-transfer capacity. DTM combines the two basic
technologies used to build high-capacity networks—circuit and packet
switching—and therefore offers many advantages. It also provides
several service-access solutions to city networks, enterprises,
residential and small offices, content providers, video production
networks, and mobile network operators.
WHY DTM?
Over the last few years, the demand for network-transfer
capacity has increased at an exponential rate. The impact of the
Internet; the introduction of network services such as video and
multimedia that require real-time support and multicast; and the
globalization of network traffic enhance the need for cost-efficient
networking solutions with support for real-time traffic and for the
transmission of integrated data, both audio and video. At the same time,
the transmission capacity of optical fibers is today growing
significantly faster than the processing capacity of computers.
Traditionally, the transmission capacity of the network links has been
the main bottleneck in communication systems. Most existing network
techniques are therefore designed to use available page link capacity as
efficiently as possible with the support of large network buffers and
elaborate data processing at switch points and interfaces. However,
with the large amount of data-transfer capacity offered today by fiber
networks, a new bottleneck problem is caused by processing and
buffering at switch and access points on the network. This problem has
created a need for networking protocols that are not based on computer
and storage capacity at the nodes but that instead limit complex
operations to minimize processing on the nodes and maximize
transmission capacity.
Against this background, the DTM protocol was developed.
DTM is designed to increase the use of fiber's transmission capacity
and to provide support for real-time broadband traffic and multicasting.
It is also designed to change the distribution of resources to the network
nodes dynamically, based on changes in transfer-capacity demand.
DTM BASICS
CIRCUIT SWITCHING vs. PACKET SWITCHING
In principle, two basic technologies are used for building highcapacity
networks: circuit switching and packet switching. In circuitswitched
networks, network resources are reserved all the way from
sender to receiver before the start of the transfer, thereby creating a
circuit. The resources are dedicated to the circuit during the whole
transfer. Control signaling and payload data transfers are separated in
circuit-switched networks. Processing of control information and
control signaling such as routing is performed mainly at circuit setup
and termination. Consequently, the transfer of payload data within the
circuit does not contain any overhead in the form of headers or the like.
Traditional voice telephone service is an example of circuit switching.
Circuit-Switched Networks
An advantage of circuit-switched networks is that they allow for
large amounts of data to be transferred with guaranteed transmission
capacity, thus providing support for real-time traffic. A disadvantage of
circuit switching, however, is that if connections are short-lived—when
transferring short messages, for example—the setup delay may
represent a large part of the total connection time, thus reducing the
network's capacity. Moreover, reserved resources cannot be used by
any other users even if the circuit is inactive, which may further reduce
link utilization.
Packet-Switched Networks
Packet switching was developed to cope more effectively with
the data-transmission limitations of the circuit-switched networks
during bursts of random traffic. In packet switching, a data stream is
divided into standardized packets. Each contains address, size,
sequence, and error-checking information, in addition to the payload
data. The packets are then sent through the network, where specific
packet switches or routers sort and direct each single packet.
Packet-switched networks are based either on connectionless
or connection-oriented technology. In connectionless technology, such
as IP, packets are treated independently of each other inside the
network, because complete information concerning the packet
destination is contained in each packet. This means that packet order is
not always preserved, because packets destined for the same receiver
may take different paths through the network. In connection-oriented
technology such as asynchronous transfer mode (ATM), a path through
the network—often referred to as a logical channel or virtual circuit—is
established when data transfer begins. Each packet header then contains
a channel identifier that is used at the nodes to guide each packet to the
correct destination. In many aspects, a packet-switched network is a
network of queues. Each network node contains queues where
incoming packets are queued before they are sent out on an outgoing
link. If the rate at which packets arrive at a switch point exceeds the
rate at which packets can be transmitted, the queues grow. This
happens, for example, if packets from several incoming links have the
same destination link. The queuing causes delay, and if the queues
overflow, packets are lost, which is called congestion. Loss of data
generally causes retransmissions that may either add to the congestion
or result in less-effective utilization of the network. The ability to
support real-time traffic in packet-switched networks thus calls for
advanced control mechanisms for buffer handling and direction. As a
result, the complexity and necessary ability to process information, and
therefore the need for computer power, increases sharply when striving
for high transmission capacity.
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