New broadband technology boosts backbone convergence

Dynamic synchronous transfer mode (DTM) is a new broadband network technology that helps enterprise networks efficiently carry voice, data and streaming video on a single, integrated network.

Diagram of how it works

Developed by the Swedish Royal Institute of Technology, Ericsson and other companies, DTM can automatically set up connections and assign a certain number of channels or bandwidth to each connection, depending on traffic type and bandwidth requirements - all with little or no delay.

Specifically, DTM divides fiber pipe capacity into frames of 125 microseconds, which are further divided into 64-bit slots. The number of slots per frame depends on the bit rate. For example, with a bit rate of 2.5G bit/sec, the number of slots is around 4,800. DTM uses a distributed algorithm for slot reallocation - the pool of capacity, or free slots, that is distributed among DTM nodes (see graphic).

DTM is similar to SONET technology in terms of low complexity and overhead, but DTM includes signaling and switching to increase the flexibility and avoid the hierarchical structure of SONET.

One can think of DTM as a next-generation SONET with switching features and flexible, configurable and on-demand bandwidth. Once established, a DTM channel provides guaranteed service, and, depending on the channel's traffic load, the capacity of the channel can be altered during operation. DTM also provides the means to log resource usage for billing, management and administrative purposes.

As a link-layer technology that works directly on fiber, DTM may be used on top of, or in parallel with, existing SONET infrastructures. While it was primarily developed to efficiently handle IP traffic, DTM's synchronous properties also allow it to carry legacy telephony and leased-line traffic effectively over the same infrastructure as IP and streaming video. DTM maintains full isolation between the different transmission streams and produces little overhead. These synchronous properties are more important today, as IP is also carrying more real-time applications.

DTM combines the reliability and guaranteed service of SONET, and the flexibility and traffic-engineering capabilities of ATM, but at a lower cost and with less complexity. It is designed to fully utilize the almost unlimited capacity of optical fiber by emphasizing simplicity and avoiding computation-intensive policing, queuing, buffering and control mechanisms.

High logical connectivity is another advantage of DTM's multichannel interface and switching features; network nodes can easily be connected with nodes other than their physical neighbors. Together with DTM's ability to reallocate channel and on-demand bandwidth, this provides traffic-engineering capability equivalent to that of ATM networks.

DTM consolidates the number of layers between IP and fiber by combining transport, add-drop multiplexing and switching in a single box. The idea is to provide data-oriented transport and efficient and dynamic resource allocation for IP nets.

The protocol also works very well over a dense wavelength division multiplexing (DWDM) infrastructure, where DTM switches provide dynamic add-drop support between nodes sharing a wavelength, and switching of traffic between nodes situated on different wavelengths. DWDM uses light to transmit data.

Three start-ups are now designing, marketing and selling DTM products: Net Insight AB, Dynarc AB, and Effnet AB. All three are based in Sweden and have U.S. offices. Their products are interoperable on the physical and link level but not in terms of signaling. The goal is to make DTM products fully interoperable. An initiative to standardize DTM is before the European Telecommunications Standards Institute.

Toshiba and Net Insight are jointly developing a DTM chipset, integrating full switching and access features in a cascadable eight-port DTM chip with 8G bit/sec nonblocking capacity. It will be available for third-party vendors by year-end. The chipset will make products more cost-effective and will be licensed with DTM protocol software.

Several DTM networks are up and running. For example, VLT, the regional telephone and cable TV operator in Vasa, Finland, has been operating a DTM voice and data backbone since November. ICG Communications in Denver, as well as competitive local exchange carriers in Sweden, are testing DTM equipment.

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