New time division technology flattens SDH

Beleagured multiplexing transmission technology could be reinvented as a way to improve service in Internet Protocol networks.

Time division multiplexing (TDM)--eclipsed in mobile phone networks after AT&T Wireless decided to build a GSM overlay across its TDMA platform in the United States--could get a new least of life in wireline networks.

A new high bandwidth management technology called dynamic synchronous transfer mode (DTM) is being tested in Europe and the U.S. by alternative carriers looking to get an edge in service standards on all-IP networks. The technology could soon get its stamp of approval from the U.S. standards body, the Institute of Electrical and Electronics Engineers (IEEE), of Piscataway, New Jersey.

Per Lembre, of Dynarc AB of Stockholm, which has developed proprietary IP ring-technology based on DTM, claims the approach makes efficient use of bandwidth and supports multiservice "everything over IP."

DTM allows bandwidth to be reserved in increments of 512 kilobits per second between a source and one or more destinations in a ring. This can be changed dynamically by adding or removing time slots, based on synchronous time division multiplexing. Once set up the bandwidth link is guaranteed.

Some view DTM as a reinvention of synchronous digital hierarchy (SDH). "It is what TDM should have been if designed to carry IP in the first place," said David Brown, chairman of telecoms consultancy Sche-ma Ltd., of London. Unlike the hierarchical SDH, Dynarc's DTM is based on a single layer: "They have flattened it but at a price of some complexity."

Already, Nordic competitive local exchange carrier Tele1 Europe Holding AB, of Stockholm, is testing Dynarc's optical networking equipment and plans to use the technology for an IP ring to link all four Nordic capitals.

"It has a lot of advantages that you can't get with normal router technology," said Ingvar Eriksson, director networking at Tele1 Europe Holding AB.

Eriksson cited as an example the difficulty in guaranteeing customers a 100 megabit per second link between offices all year round. "If you put it on a Cisco router you have to over-dimension a lot," he said. DTM's ability to reserve bandwidth slots solves this.

Other advantages Tele1's Eriksson highlights include DTM's support for billing of services. "You can charge for volume, or volume and time," he said, stressing its similarity to a telephone switch. Its protection switching mechanism, the time it takes to recover a network failure, is something else he commends: "With the DTM technique, the protective switching is 50 milliseconds." Tele1's Nordic ring will include six cities in total linking 60 business areas by mid-2001.

Existing IP-oriented networks, based on best effort technology, require operators to provision extra network capacity to ensure a customer's service agreement is met. As operators seek to boost their revenues and stay ahead of the competition, they are introducing differentiated services using rigorous quality of service techniques.

How best to do this is open to debate. Traffic management technology such as diffserv and multi-protocol label switching are being adopted by the operators. But they admit that no one scheme meets all service requirements.

Interest in Dynarc's DTM switch equipment is not confined to Europe: another adopter is Airpower communications Inc., a metropolitan network provider based in Los Angeles.

Dynarc downplays the fact that its switch is based on proprietary DTM technology and that as a startup, it cannot expect its technology to be employed end-to-end in a network.

Dynarc's Lembre said that a new IEEE 802.17 working group will be launched: "DTM will be a standard, and we will be compliant."

Tele1 is still testing Dynarc's switch technology and Eriksson admitted that Tele1 has devised an "escape policy" should it still be required. But for new services, such as streaming video and video on demand, "this technology is ideal," he said.

COPYRIGHT 2001 EMAP Media Ltd.
COPYRIGHT 2001 Gale Group