Boosting SONET's high-speed capacity

SONET has long been the medium of choice for delivering services over metropolitan-area networks (MAN). At its core, SONET uses time-division multiplexing (TDM) - a technology many experts think is outdated and incapable of delivering the high-speed data services users are demanding.

Diagram of how it works

That's where Bellcore's GR-2837 specification, which defines the use of ATM virtual paths in SONET rings, comes in. In a nutshell, GR-2837 specifies a way for service providers to fill SONET pipes with traffic on an as-needed basis rather than having to provision TDM's set 64K bit/sec rates.

The result: Access equipment is emerging that enables carriers to deliver a new class of integrated customer services over a single link that combines data, voice and video. Early examples of networks that deliver integrated services include Sprint's ION, AT&T's Integrated Network Connect and MCI WorldCom's OnNet.

Over the past 10 years, carriers have installed fiber-optic cable throughout every major metropolitan area in the U.S. and have standardized the use of SONET equipment. However, some experts say SONET's TDM structure is too rigid for flexible provisioning or bandwidth sharing.

For instance, to provision a 10M bit/ sec LAN connection across a SONET ring, a carrier must dedicate an entire STS-1 pipe (51M bit/sec, see graphic) to the connection. Once the STS-1 is dedicated, no other sources can use that bandwidth. This "stranding" of bandwidth makes SONET's TDM structure unsuitable for handling today's datacentric network traffic.

Even though packet and cell technologies now dominate nearly every segment of the network - Ethernet in the LAN, frame relay and ATM in the WAN, and ATM in the core - SONET has prevented carriers from offering high-speed end-to-end data services at attractive rates.

The GR-2837 standard provides a platform for developing multiservice transport systems based on ATM. The systems allow carriers to offer more high-bandwidth data services over their existing fiber-optic access and interoffice networks. Using GR-2837, equipment vendors can map ATM cells into SONET for maximum bandwidth utilization.

The standard describes several ways of structuring the SONET interface, including an approach that mixes SONET and ATM STS-1 pipes on the same high-speed interface, and an approach that mixes, or concatenates, the full bandwidth of the SONET facility into a single ATM pipe.

The most efficient approach is called full concatenation, which involves joining the multiple pipes that make up a SONET link. The larger the concatenated signal, the greater the potential statistical gain from using ATM bandwidth management.

The full-concatenation approach allows ATM cells to be added or dropped at any node and lets carriers statistically multiplex traffic over the full bandwidth of the link. Freed from the rigid provisioning required by SONET, carriers can dynamically allocate bandwidth using ATM virtual paths and virtual circuits on a temporary or permanent basis.

To illustrate the gains of statistically multiplexing traffic, consider that, using ATM, a carrier could offer several hundred LAN internetwork services with a 2M bit/sec sustainable rate and a burst capacity up to 10M bit/sec over an OC-12 fiber-optic access ring. In comparison, a standard OC-12 SONET ring could only support up to 12 LAN internetwork services with a burst capacity of 10M bit/sec.

The combination of full concatenation of the SONET payload and ATM bandwidth management also gives carriers the ability to offer new data services, such as virtual private network services at native LAN speeds and a range of service-level agreements.

The GR-2837 approach for delivering services over SONET also allows carriers to sell a raw bandwidth envelope - within which customers can configure bandwidth allocation and quality of service to meet specific application, user and workgroup needs.

As an example, a customer could purchase a 3M bit/sec variable bit rate service from a carrier, and within that contract provision 1.5M bit/sec for Web traffic and 500K bit/sec for e-mail. Bandwidth allocation can also be adjusted dynamically based on the application being supported. For instance, launching a videoconferencing application could trigger a bandwidth-on-demand connection that terminates at the end of the videoconference.

Faber is director of product planning and management at Omnia Communications, a maker of ATM equipment in Marlborough, Mass. He can be reached at (508) 229-8444.

The Next Step in Metropolitan Area Networking
Omnia overview of its GR-2837 gear.

Sonet speed limits
If Sonet is to survive next-generation demand, it will need a little help from some new friends. Anerica's Network, 5/15/98.