DSL vendors look to extend service reach

By Tim Greene
Network World, 04/03/00

SAN JOSE - This week, the premiere DSL show of the year will feature products from a host of vendors looking to address one of the knottiest problems facing companies trying to build far-flung DSL networks - lack of service availability.

A company called Net to Net Technologies will announce at DSLCon an access multiplexer small enough to fit into phone company remote terminals, confined structures that have been major obstacles to broader DSL deployment.

In addition, several vendors, including Newbridge Networks and Adtran, will tout the advantages of yet another new DSL variety. Symmetric high-bit-rate DSL (SHDSL) is designed to reach farther than other versions of DSL to deliver high bandwidth without disrupting services running on other wires in phone company networks.

Solving the problem of running DSL to customers served through remote terminals could reduce the need to fall back on more expensive access technologies, says Scott McKown, administrator of Marin County Information and Data Access System (MCIDAS), a consortium that brings WAN services to nonprofit organizations in Marin County, Calif.

MCIDAS is switching its access network from frame relay to ATM over DSL but has been forced to keep some frame links or upgrade to ATM service at sites served by remote terminals, McKown says.

These terminals, also called digital loop carriers, don't allow DSL circuits to pass through. Of 80 million customers served by switching offices equipped to deliver DSL services, a quarter are blocked by a remote terminal from accessing the DSL gear, according to DSL Prime, an industry newsletter.

Remote terminals extend into neighborhoods at the end of optical fiber, and copper wires run from them to customer sites. Because DSL runs only on copper wires, DSL traffic would have to be terminated on a Digital Subscriber Line Access Multiplexer in a remote terminal and back-hauled on fiber. Net to Net's Mini DSLAM measures 1.75 by 17 by 14 inches, making it small enough to squeeze into remote terminals.

That could solve the technical challenge of extending DSL to more customers, says John Smart, director of operations for Vitts Networks, a Manchester, N.H., DSL provider that uses Net to Net gear. The carriers that own the remote terminals - largely the regional Bell operating companies - could use these devices to expand DSL services.

However, Smart says carriers competing with the RBOCs still face a problem: the RBOCs have so far denied them access to remote terminals, saying space is too tight.

"That's an industry issue with the incumbent local exchange carriers. I hope the FCC will open it up," Smart says.

SBC Communications, for example, is pushing fiber closer to its customers and installing DSLAMs in remote terminals, but the company says it will have little space for CLECs to use (NW, March 27, page 1).

McKown says Pacific Bell is already upgrading its remote terminals to include DSLAMs as it extends fiber farther out toward customers. The work is part of a $6 billion network upgrade undertaken by SBC, which owns Pacific Bell.

Another factor blocking DSL availability - the distance DSL signals travel down a copper line and still deliver high bandwidth - is addressed by the emerging DSL variety, SHDSL.

SHDSL can deliver 192K bit/sec over 40,000 feet of wire under ideal conditions, more than twice the effective range of other DSL flavors.

SHDSL is a variant of the technology behind two-wire, high-bit-rate DSL (HDSL2), which is a replacement for T-1 circuits, delivering a full 1.5M bit/sec bandwidth over just two wires rather than four. Whereas HDSL2 will only operate at 1.5M bit/sec, SHDSL's rate can be adjusted as high as 2.3M bit/sec, depending on the distance.

Adtran will push the technology, showing chips and promising SHDSL gear by the end of the year. Newbridge Networks says it will have programmable DSL line cards that fit in carrier DSLAMs in November. Any port on the 48-port card can be software-configured to handle SHDSL or three other DSL varieties.