Chipset Charges Next-Generation HDSL

Originally developed to cut the cost of T1 data service deployment over standard copper telephone wiring, High Bit-Rate Digital Subscriber (HDSL) technology has become the most widely deployed of the industry's many xDSL datacomm variants. Supporting this development is news of second generation HDSL silicon from Conexant Systems Inc. (Newport Beach, CA).

If you like to design with Rockwell modems, and recognize them as a de facto industry standard, you'll appreciate Conexant. The firm is a Rockwell International spinoff and now the world's largest independent company focused on ICs for communications.

Supporting the upcoming next-generation HDSL2 standard, Conexant is readying a highly-integrated two-chip set that's poised to offer OEM computer and communications systems developers a route to selectable data rates--up to a whopping 4.6 Mbits/second.

This speed will give Internet service providers (ISPs) and phone companies a way to meet rapidly increasing demands for high-speed—especially in geographic areas where copper pair shortages exist. In fact, HDSL2 promises up to 29 times faster raw data thoughput than ISDN lines. It also promises to be up to 83 times faster than analog modems. Hang onto your hats!

HDSL Means Business

This news comes at a time when many observers see business connectivity as the fastest-growing application area for existing xDSL technology. According to industry researchers at Cahner's In-Stat Group, xDSL chipset shipments are expected to grow by over 80% this year, and continue growing at that rate in 2000 and 2001.

That's a nice healthy marketplace that Conexant hopes to tap into. The firm expects OEMs will choose its chipset based on its on-board programmable clock synthesizer that enables software-selectable data rates from 144 kbits/second up to the 4.6 Mbits/second.

Putting It In Perspective

To understand the significance of HDSL2 and that kind of speed, look at the recent history of landline data transmission. Prior to HDSL, the highest data rate connection you could easily get a hold of was 1.544 Mbit/second T1 service. But, T1 was often difficult and expensive for phone companies to deploy because it required signal regeneration and line conditioning equipment.

Regenerators typically had to be installed every 3,000 feet between central office (CO) and customer premises. When RF-based HDSL hit the scene a few years ago, it eliminated this snag. A phone company could simply install an HDSL transceiver at either end of a twisted-pair line. That meant a line could carry T1-style traffic to customers as far as 12,000 feet away from the CO.

However, the plot thickens. Like the original T1, HDSL relied on two copper pairs, but instead of a transmit pair and a receive pair, it created two bidirectional pairs. Each ran at 784 kbits/second for T1 (1,168 kbits/second for E1). Regardless of this limitation, HDSL still exhibited lower transmission latency for voice traffic, and better resilience to crosstalk from other signals (such as telephone service or ISDN data) that might be in the same cable bundle.

Nowadays, because everyone is seeking higher speed Internet access, there's a rapidly growing market for HDSL-enabled business connections which, at $90 to $200 per month for basic service, are significantly less expensive than the typical $900 to $2,000-per month T1 connection.

Only One Pair Needed

Enter HDSL2. It delivers the same basic performance as HDSL, but over a single pair of copper wires instead of two! It also promises universal interoperability; that will eventually be possible with the forthcoming ratification of a worldwide standard.

Underscoring a means of providing a wider variety of services at tiered price points, Conexant's ZipWire2 chipset supports HDSL2. The silicon is also capable of operation in 2B1Q mode. 2B1Q HDSL line code is the industry's original line coding system.

This chipset also functions as a conventional HDSL transceiver using 2B1Q to ensure interoperability with legacy remote terminals. "Many OEMs designed terminals with our first ZipWire RS8973 chipset," notes Ron Cates, director of commercial DSL products for Conexant's Network Access Division. "Our HDSL chips are used by more than 30 telecommunications manufacturers. These same OEMs now have an approach to implement the forthcoming HDSL2 standard."

Upgradable In Software

Unlike many competing multi-rate alternatives, Cates also points out that Conexant ZipWire2 implements its functions in software. "That can eliminate the need to change components to accommodate different data rates or new features," he says.

Moreover, Cates notes that the ZipWire2 spin provides conventional 1.544 Mbit/second T1 transport and packs features that make it easier to build Internet access devices for other business connectivity applications. "These include telecommuting and branch office support," he adds.

CLEC Oriented

Cates also claims his company is the only HDSL chip supplier that caters to the needs of the competitive local exchange carrier (CLEC) business access market segment. "CLEC is a key growth segment," he explains.

"Businesses want both upload and download speed for connecting telecommuters and branch offices. They also want data services to coexist with multiple voice and fax channels, and they generally don't have access to cable or other high-speed residential data-access services."

Cates claims Conexant's silicon supports extensive loop range, and offers favorable latency characteristics, as well as high noise-immunity. "Those attributes make it coexist with other services--and transmission technologies--in the same cable binder," he says. A cable binder is a wire bundle typically containing between 25 and 50 twisted pair wires, each carrying a different telephony service.

"Local exchange carriers are expected to quickly adopt HDSL2 for T1 service because it immediately doubles the capacity of the copper wire loop plant," concurs Shannon Pleasant, a market analyst with In-Stat. "Conexant's large installed base among CLECs will make its HDSL2 products a natural migration."

Inside The Chipset

The ZipWire2 modem itself consolidates a framer, controller, DSP, analog front end (AFE), and line driver into two small, power-efficient packages that can be integrated into small high-density line cards. The chipset also complies with the forthcoming American National Standards Institute (ANSI) standard for HDSL2 T1 transport. It's based on the Overlapped Pulse Amplitude Modulation Transmission with Interlocking Spectra (OPTIS) line code developed in 1997 by the industry's multi-vendor HDSL2 alliance (Conexant is a member of the HDSL2 Alliance).

Smart Silicon

The ZipWire2 chipset also includes an on-chip microprocessor core that can be used to control its DSP, or to administer off-chip circuits such as E1 or T1 framers, codecs, HDLC (high-level data link control) controllers, or Ethernet controllers. Full compliance with the HDSL2 standard includes at least 5 dB of noise-margin on test loops. That's a line impairment and noise source spec specifically chosen to model the North American twisted pair loop plant. The chipset meets these requirements.

Packaged in a 27 mm ball grid array and an 80-pin PQFP, Conexant says its ZipWire2 will be available this quarter for about $40 a pop in OEM quantities of 50,000 units per year (prices stated at time of publication). For literature and detailed product information, contact Conexant Systems Inc., 4311 Jamboree Rd., PO Box C, Newport Beach, CA 92658. Phone: (800) 854-8099, or (949) 483-6996.