Cable modems bump into network roadblocks

Cable modems bump into network roadblocks


Cable-modem manufacturers are pushing to standardize their devices and to overcome technical problems posed by hybrid/fiber coaxial-cable (HFC) networks. In an ideal HFC cable-modem network, fiber carries data, video and telephone signals downstream from the cable operator`s headend office to customers` neighborhoods. There, light signals are converted to electrical signals and enter subscriber homes over coaxial cable. A cable-access unit in the home splits incoming signals off to telephone, computer and video systems.

This downstream link from the cable provider`s headend equipment to the user`s personal computer can transmit from 500 kbits/sec to 10 Mbits/sec. Future modems are being designed to reach 30-Mbits/sec rates. The in-home cable-access unit also injects outgoing telephone signals into the cable modem`s data over an upstream path back to the headend. Ideally, this path can support 20-kbit/sec to 10-Mbit/sec transmissions. The headend systems concentrate this traffic for transmission over fiber back to a regional content hub serving tens of thousands of subscribers.

This is an ideal scenario. However, in the real world, cable-TV operators and cable-modem manufacturers must crack some technical problems if cable modems are to catch fire with end-users. These problems have caused Lennox Research--a Boulder, CO-based market-research and consulting firm studying cable modems--to label cable modems` present performance as an "iffy kind of thing." Nevertheless, some market analysts see a booming future for the products.

The first problem for cable modems is that most of the nation`s cable infrastructure remains largely coaxial and one-way. The Boston-based Yankee Group estimates that 90% of U.S. cable-TV networks couldn`t carry data back upstream from modem users to headends. To correct this, cable operators must install two-way HFC cable and upstream signal amplifiers, at a cost variously estimated at up to $2000 per subscriber home. With 500 to 2500 homes supported by each headend node, this means an investment of as much as $5 million to upgrade a single coaxial-cable node to HFC. Regardless of where the real figure falls, this is a significant capital cost. Cable-TV operators are likely to incur this cost only when and where the user market justifies it.

Another problem is noise. Many cable-TV networks remain largely coaxial and prone to induced electrical noise between coaxial segments. And HFC`s upstream coaxial leg is also subject to "ingress noise" originating in the user`s home.

A former headend technician for Time Warner Cable lists home-based paging systems, electric motors, and radio transmitters as common sources of noise. According to modem vendor Terayon Corp., the ingress noise from multiple homes can accumulate and interfere dramatically with transmission quality.

This problem is reportedly so serious that one anonymous World Wide Web (WWW) user says, "Someone with a hair dryer and a bad hair day could bring down the block." Jim Albrycht, technical consultant for LANcity, an Andover, MA, cable-modem manufacturer, confirms this observation. He adds that ingress noise "can originate from the other side of the living room or the planet."

Current-generation cable modems try to work around cable noise using many incompatible techniques. These include wireless spread-spectrum transmissions, dynamic frequency hopping to cleaner channels, or falling back to slower dial modems designed to cope with wire noise. LANcity`s Albrycht cautions, "Noise is always there. ... Cable modems must work above the noise floor just as aircraft fly above bumpy clouds."

Bandwidth bottleneck

Users may also encounter technical troubles unrelated to cable and modems. One problem is a bandwidth bottleneck at both ends of the cable link. Robert Wells, a Lennox Research senior analyst, describes a field test during which a 10-Mbit/sec cable modem`s throughput dropped below 3 Mbits/sec. "In this demonstration, the bottlenecks hurting performance weren`t in the cable system or the modem," Wells explains, "but in slow, remote content servers and in the comparatively sluggish graphics-processing speeds of the PC at the end of the pipeline."

One cable modem vendor, Motorola Inc., is attacking this problem by partnering with Cascade Communications Corp., a Westford, MA, maker of network switches that can help cable operators manage and balance subscribers` bandwidth needs.

Another bandwidth problem may appear as more and more cable-modem users come online simultaneously. Each will enjoy a smaller piece of the cable networks` fixed bandwidth. This will slow users` online sessions regardless of the modem`s maximum speed (see "Sharing the Surf," page 8). As Communications Engineering & Design`s Michael Lafferty puts it, "Thousands of consumers sucking down all sorts of multimedia data on digital highways that are 10 Mbits/sec wide or larger could put a real strain on the system as it`s constructed today."

To eliminate such problems, Wells adds, cable-network operators may have to create "islands of peak performance within the generally sluggish Internet." Some providers have begun to do this by mirroring external Internet content at servers located inside the cable network.

Standards to the rescue?

To overcome some of these problems and prepare for an anticipated huge market, cable-modem vendors are pursuing a crash course in standards-setting. The following groups, among others, are pushing to standardize modem functions and protocols:

ATM Forum Residential Broadband Working Group

Broadband Applications Forum

Broadband Link Team

Broadband PC Council

Broadband Digital Communications Committee

Cable Television Laboratories (CableLabs)

Data Over Cable Interface Specification Project (Docis)

Digital A/V Interop Committee (Davic)

Ieee Cable-TV Protocol Working Group for Standard 802.14

Interactive Television Association

International Telecommunications Union

Multimedia Cable Network System Partners Limited (Mcns).

But while these groups debate compatibilities and protocols, cable-TV operators have already bought hundreds of thousands of current-generation modems from a handful of vendors. "These modems are not interoperable," warns Dr. John C. Malone, chairman of CableLabs, a leading cable-modem standards group.

But getting the industry to agree on modem standards is a difficult job, as one learns from the WWW-based minutes of the 802.14 group`s March meeting. While vigorously debating the merits of different protocols for upstream links, participants also struggled with the cable industry`s "needing to do a lot of cleanup work in the cable plant . . . and not understanding the nature of [Ethernet/Tcp/ip] collisions at the headend receiver."

Thus, cable-modem standards may take many months to crystallize. The Ieee 802.14 working group had hoped to release specifications this past April, but according to LANcity`s Albrycht, this may have to wait until late next year.

Trials and installations

Vendors, however, are not waiting for standards--they are moving their wares into the field. Cyberspace-based market trackers list at least 49 modem trials across the United States. And at least a quarter of these proofs-of-concept are scheduled to end by 1997.

In one of the largest trials to date, LAN city and Continental Cablevision successfully wired up a 10-Mbit/sec network serving Boston College students, professors and classrooms using LANcity Personal Cable Modems. Three years in the making, the HFC network includes 18 fiber nodes serving 2500 classrooms, 400 offices and 6700 dorm rooms.

According to C. Jeff Jeffers, Boston College director of network services planning and design, in Chestnut Hill, MA, BC students now enjoy continuous access to cable TV, campus data servers and the Internet at "400 to 500 times the speed of conventional modems." However, the network`s success has also increased demand for off-campus access. This reportedly has left some off-campus staff "stranded at home, faced with busy phone lines as the volume of telecommuters with traditional modems slowed communications to a crawl."

In testimony to the new technology, Boston College plans to widen its cable-modem network to de-clog this telecommuter traffic jam. q

Dave Powell writes from Winchester, MA.

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