The Cable Modem Traffic Jam

An early example of this principle was deployed in the Time Warner Cable television system in Queens, New York City. Queens is a melting pot of nationalities. The immigrants there tend to cluster in neighborhoods where they have relatives and friends who can help them make the transition to the new world. The fiber paths to these neighborhoods can use the same 6-MHz channel for programs in different languages. So a given channel number can carry Chinese programming on the fiber serving that neighborhood, Korean programming on another fiber, and Japanese programming on still another fiber. As the 747s fly into the John F. Kennedy International Airport in Queens each night, they bring tapes from participating broadcasters in other countries that become the next day’s programming for the various neighborhoods. (Note that this technique is impossible in a broadcast or satellite transmission system since such systems serve the entire broadcast area and cannot employ nodalization.)

The same concept of spectrum reuse is applied to the cable modem. A 6-MHz channel set aside for this purpose carries the cable modem traffic for the neighborhood served by its respective node. While most channels carry the same programming to all nodes, just the channel(s) assigned to the modem service carry specialized information directed to the individual nodes. Importantly, nodalization reuses the upstream spectrum as well as the downstream spectrum. So, given enough nodes, traffic jams are avoided in both directions.

However, nodalization is costly. Optical-fiber paths must be installed from the headend to the individual nodes. The fiber paths require lasers and receivers to convert the optical signals into electrical signals for the coaxial cable in the neighborhood. Additional modulators per node are required at the cable headend , as well as routers to direct the signals to their respective lasers. The capital investment is substantial. However, it is technically possible to solve the problem. (In principle, nodalization could be implemented in a fully coaxial cable system. But in practice coaxial cable has a lot higher losses than fiber and incurs even greater expense in the form of amplifiers and their power supplies.)

Other techniques for alleviating the traffic jam include upgrading the cable system so that 256 QAM can be used instead of 64 QAM downstream and 16 QAM can be used upstream instead of QPSK. If the ISP’s connection to the Internet is part of the problem, a larger data capacity connection to the Internet backbone can be installed.

Also, non-Docsis high-speed access systems are under development for very heavy users. These systems will provide guaranteed ultrahigh speeds of multiple megabits per second in the downstream direction while avoiding the loading of the Docsis cable modem channels. The service can then be partitioned into commercial and residential or small business services that do not limit each other’s capabilities.

Speculations on the future

The cable modem traffic jam is due to rapid growth that sometimes outpaces the resources available to upgrade the cable system. But solutions may be near at hand.

The next wave of standardization, Docsis 1.1 released in 1999, provides for quality-of-service segmentation of the market. Now that the standard is released, products are in development by suppliers and being certified by CableLabs. Release 1.1 products will migrate into the subscriber base over the next several years. Subscribers will then be able to choose the capacity they require for their purposes and pay an appropriate fee. The effect will be to discourage bandwidth hogs and ensure that those who need high capacity, and are willing to pay for it, get it. And market segmentation will provide financial justification to implement even more comprehensive nodalization. After enough time has passed for these system upgrades to be deployed, the traffic jam should resolve itself.

Tekla S. Perry, Editor

About the Author

WALTER S. CICIORA (F) is a consultant specializing in cable, telecommunications, and consumer electronics, based in Southport, Conn. He serves on several corporate and advisory boards and does expert witness work on patents. He cofounded EnCamera Sciences Corp., which was sold last year.

To Probe Further

For more on cable modems, see Delivering Internet Connections Over Cable: Breaking the Access Barrier, by Mark Laubach, David Farber, and Stephen Dukes (John Wiley & Sons, New York, 2001 ).

Cable Modems: Current Technologies and Applications (IEEE Press, 1999) is a collection of papers by early contributors to the technology. Its executive editor is Venkata C. Majeti.

Standardized cable set-top boxes are discussed in Open Cable Architecture, by Michael Adams (Cisco Press, San Jose, Calif., 2000).

For a comprehensive treatment of cable technology, see Modern Cable Television Technology: Video, Voice, and Data Communications, by Walter Ciciora, James Farmer, and David Large (Morgan Kaufmann, San Francisco, 1999). An easy-to-follow development of digital technology fundamentals appears in the second volume of Digipoints, by Justin J. Junkus (Society of Cable Telecommunication Engineers).


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