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Cable Modems

Cable data modems are a recent entry into the data bandwidth delivery arena. Only in the last several years have any real standards emerged in cable modem interoperability. Whatever the cable provider was using for equipment, you had to use as well. (This is somewhat common in the IT business.)

CableLabs created an interoperability standard known as Data Over Cable Service Interface Specification (DOCSIS). CableLabs also is responsible for interoperability and conformance testing of cable data modem products.

Cable companies realized when the Internet started to become popular that they had an enormous opportunity to expand their revenue bases. New media-rich services available on the Internet required more bandwidth than the plain old telephone system could deliver. The telephone companies were having difficulty, or lack of vision, in deploying ISDN. The raging hot new xDSL technologies were not even on the reading edge of trade rags. There was definitely a bandwidth void that needed to be filled.

The cable companies had broadband wire already in the ground to a large percentage of the population of North America. All they had to do was upgrade their infrastructure to ensure data reliability; after all, some snow in your cable TV reception is significantly different than lost data in a computer transaction.

The first order of business for the cable companies was to upgrade the cable from coaxial to fiber at the head end, the endpoint at the cable provider's facility. With the installation of fiber there and coaxial at the customer's premises, the cable plant network became known as hybrid fiber-coaxial, or HFC networks.

Digital data is carried over radio frequency (RF) carrier signals on the HFC network. Cable data modems convert digital information into a modulated RF signal and convert RF signals back to digital information. The conversion is performed by a cable modem at the subscriber's premises and again by head-end equipment handling multiple subscribers.

Shared Network Technologies

Broadcast video is highly bandwidth-intensive. If you recall, telephone quality voice requires only 3KHz of frequency spectrum to be reproduced. CD-quality stereo audio requires 44KHz of the frequency spectrum. Current broadcast video requires 4.2MHz, which is 10 times more. The next generation of broadcast video, HDTV, requires 30MHz for each color signal. That would be 90MHz if not for high compression ratios, interframe differential predictive encoding, and other niceties found in the MPEG-2 standard. After HDTV is compressed and encoded, it fits neatly into the 6MHz allocated for channels by the Federal Communication Commission (FCC).

For normal television operation, the customer's television receiver selects the channel to watch by tuning to a 6MHz portion of the assigned spectrum. The FCC allocated frequency channels in such a way to prevent interference with each other. The end result of this allocation scheme is that most of the terrestrial broadcast television spectrum is vacant.

Between the poor allocation of the terrestrial broadcast television spectrum and the burgeoning number of available channels to watch, there is insufficient spectrum to accommodate the appetite of today's viewer. The large number of available channels on cable television is made possible by the use of coaxial cable. Coaxial cable is able to separate the frequency spectrum from the terrestrial broadcast spectrum while maintaining the properties of the spectrum that allow it to work with existing equipment. This means that a television receiver connected to a cable signal will behave as it does when connected to an antenna. The frequencies that in the broadcast world are reserved for use by air traffic, commercial, and military communication are now available to carry additional channels because those frequency channels are limited to the coaxial cable and do not interfere with the terrestrial broadcast television spectrum.

A single 6MHz channel can support multiple data streams or multiple users through the use of shared network technologies. Different modulation techniques can be used to maximize the data speed that can be transmitted through a 6MHz channel. Modulation techniques that you're already acquainted, if not familiar, with are quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM), and vestigial side band (VSB) amplitude modulation. VSB-AM is an artifact of the NTSC broadcast protocol and the fact that the original televisions were built using expensive tubes.

More on Sharing

Sharing usually implies taking a whole product and dividing it up equally among all participating members. This same reasoning does not apply to the world of shared bursty packetized data networks. Five users sharing a 10Mbps Ethernet do not effectively have 2Mbps of throughput each. Ethernet networks, LANs, and data communications in general would not scale very well if this line of reasoning held true.

A data-over-cable-shared-access network provides roughly 30Mbps burst capacity. If you remember from Frame Relay, the capability to deliver requested data at the port speed is known as the network's burst capacity. On standard telecom with dedicated circuits, such as a 64Kbps DS0, there are only 64Kbps of throughput burst capacity available to a single user. It is impossible for that user to borrow additional unused capacity from other idle users.

Contrast that with a shared access network. A user can use the full bandwidth of the shared resource and then release that resource for allocation to the other users. If you remember, as with Frame Relay and ATM, the ability to use a resource only when needed provides great performance benefit, as well as inherent economic benefit to both the service provider and the customer.

The portion of bandwidth reserved for upstream traffic (from the customer to the cable network) is usually in the 5–40MHz portion of the spectrum. This portion of the spectrum can be subject to interference, so if you see snow or other interference patterns on your lower six channels, most likely your data is also being affected. Upstream bandwidth is usually asymmetric to the downstream. Most cable providers provide 128–768Kbps upstream bandwidth to a nominal 3Mbps downstream bandwidth. This situation should eventually change after cable companies convert the rest of their cabling and equipment infrastructure to accommodate the return traffic. This is obviously driven by demand, so if customers don't demand fast or at least symmetric sending speeds, the cable companies are not likely to provide them.

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