Cable Modem Technology and the Emerging DOCSIS 3.0 Standard

Cable Modem Technology and the Emerging DOCSIS 3.0 Standard

In the beginning (the 1950s), digital modems were developed for data transmission requirements in support of North American air defense. In 1962, AT&T fielded the first commercial modem with a speed of 300 bits per second. By 1991, 14.4 kilobits-per-second modems were available. In 1994, modem speeds doubled to 28.8 kilobits per second. The 56 kilobits-per-second modem dates from 1996. Nowadays, the majority of Internet users connect via cable modem. Dial-up modems, although still in use (especially in rural areas), have fallen by the wayside due to the design restrictions of telecommunications networks and the advent of broadband Internet access that takes advantage of existing cable TV infrastructure.

The first cable modem deployments did not adhere to interoperable hardware and software standards. Therefore a cable company had to deploy cable modems to customers from the same manufacturer that supplied the cable company s network equipment. In 1996, an industry trade organization called CableLabs developed a standard that would allow equipment from various vendors to work together. The standard, called DOCSIS (Data Over Cable Service Interface Specification), has accelerated the rate at which cable access can be rolled out. Furthermore, standardization provides ISPs and consumers with competitive choices and prices so that nobody is locked into a proprietary solution.

Now that it is no longer necessary to know which vendor supplies your ISP s networking equipment, if you want to purchase your own cable modem you just need to know which version of DOCSIS your ISP supports. If your cable modem supports an earlier version of DOCSIS than your ISP currently delivers, it will work, but you won t be able to take advantage of the latest features.

If you buy a cable modem with a newer version of DOCSIS than your ISP delivers, it is technically backward compatible. Be warned, however, that some ISPs artificially impose extra fees if you use a DOCSIS 3.0 cable modem with an ISP that supports DOCSIS 2.0. The ISP reasons that because you are achieving faster data throughput by using a DOCSIS 3.0 cable modem connected to the ISP s DOCSIS 2.0 cable modem termination system, you should pay more for the higher speed tier. The speed is accomplished by more intelligent use of the available bandwidth. The free market will eventually sort out the pricing models.

How does DOCSIS 3.0 make more efficient use of available bandwidth? In the United States, cable TV systems allocate 6MHz slices of the available cable bandwidth to carry the signal for each TV channel. Cable companies that offer Internet access allocate a 6MHz slice to carry the downstream Internet data as if it were a cable TV channel (with an additional 2MHz channel to carry upstream data from the user to the Internet). A single 6MHz channel has a theoretical downstream data throughput capacity of 40Mbps (megabits per second), a rate supported by DOCSIS 2.0 cable modems. In Europe, the norm is to assign 8MHz channels, each with a theoretical carrying capacity of 50Mbps. Actual data throughput on either side of the pond falls short of theoretical rates due to many real-world variables. For example, ISPs assign as many as 1,000 users to a single 6MHz channel, which means that concurrent user number 1000 is not going to experience the same speed that the first ten users of an otherwise clear channel enjoy.

DOCSIS 3.0 quadruples the theoretical data throughput over the DOCSIS 2.0 standard with a concept called  channel bonding,  which binds multiple channels (often four or eight) together to deliver a single data stream. DOCSIS 3.0 enables theoretical data rates of 160Mbps downstream and 120Mbps upstream. DOCSIS 3.0 also makes better use of the Internet Protocol (IP), a necessary improvement to allow cable companies to deliver IP TV instead of TV signaling. DOCSIS 3.0 support for bandwidth-saving technology called advanced multicasting will allow ISPs to efficiently deliver IP TV and other IP applications.

If your ISP is not yet on the DOCSIS 3.0 standard, it is just a matter of time. DOCSIS 3.0 builds on DOCSIS 2.0 to advance the standard and enable ISPs to deliver new services, while remaining backward compatible with DOCSIS 2.0. You can upgrade your cable modem from DOCSIS 2.0 to 3.0 when your ISP makes the improvement or when you feel the need to avail yourself of faster speed and more interesting services. If you are in the market, consider purchasing one of these cable modems.

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