Cable Modem FAQ
cabledatacomnews.com
What is a Cable Modem?
How Fast is a Cable Modem?
Who Makes Cable Modems?
How Does a Cable Modem Work?
What Services Are Offered with a Cable Modem Connection?
How Much Does Cable Modem Service Cost?
When Will Cable Modems Start Being Deployed?
What About "Telco-Return" Cable Modems?
Can Cable Modems be Purchased at Retail?
Will Cable Modems Support Internet Access for Multiple PCs?
FAQ Prepared By Cable Datacom News, Kinetic Strategies, and Cox Communications
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What is a Cable Modem?
A "Cable Modem" is a device that allows high-speed data access (such as to the Internet) via a cable TV network. A cable modem will typically have two connections, one to the cable wall outlet and the other to a computer (PC). Most cable modems are external devices that connect to the PC through a standard 10Base-T Ethernet card and twisted-pair wiring. External Universal Serial Bus (USB) modems and internal PCI modem cards are also under development.
How Fast is a Cable Modem?
Cable modem speeds vary widely, depending on the cable modem system, cable network architecture, and traffic load. In the downstream direction (from the network to the computer), network speeds can be anywhere up to 27 Mbps, an aggregate amount of bandwidth that is shared by users. Few computers will be capable of connecting at such high speeds, so a more realistic number is 1 to 3 Mbps. In the upstream direction (from computer to network), speeds can be up to 10 Mbps. However, most modem producers have selected a more optimum speed between 500 Kbps and 2.5 Mbps.
An asymmetric cable modem scheme is most common. The downstream channel has a much higher bandwidth allocation (faster data rate) than the upstream, primarily because Internet applications tend to be asymmetric in nature. Activities such as World Wide Web (http) navigating and newsgroups reading (nntp) send much more data down to the computer than to the network. Mouse clicks (URL requests) and e-mail messages are not bandwidth intensive in the upstream direction. Image files and streaming media (audio and video) are very bandwidth intensive in the downstream direction.
Who Makes Cable Modems?
There are many companies who are producing or have announced cable modem products. Included are: 3Com, Cisco Systems, Com21, General Instrument, Motorola, Nortel Networks, Phasecom, Samsung, Terayon, Toshiba, and Zenith.
How Does a Cable Modem Work?
The fact that the word "modem" is used to describe this device can be a little misleading only in that it conjures up images of a typical telephone dial-up modem. Yes, it is a modem in the true sense of the word; it MOdulates and DEModulates signals. But the similarity ends there because cable modems are practically an order of magnitude more complicated than their telephone counterparts. Cable modems can be part modem, part tuner, part encryption/decryption device, part bridge, part router, part network interface card, part SNMP agent, and part Ethernet hub. Typically, a cable modem sends and receives data in two slightly different fashions. In the downstream direction, the digital data is modulated and then placed on a typical 6 MHz television channel, somewhere between 50 MHz and 750 MHz. Currently, 64 QAM is the preffered downstream modulation technique, offering up to 27 Mbps per 6 MHz channel. This signal can be placed in a 6 MHz channel adjacent to TV signals on either side without disturbing the cable television video signals.
The upstream channel is more tricky. Typically, in a two-way activated cable network, the upstream (also known as the reverse path) is transmitted between 5 and 42 MHz.
This tends to be a noisy environment, with RF interference and impulse noise. Additionally, interference is easily introduced in the home, due to loose connectors or poor cabling. Since cable networks are tree and branch networks, all this noise gets added together as the signals travel upstream, combining and increasing. Due to this problem, most manufacturers use QPSK or a similar modulation scheme in the upstream direction, because QPSK is more robust scheme than higher order modulation techniques in a noisy environment. The drawback is that QPSK is "slower" than QAM.
What Services Are Offered with a Cable Modem Connection?
The dominant service is high-speed Internet access. This enables the typical array of Internet services to be delivered at speeds far faster than those offered by dial-up telephone modems. Other services will include access to streaming audio and video servers, local content (community information and services), access to CD-ROM servers, and a wide variety of other service offerings. New service ideas are being developed daily.
How Much Does Cable Modem Service Cost?
In North America, cable operators are packaging high-speed data services much like they do basic cable television service. MSOs are typically charging $40 - $60 per month for an Internet service package that includes software, unlimited Internet access, specialized content and rental of a cable modem. At the low end of this pricing scale, a very robust Internet service is available to consumers for about the cost of a dial-up account with a local Internet service provider and a second telephone line. Even at $60 per month, cable is a far better value than ISDN.
When Will Cable Modems Start Being Deployed?
A number of large commercial cable modem deployments are already underway. Cable Datacom News publisher Kinetic Strategies Inc. estimated cable modem service was commercially available to 25 million homes in North America on April 15, 1999 and that cable operators had landed more than 700,000 subscribers. More than 85 percent of these subscribers were receiving service with two-way cable modems and the remainder are served by telco-return products.
Although cable operators are now beginning large-scale deployments, cable-delivered Internet services will only be available to a minority of consumers for some time, due to a lack ubiquitous cable system plant upgrades. For at least the next three years, local cable systems and their subscribers will be divided into a world of cable modem haves and have nots. However, it is likely cable modems will prove successful in those markets where they are deployed due to a compelling combination of fast throughput speeds, reasonable pricing and integrated content.
What About "Telco-Return" Cable Modems?
With telco-return cable modems, cable lines are used for fast downstream transmission and a telephone modem handles upstream communication over the public telephone network.
Since consumer demand for high-speed data services is being driven primarily by the need for faster downstream speeds, telephone-return path cable modems could prove to be a viable means for cable operators without two-way plant to rapidly enter the residential high-speed data market. A number of vendors are offering telco-return cable modems, including 3Com, GI, Hybrid Networks, and New Media Communication.
Can Cable Modems be Purchased at Retail?
Not typically, because most cable modems deployed to date have been proprietary products. That is, if a cable company is using proprietary Motorola network equipment, only a Motorola modem will work. For this reason, modems are usually leased to subscribers so they do not have to face the risk of product obsolescence.
In late 1998, cable companies started deploying standards-based DOCSIS cable modems, which will be sold at retail in some markets by the end of 1999. However, widespread retail availability is not expected until well into the year 2000.
Will Cable Modems Support Internet Access for Multiple PCs?
Yes, a cable modem can provide Intenet access to multiple PCs, assuming they are connected via a local area network (LAN). Cable modems typically have an Ethernet output, so they can connect to the LAN with a standard Ethernet hub or router. That said, each PC must have an assigned IP address, which the cable ISP ususally sells at a premium of $5-$10 a month per PC.
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