
		Cable Modem FAQ's

Broadband FAQ's

Cable Modem FAQ's

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, cable modems transmit data over specially provision cable lines that are offered from traditional cable companies, the same ones that offer cable TV services. 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 available.

How Fast is a Cable Modem?

Cable modem speeds vary, 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 reach 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. Some service providers limit upstream access speeds to 256 Kbps or less.
An asymmetric cable modem scheme is most common and is specified in the DOCSIS, EuroDOCSIS and DVB EuroModem standards. 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 more than 40 companies who are producing or have announced cable modem products. Included are: 3Com, Cisco Systems, Com21, Ericsson, Motorola, Samsung, Terayon, Toshiba, Thomson, and Zoom.

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 since 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 preferred 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 include access to streaming audio and video servers, local content (community information and services), and CD-ROM servers. 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.

Will Cable Modems Support Internet Access for Multiple PCs?

Yes, a cable modem can provide Internet 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 in some cases, depending on the cable ISP usually sells at a premium of $5-$20 a month per PC.