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 (CATV) network. A cable modem will typically have two connections, one to the cable wall outlet and the other to a computer (PC).
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How Fast is a Cable Modem?
Cable modem speeds vary widely. In the downstream direction (from the network to the computer), speeds can be anywhere up to 36-Mbps. Modems this fast are not currently on the market, but should appear some time in 1997. Few computers will be capable of connecting at such high speeds, so a more realistic number is 3 to 10-Mbps. In the upstream direction (from computer to network), speeds can be up to 10-Mbps. However, most modem producers will probably select a more optimum speed of between 200 Kbps and 2 Mbps.
In the first few years of cable modem deployment, an asymmetric setup will probably be more common than a symmetric setup. In an asymmetric scheme, the downstream channel has a much higher bandwidth allocation (faster data rate) than the upstream. One reason is that the current 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.
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 pratically 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 NIC 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 carrier, somewhere between 42MHz and 750 MHz.
There are several modulation schemes, but the two most popular are QPSK (up to ~10-Mbps) and 64QAM (up to ~36-Mbps). This signal can be placed in a 6MHz 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 40 MHz.
This tends to be a noisy environment, with lots of interference from HAM radio, CB radios and impulse noise from home appliances. 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 will be using 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.
How Does a Cable Modem Connect to a Computer?
There are several methods for computer connection, but it appears that Ethernet 10BaseT is emerging as the most predominant method. Although it probably would be cheaper to produce the cable modem as an internal card for the computer, this would require different printed-circuit cards for different kinds of computers, and additionally would make the demarcation between cable network and the subscriber's computer too fuzzy.
What Services Will be Offered with a Cable Modem Connection?
The most popular service will undoubtably be high-speed Internet access. This will enable the typical array of Internet services to be delivered at speeds hundreds of times faster than those offered by dial-up telephone modems. Other services may 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 born daily.
When Will Cable Modems Start Being Deployed?
Cable modem pilot tests and a handful of large commercial deployments are already underway. Wide scale deployments probably won't start until some time in 1997.
Who Makes Cable Modems?
There are many companies who are producing or have announced cable modem products. Included are: ADC Telecommunications, Com21, First Pacific Networks, General Instrument, Hewlett-Packard, Hybrid Networks, LANCity, Motorola, NetGame, Nortel, Phasecom, Pioneer, Scientific Atlanta, Terayon, Toshiba, and Zenith.
What Advantages Do Cable Modems Have Over ISDN?
As mentioned above, cable modems will enable data connections of much higher speeds than ISDN. ISDN transmits and receives at speeds of 64 Kbps and 128 Kbps. Cable modems will be able to receive data at up to 10 Mbps and send data at speeds up to 2 Mbps (some up to 10-Mbps). However, this is not the only advantage of a cable modem. It is well known that the installation of an ISDN data connection for a residential subscriber is a very complicated process. The home user often has to act as his or her own system integrator. Installation requires careful integration of the telephone company service, the terninal adapter, the computer system, and the software. Service from the cable company will likely result in a technician bringing the modem to your home, installing the modem, installing the necessary software, and when the technician leaves your house, you will be up and operating. This places the installation and activation burden on the cable company rather than on the subscriber.