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Cable modems are really not modems in the conventional sense. They modulate and demodulate signals like a conventional modem, but otherwise they are more like routers that are designed for installation on CATV networks, which themselves operate much like Ethernet LANs. These cable modems, as well as the cable operators plant equipment, are even being managed using the familiar simple network management protocol (SNMP).
Upstream Interference. Typically, a cable modem sends and receives data in two slightly different fashions. In the downstream direction (from the network to the user), the digital data is modulated and then placed on a typical 6-MHz television carrier. There are several modulation schemes, but the two most popular are quadrature phase-shift keying (QPSK), which provides up to 10M bps, and quadrature amplitude modulation (QAM), which can provide up to 36M bps. 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 (from the user to the network) is more complicated. Typically, in a two-way activated cable network, the upstream channel 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 or office machines. In addition, interference is easily introduced in the home, due to loose connectors or poor cabling. Because cable networks are tree and branch networks, all this noise gets added together and increases as the signals travel upstream.
Most manufacturers will use QPSK or a similar modulation scheme in the upstream direction because it is a more robust scheme than higher-order modulation techniques in a noisy environment. The drawback is that QPSK is slower than QAM.
Multifunction modems use programmable digital signal processing (DSP) technology to turn a computer into a complete desktop message center, allowing the user to control telephone, voice (recording and playback), fax, data transfers, and E-mail. Typical features include multiple mailboxes for voice mail, caller ID support, call forwarding, remote message retrieval, phone directory, and contact database.
In some cases, the modem is actually on a full-duplex sound card. By plugging in speakers and a subwoofer, the user can even enjoy a stereo-sound speakerphone. A separate connection to a CD-rom player allows the user to work at the computer while listening to music. However, these DSP-based products cannot be used as modems and sound cards simultaneously because the processor can take on only one identity at a time.
With DSP, the modem can be easily upgraded to the latest communications standards and new capabilities can be added simply by loading additional software. For example, a 14.4K-bps modem can be upgraded to 28.8K bps by installing new software instead of having to buy new hardware. Likewise, a 28.8K-bps modem can be upgraded to 33.6K bps in the same way, often at no extra charge from the vendor.
A new type of modem digital simultaneous voice and data (DSVD) allows the user to send voice and data at the same time over a single telephone line. Interference is avoided by having voice and data use different frequencies.
The biggest advantage of DSVD is that users no longer need to interrupt telephone conversations or install a separate line to transmit data or receive faxes. Multimedia modems typically include full-duplex speakerphone, fax, and 16-bit stereo audio capabilities, in addition to advanced modem functionality.
Until now, 33.6K was thought to be the practical modem speed over standard phone lines. A new class of modems that can transmit data at 56K bps employs technology that takes advantage of the fact that for most of its length, an analog modem connection is really digital.
When an analog signal leaves the users modem, it is carried to a phone company central office where it is digitized. If it is destined for a remote analog line, it is converted back to an analog signal at the central office nearest the receiving user. However, if the receiving user has a digital connection to the carriers network, the modem traffic is converted at only one place, where the analog line meets the central office. When the traffic is converted from analog to digital, noise is introduced that cuts throughput. But the noise is less in the other direction, from digital to analog, allowing the greater downstream bandwidth (see Exhibit 2).
Exhibit 2. 56K-bps Modem.
Because it is already in digital form, the traffic is free of impairments from noise introduced when an analog modem signal is made digital within the carrier network. From an analog source, the top speed is quite a bit lower than 56K bps because the traffic is subject to impairment from noise.
A fully digital ISDN basic rate interface line offers up to 128K bps in both directions, but the service is far more expensive than an analog phone line. In addition, ISDN is not universally available and can still be difficult for users to set up with their equipment. Although the new 56K-bps modems are not meant to challenge ISDN, they will fulfill a role as a more economical alternative and significantly cut into the ISDN market.
Among the vendors of 56K-bps modems are U.S. Robotics and Rockwell International. Although U.S. Robotics was the first to introduce a 56K bps modem using its X2 technology, the semiconductor division of Rockwell International followed shortly with a different technique to obtain a 56K bps operating rate. In early 1998 U.S. Robotics held approximately 60 percent of the 56K bps modem market and an International Telecommunications Union (ITU) standard for 56K bps modem technology was being finalized. Until that standard is actually implemented on a worldwide basis, either the U.S. Robotics or Rockwell modem should be considered, based upon the type of modem supported by the destination to be dialed. For example, if the Internet service provider uses Rockwell technology, Rockwell 56K bps modems should be acquired.
Once thought to be an outdated technology that would be supplanted by ISDN terminal adapters connected to digital lines, modems are not only growing in use, they are undergoing a surge in innovation as well. Not only has ISDN not met industry projections in terms of availability, but the carriers appear not to be encouraging its use, as evidenced by significant price hikes.
Higher-speed modems, the advent of cable modems that work over CATV networks, and new technologies that rely more on a computers CPU for carrying out modem functions have all combined to breathe new life into this market segment. Although todays new-generation modems will not replace the need for ISDN in all cases, they certainly will give many potential ISDN users reason to consider the modem as a practical and economical near-term alternative.
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