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Gilbert Held
As the Internet gains in popular usage within corporate America, organizations are trying to establish ways to connect to the Information Superhighway at speeds that make economic sense. This chapter describes the operation of cable modems, a device that provides high-speed connectivity and enables the organization to take advantage of transmission rates of up to tens of millions of bits per second. Cabling infrastructure the cabling within buildings is another key component defined and discussed, to introduce the data center operations manager to a rapidly evolving new technology that can help keep the organization competitive.
During 1995, the use of the Internet expanded considerably, with tens of thousands of corporations, universities, government agencies, and individuals creating home pages on servers, while tens of millions of users surfed the World Wide Web. As corporations began to recognize the value of the Internet for building software applications, promoting products and services, and locating as well as disseminating information, the addition of graphics to World Wide Web home pages literally slowed Web surfing operations to a crawl, adversely affecting user productivity. Whereas the replacement of 14.4K bps modems by state-of-the-art 28.8K bps devices has assisted many users in speeding up their Internet search operations, even at that operating rate the display of a typical Web page containing one or two graphic images can result in a delay of 10 seconds to 15 seconds as the picture is painted on a monitor.
Recognizing the operating limitations associated with transmissions via the public switched telephone network, as well as looking for an additional source of revenue, several cable television (CATV) companies initiated broadband access trials to the Internet during 1995. Each of these trials involves the use of cable modems, which enables a personal computer (PC) to access the Internet via a common CATV coaxial cable at operating rates up to tens of millions of bits per second. Although cable modems are in their infancy, both independent market research organizations and many cable operators predict that the installed base of this new type of communications device will rapidly grow to over 10 million modems within a few years.
Due to the advantages associated with obtaining high-speed Internet access, as well as the potential economics associated with the use of cable modems to obtain such access, data center managers should consider preparing their facility for the infrastructure required to use cable modems.
This chapter discusses the nature of cable modems and describes their operation. The scope of the discussion also includes the cabling infrastructure being developed to provide a megabit transmission facility to residences and businesses. The chapter outlines the cabling requirements for installation within buildings, and the requirements that are necessary to access this new high speed information highway via the use of cable modems. The data center manager should have a background of knowledge concerning a rapidly evolving new technology and be able to support its use when corporate policy begins to include Internet issues.
The ability to appreciate why cable modems are able to provide a transmission capability that is an order of magnitude or more than conventional modems, used for transmission on the switched telephone network, requires knowledge of certain transmission concepts, including the Nyquist theorem. This section concentrates on the operation of conventional analog modems that are used on the switched telephone network. This can provide the data center manager with an understanding of why analog modems operating rate is limited and how they may be able to overcome that operating rate limitation.
A conventional analog modem commonly used to transmit information over the switched telephone network is limited to a maximum operating rate of between 28.8K bps and 33.6K bps, with the rate achievable dependent upon the quality of the connection and according to the modulation technique employed. In theory, the maximum operating rate of an analog modem that has been designed for use on the switched telephone network is limited by the 4K Hz bandwidth provided by the communications carrier for a switched telephone channel.
In 1924 Nyquist proved, in what is now referred to as the Nyquist theorem, that the maximum signaling rate of a device is limited to twice the available bandwidth; beyond that rate, inter-symbol interference starts to occur and adversely affects the transmission. As an example, for the 4K Hz telephone channel, this means the maximum signaling rate of a modem used to transmit on that medium is limited to 8,000 baud. Baud is a term used to indicate signal changes per second.
The most commonly used modem modulation technique, quadrature amplitude modulation (QAM), uses a combination of phase and amplitude to convey the settings of a group of bits in one signal change, enabling four bits to be represented by one baud change. This in turn enables an 8,000 baud signaling rate to transport data at a rate of 32K bps when QAM is used for modulation.
Due to the 4K Hz telephone channel limitation, however, data transmission rates are limited to approximately 32K bps, with a slightly higher rate of 33.6K bps recently achieved by a few modem vendors using a modified QAM technique. Although the incorporation of data compression into modems provides a potential doubling to quadrupling of modem throughput, to between 67.2K bps and 134.4K bps, the ability of a modem to compress data depends upon the susceptibility of data to the compression algorithm being used. Because that susceptibility varies considerably as a modem user performs different operations, the end result is a variable compression rate; even though it is not noticeable during file transfer operations, that variable rate becomes extremely noticeable during interactive operations. In addition, even with the ability to compress data at a high rate, the resulting information transfer rate of 134.4K bps pales by comparison to the operating rate obtainable through the use of cable modems. It is clear that advances in modem and cabling technology are limited with respect to increasing the performance of modems used to communicate via the switched telephone network, however.
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