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`1 of 100 DOCUMENTS
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`Copyright 1987, Network World, Inc.
`Network World
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`February 2, 1987
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`SECTION: FEATURE FOCUS; Special Section: Bypass; Pg. 1
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`LENGTH: 1711 words
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`HEADLINE: Conjuring up data over CATV
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`BYLINE: BY RODNEY MITCHELL JR., Special to Network World; Mitchell is senior programmer for Anne Arundel
`County Public Schools in Annapolis, Md.
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`HIGHLIGHT:
`The wizardry of bypass works wonders over broadband.
`
`BODY:
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`Can the same cable system that brings that costly brew of Home Box Office, Music Television and other
`entertainment to your television set also bring data to your office workstation? The answer is yes, and it is probably
`easier than you think.
`
`Broadband cable communications technology has been around since 1949. Its most popular application is in
`community antenna television. But CATV is not widely used as a data communications bypass medium, mainly
`because most users are unaware that use of CATV systems rather than conventional bypass methods can save them a
`considerable amount of money.
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`Despite continually increasing user demand for data communications alternatives, most CATV franchises have
`been too engrossed in broadcasting television signals to become interested in the networking market. Thus, it is up to
`communications managers to investigate broadband and determine whether it is appropriate for their applications.
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`Not just a one-way street
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`Perhaps the most common misconception about CATV is that it can't be used for data communications because it is
`most commonly used for one-way transmission. In fact, cable systems are capable of two lanes of traffic, and the
`second lane already exists.
`
`This misconception stems from the way most CATV systems were originally installed (see Figure 1). Video
`signals originated at the head-end transmitter and traveled down through the cable network to the subscribers' sets
`below.
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`Apple Exhibit 4178
`Apple v. SightSound Technologies
`CBM2013-00020
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`Conjuring up data over CATV Network World February 2, 1987
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`These signals are termed "outbound" signals, whose sets are referred to as being "downstream." Amplifiers are
`required periodically to boost signal strength over long distances. However, amplifiers perform other functions, and
`that is what causes the misconception.
`
`Just as ripples in a pond can be reflected back to their source, so can the CATV signal. If allowed to pass back
`through the system, the amplified signals will interfere with picture quality. To prevent this, line amplifiers also act as
`filters and block reflected signals to keep them from traveling back upstream. Therefore, any signal sent from one
`terminal to another would be an inbound, or upstream, signal and would be blocked by the amplifiers. Another lane is
`needed to carry signals that are independent of the existing downstream traffic.
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`A basic cable system provides 55 channels, each six MHz wide, but for various reasons, many channels are not
`used. One reason is that the Federal Communications Commission prohibits the use of any frequency that corresponds
`to aircraft navigation frequencies. Other channels are not used because the FCC set them aside for upstream traffic flow
`in an attempt to standardize channel pairs for modem makers. And it is here that the other lane is found.
`
`Using subband
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`Occupying the low end of the frequency spectrum is the subband. Within it are eight channels that have been
`reserved for upstream traffic. Known as T-7 through T-14, they are available for data traffic. To use the subband
`channels, two things must be done.
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`The first is to modify the amplifiers to allow the subband frequencies to pass upstream. Because this is a common
`procedure, most amplifiers are designed to accept a plug-in module that can split the CATV band into the upstream and
`downstream lanes.
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`The second is to install a device known as a frequency translator at the head end. The sole purpose of this piece of
`equipment is to convert inbound frequencies.
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`The resulting change involves transmitting on channel T-7 and receiving on an unused outbound, or downstream,
`channel (see Figure 2). When the terminal operator hits the enter key, the terminal transmits on channel T-7.
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`The amplifiers have been conditioned to allow the subband frequencies to pass upstream to the head-end
`transmitter. The inbound T-7 signal is then converted by the translator to an outbound signal to be received by the
`computer downstream. The return message from the computer would follow the same procedure: The transmitted T-7
`signal would travel upstream to the translator, be converted to an outbound signal and be received by the terminal.
`Because a frequency translator is needed to convert the inbound frequency to an outbound frequency, this system of
`broadband communications is known as a "translated" system.
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`The cost involved to make this lane available is about $1,500 for the frequency translator and $200 per amplifier for
`the plug-in modules. The total cost would depend on how many terminals or nodes are in the network and how close
`they are to each other. Only those amplifiers leading to communications devices need to be converted. The remainder
`of the cable system is unchanged.
`
`To increase the number of data lines, a user doesn't necessarily have to go to the cable company to obtain more
`channel pairs. At the lowest transmission rate of 19.2K bit/sec, a single pair of CATV channels can accommodate up to
`120 duplex data lines. This number is usually more than enough for most users' needs.
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`CATV reliability
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`The second most common misunderstanding concerning CATV communications is its reliability. Most people tend
`to equate reliability with the quality of the picture on their television sets, but data signals are quite different from TV
`signals.
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`The device responsible for sending and receiving data signals is a radio frequency (RF) modem that is programmed
`to look for a specific signal pattern and ignore the rest. Picture anomalies like snow, ghosts and herringbone patterns,
`which annoy the average television viewer, are ignored by the RF modem. In fact, the RF modem is capable of
`functioning in an environment many times more hostile than a TV viewer would tolerate.
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`Another characteristic that distinguishes data signals from TV signals is the origin of the signal itself. The sole
`source of a television signal is the head end transmitter. Data signals, however, are not only received by the RF modem
`but are transmitted by it as well. The head end is only needed to house the frequency translator, which operates
`independently. Thus, the TV tube can be blank, and the terminal tube will be unaffected.
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`Broadband data reliability, while not as good as the local telephone company can provide, is still somewhat better
`than a CATV viewer might expect. Balanced against an increase in performance and a decrease in operating costs,
`occasional interruptions in service might be tolerated. And that is what CATV communications excels at: providing
`high performance at low cost.
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`The advantages of cable channels
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`Data communications traffic is like automobile traffic in that the wider the lane, the more efficiently it can operate.
`The speed and accuracy of data communications are proportional to the size of the lane as well. Because a CATV
`channel is 2,000 times wider than a telephone channel, it has several benefits.
`
`The first benefit is accuracy. If someone were to drive his car at high speeds along narrow streets, his error rate
`would increase. In communications, this quantity is the bit-error rate, or the number of erroneous bits of information
`per million transmitted. Because of the wider lanes in CATV, the bit-error rate is 1,000 times better than with a
`telephone line.
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`Another benefit involves the transmission speed and the number of data lines users can get from a transmit/receive
`pair of CATV channels. Higher transmission rates require wider subchannels and reduce the number of available data
`lines; lower rates have the opposite effect. In a single transmit/receive channel pair, there can be several different speed
`subbands, depending on the data. The benefit for the manager is that he has some flexibility in determining how to
`arrange various speeds on the same channel pair.
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`Channel size also affects the cost of modems. Telephone modems must use sophisticated techniques such as
`quadrature phase shift keying, bit compression/decompression and constant line-quality sampling to achieve their high
`data rate transfer. Because of the larger channel size in broadband communications, it is must easier for the RF modem
`to do its work. The simpler design is reflected in lower modem costs -- normally a third to a half as much as their
`telephone counterparts.
`
`Some companies manufacturing RF modems today are Fairchild Data Corp. in Scottsdale, Ariz., General
`Instrument Corp. in New York, Kee, Inc. in Beltsville, Md., and Zeta Laboratories, Inc. in Santa Clara, Calif. Most
`divide the CATV channel into smaller subchannels. The modems are transparent to computer protocols and support
`either point-to-point or multipoint communications.
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`However, some products like Kee's Bus Interface treat the entire CATV channel as one big pipeline. Using an
`X.25 protocol, they allow all nodes access to one another in a high-speed, packet-switching network.
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`Legalities of data over CATV
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`Can a local franchise offer a bypass service to the public or private sector? Yes, says Syd Bradfield, supervisor of
`electronic engineers in the cable television branch of the FCC, as long as the cable operator observes restrictions
`involving aeronavigation frequencies.
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`Conjuring up data over CATV Network World February 2, 1987
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`On a larger scale, the FCC recently ruled that Cox Communications Co., a CATV franchise in Omaha, Neb., will
`be permitted to operate a microwave link between two of its cable systems. This opens the door to interstate
`transmission of broadband data.
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`The key to broadband data transmission is procuring transmit/receive channel pairs from the CATV franchise, and
`local governments and public school systems have an advantage in this regard. Most franchise agreements that local
`governments have with CATV companies require the cable company to provide a community service by reserving
`certain channels, usually the T channels, for institutional use.
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`Whatever means are used, the communications manager must be the insigator and stir up the notion of data over
`cable. Cable franchises are not eager to get into this mysterious caldron of data communications. Before meeting with
`CATV officials, a manager should assemble as much supporting information as possible. The reward can be a
`communications system that will pay for itself in a few years, accommodating network growth at the same time.
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`GRAPHIC: Illustration, no caption, TOM BARRETT; Figure 1, Typical CATV installation, SOURCE: RODNEY
`MITCHELL JR., ANNAPOLIS, MD.; Figure 2, Data over CATV, SOURCE: RODNEY MITCHELL JR.,
`ANNAPOLIS, MD.
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