334 lines
19 KiB
INI
334 lines
19 KiB
INI
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Not long ago, many data communicators thought that dial-up modem manufacturers
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had pushed transmission speeds to the limit with the introduction of 2400 bit
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per second (bps) modems. Recently, however, several manufacturers have
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creatively combined relatively mature techniques of data transmission with
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newer technology and have introduced 9600 bps modems.
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Unfortunately, a widely accepted standard for full duplex 9600 bps
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transmission as defined by the International Consultative Committee for
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Telegraphy and Telephony (CCITT) does not yet exist (the CCITT is currently
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considering proposals for a new 9600 bps dial-up standard). This means that
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today's 9600 bps modems do not offer cross-manufacturer compatibility. The
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CCITT HAS endorsed a half duplex and a full duplex 9600 bps standard, but to
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date implementations of these relatively flexible standards have been
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proprietary, i.e., even the "standardized" modems from different manufacturers
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are not compatible.
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All this means that modem users who want to enjoy the dream speed of 9600 bps
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must weigh the pros and cons of each 9600 bps technique before committing to a
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particular 9600 bps design. This paper was written in an effort to provide
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typical modem users with enough technical information and insight that they
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will be able to consider the new 9600 bps modems from the position of an
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educated consumer and not have to rely on information gleaned from sales
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brochures and advertisements. It should be noted that the author, Wes Cowell,
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is an employee of USRobotics.
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THE ROAD TO 9600
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High speed data communications via the dial-up phone network is limited by the
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available phone line bandwidth and by random channel impairments. Just as the
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diameter of a pipe limits its liquid flow capacity, so does the telephone
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channel bandwidth limit its data flow capacity.
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The roughly 3000-Hz available in the telephone bandwidth poses few problems
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for 300 bps modems, which only use about one fifth of the bandwidth. A full
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duplex 1200 bps modem requires about half the available bandwidth,
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transmitting simultaneously in both directions at 600 baud and using phase
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modulation to signal two data bits per baud. "Baud rate" is actually a
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measure of signals per second. Because each signal can represent more than
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one bit, the baud rate and bps rate of a modem are not necessarilly the same.
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In the case of 1200 bps modems, their baud rate is actually 600 (signals per
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second) and each signal represents two data bits. By multiplying signals per
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second with the number of bits represented by each signal one determines the
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bps rate: 600 signals per second X 2 bits per signal = 1200 bps.
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In moving up to 2400 bps, modem designers decided not to use more bandwidth,
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but to increase speed through a new signalling scheme known as quadrature
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amplitude modulation (QAM).
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In QAM, each signal represents four data bits. Both 1200 bps and 2400 bps
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modems use the same 600 baud rate, but each 1200 bps signal carries two data
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bits, while each 2400 bps signal carries four data bits:
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600 signals per second X 4 bits per signal = 2400 bps.
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A technique known as adaptive equalization enables 2400 bps modems to adapt to
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phone line impairments call-by-call. Essentially, if the modem is experiencing
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problems with a noisy line, it looks for a "sweet spot" in the bandwidth and
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attempts to avoid troublesome frequencies. This technique makes 2400 bps
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modems more tolerant of line noise than their 1200 bps counterparts that use
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compromise equalization (a one-size-fits-all approach).
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While these advanced modulation and equalization techniques in 2400 bps modems
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provide for double the data rate of 1200 bps modems, they also result in a
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design at least four times more complex than 1200 bps modems.
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Which brings us to the problem of designing a 9600 bps modem.
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Jumping to 9600 from 2400 bps is several orders of magnitude more complicated
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than going to 2400 from 1200 bps. Telephone network characteristics make it
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highly unlikely that success will be had in extending the "data signal
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alphabet" (number of bits represented by each signal) beyond four bits per
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signal.
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Instead, modem designers must increase the bandwidth that is to carry the
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signal, and this presents a very big problem. In fact, at speeds of 4800 bps
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(1200 signals per second), the transmit and receive channels must be expanded
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to the point where they actually begin to overlap. A 9600 bps "band"
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requires roughly 90 percent of the available bandwidth, making it impossible
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to have two-way communication without the bands interfering with each other.
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A helpful analogy to the problem might be to consider a two lane highway:
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traffic must flow in both directions simultaneously, but to carry more cars
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per unit of time, highway designers must either increase the number of lanes
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in each direction or widen the two lanes to accommodate driver error with a
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margin of safety. Unfortunately, these options are not available to modem
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designers as the available bandwidth is of a fixed size.
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With these considerations and limitations in mind, let's examine three basic
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ways to accomplish full duplex (two-way) 9600 bps communications: echo
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cancellation, virtual full duplex (achieved by half duplex systems), and
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asymmetrical frequency division.
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ECHO-CANCELLATION
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This method solves the problem of overlapping transmit and receive channels.
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Each modem's receiver must try to filter out the echo of its own transmitter
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and concentrate on the other modem's transmit signal. This presents a
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tremendous computational problem that significantly increases the complexity
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-- and cost -- of the modem. But it offers what other schemes don't:
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simultaneous two-way transmission of data at 9600 bps.
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The CCITT "V.32" recommendation for 9600 bps modems includes echo-
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cancellation. The transmit and receive bands overlap almost completely, each
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occupying 90 percent of the available bandwidth. Measured by computations per
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second and bits of resolution, a V.32 modem is roughly 64 times more complex
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than a 2400 bps modem. This translates directly into added development and
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production costs which means that it will be some time before V.32 modems can
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compete in the high- volume modem market.
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Despite the fact that V.32 is a recognized standard, it is uneconomical and
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unnecessarily complex for personal computer datacomm applications that simply
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don't require simultaneous two-way 9600 bps transmission.
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HALF DUPLEX SYSTEMS
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(Virtual Full Duplex)
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Half duplex solutions devote the entire bandwidth to 9600 bps in one direction
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at a time, and "ping-pong" the data flow back and forth to simulate full
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duplex. This is potentially the simplest scheme. Its performance is
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acceptable in data transfer applications that don't involve user interaction,
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i.e. file transfers. Even so, advanced error-control protocols that require
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ACKnowledgments to be sent in response to received data blocks generate a high
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number of "line reversals" which greatly impair overall data throughput. In
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short, the benefit of higher speed is so significantly compromised by line
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reversals in half duplex sessions that the net gain in data throughput may be
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marginal at best.
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If users want to operate in an interactive mode, their data must be sent to
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the remote computer, the data channel must be reversed, and then the data must
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be echoed back. This process results in significant turn-around delays which
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can be very frustrating to users.
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Half duplex modems of this kind are most often based on CCITT recommendation
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V.29 for half duplex 9600 bps transmission on the dial-up network. V.29 based
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data pumps used in facsimile systems are available as LSI chip sets, providing
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a short-cut to modem manufacturers, particularly to companies that don't
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develop their own modem technologies. But the major problem is that the V.29
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modulation scheme has been outdated by the fact that it operates in a half
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duplex mode and doesn't provide good signal to noise performance. The V.32
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recommendation, which operates in a full duplex mode and employs Trellis
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Coding Modulation offers greater throughput and a greater immunity to channel
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impairments.
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To the best of my knowledge, modems employing V.29-based modulation include
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products from Racal-Vadic, Comspec, Develcon, Gamma Technology, Microcomm, and
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Electronic Vaults, Inc. (EVI). These modems, however, are NOT mutually
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signal compatible -- cross-manufacturer compatibility does not exist.
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Another modem in the half duplex category, but not based on V.29 modulation,
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is the Telebit Trailblazer (R), which uses a proprietary modulation method.
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Trailblazer is based on a multi-carrier technique. Conceptually, the
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transmission channel is divided into many (512), independent, very narrow
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channels (think of our two-lane highway and imagine it as having 512 very
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narrow lanes (say, for bicycles) going in one direction and you've got a fair
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idea of how Trailblazer divides the bandwidth). The main advantage is that no
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receiver adaptive equalizer is needed because each channel is very narrow
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compared to the overall channel bandwidth.
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Further, in the Trailblazer modulation scheme, the modulation rate in each
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narrow channel can be changed somewhat independently. Trailblazer is
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different from many other modems in that the decision to fall back to lower
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speeds is built into the modem protocol, rather than controlled by the user's
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computer port. It is claimed that in the face of channel impairments,
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throughput can be adapted gracefully to channel conditions. Traditional
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modulation systems would have to fall back in larger steps. But there are
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three inherent MAJOR problems:
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1) The turn-around delay is very long compared to conventional modulation
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techniques because data must be sent in large blocks. A typed character may
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take several seconds to be echoed back to the system that sent it. As a
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result, the system fails to achieve the illusion of full duplex and is not
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really suited to interactive online sessions.
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2) The Trailblazer receiver cannot "track" carrier "phase jitter" (phase
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jitter can be thought of in terms of "phase shift": think of how the whine of
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a race car goes from higher to lower as it passes the viewer -- the frequency
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of the sound is said to be "shifted" or "jittered"). Instead of cancelling
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out phase jitter (which is commonly encountered on long distance calls) the
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Trailblazer can only respond by lowering throughput to gain more immunity to
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phase jitter.
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3) The ability to transmit at the maximum rate when subject to channel
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impairment is considerably less than for conventional modems. There is one
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notable exception: the multiple channel technique offers extremely good
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immunity to impulse noise because the impulse energy is distributed over
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narrow channels. While conventional modems can achieve similar results
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through special coding or filtering techniques they rarely implement such
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methods.
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ASYMMETRICAL FREQUENCY DIVISION
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When one considers the nature of most PC datacomm applications, it is realized
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that most applications are interactive, involving manual (typed) data entry
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from one end and data file transmission from the other end.
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Few, if any, PC users can justify using an expensive 9600 bps channel to carry
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their typed characters when they realize that 300 bps translates to 360 words
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per minute. Assuming one could type 100 words per minute, even a 100 bps
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transmission channel would be sufficient.
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On the other hand, file transfer should take advantage of the tremendous speed
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of the microprocessor. Serial ports are often set at data rates in excess of
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19,000 bps.
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Considering these inherent characteristics, a communications scheme that
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incorporated a high speed and a low speed channel would be best suited for
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most PC datacomm applications.
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Remembering the highway analogy (higher speeds mean wider lanes), one can see
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how such a method would grant modem designers a large portion of the
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available bandwidth for a 9600 bps channel and still leave enough room to
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accommodate a narrow 300 bps channel without any channel overlap.
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By utilizing two discreet channels, such a modem would avoid costly, complex
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echo-cancellation schemes. And, because the channels carry data in both
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directions simultaneously, the communications link is a true full duplex
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connection. This means that data entered at one system would be almost
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instantaneously echoed back -- eliminating the frustrating turn-around delay
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experienced in half duplex sessions.
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USRobotics has developed just such a modem. It passes data in one direction
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using the V.32 modulation technique (a very robust method that is very immune
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to phone line impairments) but employs only a 300 bps channel in the opposite
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direction so that the channels do not overlap and echo-cancellation is not
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necessary.
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The use of the high-speed channel by the two modems is based on data demand.
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In most applications, however, "channel swapping" will not be required. For
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interface elegance, the modems employ a 4K buffer that allow them to perform
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data rate conversion: sending and receiving speeds remain constant between the
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modem and the computer -- it is only in between the modems that transmitted
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and received data run at different speeds.
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For interactive sessions, users are assigned the low-speed channel while the
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data sent to them (long mail messages, menus, files, etc.) in the 9600 bps
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channel.
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For file transfer sessions, the data blocks that make up a file are sent in
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the 9600 bps channel while the corresponding ACKnowledgments are returned in
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the 300 bps channel. An asymmetric frequency division scheme is ideal for
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file transfer where large data blocks (usually several hundred bytes in
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length) are transmitted in the high-speed channel and the ACKs (usually only
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a few bytes in length) are carried in the low-speed channel.
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If a user switches from an interactive mode to file transfer and then back to
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interactive mode, the high speed channel is dynamically and automatically
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assigned to the system with the greatest data demand.
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A BRIEF COMPARISON
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Three options exist for data communicators who desire to operate at 9600 bps:
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1) V.32-type modems offer a full duplex connection but do so by virtue of
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echo-cancellation. This technique is so complex, and has proven so difficult
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to employ, that the cost for such modems will remain prohibitively high and
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their implementation a delicate task for some time to come.
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2) Half duplex modems (either V.29 or multi-carrier) offer 9600 bps but the
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turn-around delay inherent in half duplex links severely compromise overall
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throughput. This degradation of throughput, however, can be more than offset
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by data compression techniques assuming the modems in question support
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identical compression protocols and are operating on relatively "clean" phone
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lines. Both half duplex methods suffer disproportionate degradation on
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"noisy" lines: the V.29 modems must spend more and more time in line reversals
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as detected data errors increase, and the multi-carrier modems must sacrifice
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throughput to gain noise immunity.
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3) Asymmetrical Frequency Division offers 9600 bps communications in a true
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full duplex implementation. By efficiently utilizing the available bandwidth,
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these modems provide users with high speed file transfer capabilities and fast
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response in interactive sessions. Because the transmit and receive data
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channels do not overlap, expensive echo-cancelling techniques are unnecessary
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making these modems economically efficient.
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IN CONCLUSION
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Until a widely recognized standard is agreed upon by the standards community,
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and implemented by several manufacturers, modem buyers must weigh the benefits
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and detriments of each 9600 bps scheme.
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V.32 would be best where symmetrical, full duplex, synchronous communication
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is desired (for example, dial-up HDLC links between multiplexers) and where
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the user can modify his software to accommodate non-"AT" command-driven
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modems.
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V.29 modems would be likely solutions where absolute lowest price is required
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and conformance to an international standard (in a very limited sense) is
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desired.
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Multi-carrier transmission schemes are well-suited to applications that
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require maximum one-way throughput and where circuit conditions are known to
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be good. This transmission method is also ideally suited for circuits where
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immunity to impulse noise is paramount.
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Users who most often work with one-way file transfers (PC-to-PC) or with real-
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time applications may opt for an Asymmetrical Frequency Division scheme, which
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is suited equally well for either application. The elegant approach to the
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frequency division (avoiding overlapping bandwidths) also allows these modems
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to present a very economical ratio between dollars and bps.
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Potential high-speed-modem buyers should also consider the aspects of ease-of-
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use, ease-of-implementation, and downward compatibility with existing
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implemented standards (the CCITT's V.22bis for 2400 bps, Bell 212A for 1200
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bps, and Bell 103 for 200 bps).
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POST SCRIPT
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Many modem users have voiced confusion and consternation about the lack of
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compatibility between modem manufacturers at speeds greater than 2400 bps.
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Modem manufacturers have embraced the Bell 212A and 103 standards for 1200 and
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300 bps. In these post-divestiture days, however, Bell no longer sets modem
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standards in the U.S. and hence, U.S. modem manufacturers have turned to the
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CCITT as a definitive source for standards. The industry-wide acceptance of
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the CCITT's V.22bis standard for 2400 bps is the best example of this shift.
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The CCITT recommendations V.29 and V.32 for 9600 bps have not resulted in
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compatible implementations. It is important to remember that V.29 was
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originally developed as a four-wire full duplex leased-line modem and has
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since been adapted by various manufacturers to encompass half duplex dial up
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applications. Other problems with V.29 are that it compromises transmission
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speed and is poor for interactive sessions. V.32 is proving to be
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prohibitively complex and exceptionally difficult to implement (driving
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development and production costs up).
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Recognizing the need for an alternative to the V.32 recommendation, the CCITT
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has requested proposals from modem manufacturers.
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Presently, two proposals are being considered by the CCITT. One is the multi-
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carrier scheme developed and sponsored by Telebit. The other is an
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Asymmetrical Frequency Division scheme developed and sponsored by USRobotics.
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and s |