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Article #214 (419 is last):
Newsgroups: freenet.sci.comp.atari.questions
From: ae302@cleveland.Freenet.Edu (Peter Haller)
Subject: FYI:9600 bps Modems
Date: Sun Jan 13 22:59:45 1991

I downloaded this artical from a local BBS, and though it might prove
to be interesting reading....I thought so.

 

Not long ago, many data communicators thought that dial-up modem manufacturers
had pushed transmission speeds to the limit with the introduction of 2400 bit
per second (bps) modems.  Recently, however, several manufacturers have
creatively combined  relatively mature techniques of data transmission with
newer technology and have introduced 9600 bps modems.

Unfortunately, a widely accepted standard for full duplex 9600 bps
transmission as defined by the International Consultative Committee for
Telegraphy and Telephony (CCITT) does not yet exist (the CCITT is currently
considering proposals for a new 9600 bps dial-up standard).  This means that
today's 9600 bps modems do not offer cross-manufacturer compatibility.  The
CCITT HAS endorsed a half duplex and a full duplex 9600 bps standard, but to
date implementations of these relatively flexible standards have been
proprietary, i.e., even the "standardized" modems from different manufacturers
are not compatible.

All this means that modem users who want to enjoy the dream speed of 9600 bps
must weigh the pros and cons of each 9600 bps technique before committing to a
particular 9600 bps design.  This paper was written in an effort to provide
typical modem users with enough technical information and insight that they
will be able to consider the new 9600 bps modems from the position of an
educated consumer and not have to rely on information gleaned from sales
brochures and advertisements.  It should be noted that the author, Wes Cowell,
is an employee of USRobotics.



                              THE ROAD TO 9600



High speed data communications via the dial-up phone network is limited by the

available phone line bandwidth and by random channel impairments.  Just as the

diameter of a pipe limits its liquid flow capacity, so does the telephone

channel bandwidth limit its data flow capacity.



The roughly 3000-Hz available in the telephone bandwidth poses few problems

for 300 bps modems, which only use about one fifth of the bandwidth.  A full

duplex 1200 bps modem requires about half the available bandwidth,

transmitting simultaneously in both directions at 600 baud and using phase

modulation to signal two data bits per baud.  "Baud rate" is actually a

measure of signals per second.  Because each signal can represent more than

one bit, the baud rate and bps rate of a modem are not necessarilly the same.

In the case of 1200 bps modems, their baud rate is actually 600 (signals per

second) and each signal represents two data bits.  By multiplying signals per

second with the number of bits represented by each signal one determines the

bps rate: 600 signals per second X 2 bits per signal = 1200 bps.



In moving up to 2400 bps, modem designers decided not to use more bandwidth,

but to increase speed through a new signalling scheme known as quadrature

amplitude modulation (QAM).



In QAM, each signal represents four data bits.  Both 1200 bps and 2400 bps

modems use the same 600 baud rate, but each 1200 bps signal carries two data

bits, while each 2400 bps signal carries four data bits:

600 signals per second X 4 bits per signal = 2400 bps.



A technique known as adaptive equalization enables 2400 bps modems to adapt to

phone line impairments call-by-call. Essentially, if the modem is experiencing

problems with a noisy line, it looks for a "sweet spot" in the bandwidth and

attempts to avoid troublesome frequencies.  This technique makes 2400 bps

modems more tolerant of line noise than their 1200 bps counterparts that use

compromise equalization (a one-size-fits-all approach).



While these advanced modulation and equalization techniques in 2400 bps modems

provide for double the data rate of 1200 bps modems, they also result in a

design at least four times more complex than 1200 bps modems.



Which brings us to the problem of designing a 9600 bps modem.



Jumping to 9600 from 2400 bps is several orders of magnitude more complicated

than going to 2400 from 1200 bps.  Telephone network characteristics make it

highly unlikely that success will be had in  extending the "data signal

alphabet" (number of bits represented by each signal) beyond four bits per

signal.



Instead, modem designers must increase the bandwidth that is to carry the

signal, and this presents a very big problem.  In fact, at speeds of 4800 bps

(1200 signals per second), the transmit and receive channels must be expanded

to the point where they actually begin to overlap. A  9600 bps "band"

requires roughly 90 percent of the available bandwidth, making it impossible

to have two-way communication without the bands interfering with each other.



A helpful analogy to the problem might be to consider a two lane highway:

traffic must flow in both directions simultaneously, but to carry more cars

per unit of time, highway designers must either increase the number of lanes

in each direction or widen the two lanes to accommodate driver error with a

margin of safety.  Unfortunately, these options are not available to modem

designers as the available bandwidth is of a fixed size.



With these considerations and limitations in mind, let's examine three basic

ways to accomplish full duplex (two-way) 9600 bps communications:  echo

cancellation, virtual full duplex (achieved by half duplex systems), and

asymmetrical frequency division.



                              ECHO-CANCELLATION



This method solves the problem of overlapping transmit and receive channels.

Each modem's receiver must try to filter out the echo of its own transmitter

and concentrate on the other modem's transmit signal.  This presents a

tremendous computational problem that significantly increases the complexity

-- and cost -- of the modem.  But it offers what other schemes don't:

simultaneous two-way transmission of data at 9600 bps.



The CCITT "V.32" recommendation for 9600 bps modems includes echo-

cancellation.  The transmit and receive bands overlap almost completely, each

occupying 90 percent of the available bandwidth.  Measured by computations per

second and bits of resolution, a V.32 modem is roughly 64 times more complex

than a 2400 bps modem.  This translates directly into added development and

production costs which means that it will be some time before V.32 modems can

compete in the high- volume modem market.



Despite the fact that V.32 is a recognized standard, it is uneconomical and

unnecessarily complex for personal computer datacomm applications that simply

don't require simultaneous two-way 9600 bps transmission.



                              HALF DUPLEX SYSTEMS

                             (Virtual Full Duplex)



Half duplex solutions devote the entire bandwidth to 9600 bps in one direction

at a time, and "ping-pong" the data flow back and forth to simulate full

duplex.  This is potentially the simplest scheme.  Its performance is

acceptable in data transfer applications that don't involve user interaction,

i.e. file transfers. Even so, advanced error-control protocols that require

ACKnowledgments to be sent in response to received data blocks generate a high

number of "line reversals" which greatly impair overall data throughput.  In

short, the benefit of higher speed is so significantly compromised by line

reversals in half duplex sessions that the net gain in data throughput may be

marginal at best.



If users want to operate in an interactive mode, their data must be sent to

the remote computer, the data channel must be reversed, and then the data must

be echoed back.  This process results in significant turn-around delays which

can be very frustrating to users.



Half duplex modems of this kind are most often based on CCITT recommendation

V.29 for half duplex 9600 bps transmission on the dial-up network.  V.29 based

data pumps used in facsimile systems are available as LSI chip sets, providing

a short-cut to modem manufacturers, particularly to companies that don't

develop their own modem technologies.  But the major problem is that the V.29

modulation scheme has been outdated  by the fact that it operates in a half

duplex mode and doesn't provide good signal to noise performance.  The V.32

recommendation, which operates in a full duplex mode and employs Trellis

Coding Modulation offers greater throughput and a greater immunity to channel

impairments.



To the best of my knowledge, modems employing V.29-based modulation include

products from Racal-Vadic, Comspec, Develcon, Gamma Technology, Microcomm, and

Electronic Vaults, Inc.  (EVI).  These modems, however, are NOT mutually

signal compatible -- cross-manufacturer compatibility does not exist.



Another modem in the half duplex category, but not based on V.29 modulation,

is the Telebit Trailblazer (R), which uses a proprietary modulation method.



Trailblazer is based on a multi-carrier technique.  Conceptually, the

transmission channel is divided into many (512), independent, very narrow

channels (think of our two-lane highway and imagine it as having 512 very

narrow lanes (say, for bicycles) going in one direction and you've got a fair

idea of how Trailblazer divides the bandwidth).  The main advantage is that no

receiver adaptive equalizer is needed because each channel is very narrow

compared to the overall channel bandwidth.



Further, in the Trailblazer modulation scheme, the modulation rate in each

narrow channel can be changed somewhat independently.  Trailblazer is

different from many other modems in that the decision to fall back to lower

speeds is built into the modem protocol, rather than controlled by the user's

computer port.  It is claimed that in the face of channel impairments,

throughput can be adapted gracefully to channel conditions.  Traditional

modulation systems would have to fall back in larger steps.  But there are

three inherent MAJOR problems:



1)  The turn-around delay is very long compared to conventional modulation

techniques because data must be sent in large blocks.   A typed character may

take several seconds to be echoed back to the system that sent it.  As a

result, the system fails to achieve the illusion of full duplex and is not

really suited to interactive online sessions.



2)  The Trailblazer receiver cannot "track" carrier "phase jitter" (phase

jitter can be thought of in terms of "phase shift": think of how the whine of

a race car goes from higher to lower as it passes the viewer --  the frequency

of the sound is said to be "shifted" or "jittered").  Instead of cancelling

out phase jitter (which is commonly encountered on long distance calls) the

Trailblazer can only respond by lowering throughput to gain more immunity to

phase jitter.



3)  The ability to transmit at the maximum rate when subject to channel

impairment is considerably less than for conventional modems.  There is one

notable exception:  the multiple channel technique offers extremely good

immunity to impulse noise because the impulse energy is distributed over

narrow channels.  While conventional modems can achieve similar results

through special coding or filtering techniques they rarely implement such

methods.



                       ASYMMETRICAL FREQUENCY DIVISION



When one considers the nature of most PC datacomm applications, it is realized

that most applications are interactive, involving manual (typed) data entry

from one end and data file transmission from the other end.



Few, if any, PC users can justify using an expensive 9600 bps channel to carry

their typed characters when they realize that 300 bps translates to 360 words

per minute.  Assuming one could type 100 words per minute, even a 100 bps

transmission channel would be sufficient.



On the other hand, file transfer should take advantage of the tremendous speed

of the microprocessor.  Serial ports are often set at data rates in excess of

19,000 bps.



Considering these inherent characteristics, a communications scheme that

incorporated a high speed and a low speed channel would be best suited for

most PC datacomm applications.



Remembering the highway analogy (higher speeds mean wider lanes), one can see

how such a method would grant modem designers a  large portion of the

available bandwidth for a 9600 bps channel and still leave enough room to

accommodate a narrow 300 bps channel without any channel overlap.



By utilizing two discreet channels, such a modem would avoid costly, complex

echo-cancellation schemes.  And, because the channels carry data in both

directions simultaneously, the communications link is a true full duplex

connection.  This means that data entered at one system would be almost

instantaneously echoed back -- eliminating the frustrating turn-around delay

experienced in half duplex sessions.



USRobotics has developed just such a modem.  It passes data in one direction

using the V.32 modulation technique (a very robust method that is very immune

to phone line impairments) but employs only a 300 bps channel in the opposite

direction so that the channels do not overlap and echo-cancellation is not

necessary.



The use of the high-speed channel by the two modems is based on data demand.

In most applications, however, "channel swapping" will not be required.  For

interface elegance, the modems employ a 4K buffer that allow them to perform

data rate conversion: sending and receiving speeds remain constant between the

modem and the computer -- it is only in between the modems that transmitted

and received data run at different speeds.



For interactive sessions, users are assigned the low-speed channel while the

data sent to them (long mail messages, menus, files, etc.) in the 9600 bps

channel.



For file transfer sessions, the data blocks that make up a file are sent in

the 9600 bps channel while the corresponding ACKnowledgments are returned in

the 300 bps channel.  An asymmetric frequency division scheme is ideal for

file transfer where large data blocks (usually several hundred bytes in

length) are transmitted in the high-speed channel and the ACKs (usually only

a few bytes in length) are carried in the low-speed channel.



If a user switches from an interactive mode to file transfer and then back to

interactive mode, the high speed channel is dynamically and automatically

assigned to the system with the greatest data demand.



                              A BRIEF COMPARISON



Three options exist for data communicators who desire to operate at 9600 bps:



1)  V.32-type modems offer a full duplex connection but do so by virtue of

echo-cancellation.  This technique is so complex, and has proven so difficult

to employ, that the cost for such modems will remain prohibitively high and

their implementation a delicate task for some time to come.



2)  Half duplex modems (either V.29 or multi-carrier) offer 9600 bps but the

turn-around delay inherent in half duplex links severely compromise overall

throughput.  This degradation of throughput, however, can be more than offset

by data compression techniques assuming the modems in question support

identical compression protocols and are operating on relatively "clean" phone

lines.  Both half duplex methods suffer disproportionate degradation on

"noisy" lines: the V.29 modems must spend more and more time in line reversals

as detected data errors increase, and the multi-carrier modems must sacrifice

throughput to gain noise immunity.



3)  Asymmetrical Frequency Division offers 9600 bps communications in a true

full duplex implementation.  By efficiently utilizing the available bandwidth,

these modems provide users with high speed file transfer capabilities and fast

response in interactive sessions.  Because the transmit and receive data

channels do not overlap, expensive echo-cancelling techniques are unnecessary

making these modems economically efficient.



                                  IN CONCLUSION



Until a widely recognized standard is agreed upon by the standards community,

and implemented by several manufacturers, modem buyers must weigh the benefits

and detriments of each 9600 bps scheme.



V.32 would be best where symmetrical, full duplex, synchronous communication

is desired (for example, dial-up HDLC links between multiplexers) and where

the user can modify his software to accommodate non-"AT" command-driven

modems.



V.29 modems would be likely solutions where absolute lowest price is required

and conformance to an international standard (in a very limited sense) is

desired.



Multi-carrier transmission schemes are well-suited to applications that

require maximum one-way throughput and where circuit conditions are known to

be good.  This transmission method is also ideally suited for circuits where

immunity to impulse noise is paramount.



Users who most often work with one-way file transfers (PC-to-PC) or with real-

time applications may opt for an Asymmetrical Frequency Division scheme, which

is suited equally well for either application.  The elegant approach to the

frequency division (avoiding overlapping bandwidths) also allows these modems

to present a very economical ratio between dollars and bps.



Potential high-speed-modem buyers should also consider the aspects of ease-of-

use, ease-of-implementation, and downward compatibility with existing

implemented standards (the CCITT's V.22bis for 2400 bps, Bell 212A for 1200

bps, and Bell 103 for 200 bps).



                                  POST SCRIPT



Many modem users have voiced confusion and consternation about the lack of

compatibility between modem manufacturers at speeds greater than 2400 bps.



Modem manufacturers have embraced the Bell 212A and 103 standards for 1200 and

300 bps.  In these post-divestiture days, however, Bell no longer sets modem

standards in the U.S. and hence, U.S. modem manufacturers have turned to the

CCITT as a definitive source for standards.  The industry-wide acceptance of

the CCITT's V.22bis standard for 2400 bps is the best example of this shift.



The CCITT recommendations V.29 and V.32 for 9600 bps have not resulted in

compatible implementations.  It is important to remember that V.29 was

originally developed as a four-wire full duplex leased-line modem and has

since been adapted by various manufacturers to encompass half duplex dial up

applications.  Other problems with V.29 are that it compromises transmission

speed and is poor for interactive sessions.  V.32 is proving to be

prohibitively complex and exceptionally difficult to implement (driving

development and production costs up).



Recognizing the need for an alternative to the V.32 recommendation, the CCITT

has requested proposals from modem manufacturers.



Presently, two proposals are being considered by the CCITT.  One is the multi-

carrier scheme developed and sponsored by Telebit.  The other is an

Asymmetrical Frequency Division scheme developed and sponsored by USRobotics.



 


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