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Text File | 1992-08-21 | 30KB | 487 lines

The Issue of the Reliable Modem Escape Sequence

Modems allow personal computers to communicate with other computers over
telephone lines by transmitting data and files.  There are two states of
communication between an intelligent modem and the computer to which it is
connected, the Command Mode and the Receive/Transmit Mode.  The Command Mode
allows that modem to receive instructions or commands from the computer, such
as directing the modem to dial the phone, change modem settings, and hang up
the telephone after a transmission is completed.  The other state of
communication is the Receive/Transmit Mode.  It is during this state of
operation that the modem sends or receives data or files which are exchanged
between a personal computer and a remote computer.

When the modem dials the telephone number and makes the connection, it goes
from Command Mode to the Receive/Transmit Mode.  The mechanism that causes the
modem to go from the Receive/Transmit Mode back to the Command Mode is called
an "Escape."  The reliability of the escape mechanism is vital to the integrity
of the system and the ability to predictably deliver the data or file being
transmitted.

When the escape occurs in the middle of a data transmission an aborted
transmission results.  This unintentional escape causes the interruption of the
data flow, loss of time, increased  telephone toll charges because the
telephone line stays open even though no data is being transmitted, the
inability of the software to use the modem until it has been manually reset,
and most importantly, the file not being transferred.  In computer jargon, the
transmission "blows up" or "crashes" and the file transfer "aborts" or "bombs."

For the personal user, this creates at the very least a significant
inconvenience.  Most users wonUt recognize the problem for what it is when it
occurs.  In businesses with large numbers of modems, this can create
significant problems in operations for any company which relies on the
transmission of data by modems to conduct its operations and make money.
Preventing this inadvertent escape is a major design goal in reliable modem
design.


The Problem

Prior to 1981, modems used a sequence of characters to escape from
Receive/Transmit Mode to Command Mode.  The state-of-the-art at the time used a
series of control characters, sometimes "QUIT," to tell the modem to escape.
Because these characters could appear in the data stream, this escape mechanism
did not provide the user with a fail-safe method of escape.  It is easy to see
how a modem like this could inadvertently escape, creating just the problems
described.  The best idea at the time was to increase the length of the escape
code and reduce the probability of it appearing inadvertently in the data.  You
could never be sure a file you were about to send would not contain the
sequence of escape characters which would block the transmission.  In order to
alleviate this problem of inadvertent escape, it is necessary that the escape
mechanism be transparent to data.  That is, the possibility of the data alone
triggering the escape should not exist.


The Solution Was Found in 1981

Designing a modem completely transparent to data was the goal behind Dale
HeatheringtonUs efforts during the development of the first Hayes Smartmodem in
the early 1980s.  Dale was not satisfied with an escape mechanism which caused
some data to be UNSENDABLE.  Dale's belief was that it was unacceptable to
build a system where the modem was working as designed, i.e. NOT BROKEN, and
the computer was not broken, and the software was not broken, but at some time,
eventually, regardless of how improbable, the modem would fail to do its job.
In effect, Dale redefined the problem as understood by the experts at that time
and, by understanding the problem a different way, was on the path to a new
solution.

As part of his intensive research in the development of the original Hayes
Smartmodem, Dale Heatherington solved this problem by surrounding the escape
code, a sequence of characters, with guard times on both sides to alert the
modem that the sequence is distinguished from a typical string of characters in
a file transmission.  This escape sequence

                <guard time>    <escape code>    <guard time>

eliminates the problems inherent in a data-dependent escape sequence because
of its use of time and because it does not depend on the probability of
character occurrence in a stream of data.  It is virtually impossible for the
Hayes escape sequence with guard time to appear in a file transfer and cause an
unintentional escape using the common file transfer protocols.  Computers just
donUt work that way.  The problem of inadvertent escapes was solved.

Dale Heatherington's invention led to the issuance of United States Patent
Number 4,549,302, the Improved Escape Sequence With Guard Time, often called
the Hayes '302 Patent, and corresponding patents in a number of countries.  The
Hayes '302 Patent ensures that modems escape or change to the Command Mode of
operation reliably and without the possibility that data alone could ever
trigger the escape.  In over eleven years of use of the Hayes '302, Hayes has
never received a complaint about an unintentional escape.  In addition, this
method was copied by almost everyone in the industry making it one of the most
widely adopted defacto standards.

With the Improved Escape Sequence With Guard Time you have reliable modem
control.  With reliable modem control, you donUt have to know how to:

o     Understand and know how to modify file and document data formats.
o     Understand the details of how your communications software controls
          your modem.
o     Learn AT commands, syntax and escape sequence parameters.
o     Bypass your communications software and directly communicate with
          your modem.
o     Deal with serial cable pin outs and control signals.


The New/Old Problem

In September of 1991, another modem manufacturer requested that Hayes examine
and test a particular modem to determine if the escape mechanism incorporated
by the modem was covered by the Hayes '302 Patent.  During the testing process,
Hayes discovered that the modem incorporates a new escape sequence that is
based purely upon a series of characters which appear in the data stream.

This new escape mechanism is called Time Independent Escape Sequence or
TIES.  The name appears to derive from the way in which the escape sequence
works because it does not make use of time as the Hayes '302 does.  TIES
depends entirely upon the appearance of the escape sequence in the stream of
data being received by the modem.  The TIES escape mechanism is very similar to
the escape mechanism in use at the time of the invention of the Hayes '302
because an escape can be triggered by the data being sent as part of a file
transfer.  Historically, that old escape sequence was unreliable, and it is
very likely that TIES will fall to a similar fate.  TIES cannot be considered
by anyone who understands it to be progress in the ongoing development of modem
design.


TIES - What Is It?

The simplest escape sequence for TIES is "+++AT<CR>" where "+++" stands for
any escape character and "<CR>" represents carriage return or the character set
in the modem registers by the AT command set which designates the end of the
command. Whenever that series of characters appears in the data stream, the
modem "escapes" or changes from Receive/Transmit Mode to Command Mode of
operation.  In effect, what happens at that point in the transmission is that
the flow of data stops!  The flow of data has halted simply because the
characters which make up the escape sequence have appeared in the data being
transmitted.

The appearance of these characters does not necessarily mean that you can pick
up a file and read until you find the escape characters.  The escape characters
may appear as a mixture of file data and file transfer protocols which would
not appear in printed text.  In either event, the receipt of those characters
by the modem will cause it to escape.

Since software compatibility was an important criteria, the TIES escape
sequence was selected to use the same sequence of characters that a computer
sends to a Hayes modem to cause it to first escape and then execute a command.
However, the TIES escape mechanism does not use guard times.  The TIES modem
appears to work with most existing software, but by disregarding the guard
times the data transparency of the TIES modem is fatally flawed.  In essence,
the TIES escape sequence was designed to fool computers into thinking that they
were communicating with a Hayes modem or a modem incorporating the Hayes '302
Patent, but TIES modems cannot fool the data being sent to prevent the TIES
block from stopping the data transmission.


TIES - What does this mean to you?

Depending on where the escape characters appear in the data, the receipt of
those characters by the modem could create unintentional escapes, prematurely
halting the flow of data.  For instance, if the file being transferred has been
completely transferred and the TIES escape characters appear at the end of the
data, there is no problem.  The receipt by the modem of the escape sequence at
that time would not interrupt the flow of data as all the data in that file has
already been transmitted.  However, if the TIES escape characters appear at any
place during the transfer of a file other than at the end of the data, when
those characters are received by the modem, the modem will escape, blocking the
transmission of the file Q the TIES block.  At that time, the modem may need to
be reset, and the transmission of that file must begin again.

Every time the file reaches the point where the TIES escape characters occur,
the modem will experience a TIES block and escape.  As long as those characters
appear in the data, at any point other than at the end of the data, the file
can never be successfully transmitted through a TIES modem.  No matter how many
times you try to transmit the file containing those escape characters in the
data stream, there is a 100% probability of a false escape.

When the TIES block occurs, the typical modem user operating that modem will
not be able to determine why the transmission of the data has stopped.  Each
and every time that the user attempts to send the file containing the escape
sequence characters, the transmission of data will halt, and the user will be
unable to determine the problem.  In some instances, depending on the data
following the inadvertent escape, the modem may actually have to be turned off
and back on again before the modem will be able to operate.

Further, a modem incorporating TIES cannot be repaired because it is not
broken.  There is an inherent limitation engineered into the product, i.e. the
absolute inability to transmit the sequence of data that TIES defines as its
escape sequence.  The only practical solution to this problem of unreliability
is to replace the TIES modem with one that does not incorporate the TIES escape
sequence, unless of course you are willing to stand by the modem and reprogram
the escape character and retransmit the data every time the TIES block occurs.

Since many modems are used in unattended operation, this is completely
unacceptable.  And why would anyone who knows about this want to take the risk?


TIES - Why You Haven't Heard About it?

In December of 1991, Hayes Customer Service received a telephone call from a
user of a modem manufactured by another modem company.  This individual was
using Hayes Smartcom software and was having a transmission-related problem.
While attempting to assist this individual in solving this problem, Hayes
learned that the modem was using the TIES escape sequence.

This was the first time that Hayes had actual knowledge that the TIES escape
sequence was being introduced into the marketplace.  Manufacturers who are
incorporating this technology have not been publicizing their use of TIES.
There have been few articles written about this new escape sequence, and the
majority of those articles are the result of Hayes expressions of concern over
the reliability of TIES.  Hayes was surprised to learn of the commercial
implementation of this escape sequence because of those reliability questions.
In fact, TIES appears to be the reincarnation of the problem that was solved by
the invention of the Hayes '302 patented technology in 1981.

Why are modem manufacturers not publicizing their use of this new technology?
If there are no inherent reliability problems, why does it seem that modem
manufacturers using this technology are hiding it and not touting its benefits
from the rooftops?  Why is it that modem manufacturers who use TIES want to
keep consumers in the dark about their adoption of this new escape sequence?
The answers to all these questions are rooted in financial concerns.  Hayes, on
the other hand, knows that consumers want to be completely and accurately
informed about the reliability of the communications products they purchase.
Modem manufacturers who use TIES should know that informed consumers will not
knowingly want to buy products with TIES.


The Philosophy of Reliable Modem Design

Data alone should never cause a modem to escape.  Users rely on the
manufacturers of computer equipment and modems to build the equipment in such a
way that users can count on its operation to conduct business and increase
personal productivity.  When the data being sent by a computer user can cause
an interruption of use, the user does not get the performance expected.  For a
manufacturer to intentionally include a mechanism in a modem where data
transmission alone can cause the escape is a great disservice to the computer
industry, the public using the products, and eventually the manufacturer.  The
manufacturer should be concerned that dissatisfied customers who learn about
this defect will at least have some serious questions about buying such a
product.

Once customers understand the issue they will no more want a modem that
escapes when data is being sent than they would want a computer which
miscalculates when processing a particular program sequence.  The defect is
simply unacceptable if there is a readily available, well-understood way to
avoid the problem.  Manufacturers who care about their responsibility to their
customers will avoid products which contain deliberate defects since their
products may create legal liability for that manufacturer.


Development of the TIES Test

Anticipating that there would be other telephone calls to Hayes Technical
Support about the first known TIES modem and realizing that there may be other
modems which incorporate this new escape sequence, Dr. John Copeland of Hayes
was asked to formulate a test for use by Hayes Technical Support to determine
if a modem being used by a caller was a modem incorporating the TIES escape
mechanism.

The test was easy to develop because TIES will cause a file to abort whenever
a certain sequence of characters is present, and this sequence of characters is
contained in the data stream.  In fact, the characters themselves are nothing
more than data.  The test data file developed by Dr. Copeland includes all
possible sequences of the form "xxxATy" (the TIES escape sequence).  The
character "x" can be any one of 128 different data characters.  The character
"y" similarly can be any one of 128 different data characters.  This results in
16,384 possible sequences which are each repeated twice in the test file.

The TIES test data file was developed only to determine whether a modem would
escape on a TIES data sequence.  Contrary to the statements of some members of
the press, the file is not a software package.  The test file is simply an
ASCII data file to assist users in determining if their modem implements TIES.

Some people have charged that the test data file is rigged.  This is not a
rigged test.  It is necessary that the test contain all possible combinations
of characters in order to be effective because some software packages you use
may set the escape code to be of different values.


Not "Hayes Compatible"

Since the introduction and overwhelming success of the first Hayes Smartmodem,
other modem manufacturers have touted their products as "Hayes compatible" and
have assured purchasers that the modems they buy are "Hayes compatible."
Consumers have come to rely on "Hayes compatibility" as a standard
consideration in their purchases of communications equipment.  Consumers who
buy a TIES modem might assume that the modem is "Hayes compatible" because it
uses AT commands, only to learn later that the modem might have been designed
with a serious defect.  TIES modems are not "Hayes compatible."


Why Is This Issue So Important?

Hayes believes that the introduction of TIES into the installed base of modems
will seriously compromise the integrity of the industry.  The degree of damage
to the industry and to the confidence in the installed base of modems depends
on the frequency that this problem will occur if the use of modems with TIES
becomes widespread.

There is no way of accurately predicting the probability of inadvertent
escapes occurring in a TIES modem.  One manufacturer of a chip set
incorporating the TIES technology has admitted that when using a 9600 bps modem
in continuous operation, there is a probability of an inadvertent escape every
seven years while transmitting random data.  The problem with these
calculations is that data in the real world is never random.  We know from
research in data compression that data is not random.  The fact the data is
compressible demonstrates that it is not random.  As a result the statistical
probability is even greater that the ASCII characters used in the TIES escape
sequence would occur in data.  Further, once a byte sequence has special
meaning, it appears much more frequently than would a truly random sequence of
the same length.  For example, Dr. Copeland scanned the hard disk in his office
and found three files that would escape a TIES modem: a communications program
EXE file, an Internet message discussing modems, and a log file from a
telephone-line security program.  Based on these results, the mean time to
blockage at 9600 bps for a TIES modem would only be 14 hours for Dr. Copeland's
data.

The probability of an inadvertent escape may either increase or decrease
depending on the data being transmitted.  Probability figures are easily
manipulated to reflect the outcome desired by the entity doing the
calculations.  For this reason, Hayes has not calculated the probability of
inadvertent escape by a TIES modem, and we believe the calculations made by the
modem chip manufacturers selling TIES in their chip sets represent the absolute
minimum under the best conditions.

For those who require an answer to the question of "how often," they can turn
to the figures provided by the chip manufacturer itself, in the attempt to
minimize the importance of this problem.  Placing that calculation in terms of
the "real world," in a company using 700 TIES modems communicating at 9600 bps,
there would be an inadvertent escape twice a week.  Twice a week a data file
important to the business would not be sent because it could not get through
the TIES modem.  Hayes maintains that any inadvertent escape is unacceptable.
Two inadvertent escapes each week is outrageous!  And if the data is being sent
at 38.4 kbps, as many new modems are now capable of doing, this TIES block will
occur more than once per day, only you cannot predict which of the systems will
be hit.

The following chart represents the minimum frequency of TIES blocks that you
should expect with a TIES modem depending on the number of modems and the speed
at which they are operating:


If Data Was Random

Type of Use     Hours/Day   Modem Speed     Number of Modems   TIES Aborts/Year

Single User         1          2400                 1                    0.001
Small BBS          12          9600                 2                    0.12
Large BBS          12         14400                12                    1.1
Small Company       2         14400                50                    0.8
Medium Company      2         14400               500                    7.6
Large Company       2         14400              5000                   76
Dialup WAN         24         14400               200                   36
Dialup WAN         24         38400               200                   97

Data with 1 TIES per 50 Mbytes

Type of Use     Hours/Day   Modem Speed     Number of Modems   TIES Aborts/Year

Single User         1          2400                 1                    6
Small BBS          12          9600                 2                  605
Large BBS          12         14400                12                5,449
Small Company       2         14400                50                3,784
Medium Company      2         14400               500               37,843
Large Company       2         14400              5000              378,432
Dialup WAN         24         14400               200              181,647
Dialup WAN         24         38400               200              484,393


TIES Impact on Business - From Personal Frustration to Corporate Chaos

Fortunately, TIES blocks have yet to show up in great volume.  However, as
more and more TIES modems are sold, it is predictable that TIES blocks will
start to occurr with ever-increasing frequency in businesses where numbers of
modems exist.

Personal Frustration.  Take for example, your remote sales staff.  What is the
traveling salesperson going to do when he or she tries to log an order into the
system at 9:00 at night and suffers a TIES block?  Is that person going to
attempt to adjust the RS232 serial cable pin outs or reconfigure the modem
control from a hotel room?  No, that is unreasonable to expect.  Or say, for
instance, sales support is attempting to send the monthly sales forecast
spreadsheet to the regional offices.  Only this time the transmission keeps
crashing because one of the fields happens to include a number that when
translated for transmission happens to be the TIES escape sequence.   No matter
how many times the transmission is attempted, it will never go through.

Paralysis.  TIES could also have a dramatic impact on entire computer systems
that had incorporated software which is incompatible with the TIES escape
sequence.  Imagine the scenario where over the weekend your company upgrades
its software using a program that includes some data that is incompatible with
TIES.  On Monday morning, when employees try to send material to remote offices
of the corporation, the transmissions all fail. Your entire company will suffer
a TIES block.  Think of the time and money required to correct such a problem,
not to mention the lost productivity while the problem is being found and fixed.

Sabotage.  The introduction of TIES into the installed base of modems has
far-reaching consequences beyond TIES blocks.  TIES is a threat to the security
of your business.  Its implementation  makes it easier for an individual to
sabotage the work of a company.  If a company is using TIES modems to transmit
data in its day-to-day operations, it would be very easy for a disgruntled
employee to insert the TIES escape mechanism into the companyUs data.  The
employee could even change the software used by the company to insert this
sequence of characters in every file being created at the company.  Even more
problematic is the ability of an employee to sabotage the employerUs business
by the insertion in spreadsheets of a few numbers which will cause the TIES
modem to fail.  The TIES escape sequence can appear in a transmission of either
binary numbers or ASCII characters.  There are many number combinations in data
that will cause an inadvertent escape in a TIES modem.

Chaos.  TIES creates a window of opportunity for a new type of computer virus.
In the last year, we have seen the business world shaken by the existence of a
number of computer viruses that attack data directly in the computer.  TIES
offers illegal hackers the ability to attack the integrity of data by inserting
a character string in unused parts of the data file to prevent its future
transmission.  Because TIES relies entirely on the existence of its escape
characters in the data stream, it would be possible for a hacker to introduce a
virus that would carry those escape characters into files without changing the
program data itself.  The file would be incapable of transmission because the
TIES modem would inadvertently escape when it reached those characters every
time it was transmitted.

With this perspective in mind it is clear that TIES is a step backward that
also could potentially create chaos in computer communications if it is
deployed to any great extent.  On many levels, TIES could actually affect
productivity throughout the business community.  There is an installed base of
modems estimated at over 25 million.  Assuming that these modems were TIES
modems and using the probability calculations provided by the TIES chip
manufacturer, there would be almost 3 million inadvertent escapes a year.  Even
assuming typical modem usage and a mixture of modem speeds,  there would be
more TIES blocks in one year than the number of people killed on U.S. highways.
As modems with this inferior technology are introduced into the marketplace,
their effect on productivity has the potential to be far-reaching.  Keep in
mind these calculations of probability are based upon what we believe to be the
theoretical absolute minimum. Hayes fully believes that calculations using
random data grossly underestimate the problem presented by the introduction of
TIES modems into the installed base of computing equipment used around the
world.

Arguments for Acceptable Levels of Failure
Because manufacturers have thus far refused to tell the public that they are
using this new technology, it is very difficult to identify who is using TIES.
However, the manufacturers of the chip sets necessary to implement TIES and the
modem manufacturers who admit that they use this new escape sequence concede
that "it is not as reliable as the technology offered by the Hayes '302."

There are repeated instances of these companies making statements to this
effect.  The question remains; what level of reliability is acceptable?  Hayes
believes that the industry should not accept any reduced level of performance
if there is readily-available technology that offers proven reliability.


Why is Hayes Interested In Educating the Public About Escape Sequence
Reliabity?

Hayes developed an education campaign targeted to corporate and business
America because we know this is where the greatest damage is likely to occur if
TIES control modems become widely installed.  Without action the problem
eliminated by Dale Heatherington in 1981 will be in the future of the modem
industry.  Data alone should not cause a transmission to abort!  Modem
manufacturers who have made the decision to begin implementing TIES have not
informed the consuming public about TIES, their decision to use this escape
mechanism, nor the inherent problems associated with the use of data alone as
an escape signal.  We believe that many of the manufacturers who are using TIES
do not even understand the true nature of the problem and some just donUt care
and want the public to believe it doesnUt matter.

Since business, government, and industries increasingly rely on information
technology infrastructure to conduct their business with applications like
electronic mail, electronic data interchange for orders and invoicing,
telecommuting to reduce traffic and fossil fuel consumption, and the rapid
growth of information services for education and entertainment, it is becoming
a regular part of our everyday lives.  When major events occur that affect the
infrastructure like the telephone system crashing in the Northeast U.S., the
major scare over computer viruses, or similar threats to the use, availability
or reliability of this strategic and tactically critical resource, the public
confidence severely diminishes.

Since its founding in 1978, Hayes has sought to educate consumers about many
topics related to modems and their usage.  TIES is just one more of these
issues.  Hayes has learned through the years that consumers want to know about
these issues and appreciate any information provided that will enable them to
make informed choices in their purchase of communications products.  If someone
wants to buy a modem with TIES, they should be in a position to be an informed
consumer and should be prepared to deal with the consequences when the TIES
block happens to them.