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This is the third in a series of tutorials that I hope will be
found to be useful to both new and experienced users of
communications facilities.
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Q: Why is it that I experience so much more line noise than the
people I call? It seems that I see noise on my screen with some
frequency, but if I ask the party that I have called if he sees
it too, I'm usually told his screen is clean. Is there something
wrong with my system?
A: The odds are twice as great that you will have line noise if
you place a call to a computer than if a computer were to call
you. It is normal and easily explainable.
While it is true that the odds are twice as great that you will
experience or know about noise in the case where you have
initiated the call, the incidence of noise is the same regardless
of who places that call (assuming the same lines and circuits are
being used in both cases). The reason for this is that when you
are in Terminal mode (placing the call), your system is set to
full-duplex operation and when it is in Host mode (auto answer),
it is in half duplex.
Full duplex means that whatever you type on your keyboard does
not get sent to your screen. It is sent, instead, to the
communications port and from there it travels through your modem,
along the telephone lines to an answering modem, and then to a
host sytem. The host system then sends it back to you. In half
duplex, on the other hand, whatever you type is sent to both your
communications port and to your screen. From this it is obvious
that every character seen on your screen when you have placed a
call has gone through the telephone system while only half of
what is seen on the host system's screen has been on the
telephone circuit before it got there.
Further, line noise can be unidirectional. That is, it may
appear as data travels in only one direction or the other.
Regardless of that fact, it will be seen by the terminal mode
user (data must go both ways before it reaches the screen) and if
it appears only on the link from the host to the terminal user it
will never be seen by the host.
Q: The last tutorial you wrote told us about MNP and ARQ modems
being able to eliminate most line noise. How do they do that?
A: Part of that answer is still a mystery to me, but I know how
it does it in theory at least. I will tell you why part of the
answer remains a mystery in a moment. First, recall the
discussion we had about file transfer protocols. All of them
utilize some form of CRC mechanism to insure that the receiving
system had received all of the contents of a packet of
information without having dropped any bits or picked up any
extra bits. The CRC is a byte or a word of data that is the
result of an algoritm that 'folds' every byte in the data packet
onto itself in such a way as to result in a pattern of bits that
can be calculated by the receiving system as each byte of data is
received and then compared with the CRC that is subsequently
received. If there is a mismatch then the data (or CRC byte) did
not get received correctly. The MNP and ARQ modems implement
this strategy within themselves. All data that is transmitted
from one of these modems is re-packaged into what the modem
manufacturers call 'frames' (packets) before being transmitted.
Each frame is followed by a CRC byte or word that is stripped off
by the receiving modem and used to determine if the frame was
received correctly. Line noise simply makes that CRC check fail
and the result is an automatic retransmission of the frame.
As you can see from the above, the modem is now acting just like
your computer does during file transmissions using a protocol
transfer method. This is not done for 'free'. The overhead of
doing so results in less than rated speeds in every case. That
is, the theoretical maximum data rate of a 1200 baud modem is 120
characters per second, but MNP and ARQ modems are sending more
characters between themselves than the sending system itself. If
there are errors and, thus, an automatic retransmission of a
frame, the sending modem is very likely to have to ask the
sending computer to wait for it. It is estimated that this
overhead (even without errors) results in a degradation of about
12% in terms of the maximum possible performance of the modem
yielding about 106 characters per second possible throughput. To
counter that built in degradation, the modems strip the start and
stop bits from each byte and send only 8 bits rather than the 10
(or eleven) that are sent by non-error-correcting modems. This
increases the efficiency by about 20%. The net effect is that,
assuming no errors, the possibility of about 108% of rated
performance. (It is possible to get about 130 characters per
second rather than 120 if there are no errors - this also fails
to account for additional 'compression' methods built into some
of these modems).
So, where is the confusion? Well, the above assumes there is a
stream of data being sent that can be 'framed'. How the modems
function when a user is merely typing one or two characters or
words at a time before the other side responds is a mystery.
Indeed, as each character is typed it is sent down-line.
Presumably there is a timeout of some kind in the modem that says
that if another character is not entered within x milliseconds it
is presumed that the frame is complete and it is sent along with
its CRC. However it does it in practice, it does seem to be
effective at eliminating line noise.
Q: So MNP and ARQ modems are faster and eliminate line noise.
Sounds like the way to go. Are there any negatives to their
usage?
A: Interesting question. Assuming that you use protocol
transfer methods in addition to the error detection and
correction logic of the modems themselves, I can only think of a
couple of negatives at the moment. The first, of course, is the
lack of standards, particularly at the higher baud rates. Second
is the fact that every time you use one to call a system that
does not use MNP or ARQ (the vast majority of them do not) then
you automatically lose part of their opening screens.
Let me explain that. When an MNP or ARQ modem first connects
with another modem the calling modem issues a sequence of bytes
that is asking the answering modem if it is also MNP or ARQ.
These bytes include an id and an indication of the level of MNP,
for example, that the caller is using. The first set of
characters that come back from the called modem are then consumed
by the modem rather than passed through to the user's screen.
Thus, they are lost to your system. Very often it is necessary
for the calling system user to press his Enter key in order to
cause subsequent characters to be passed through the modem
(telling it in effect, to turn off MNP or ARQ). This is an
annoyance to the terminal mode user but it can be worse for the
host system.
With the introduction of release 12.20 of GT PowerComm there has
been some controversy as to the existence of the opening prompt
that it issues in which it asks if the caller wants to use ANSI
graphics or not. Many users seem mildly annoyed that their
selection is not recorded somewhere so they don't have to answer
that prompt more than once. What they fail to understand is that
the prompt is there for several reasons. MNP is a good example
of what I mean as is the possibility of noise on the line.
When an MNP call comes in, those initial characters I just
mentioned 'hit' the prompt and result in reissuance of it. We do
not permit a default to that prompt so that we do not go past it
with noise or MNP. By the time a Y or N is entered, the MNP
sequence of handshake signalling is done. If we did not have
that initial prompt then the first question the user would be
asked would be his first name. Ask any Sysop how many garbage
names he has in his user base. If there are any then I can
reasonably assure you that his system does not have a leading
prompt such as ours to protect him from noisy incoming calls (or
MNP).
Q: Is 9600 baud the theoretical limit to technology in terms of
modems?
A: Hardly. It appears that 9600 'baud' stretches the
reliability limits of today's unconditioned telephone system, but
modems exist that are much, much faster than that already.
19,200 bits per second modems are functional on conditioned lines
even now. As to limits, well, did you know that satellite
communications capabilities exist that already permit the
transfer of over a million bits per second?
Over the past 20 years there has been a rather constant rate of
improvemnt in all aspects of data processing technology. As a
rule of thumb that is pretty close consider this: Every four
years there has been a three fold improvement in
performance/capacity for only a two fold increase in price.
Sometimes we forget how long this trend has been in effect, but
an IBM advertisement a few years back made it pretty clear. At
that time the ad suggested that if the automobile industry had
enjoyed the same rate of improvements over the past 20 years that
the data processing industry has enjoyed, then every adult in
this country could afford to own a Rolls Royce, as it would cost
only about $20 and, incidently, it would get about 2,000,000
miles to the gallon of gasoline. For a more contemporary
example, we need only look back at the original IBM PC. That
machine had 320K disk drives and a clock speed of 4.77 micro
seconds. Today you can buy a Compaq 386 that is 17 times faster
than the original PC (throughput) and you can get it off the
shelf with 130 megabyte hard disk. The price of this newer
machine is less than three times the original PC, closer to twice
the price. No, we are not at the limit of technology, not by a
long shot.
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