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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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