Almathera Ten Pack 3: CDPD 3/Almathera Ten on Ten - Disc 3: CDPD3.iso/scope/001-025/scopedisk1/commtut/gttutor3.txt

Text File | 1995-03-18 | 11KB | 159 lines

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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  resultof 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 calla  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.