

                High Speed Modem Info for PC users


Written by Mike Benna, January 1991.

Because of the confusion people seem to have related to all the new
advances in modem technology I thought it was about time that somebody
compiled the relevant information into a single document.  I have now
done so with this article in the hopes of clearing up many of the
mysteries and misconceptions about modems which operate using MNP and
V.42 style protocols.

Within this article, please note that some of the table values have been
derived from actual testing whereas others have been calculated based on
the theory behind the operation of all the protocols involved.  All
table values assume clean (i.e. noise free) telephone connections and a
computer which is fast enough to feed the modem all the data it wants
and can accept all the data the modem gives to it.

First of all, a short glossary might be in order to clear up the most
common terms:

  cps : Characters per second (usually used to measure the effective
        throughput from the source computer, through the modems, and
        into the receiving computer).  In the end, this is the only
        number which you should really care about.

  bps : Bits per second (usually used to describe the raw data link
        between two modems or between the modem and the computer). Note
        that this is _not_ the baud rate.

 baud : The single most often misused word in telecommunications.  Most
        people really mean "bits per second" when they use this term.
        For example 2400bps modems only run at 600 baud and 9600bps V.32
        modems only run at 2400 baud.  If you don't know what a baud is
        then you should probably be using 'bits per second' instead.

Next, you can probably forget most of what any salesman has told you;
most PC salesmen do not fully understand this stuff either.  Don't blame
them however, they usually have the disadvantage of having read sales
literature from the manufacturers which was designed to be misleading in
the first place.  Your best bet is to read this article and then go to
the store and test the hardware you think you want to purchase.


Link Protocols
--------------

In trying to explain how the various modems communicate I will start at
the lowest layer you care about: the link protocol.  These protocols
describe the standard methods which the modems use to talk to each
other; they have nothing to do with error correction or data
compression.  Note that there are many other protocols in use but it is
unlikely that you will encounter them in day-to-day PC use.

The rates described in the link protocol descriptions are often called
the 'link rates'; they are not necessarily the same as the rate between
the modem and the computer.

  V.22bis: 2400bps (this applies to _most_ 2400bps modems).  Normal
           V.22bis (i.e. without MNP or V.42) is capable of a maximum
           sending rate of approx. 240cps in both directions at the same
           time.

  V.32:    One of the 9600bps standards.  It is not compatible with the
           HST protocol.  V.32 modems can send at full speed in both
           directions at the same time.  Most V.32 modems come with MNP
           and/or V.42.

  V.32bis: The upgraded V.32 standard which runs at 14400bps in both
           directions at the same time.  In all other ways it is similar
           to V.32.  V.32bis is newly emerging (I don't think the
           standard has even been totally finalized yet).  When V.32bis
           modems become available you can expect all of them to offer
           V.32 compatibility.

  HST9600: The U.S. Robotics 'High Speed Transfer' protocol.  It offers
           9600bps in one direction and 300bps in the opposite
           direction.  It is only available from USR in their Courier
           HST and Courier Dual Standard modems.  The USR Courier
           HST9600 modems are no longer for sale from USR (they only
           sell the 14400 models).

  HST14400: The upgraded HST9600 standard which runs at 14400bps in one
           direction and 450bps in the other.  Again, it is only
           available from USR in their Courier HST and Courier Dual
           Standard modems.  These modems are still compatible with
           HST9600 modems but not with any others in HST mode.

note: None of the above standards are compatible with FAX machines.


Error Correction
----------------

Once two of these modems get talking to one another they may try to
establish an error free connection using either MNP4 or V.42 (V.42 is
also known as LAP-M for Link Access Protocol for Modems).  There are
other protocols as well but these two are by far the most common.  Both
of these protocols perform an asynchronous to synchronous conversion
which allows them to avoid sending start and stop bits.  In general this
increases throughput for all data (even compressed files) by about 20%
(25% increase due to not sending the start/stop bits and approximately
5% decrease due to error correction and sync data).  There is only a
slight difference in throughput between these two protocols with MNP4
coming out just barely ahead of V.42.  It should also be noted that
these two protocols are not compatible with each other and therefore
many of the newer modems on the market support both standards.

By getting two MNP4 or V.42 modems talking together you can expect to
get throughputs such as these:

        Link Rate   Without MNP4/V.42    MNP4     V.42
        ---------   -----------------   -------   -------
         2400bps          240cps         287cps    285cps
         9600bps          960cps        1138cps   1124cps
        14400bps         1440cps        1707cps   1686cps



Data Compression
----------------

The next layer of standards which can be added is data compression.
The two common data compression protocols are MNP5 and V.42bis (not to
be confused with V.42 which is an error correction protocol).

Data compression works similarly to programs like PKZIP except that they
compress 'on-the-fly' as you send the data to the modem.  In general, if
you are sending files which are already compressed with a program such
as PKZIP then there is no advantage to turning on data compression in
your modems.  In fact, if you are using MNP5 then you should disable
data compression (go back to MNP4) before sending compressed files
because it will actually take longer to send with MNP5 than it will with
MNP4. V.42bis on the other hand is smart enough to realize that it can't
compress the data any further and it turns itself off until it decides
that it will be useful again.

Data compression has its biggest advantage when you are reading text
which may repeat itself frequently (e.g. ANSI codes and menu boxes have
a lot of redundancy and consequently they compress very well).  Because
different types of data have different amount of redundancy, I've broken
the throughput table into several types of data:

A) Compressed data (e.g. .ZIP, .ARC, .SIT, etc. files).
B) Regular text (e.g. this article).
C) Typical BBS Menus.

Typical throughput table for MNP5 and V.42bis (in cps):

               Protocol:     MNP5      |     V.42bis
   Link Rate  Data Type:  A    B    C  |   A    B    C
   ---------            ---- ---- ---- | ---- ---- ----
     2400bps             254  489  609 |  285  768  928
     9600bps            1013 1956 2440 | 1124 3072 3718
    14400bps            1520 2934 3658 | 1686 4608 5574


As you can see V.42bis does a better job than MNP5 for all types of
data and has the advantage that you can always leave it on (even if you
are going to be doing file transfers of compressed data).


Software MNP5
-------------

Some 2400bps modems for sale today offer MNP5 compatibility in software,
not in hardware (read the box carefully).  If the modem manufacturer is
offering software MNP5, he is really selling you a regular 2400bps modem
(without any MNP capability) and including a terminal program for your
PC which allows any modem to perform some of the functions of the MNP
protocols.  In fact, if you were to buy any old 2400bps modem you could
then go out and purchase a terminal program which had software MNP
support.

Confused?  I'll try to clear this up further...

The MNP protocol cannot be implemented fully from the computer side of
things.  In order to run at full speed it must be able to do the
asynchronous to synchronous conversion and this cannot be done from the
computer, it must be done inside the modem.  At 2400bps these are some
of the typical throughput speeds you might expect to encounter for
software MNP5:

                    No MNP    Hardware MNP5    Software MNP5
                    -------   -------------    -------------
Compressed data     240 cps      254 cps          193 cps
Regular text        240 cps      489 cps          371 cps
Typical BBS Menus   240 cps      609 cps          487 cps

As you can see from the table, software MNP5 is not nearly as efficient
as hardware MNP5 and it also means that you cannot choose your terminal
program - you must use the one which supplies the software MNP5 support.

Since software MNP4 does not benefit from the async to sync conversion
it will offer you an error free line but it will only run at about 228
cps (instead of the regular 240 cps you will get with no MNP support).


Not Getting the Throughputs I claim?
------------------------------------

The throughput numbers I've provided in this document are the raw total
throughput numbers.  Please note that this is not the same as what you
would measure using a typical file transfer protocol.  For example,
Zmodem normally gets about 234cps on a 240cps link, or to put it another
way, Zmodem runs at 97.5% efficiency.  Therefore to calculate your
expected throughput using Zmodem you simply need to multiple the numbers
I've provided by 0.975.  Of course in this complicated world of
communications no single number is enough:  Zmodem with the Moby-Turbo
option usually offers about 99.2% efficiency (238cps on a 240cps link).
Also note that non-streaming protocols (such as Xmodem and Ymodem) do
not fair as well as streaming protocols (such as SEAlink, Ymodem-G, and
Zmodem) on higher speed links because propogation delays and response
times do not necessarily decrease when the link rate increases.


Which to buy: an HST or V.32?
-----------------------------

Many people in the PC world who want to upgrade to a modem which goes
faster than 2400bps are faced with the question of which standard to go
with.  As of this writing (Dec '90) the only two standards which are
popular in the PC world are the USR HST standard and the V.32 (and soon
to come V.32bis) standard.

In this section I won't tell you which to buy but I will give you some
information which may help you to make your choice:

- Only USR is currently building HST modems.  If you wish to get an HST
  modem it must come from USR.  The advantage of this is that you aren't
  likely to run into compatibility problems when using the HST standard.
  The disadvantage is the price: USR modems aren't cheap (but they are
  of good quality).

- Many other manufacturers are supporting the V.32 standard. Competition
  seems to be driving the price of V.32 modems below that of the HST
  modems (this was not always the case).  In the future we can expect a
  big difference between the two standards; V.32 modems will likely be
  much cheaper in the long run than HST modems (even though V.32 modems
  are more complicated to build).

- USR makes a modem called the 'Courier Dual Standard' which supports
  both HST and V.32 protocols in the same modem.  It's disadvantage is
  cost.

- Most public BBS's which support speeds higher than 2400bps only
  support the HST standard.  This is because USR used to offer Courier
  HST modems to BBS operators at a reduced cost.

- HST modems are only high speed in one direction at a time.  This
  causes severe speed degradation problems during some kinds of file
  transfers.  In all cases it is best to try before you buy.

- One very popular type of modem in the Unix world is the Telebit.  The
  older Telebit modems are not compatible with either HST or V.32 modems
  but many of the newer ones (if not all) have V.32 support added to
  them (in addition to their native modes).  This is further evidence
  that V.32 has more long term potential than HST.

- Compuserve recently purchased a bunch of rack-mount USR Dual Standard
  modems but is unwilling to enable the HST mode on them because once
  they start supporting HST mode they feel they must continue to do so
  for years into the future and they do not want to be locked into
  purchasing more modems from only one supplier.  It seems they also
  feel V.32 is going to be the high speed modem choice of the future.
  (They apparently purchased Dual Standard modems because they are the
  only rack mount V.32 modems currently available.)

My feelings are that V.32 modems are technically superior to HST modems
in many ways and are likely to become common in the next few years. The
problem with this is that very few public BBS's support V.32 making a
V.32 modem almost useless at anything over 2400bps if the only places
you call are BBS's (you should check your favorite BBS yourself).


Disclaimer
----------

The information provided in this document is for the convenience of the
BBS and uunet community.  It may be freely distributed but may not be
modified.  There are no warranties as to the accuracy of anything which
has been written here.  In all cases if you are buying any computer
equipment (including modems) it is best to test the setup you wish to
purchase before purchasing anything.  Remember: if it won't work in the
store then why would you expect it to work at home?

This document is Copyright (C) 1991 Mike Benna.
--
   ---> Mike Benna, Vancouver, B.C., Canada
        MindSpan Technologies Corp - Video Game Design and Development
  UUCP: Mike_Benna@mindlink.UUCP  or  uunet!van-bc!rsoft!mindlink!Mike_Benna


