Simtel MSDOS 1992 September/Simtel20_Sept92.cdr/msdos/telix/tlx_hst.arc/TLX_HSTI.DOC

Text File | 1988-12-15 | 11KB | 239 lines

The USRobotics Courier HST Modem

(c) Andrzej Taramina, 1988,1989

USR, Courier, HST are trademarks of USRobotics, Inc.
Telix (c) Colin Sampaleanu, Exis Inc.


INTRODUCTION

This document discusses some of the unique features of the HST
modem and gives more specific information on using the HST with
Telix 3.11 for Bulletin Board operations.  It is not deeply
technical, but major differences between the HST and other, more
conventional modems are covered.


HIGH SPEED PROTOCOLS AND MODEM COMPATIBILITY

The HST is fast, which is probably why you bought it in the
first place.  Without going into a philosophical discussion,
suffice it to say that there are many 9600bps modems available
commercially, and most of these modems use totally incompatible
high-speed communications protocols.  Your HST will NOT
communicate at 9600bps with 9600 modems made by Hayes, Microcom,
Fastcomm, etc... Although a 9600bps standard exists, modems
adhering to it are few, far between, and still quite expensive.
You may not like this, but that's the way it is.  The HST will,
however, communicate at lower speed with all modems supporting
the 1200/2400 standards, including other modems incompatible at
the higher speeds.  Don't waste the Sysop's time by asking him
why you can't download at 9600bps if his board is equipped with
a Hayes modem.  Chances are you'll end up in a heated debate as
to which modem is best. Progress is being made, however, on the
creation of an industry-wide 9600bps standard.  USR expect to
deliver a new modem early next year (1989) that will talk to all
common 9600 baud modems. Upgrades will be available to HST owners.

Also, when connected to another HST, the effective
modem-to-modem speed ("link rate") may exceed the nominal
9600bps rate by 10 to 30%, depending on line conditions and the
ability of your computer to send/receive data at the higher
speed.  File transfer rates of 1100-1200 cps between two HST
modems (characters per second) are not uncommon.


ERROR CORRECTION

At 9600bps, when communicating with another HST,  both modems
use sophisticated (and proprietary) error correction techniques
which insure that the data which was received is identical to
the data which was sent.  At lower speeds (1200/2400), the HST
relies on its built-in MNP firmware (Microcom Networking
Protocol) for error-free communications with other MNP-equipped
modems.  This effectively eliminates the need for conventional
"Software" error-correction implemented in software-only file
transfer protocols such as Xmodem, Xmodem/CRC, 1k-Xmodem (a.k.a.
Ymodem), Zmodem and many others.

To take advantage of the much faster and more efficient built-in
error correction, special file transfer protocols must be used.
These protocols do not perform any error-checking and
correction; they rely entirely on the modem to do so.  Such
protocols include Ymodem-G, Imodem, and the registered version
of Zmodem (DSZ, authored and copyrighted by Chuck Forsberg).

Remember that error-free transmissions take place ONLY if you
are connected to an HST (at 9600bps) or to an MNP-compatible
modem of any brand at 1200 or 2400bps.  If setup properly, the
HST will return a special "CONNECT xxxxx/ARQ" message upon
connect (where xxxxx is the connect speed)  which reflects the
fact that an error-free connection has been established with the
other modem.


FACTORS AFFECTING TRANSFER SPEED

There seems to be some widespread misunderstanding of the term
"Speed" as it applies to the HST and other "high-speed" modems.
How fast does it really go?  What is its limit?  More important,
what are the factors that limit the modem's speed?

It is all very simple, really.  One end of the modem talks to
the computer, while the other end talks to another modem.  The
speed you are really interested in is the LINK RATE, which is
the speed at which the two modems transmit data.

If the HST is setup to transmit/receive at the highest possible
link rate, the effective speed will depend on the following
physical factors:

1. The maximum speed that can be achieved by the other modem,
2. The maximum speed at which your computer can communicate with
your HST modem and
3. The maximum speed at which the OTHER computer can communicate
with ITS modem.

It is intuitively obvious that the effective modem-to-modem link
rate cannot possibly exceed the slowest of 1,2, or 3 above.
Unfortunately, you can only control item #2, the speed at which
your computer communicates with your HST modem.  This speed is
NOT determined by the modem, but by the following:

1. The type of computer and communications interface you are
using and
2. The communications software you are using.


PHONE LINE NOISE

In an HST-HST connection, extraneous noise characters will not
appear on the computer's screen.  This led to the false belief
that error-correcting modems eliminate noise.  They do not.
The modem's built-in error-correction firmware (ie, MNP) insures
that the data which is received is the same as the data that was
sent in a manner similar to software-only error-corrction
protocols such as CRC (Cyclical Redundancy Check).  Thus, the
modem themselves initiate the retransmission of blocks of data
which were affected by line noise.  Although the computer does
not see it, noise affects transmissions just as negatively and
results in a slowing down of the data transfer.  On the HST
modem, erratic blinking of the RD/SD lights may be an indication
of poor line quality.


DATA FRAME OPTIMIZATION (MNP4) AND DATA COMPRESSION (MNP5)

With release level 964 of the HST's ROM code, USR implemented
MNP level 4 (Data Frame Optimization) and MNP Level 5 (Data
Compression).  Although 964-level HST modems are fully
downward-compatible with earlier releases of the ROM (ie, 961,
963), these features can only be used with other 964-level (or
more recent) HST modems.

Control of MNP level 4 operation is available via S-register 15,
a new register implemented in release 964 of the HST ROM.  If
bit 4 (decimal value 16) is ON in this register, MNP level 4 is
DISABLED.   MNP level 4 operation is only effective at speeds
of 1200bps and above.

Data Compression (MNP level 5) may be a mixed blessing,
depending mostly on the type of data being transferred by the
HST modem.  If your HST is used primarily for BBS communications
and your file transfers are mostly ARChived files (processed by
SEA's ARC, Vern Buerg's ARCA, or Phil Katz's PKARC), data
compression by the modem is not only redundant, but it may
actually SLOW DOWN your file transfers.  Effectively, the MNP 5
data compression algorithms will attempt to replicate the work
already done by the ARC utility and will fail to reduce the size
of the data any further (it will likely INCREASE the size of
your ARC'ed files), resulting in increased transfer times.
Thus, the most efficient transfer or compressed files on
964-level HST modems occurs when data compression is turned off
(to disdable MNP5, use the AT&K0 command).

ASCII, Text, .EXE and .COM files may benefit greatly from data
compression, yielding transfer rates in the 1400-1900cps range.
To activate automatic data compression, use the AT&K1 command.


HARDWARE LIMITATIONS

How fast your computer can communicate with the modem is
influenced by many factors.  The HST modem itself can
communicate with the computer at a maximum rate of 19,200bps.
Earlier models of the IBM PC and compatibles utilized an
8088 processor running at 4.77Mhz, transmitted data internally
over an 8-bit wide data path, and their serial interfaces (to
which the modem is attached) also used an 8-bit processor chip.
These factors effectively limit the serial rate to
9600-10000bps.  So-called "Turbo" machines which use a
dual-speed 8088 or NEC V20 microprocessor, or some of the 8086
or V30 machines can achieve significantly faster serial speeds.
The new series of AT-class 80286 and 80386 based machines can
provide much faster serial speeds.

The original serial processor chip is usually an 8250, and is
rated at 9600 (some 8250s can run satisfactorily at 19200,
however).  The newer 16450, a plug replacement for the 8250, is
nominally rated at 19200.

Regardless of the speed of your computer and serial interface,
what's at the other end is just as important.  Running a 20Mhz
386 system and with serial interface set at 19200bps is of no
consequence if the system you are communicating with is limited
to a 9600bps serial speed.  The SLOWEST link in the entire
communication chain sets the pace for all the rest, folks.
Also, be aware that computer networks running multi-node
bulletin board systems have many other tasks to perform besides
communicating over telephone lines.  These other processes may
interfere with reliable communications at high speeds, and the
sysops may purposely slow down the serial interface speed to
prevent possible conflicts arising from this situation.  One of
the most common problems in this environment is an aborted upload
when concurrent disk updates on the network's file server take
precedence over communications interupts and cause loss of
incoming data.

Other factors which can negatively affect high speed
transmissions are not readily apparent at lower speeds.  A slow
hard disk, for example, can drop the effective data transfer
rate by a few percentage points.  You may want to try a RAM
disk, for example, which eliminates disk access as a limiting
factor to data transfer performance.


COMMUNICATIONS SOFTWARE

How fast your serial interface operates is a function of the
serial interface itself, since it has an upper speed limit, as
well as the communication software you use, since IT physically
sets the speed of your communications port.  In fact, the
communications software you use may be your worst enemy.

While many commercial communications packages are good and
offer an outstanding array of features, few of them measure up
to Telix in terms of raw data transfer performance, especially
with an HST-class modem.  A number of communications packages
limit the maximum serial rate to 9600bps, regardless of your
computer and serial interface's inherent abilities.  Telix
release 3.11 can set you serial speed all the way up to
19,200bps.

Furthermore, some otherwise good communications packages offer a
very limited choice of transmission protocols which cannot take
advantage of your computer and communications hardware.  Not
providing protocols such as Ymodem-G and Imodem, or an easy
interface to external protocols such as DSZ, severely limits the
effectiveness of your communications setup.  Your choice of
Telix is a good one, offering internal ZModem and many others along
with the most powerful script language around...stick with it!

Andrzej Taramina
58 Aquila Court
Rexdale, Ontario
Canada M9W 5J2

12/13/1988