Shareware Gold 1/The_Golden_ROM_Series_-_Shareware_Gold_Volume_1_Number_11_-_SK_89-301_-_1989.iso/LIB022/HSTINTRO.DOC

Text File | 1988-01-01 | 8KB | 177 lines

The USRobotics Courier HST Modem

(c) Dan Moore, 1988

USR, Courier, HST are trademarks of USRobotics, Inc.
Qmodem, SST (c) John Friel III and the Forbin Project, Inc.


INTRODUCTION

This document discusses some of the unique features of the HST
modem and gives more specific information on using ther HST with
Qmodem 3.1A 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.

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.


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.
IBM or compatible AT-class machine running at 6Mhz or faster
have a 16-bit data path and use a much faster 16-bit serial
processor chip, and can communicate at the 19200bps serial rate
easily.  WARNING: some cheap AT clones may use PC-style 8-bit
serial adapter boards to cut costs.  The 8-bit serial chip
is usually a 8250-type, while the 16-bit chip is a 16250 or
16450 type.  On AT-class machines, the serial adapter card will
require a full 16-bit expansion slot if it is equipped with the
faster serial processor.  On PC/XTs and cheap ATs, the serial
adapter card will use an 8-bit slot or half of a 16-bit slot.

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 disk updates on the network's file server takes precedence
over communications 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 QModem in terms of raw 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.  QModem release 3.1A's
can set you serial speed all the way up to 38,400bps, which
is more than most personal computers or modems can handle today.

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
Qmodem is a good one; stick with it.