

               USRobotics 9600-BPS COURIER HST MODEM 

                   SETS NEW HIGH-SPEED STANDARD 


     The  new 9600-bps Courier HST modem from  USRobotics,  Inc., 
sets a new standard in high-speed modem technology.  And its $995 
list price sets a new standard in modem value.


     The Courier HST (High Speed Technology) modem,  designed for 
use on voice-grade,  dial-up telephone circuits,  provides  full-
duplex   9600-bps  data  communication  through  an  asymmetrical 
frequency  division  of  the  telephone  channel.    This  design 
provides  simultaneous 9600-bps and 300-bps data  channels,  with 
the high-speed channel direction automatically assigned according 
to data flow demand.

     At 9600-bps,  Courier HST uses Trellis Coded Modulation,  an 
advanced  technology  which enables the modem to achieve  maximum 
speed over a wide range of dial-up line conditions.  

     A   proprietary  error- and  flow-control  protocol   allows 
Courier  HST to transmit up to 1,100 characters a  second,  error 
free, over local and long-distance telephone circuits.

     Modem users will find the Courier HST easy to  operate.   It 
uses  an  extended version of the industry standard "AT"  command 
set, and works with most popular data communications software.

     The modem incorporates the same user features as  USRobotics 
Courier  2400 modem.   It also automatically falls back to  2400, 
1200  and  300  bps in both answering and  originating  calls  to 
connect with nearly any modem.

     At  2400  and  1200 bps,  Courier HST implements  Classes  1 
through  3  of MNP (an industry-standard error- and  flow-control 
protocol),  providing error-control compatibility with USRobotics 
Courier 2400e and other MNP-capable modems.



 New Standard in Value 

     Courier  HST advances a new standard in value.   On a price-
per-bit basis,  the $995 Courier HST costs about ten cents a bit.  
A competitive 2400-bps modem priced at $899 costs about 37  cents 
a bit.




     Compared  with  conventional 1200-bps  modems,  Courier  HST 
could  pay  for  itself in just eight hours  of  high-speed  data 
transmission. Sending 1,100 characters a second,  the Courier HST 
could  transmit  over 31 million characters in  eight  hours.   A 
1200-bps modem would take 73 hours and 20 minutes -- more than 65 
hours  longer -- to send the same amount of data.   At an average 
long-distance telephone rate of $15 an hour,  the time saved with 
the Courier HST modem would equate to its $995 list price.

     Viewed in yet another way,  a customer could purchase  seven 
Courier  HST  modems for the same price of just  two  competitive 
9600-bps modems that implement the V.32 recommendation from CCITT 
(the  international  communications standards  organization),  or 
five  Courier  HST  modems for about the same price  of  two  DCA 
Fastlink modems which use a proprietary multi-carrier half-duplex 
modulation technique.



 Optimum Use of Bandwidth 

     "The  asymmetrical  modulation  design responds to  the  way 
people  really  use  PCs  and  data  terminals  to  communicate," 
USRobotics  Vice  President of Engineering Dale Walsh  explained.  
"It  gives the user a high-speed channel for fast downloading  or 
uploading   of  data  files,   and  a  low-speed   channel   that 
handles  both  manual  data entry by the user  and  error-control 
coding.

     "Courier  HST represents optimum use of the  dial-up,  voice 
grade telephone circuit," he continued. "It provides simultaneous 
two-way  communication -- full duplex.   Most currently available 
9600-bps  modems  are  half-duplex devices based  on  proprietary 
modulation  techniques  or the old  CCITT  V.29  recommendation."      

     Half-duplex  modems devote the entire telephone bandwidth to 
9600-bps  in one direction at a time,  and "ping-pong"  the  data 
flow back and forth to approximate full duplex.  This solution is 
ill suited to interactive online sessions.  

     For  example,  in half-duplex a typed character will be sent 
at  9600-bps,  but  the echo of that character may  take  several 
seconds  to appear on the screen of the system that sent it,  due 
to  long  round-trip delay.   As a result,  the system  fails  to 
achieve  the  illusion  of  full-duplex  that  most   interactive 
applications require.

     The  Courier  HST modem's aymmetrical  full-duplex  approach 
also is better suited to PC datacom applications -- and much less 
expensive   -- than   the  symmetrical   full-duplex   technology 
representing CCITT's V.32 recommendation.  




     Walsh explained that V.32 modems employ echo-cancelling as a 
solution to overlapping Answer and Originate frequencies.  Simply 
stated,  echo-cancelling  allows  each  modem to ignore  its  own 
transmitter  and  pick up the remote  transmitter.   Measured  by 
computations per second and bits of resolution,  a V.32 modem  is 
roughly 64 times more complex than a 2400-bps modem,  Walsh said.  
This translates directly into higher costs.



 Trellis Coded Modulation 

     While  avoiding  the  cost  and  complexity  of  V.32  echo-
cancelling,  the  Courier  HST modem employs the V.32  modulation 
technique -- Trellis Coded Modulation (TCM).

     TCM is a multi-dimensional technique that makes transmission 
less  vulnerable  to  data errors  caused  by  telephone  network 
conditions.  

     The  Courier  HST  implementation of TCM  uses  a  2400-baud 
signalling rate,  transmitting four data bits per baud to achieve 
its  9600-bps  speed.   Unlike other modulation  techniques,  TCM 
allows migration to even high speeds, signalling five or six bits 
per baud to achieve bit rates of 12,000 and 14,400  bps.   Future 
versions  of the Courier HST may incorporate these higher speeds.

     TCM provides an inherent 4-dB signal-to-noise advantage over 
V.29 modulation.  Four dB difference might not mean much at lower 
data rates that already have large built-in margins.  But at 9600 
bps,   it  is  extremely  meaningful  in  improving  transmission 
quality.

     It  is very important to note that the  currently  available 
9600-bps  modems based on the V.29 recommendation use  Quadrature 
Amplitude Modulation (QAM).  All other factors being equal, a QAM 
modem  will experience at least twice as many block errors as   a 
TCM  modem.   This  translates to far fewer  retransmissions  and 
consistently higher data throughput for a TCM modem.

     Courier  HST  achieves its maximum speed  on  circuits  with 
signal-to-noise  conditions of 17dB or better.  It also  is  more 
resistant  to  such  impairments as low frequency  phase  jitter, 
satellite  connections and impulse noise than either V.29  modems 
or  the multi-carrier half-duplex technique used in  the  Telebit 
Trailblazer and DCA Fastlink modems.









 Enhanced Error Control 

     While   TCM   itself  provides  virtually  error-free   data 
transmission,  the  Courier  HST modem also  incorporates  a  new 
proprietary  ARQ  (Automatic Repeat Request)  block-check  error-
control protocol called USR-HST.  This protocol is essentially an 
enhanced  version of MNP that USRobotics devised specifically for 
the Courier HST modem's aysmmetric frequency division.

     Like    MNP,    USR-HST   provides    error-detection    and 
retransmission  to protect data integrity.   It divides data into 
transmission  frames  that  include  an  algorithmic  calculation 
called  Cyclic Redundancy Checking (CRC).   The  receiving  modem 
performs  the  same  CRC  algorithm on each  frame  and  responds 
positively or negatively to the sending modem.

     A  negative  acknowledgement is a request that  the  sending 
modem  retransmit  an errored data frame.   The  receiving  modem 
accepts  no more frames until the one in question is  transmitted 
successfully.   In this way,  USR-HST protects against errors and 
ensures that the data arrives in sequence.

     Adapted for asymmetrical modulation, USR-HST provides faster 
response  than MNP.   USR-HST includes nine types of data  frames 
compared  to  MNP's  five types,  including a  faster  method  of 
signalling  a  negative  acknowledgement and  a  faster  training 
algorithm  to  command  channel  turnaround.    USR-HST's  design 
efficiencies  result in less data overhead (control  information) 
than MNP requires.

     Under optimal phone line conditions,  two Courier HST modems 
can exchange data at rates up to 1,100 characters a second.  This 
fifteen  percent gain over the normal 960-cps speed for  9600-bps 
modems is achieved by removing start and stop bits from each data 
character  so  the  data  is in bit  format,  as  in  synchronous 
transmission.   The receiving modem reinserts the start and  stop 
bits  before  passing  the  data to  the  receiving  terminal  or 
computer.

     USR-HST,  like other error-control protocols,  requires that 
the modem control data flow from the terminal or computer to  the 
modem,  to prevent the possible loss of data that might otherwise 
occur.

     Line  conditions,  for  example,  might cause  a  number  of 
retransmission  requests that interrupt the normal flow over  the 
data  link.   The  modem  is equipped with a buffer  for  storing 
incoming  data from the computer or terminal,  so that  the  data 
does not get lost while the modem is retransmitting.






     The  modem  monitors the buffer and,  if it approaches  full 
capacity,  signals the computer or terminal to stop sending.  The 
modem  signals the computer to resume sending when the modem  has 
sent enough data over the link to sufficiently empty the buffer.

     USR-HST's  hardware-based flow-control protocol is known  as 
"Request to Send/Clear to Send" (RTS/CTS),  using Pins 4 and 5 of 
the  RS232C interface.   Software-based,  or command-based,  flow 
control  uses ASCII "XON/XOFF" signals to perform  the  identical 
function of RTS/CTS.

     An important user benefit of flow control is the ability  to 
set the data transfer rate of the computer or terminal as high as 
19,200 bps, regardless of the transmission speed of the modem.

     With  this arrangement,  the computer or terminal data speed 
is  the same from call to call.   The computer or terminal  sends 
data  to  the modem buffer as fast as possible,  with  the  modem 
controlling  the  flow  of data into  the  buffer.   Courier  HST 
automatically  matches  the  speed  of  the  remote  modem   when 
answering or originating a call.

     In  addition  to  USR-HST  error-control  at  9600-bps,  the 
Courier  HST features Classes 1 through 3 MNP when  connected  at 
2400 or 1200 bps to other MNP modems.



 Other Features 

     The  Courier  HST also includes Non-volatile  Random  Access 
Memory  (NRAM),  which  saves  user-defind  modem  settings  from 
session to session, even if the modem is turned off and on.

     NRAM  also  allows  the  Courier HST to  store  up  to  four 
frequently  dialed phone numbers for automatic dialing,  and  can 
instruct the modem to dial the first number stored as soon as the 
modem is powered on.

     Courier  HST  responds to the industry standard  "AT"  modem 
command   set   at  any  of  its  operating   speeds,   providing 
compatibility  with most data communications  software  programs. 

     At fallback data rates, Courier HST is compatible with CCITT 
V.22 bis at 2400, Bell 212A at 1200, and Bell 103 at 300 bps.

     Courier  HST includes automatic retraining  (resynchronizing 
with the remote modem) if it detects line disturbances that might 
affect  data reliability.   At 9600 bps,  the connection must  be
with another Courier HST modem.   Retraining also occurs at  2400 
bps with other V.22 bis compatible modems.




     Additionally,  Courier  HST incorporates the same design and 
operational  features  of  other modems  in  USRobotics'  Courier 
product  line,  including  "help screen" summaries of  the  modem 
command set and S-registers,  a printed operations summary on the 
modem's bottom panel,  externally accessible programming switches 
and an extended twelve-function LED front panel display.

     Other  features in common with the Courier modem family  are 
call-duration reporting,  call-progress detection, modem settings 
display,  the  ability to continuously repeat a command,  an  on-
board   speaker   with  volume  control  for  audio  phone   line 
monitoring,  automatic  speed detection in answer and  originate, 
analog loopback self-test in answer and originate,  and  internal 
test pattern generation.

     Courier  HST includes a telephone cord,  illustrated  user's 
manual and quick reference card.   USRobotics provides free parts 
and  service  coverage under a two-year warranty.   The  original 
purchaser also may buy a two-year warranty extension for $15.   
         


 International Standard Proposed 

     No  international  standard exists yet for  an  asymmetrical 
high-speed  modem  like  the Courier HST.   Support  for  such  a 
standard is developing,  however,  within the U.S.  Modem Working 
Party,  a committee of industry representatives that reports to a 
study group of CCITT.

     USRobotics  submitted  proposals at two 1986  Modem  Working 
Party meetings for an HST-based standard.   Concord Data Systems, 
which  manufactures  a  V.32  modem,   became  a  co-sponsor   of 
USRobotics' proposal subsequent to its submission.

     The  only  other technology being advanced  as  a  potential 
high-speed   modem  standard  is  the  multi-carrier  half-duplex 
technique used by the Telebit Trailblazer modem.

     Dale Walsh is USRobotics representative to the Modem Working 
Party.   He served from 1978 to 1984 as the group's chairman, and 
is  well-acquainted  with CCITT's standards-development  methods.  
He said it is likely that CCITT will move cautiously in  adopting 
a new standard for 9600-bps dial-up modems,  especially since the 
V.29 and V.32 standards already exist.

     "The standards community is fairly conservative in  adopting 
new  recommendations," Walsh explained.   "They favor incremental 
change  and derivative technology,  not radical  departures  from 
existing standards.





     "In that respect,   the asymmetrical modem proposal would fit 
the  expectations  of the standards community much  more  closely 
than would the multi-carrier proposal. 

     "The  asymmetrical modem represents the synthesis of several 
existing  standards  -- the  modulation  technique  of  V.32,   a 
derivative   of   an   accepted   error-control   protocol,   the 
incorporation of existing 2400 and 1200 standards,  and even  the 
concept  of asymmetrical frequency division,  which has been used 
successfully in other modems," Walsh continued.

     "The multi-carrier technique,  however,  is a complete break 
from   existing dial-up modem technology.  It remains to be  seen 
whether the standards community will support this idea."

     USRobotics also is working with other modem companies which
may be interested in developing Courier HST-compatible modems.

                      ___________________


     For  more information on USRobotics Courier HST modem,  call 
toll-free,  800-DIAL-USR (342-5877).  In Illinois, call our sales 
department at 312/982-5001.

