


                                                                COURIER HST
                                                                  CHAPTER 1
                                                               INTRODUCTION


          ADVANCED FEATURES

          The USRobotics Courier HST modem represents powerful, advanced
          electronic design that gives you optimal speed and accuracy.

          Trellis Coded Modulation

          Trellis coded modulation is a convolutional coding technique that
          makes data transmission less vulnerable to errors caused by phone
          network impairments.  It can tolerate twice the telephone channel
          noise power as conventional modulation (quadrature amplitude
          modulation, or QAM), so there are fewer error-control
          retransmissions.  Trellis coded modulation is also less
          susceptible to impulse-type noise.

          Asymmetrical Modulation

          At top speed, data flows in one direction at 9600 bits per second
          and at 450 bits per second in the other.  The modems
          automatically switch the high-speed channel on demand, i.e.,
          depending on which transmitter has the most data to transmit.

          In practice, the modems seldom need to reverse channels, since
          the asymmetric design reflects typical communications sessions --
          brief messages typed at one end of the link, files sent from the
          other.  The asymmetrical approach provides the most efficient and
          economical strategy for using ordinary phone channels at high
          speeds.

          Error Control

          The MNP protocol is a widely used, sophisticated method of
          ensuring data integrity at 2400 and 1200 bps.  A USRobotics
          proprietary protocol similar to MNP is used at 9600, 7200 and
          4800 bps.  With error detection and retransmission of errored
          data blocks, plus flow control, you can be assured that your data
          is sent and received with reliability.

          Data Compression/Throughput Gains

          The Courier HST compresses data to pack more information into
          each second of your calls, enabling throughput of up to 19.2K
          bits per second.  In addition, the implementation of larger data
          blocks eliminates much of the added overhead (protocol
          information) associated with less powerful error-control
          protocols.

          Rate Selection/Flow Control

          The Courier HST modem's flow control buffer allows you to set
          your computer/modem rate as high as 19.2k bits per second, for




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                                                                COURIER HST
          even greater performance. Flow control settings protect
          transmissions from data loss.

          Online Fallback/Fall Forward

          During 9600-bps communications, if severe line impairments
          threaten the integrity of data transfer, the modems fall back and
          retrain at 7200 bps.  If necessary, they fall back further to
          4800 bps.  As line improvements occur, the modems automatically
          shift up to the next higher speed.

          Programmable Nonvolatile Memory

          You can tailor your own default settings for the Courier HST and
          store them in nonvolatile random access memory (NRAM).  Each time
          the computer is powered on, the modem operates at the settings
          you've specified.

          Diagnostics

          After each call, you can display a Link Diagnostics screen
          containing information about the call.  Two S-Registers also
          store information on the reason for a call's disconnect.

          Shorter Response Time

          9600-bps modems are generally most efficient for file transfers.
          You'll find that the Courier HST also offers faster response
          times during interactive sessions.

          THE COURIER PRODUCT FAMILY

          In addition to the Courier HST's new features, the modem
          incorporates the same popular capabilities of other products in
          the USRobotics Courier line:

          Link Negotiation Fallback

          The Courier HST automatically falls back to 2400, 1200, and 300
          bits per second, in both Originate and Answer Modes, to match a
          lower rate of a remote modem.  You can optionally set local
          interface and link rates at fixed speeds.

          Inactivity Timer

          You can set the modem to automatically hang up after a specified
          number of minutes if there is no activity on the phone line.

          Call Duration Reporting

          The modem records the duration of your calls in hours, minutes,
          and seconds.  This feature enables you to display and print an
          audit of your calling activities.  You can optionally use the
          modem clock as a real-time clock.





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                                                                COURIER HST
          Call Progress Detection

          An optional set of result codes (screen messages) lets you know
          when a line is busy, a person rather than a modem has answered
          the phone, there is no dial tone, or the distant phone is
          ringing.

          Modem Settings Display

          On command, the modem displays its current settings, a handy way
          to check your transmission rate, S-registers and other
          operational controls.

          HELP Screens

          You can also display screens that summarize the command sets,
          Dial command options, and S-register functions.

          Bottom Panel Reference

          Operational summaries and other information are printed on the
          bottom of the modem case.  A Dual In-Line Package (DIP) switch
          guide makes it easy to tailor the switch settings to your
          requirements.

          Automated Redialing

          You can put the modem into Repeat Mode to automatically redial if
          a previous dial attempt fails to connect.  This is especially
          useful in dialing services whose lines are often busy.

          Quote Mode

          Set the modem to Quote Mode if you want it to dial an alphabetic
          "number," such as 800-"DIAL USR" (USRobotics' Sales Department).

          Adaptive Dialing

          You can set the modem to first try Touch-Tone dialing.  If tone
          dialing doesn't work on the line, the modem automatically
          switches to the slower type, pulse (rotary).

          Automatic Retraining

          Retraining (a resynchronization with the remote modem) occurs if
          the modem detects line disturbances that might affect data
          reliability.  Retraining occurs at 9600 and 2400 bps.  (Appendix
          E-3 contains more information.)











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                                                                COURIER HST
          COURIER HST COMPATIBILITY

          The Courier HST offers upgrading to 9600 bps while maintaining
          compatibility with most installed 2400/1200/300 bps, dial-up
          modems and existing software.  Its compatibility features include
          the following:

               o    Can be used with any computer or terminal that is
                    compatible with the RS-232C standard interface.

               o    Can be used with any computer or terminal that uses
                    ASCII, the standard character code supported by most
                    equipment manufacturers.

               o    Connects with any modem whose signal scheme is
                    compatible with the following standards at the given
                    data rate:

                         300 bps   Bell 103
                         1200 bps  Bell 212A
                         2400 bps  V.22bis
                         9600 bps  USR-HST

               o    Uses the HST error control protocol at 9600 bps and the
                    MNP error control protocol, Service Levels 1-5, at 2400
                    and 1200 bps.

               o    Is fully FCC-certified for the uses described in this
                    manual.






























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