





                                                              COURIER 1200e


                                                                   GLOSSARY
          Cross-references in the following definitions are printed in
          boldface.

          Analog Loopback

          A modem self-test in which data from the keyboard is sent to the
          modem's transmitter, modulated into analog form, looped back to
          the receiver, demodulated into digital form, and returned to the
          screen for verification.  Tests either the modem's originate or
          answer frequency.

          Analog signals

          Continuous, varying waveforms such as the voice tones carried
          over phone lines.  Contrast with digital signals.

          Answer Mode

          A state in which the modem transmits at the pre-defined high
          frequency of the communications channel and receives at the low
          frequency.  The transmit/receive frequencies are the reverse of
          the calling modem which is in Originate Mode.

          ARQ

          Automatic Repeat Request.  A general term for error control
          protocols which feature error-detection and automatic
          retransmission of defective blocks of data.  See HST and MNP.

          ASCII

          American Standard Code for Information Interchange.  A 7-bit
          binary code (0s, 1s) used to represent letters, numbers, and
          special characters such as $, !, and /.  Supported by almost
          every computer and terminal manufacturer.  See Appendix B-6 for
          decimal and hexadecimal code equivalents.

          Asynchronous Transmission

          Data transmission in which the length of time between transmitted
          characters may vary.  Because the time lapses between transmitted
          characters are not uniform, the receiving modem must be signaled
          as to when the data bits of a character begin and when they end.
          The addition of Start and Stop bits to each character serves this
          purpose.  Compare Synchronous Transmission.

          Auto Answer

          A feature in modems enabling them to answer incoming calls over
          the phone lines without the use of a telephone receiver.

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          Auto Dial

          A feature in modems enabling them to dial phone numbers over the
          phone system without the use of a telephone transmitter.

          Baud Rate

          The number of discrete signal events per second occurring on a
          communications channel.  Although not technically accurate, baud
          rate is commonly used to mean bit rate.

          Binary Digit

          A 0 or 1, reflecting the use of a binary numbering system (only
          two digits).  Used because the computer recognizes either of two
          states, OFF or ON.  Shortened form of binary digit is "bit."

          Bit Rate

          The number of binary digits or bits transmitted per second (bps).
          Communications channels using telephone channel modems are
          established at set bit rates, commonly 110, 300, 1200, 2400,
          4800, 9600, and 144000.

          BPS

          The bits (binary digits) per second rate.

          Buffer

          A memory area used as temporary storage during input/output
          operations.  Two examples in the Courier HST are the command
          buffer (last issued command) and the flow control buffer (for
          data flow control and storing copies of transmitted frames until
          they are positively acknowledged by the receiving modem).

          Byte

          A group of binary digits stored and operated upon as a unit.  A
          byte may have a coded value equal to a character in the ASCII
          code (letters, numbers), for example, or have some other value
          meaningful to the computer.  In user documentation, the term
          usually refers to 8-bit units or characters.  1 kilobyte = 1,024
          bytes or characters; 64k = 65,536 bytes or characters.

          Carrier

          A continuous frequency capable of being either modulated or
          impressed with another information-carrying signal.  Carriers are
          generated and maintained by modems via the transmission lines of
          the telephone companies.

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          CCITT

          An international organization that defines standards for
          telegraphic and telephone equipment.  For example, the Bell 212A
          standard for 1200 bps communication in North America is observed
          internationally as CCITT V.22.  For 2400 bps communication, most
          U.S. manufacturers observe V.22bis.  The initials CCITT represent
          the French name; in English it's known as the International
          Telegraph and Telephone Consultative Committee.

          Character

          A representation, coded in binary digits, of a letter, number, or
          other symbol.

          Characters Per Second

          A data transfer rate generally estimated from the bit rate and
          the character length.  For example, at 2400 bps, 8-bit characters
          with Start and Stop bits (for a total of ten bits per character)
          will be transmitted at a rate of approximately 240 characters per
          second (cps).  Some protocols, such as USR-HST and MNP-Service
          Class 3, employ advanced techniques to increase cps.  When the
          Courier HST is set for maximum throughput, as described in
          Chapter 3, 140 characters in addition to the standard 960 can be
          transmitted for a total of 1100 cps.

          Cyclic Redundancy Checking (CRC)

          An error-detection technique consisting of a cyclic algorithm
          performed on each block or frame of data by both sending and
          receiving modems.  The sending modem inserts the results of its
          computation in each data block in the form of a CRC code.  The
          receiving modem compares its results with the received CRC code
          and responds with either a positive or negative acknowledgment.
          In the ARQ protocol implemented in the Courier HST, the receiving
          modem accepts no more data until a defective block is received
          correctly.

          Data Communications

          A type of communications in which computers and terminals are
          able to exchange data over an electronic medium.

          Data Compression Table

          A table of values assigned for each character during a call under
          data compression.  Default values in the table are continually
          altered and built during each call:  the longer the table, the
          more efficient throughput gained.


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          If a destructive Break is sent during a call (see the &Y
          command), causing the modems to reset the compression tables, you
          can expect diminished throughput.

          DCE

          Data Communication (or Circuit-Terminating) Equipment.  In this
          manual, the dial-up modems that establish and control the data
          link via the telephone network.

          Dedicated Line

          A user-installed telephone line used to connect a specified
          number of computers or terminals within a limited area, for
          example, one building.  The line is a cable rather than a public-
          access telephone line.




































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          Default

          Any setting assumed, at startup or reset, by the computer's
          software and attached devices, and operational until changed by
          the user.

          Digital Signals

          Discrete, uniform signals.  In this manual, the term refers to
          the binary digits 0 and 1.

          DTE

          Data Terminal (or Terminating) Equipment.  The device, such as a
          personal computer, terminal, or mainframe, that generates or is
          the final destination of data.

          Duplex

          Indicates a communications channel capable of carrying signals in
          both directions. See Half Duplex, Full Duplex.

          Equalization

          A compensation circuit designed into modems to counteract certain
          distortions introduced by the telephone channel.  Two types are
          used:  fixed (compromise) equalizers and those that adapt to
          channel conditions.

          Error Control

          Various techniques which check the reliability of characters
          (Parity) or blocks of data.  The Courier's error control protocol
          provides error control through error detection (CRC) and
          retransmission of errored frames (ARQ).

          Flow Control

          A mechanism that compensates for differences in the flow of data
          input to and output from a modem or other device.

          Frame

          A data communications term for a block of data with header and
          trailer information attached.  The added information usually
          includes a frame number, block size data, error-check codes, and
          Start/End data.





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          Full Duplex

          Signal flow in both directions at the same time. In microcomputer
          communications, may refer to the suppression of the online Local
          Echo.  The receiving computer may provide a Remote Echo.

          Half Duplex

          Signal flow in both directions, but only one way at a time.  In
          microcomputer communications, may refer to activation of the
          online Local Echo, which causes the modem to send a copy of the
          transmitted data to the screen of the sending computer.

          HST

          High Speed Technology, part of the trademark for USRobotics' USR-
          HST, a proprietary signaling scheme, design and protocol for 9600
          bps modems.  USR-HST incorporates trellis-coded modulation, for
          greater immunity from variable phone line conditions, and asym-
          metrical modulation for more efficient use of the phone channel.
          HST also represents the enhanced error control protocol at 9600
          bps that is similar to and compatible with MNP error control at
          2400/1200 bps.

          Local Echo

          A modem feature that enables the modem to send copies of keyboard
          commands and transmitted data to the screen.  When the modem is
          in Command Mode (not online to another system) the local echo is
          invoked through the ATE1 command.  The command causes the modem
          to display your typed commands.  When the modem is online to
          another system, the local echo is invoked through the ATF0
          command.  This command causes the modem to display the data it
          transmits to the remote system.

          MNP

          Microcom Networking Protocol.  An error control protocol
          developed by Microcom, Inc. and now in the public domain.  The
          protocol ensures error-free transmission at 1200 and 2400 bps
          through error detection (CRC) and retransmission of errored
          frames.

          Modem

          A device that transmits/receives computer data through a
          communications channel such as radio or telephone lines.  The
          Courier is a telephone channel modem that modulates, or
          transforms, digital signals from a computer into a form that can
          be carried successfully on a phone line.  It also demodulates


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          signals received from the phone line back to digital signals
          before passing them to the receiving computer.

          Intelligent modems with built-in software, such as the USRobotics
          modem described in this manual, simplify using phone networks for
          data communications.

          Nonvolatile Memory (NRAM)

          User-programmable random access memory whose data is retained
          when modem power is turned off.  Used in the Courier HST to store
          a user-defined default configuration loaded into random access
          memory (RAM) at power on.

          OFF/ON Hook

          Modem operations which are the equivalent of manually lifting a
          phone receiver (taking it off hook) and replacing it (going on
          hook).

          Originate Mode

          A state in which the modem transmits at the pre-defined low
          frequency of the communications channel and receives at the high
          frequency.  The transmit/receive frequencies are the reverse of
          the called modem which is in Answer Mode.

          Parallel Transmission

          The transfer of data characters using parallel electrical paths
          for each bit of the character, for example, 8 paths for 8-bit
          characters.  Data is stored in computers in parallel form, but
          may be converted to serial form for certain operations.  See
          Serial Transmission.

          Parity

          An error-detection method that checks the validity of a
          transmitted character.  Character checking has been surpassed by
          more reliable and efficient forms of block-checking, including
          XMODEM, a public domain protocol used in some telecommunications
          software, and the ARQ protocol implemented in the Courier HST.

          The same type of parity must be used by two communicating
          computers, or both may omit parity.  When parity is used, a
          parity bit is added to each transmitted character.  The bit's
          value is 0 or 1, to make the total number of 1s in the character
          even or odd, depending on which type of parity is used.




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          Protocol

          A system of rules and procedures governing communications between
          two or more devices.  Protocols vary, but communicating devices
          must follow the same protocol in order to exchange data.  The
          format of the data, readiness to receive or send, error
          detection, and error handling are only a few of the operations
          that may be defined in a protocol.

          RAM

          Random Access Memory.  Memory that is available for use when the
          modem is turned on and clears of all information when the power
          is turned off.  The Courier HST's RAM holds the current
          operational settings, a flow control buffer, and a command
          buffer.

          Remote Echo

          A copy of the data received by the remote system, returned to the
          sending system and displayed on the screen.  Remote echoing is a
          function of the remote system.

          ROM

          Read Only Memory.  Permanent memory, not user-programmable.  The
          Courier HST's factory settings are stored in ROM and can be read
          (loaded) into RAM as an operational configuration.

          Serial Transmission

          The transfer of data characters one bit at a time, sequentially,
          using a single electrical path.  See Parallel Transmission.

          Start/Stop Bits

          The signaling bits attached to a character before the character
          is transmitted during Asynchronous Transmission.

          Synchronous Transmission

          A form of transmission in which groups of data bits are sent at
          regular intervals.  Because the timing is uniform, no Start or
          Stop bits are required.  See Asynchronous Transmission.

          Terminal

          A device whose keyboard and display are used for sending and
          receiving data over a communications link.  Differs from a
          microcomputer in that it has no internal processing capabilities.


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          Used to enter data into or retrieve processed data from a system
          or network.

          Terminal Mode

          An operational mode required for microcomputers to transmit data.
          In Terminal Mode the computer acts as if it were a standard
          terminal such as a teletypewriter, rather than a data processor.
          Keyboard entries go directly to the modem, whether the entry is a
          modem command or data to be transmitted over the phone lines.
          Received data is output directly to the screen.  The more popular
          communications software products control Terminal Mode as well as
          enable more complex operations, including file transmission and
          saving received files.

          Throughput

          The amount of actual user data transmitted per second without the
          overhead of protocol information such as Start and Stop bits or
          frame headers and trailers.  Compare characters per second.

          Transmission Rate

          Same as Bit Rate.

          V.22bis

          The international CCITT standard for modem communications at 2400
          bps.  The standard includes an automatic data rate fallback to
          1200 bps and compatibility with Bell 212A/V.22 modems.

          Word Length

          The number of bits in a character without parity, start or stop
          bits.

          XON/XOFF

          Standard ASCII control characters used to tell an intelligent
          device to stop/resume transmitting data.  In most systems typing
          <Ctrl>-S sends the XOFF character.  Some devices, including the
          Courier, understand <Ctrl>-Q as XON; others interpret the
          pressing of any key after <Ctrl>-S as XON.









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