
   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.

   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 predefined 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, MNP, and V.42.

   ASCII

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

   Asymmetrical Modulation

      A duplex transmission technique which
      splits the communications channel into
      one high speed channel and one slower
      channel.  During a call under
      asymmetrical modulation, the modem with
      the greatest amount of data to transmit
      is allocated the high speed channel.
      The modem with less data is allocated
      the slow, or back channel (450 bps).
      The modems dynamically reverse the
      channels during a call if the volume of
      data transfer changes.

   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.

   Auto Answer

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

   Auto Dial

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

   Baud Rate

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

   Bisync

      Binary Synchronous Control.  An earlier
      protocol developed by IBM for software
      applications and communicating devices
      operating in synchronous environments.
      The protocol defines operations at the
      link level of communications, for
      example, the format of data frames
      exchanged between modems over a phone
      line.  See Protocol, HDLC, SDLC.

   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 14400.

   BPS

      The bits (binary digits) per second
      rate.

   Buffer

      A memory area used as temporary storage
      during input and output operations.  An
      example is the modem's command buffer.
      Another is the Transmit Data flow
      control buffer used for flow control and
      to store 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), or
      have some other value meaningful to the
      computer.  In user documentation, the
      term usually refers to 8-bit units or
      characters.  1 kilobyte (K) is equal to
      1,024 bytes or characters; 64K indicates
      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.

   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.22 bis.  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, employ advanced techniques such
      as longer transmission frames and data
      compression to increase 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 USRobotics high
      speed modems, 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.

      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 term applies to 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 tele-
      phone line.  The communications channel
      may also be referred to as nonswitched
      because calls do not go through
      telephone company switching equipment.

   Default

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

   Digital Loopback

      A test that checks the modem's RS-232
      interface and the cable that connects
      the terminal or computer and the modem.
      The modem receives data (in the form of
      digital signals) from the computer or
      terminal, and immediately returns the
      data to the screen for verification.

   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 that generates or
      is the final destination of data. In
      this manual, the term refers to your
      computer.

   Duplex

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

   EIA

      Electronic Industries Association, which
      defines electronic standards in the U.S.

   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.  USRobotics high
      speed modems use adaptive equalization.

   Error Control

      Various techniques which check the
      reliability of characters (parity) or
      blocks of data.  V.42, MNP and HST error
      control protocols use 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 indicators.

   Full Duplex

      Signal flow in both directions at the
      same time.  In microcomputer
      communications, may refer to the
      suppression of the online Local 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.

   HDLC

      High Level Data Link Control.  A
      standard protocol developed by the
      International Standards Organization for
      software applications and communicating
      devices operating in synchronous
      environments.  The protocol defines
      operations at the link level of
      communications, for example, the format
      of data frames exchanged between modems
      over a phone line.  See Bisync,
      Protocol, SDLC.

   HST

      High Speed Technology, USRobotics'
      proprietary signaling scheme, design and
      error control protocol for high-speed
      modems.  HST incorporates trellis-coded
      modulation, for greater immunity from
      variable phone line conditions, and
      asymmetrical modulation for more
      efficient use of the phone channel at
      speeds of 4800 bps and above.  HST also
      incorporates MNP-compatible error
      control procedures adapted to
      asymmetrical modulation.

   Hz

      Hertz, a frequency measurement unit used
      internationally to indicate one cycle
      per second.

   LAPM

      Link Access Procedure for Modems, an
      error control protocol incorporated in
      CCITT Recommendation V.42.  Like the MNP
      and HST protocols, LAPM uses cyclic
      redundancy checking (CRC) and retrans-
      mission of corrupted data (ARQ) to
      ensure data reliability.

   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.

   MI/MIC

      Mode Indicate/Mode Indicate Common, also
      called Forced or Manual Originate.
      Provided for installations where other
      equipment, rather than the modem, does
      the dialing.  In such installations, the
      modem operates in Dumb mode (no Auto
      Dial capability), yet must go off hook
      in Originate mode to connect with
      answering modems.  See MI/MIC Closure in
      Chapter 2.

   MNP

      Microcom Networking Protocol, an
      asynchronous error control protocol
      developed by Microcom, Inc. and now in
      the public domain.  The protocol ensures
      error-free transmission through error
      detection (CRC) and retransmission of
      errored frames.  USRobotics modems use
      MNP Levels 1+4 and Level 5 data com-
      pression.  MNP Levels 1+4 have been
      incorporated into CCITT Recommendation
      V.42.  Compare HST.

   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 trans-
      forms, digital signals from a computer
      into the analog form that can be carried
      successfully on a phone line.  It also
      demodulates signals received from the
      phone line back to digital signals
      before passing them to the receiving
      computer.

   Nonvolatile Memory (NVRAM)

      User-programmable random access memory
      whose data is retained when modem power
      is turned off.  Used in Courier modems
      to store a user-defined default configu-
      ration 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).

   Online Fallback

      A feature that allows high speed error-
      control modems to monitor line quality
      and fall back to the next lower speed if
      line quality degrades.  The modems fall
      forward as line quality improves.

   Originate Mode

      A state in which the modem transmits at
      the predefined 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-type
      protocols and the ARQ protocol
      implemented in Courier modems.

      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 1's in the
      character even or odd, depending on
      which type of parity is used.

   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 correction are some of the opera-
      tions that may be defined in protocols.

   RAM

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

   Remote Digital Loopback

      A test that checks the phone link and a
      remote modem's transmitter and receiver.
      Data entered from the keyboard is trans-
      mitted from the initiating modem,
      received by the remote modem's receiver,
      looped through its transmitter, and
      returned to the local screen for
      verification.

   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's
      factory settings are stored in ROM and
      can be read (loaded) into RAM as an
      operational configuration if DIP switch
      S10 is ON at power on.

   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.

   SDLC

      Synchronous Data Link Control.  A
      protocol developed by IBM for software
      applications and communicating devices
      operating in IBM's Systems Network
      Architecture (SNA).  The protocol
      defines operations at the link level of
      communications, for example, the format
      of data frames exchanged between modems
      over a phone line.  See Bisync,
      Protocol, HDLC.

   Synchronous Transmission

      A form of transmission in which blocks
      of data are sent at strictly timed
      intervals.  Because the timing is
      uniform, no Start or Stop bits are
      required.  Compare Asynchronous
      Transmission.

      Some mainframes only support synchronous
      communications unless their owners have
      installed a synchronous adapter and
      appropriate software.

   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.  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.21

      A CCITT standard for modem
      communications at 300 bps.  Modems made
      in the U.S. or Canada follow the Bell
      103 standard.  However, the modem can be
      set to answer V.21 calls from overseas.
      See Register S27 in Chapter 6.

   V.22

      A CCITT standard for modem
      communications at 1200 bps, compatible
      with the Bell 212A standard observed in
      the U.S. and Canada.

   V.22 bis    

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

   V.23

      A CCITT standard for modem
      communications at 1200 bps with a 75 bps
      back channel.  Used in the U.K.

   V.25

      A CCITT standard for modem
      communications that specifies an answer
      tone different from the Bell answer tone
      used in the U.S. and Canada.  All
      USRobotics modems can be set with the B0
      command so that they use the V.25 2100
      Hz tone when answering overseas calls.

   V.32

      A CCITT standard for modem
      communications at 9600 bps and 4800 bps.
      V.32 modems fall back to 4800 bps when
      line quality is impaired, and fall
      forward again to 9600 bps when line
      quality improves.

   V.32 bis

      A CCITT standard that extends the V.32
      connection range:  4800, 7200, 9600, 12K
      and 14.4K bps.  V.32 bis modems fall
      back to the next lower speed when line
      quality is impaired, and fall back
      further as necessary.  They fall forward
      to the next higher speed when line
      quality improves.

   V.42

      A CCITT standard for modem
      communications that defines a two-stage
      process of detection and negotiation for
      LAPM  error control.  V.42 also supports
      the MNP error control protocol, levels
      1+4.

   V.42 bis

      An extension of CCITT V.42 that defines
      a specific data compression scheme for
      use with V.42 and MNP error control.

   Word Length

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

   Xmodem

      The first of a family of error control
      software protocols used to transfer
      files between modems.  These protocols
      are in the public domain and are
      available from many bulletin board
      services.

   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.


   #NPD

