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From: Gord Hannah
To: All
Date: 2010-06-15 01:00:00
Subject: [3 of 12] Comm Primer

DUPLEX - Indicates a communications channel that is capable of carrying
signals in both directions.

   FULL-DUPLEX - Signal flow in both directions at the same time.

   HALF-DUPLEX - Signal flow in both directions, but in only one
   direction at a time.

     SYMMETRICAL/ASYMMETRICAL - Describes whether    data flow in a
     communications channel is balanced or unbalanced (ie. data flow
     rate is the same in both channels or different).

      Eg. The USR 16.8K HST protocol is full-duplex asymmetrical, it
          employs one 16800bps channel and a second 450bps channel.
          ITU-T V.32bis is full-duplex symmetrical, both channels
          operate at 14400bps.

ENCODING - The process of impressing data onto a carrier. By varying the
frequency, amplitude, and phase of the data carrier, it is possible to
generate signals which are equivalent to one or more data bits per change
of state. Devices which operate above 300bps employ modulation technology
which encodes more than one bit per transition. 1200 and 2400bps modems
operate at 600 symbols/sec using data encoding techniques that yield 2 and
4 bits per symbol element respectively. Similarly, 9600 and 14400bps modems
operate at 2400 symbols/sec using techniques which yield 4 and 6 bits per
symbol element.

ERROR-CORRECTING MODEM - describes a modem which incorporates data
integrity checking protocols in its hardware (eg. MNP4 or V.42 capable
modems). When two such modems connect, they transmit data synchronously,
obviating the necessity of framing start and stop bits for transmitted
data; such modems are thereby able to achieve a 25 per cent increase in
throughput relative to non-error-correcting modems.

ESCAPE SEQUENCE - A command sent to a modem that will place it in command

EXPRESS-96 - A proprietary high-speed modulation protocol developed by

FLOW CONTROL -  A mechanism for compensating for differences in the flow of
data input to and output from a a modem.

FOSSIL - (F)ido-(O)pus-(S)eadog-(S)tandard-(I)nterface-(L)ayer. A
specification detailing the interface between a communications application
and a PC's serial communications hardware. Originally conceived to allow
programmers to implement common low-level serial I/O functions for their
applications on platforms that were not 100% compatible.

FSK - [F]requency [S]hift [K]eying. FSK describes the modulation of a
carrier (or two carriers) by using a different frequency for a 1 or 0. The
resultant modulated signal may be regarded as the sum of two amplitude
modulated signals of different carrier frequency.

GROUP - the standards that govern the way fax devices communicate with each
other across the phone line. Group 3 is defined in ITU-T Recommendations
T.4 and T.30, and is the set of capabilities implemented in the vast
majority of present fax devices that operate on analog telephone lines
(although some recent additions will also permit Group 3 protocols to be
used over digital circuits such as ISDN).

GSTN - The (G)eneral (S)witched (T)elephone (N)etwork, also referred to as
the (P)ublic (S)witched (T)elephone (N)etwork. The PSTN is, by definition,
the analog telephone network that provides voice and data access around the
world. Digital signals can also be sent after being modulated and sent as
an analog signal. When received, the analog signals are then demodulated
back into digital signals. This modulation/demodulation by a modem is
necessary, as dial-up PSTN lines are not as suitable for digital
communications as for voice communications. In actuality, this analog
telephone network is now mostly digital, except for the lines from the end
offices to the telephone or modem. However, since the connections at both
ends are analog, the PSTN can be considered to be essentially analog in

"HAYES COMPATIBLE" - A term that implies that a modem implements the
[AT]tention command set first utilized on the Hayes Smartmodem 1200.
Standardized as EIA602, but varies between manufacturers.

    Basic "AT" Commands:

    ATA - force answer mode

    ATB - select Bell or CCITT modes (all but obsolete now)
          B or B0 = CCITT
          B1 = Bell

    ATD - dial - see your manual for details as this is a flexible and
          powerful command with lots of neat options.

    ATE - echo on/off - echo is the copying of a command from the
          terminal or computer back to it
          E1 = on
          E0 = off

    ATH - hook status, ie. line state
          H1 = off hook
          H0 = on-hook

    ATI - product code or identity code, some manufacturers put in
          options to provide more or less information about the modem
          being interrogated, for example:
          I or I0 = ID and version number
          I1 = checksum of EPROM
          I2 = OK or ERROR on validity of checksum

    ATL - speaker volume
           L0 = low volume
           L1 = medium volume
           L2 = high volume

    ATM - speaker function
           M or M0 = speaker off
           M1 = speaker on until carrier detected (normal)
           M2 = speaker on while modem on line

    ATO - originate - force originate mode or resume data state when on

    ATQ - result code output
          Q or Q0 - normal result codes sent to terminal
          Q1 - result codes not returned to terminal

    ATS - set or read S register - see below

    ATV - result code format output
          V or V0 - single digit result codes generally most suitable
          for a computer to interpret
          V1 - "English" result codes suitable for human interpretation

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