                       Communications Terms


                             Figure 1
 +========+                                               +========+
 |Computer|   DTE-              DCE-               DTE-   |Computer|
 |   A    |   Rate   +--A--+    Rate     +--B--+   Rate   |   B    |
 |        |~~~~~~~~~~|Modem|~~~~~~~~~~~~~|Modem|~~~~~~~~~~|        |
 +========+          +=====+             +=====+          +========+


Pictured above is a brief sketch of a complete signal circuit,
consisting of two computers (A & B) interconnected thru their Modems.

                          Five Definitions
                (as they relate to the above Figure 1)

BAUD RATE: The actual rate of signal transitions per second.  Since each
signal may represent more than one bit, it is not the same as BPS.

BPS or Bits Per Second: The number of data bits per second transmitted
between two modems.  Sometimes incorrectly called the baud rate.  (See
Baud Rate above)

DCE (Data Communication Equipment) rate: The transmission speed of the
interface between two modems.  The DCE rate and BPS are synonymous.

DTE (Data Terminal Equipment) rate: The transmission speed of the
interface between a computer/terminal and it's modem.  Includes the
RS-232 Serial ports.  Baud rate is frequently cited in this interface
(*as is bps*), all of which is depicted on Fig.  1. Note DTE/baud exists
on both ends, while DCE/bps is between the modems.  DTE is often called
the Port area (and Port Speed) since this is where the Serial Ports are.

THROUGHPUT: The DCE rate expressed in cps (Characters/second = bytes/sec).

   In a modern 9600 bps modem (often incorrectly called a 9600 baud
modem) the signal rate is actually 2400 baud.  It is encoded with four
data bits per baud, resulting in a DCE rate of 9600 bps.  The 14,400
bps modem is much the same, but encoded with 6 data bits per baud.  In
a 2400bps modem (often incorrectly called a 2400baud modem) the signal
rate is actually 600 baud.  It is encoded with four data bits per
baud, resulting in a DCE rate of 2400bps.  The 1200bps modem is much
the same, but encoded with only 2 data bits per baud.

Since the normal BBS communication schemes employ 8 bits (for each Byte,
or character) + 1 start bit, + 1 stop bit = 10 bits/character, it
follows that the DCE rate is 10 times the char. rate.  Therefore a DCE
rate of 2400bps results in a Thruput of ~240cps.  As this represents a
100% efficiency, this figure is not attainable (without additional
techniques), but can be very closely approached using a modern protocol
like Zmodem MobyTurbo (tm), which can achieve a thruput of 235 to 238cps
on ZIP'd files.  High speed modems usually utilize error correction to
exceed the 100% efficiency rate (~1100 cps at 9600 & ~1650 at 14400).

                        MODEM PROTOCOLS
                        ~~~~~~~~~~~~~~~
             A variety of ITU-TSS V. standards as well
            as Microcom MNP standards apply to modems

    MODULATION PROTOCOLS
    ~~~~~~~~~~~~~~~~~~~~
|   V.17         Emerging ITU-TSS standard for 14400/second FAX.
                 V.17 also supports a more robust 9600 bits/second than V.29
    V.21         Transmission at up to   300 bits/second   (300 baud)
    V.22         Transmission at 1,200 bits/second         (600 baud)
    V.22bis      Transmission at 2,400 bits/second         (600 baud)
|   V.23         1200 bps ITU-TSS operation with 75 bps back channel (Europe)
|   V.27ter      ITU-TSS standard for 2400 & 4800 bit/second (Group III) Fax
|   V.29         ITU-TSS standard for 9600 bit/second (Group III) FAX
    V.32         Transmission at 4800 & 9,600 bits/second (2400 baud)
    V.32bis      Transmission at 7200 & 12000 & 14400 bps (2400 baud)


    Note that in the United States of America, Bell 103 and Bell 212a
are the protocols that are widely used for 300 and 1200 bps
respectively. In Europe, and the rest of the world, V.21 and V.22 are
the protocols used for these speeds.

    ERROR-CONTROL PROTOCOLS
    ~~~~~~~~~~~~~~~~~~~~~~~
    MNP 2-4      Detects and corrects transmission errors
                 MNP standards do not include full V.42 compliance.
                 Each MNP level adds additional features.  The top
                 level, Version 4, strips start and stop bits and
                 optimizes data packets for line conditions.

    V.42         Handshaking procedure that determines what levels of
                 error correction are common between two modems.
                 LAP-M, a more sophisticated but similar protocol to
                 MNP Level 4, is defined as the primary error
                 correction protocol in V.42.  MNP Level 4, providing
                 backwards compatibility with all MNP levels except 1,
                 is recommended as an Annex error correction protocol.
                 Therefore, all V.42 modems should be compatible with
                 all MNP 2-4 modems as well.

    MNP 10       A set of Adverse Channel Enhancements that helps
                 modems work better with poor connections,
                 compensating for line noise, echo problems, and
                 limited bandwidth. Modems will make multiple attempts
                 to set up a transmission link, optimize the size of
                 data packets for a connection, and adjust to the
                 highest rate possible. The protocol was originally
                 developed to improve connections with cellular modem
                 connections, but it may improve land line connections
                 as well.

    DATA-COMPRESSION PROTOCOLS
    ~~~~~~~~~~~~~~~~~~~~~~~~~~
    MNP 5        Provides compression as great as 2 to 1 on some files
                 Not related to V.42bis, but they often appear
                 in the same modems.  Slows down compressed file xfers.

    V.42bis      Provides compression as great as 4 to 1 on some files.
                 Doesn't itself include MNP 5 compatibility, and will
                 not slow down transfer of compressed files.

|   T.4          Specifications for encoding FAX data according to one
                 of several possible compression algorithms

|   T.30         A standard for controlling the handshake of two Group
                 III FAX devices.  It sets negotiations between devices
                 for speed and data encoding, and sends a 1100 hertz
                 communications tone between devices.

    NOTE: Each V.nnbis complements, but is separate from, its V.nn
counterpart.  Theo Irmer, director of the ITU-TSS wrote:
"I would like you to note further, that the use of "bis", "ter" etc.
in the numbering of Recommendations (e.g. V.42bis) does not necesarily
mean a simple expansion of the Recommendation having the original
number, (e.g. V.42). In other words, V.42 and V.42bis should be
considered as independent, separate Recommendations."

A discussion of proprietary modulation protocols--Compucom CSP, Hayes
Express, Microcom MAX, Telebit PEP, & USR HST--is beyond the scope of
this primer, except to emphasize that ALL proprietary's will connect
at Hi Speed with ONLY THEIR OWN; ie HST to HST, PEP to PEP, etc. At
present, this includes v.Fast, which is not a standard of any kind,
and any current claims to support this not yet standard will likely
require firmware and/or hardware upgrades when it is approved.

UARTs: Universal Asynchronous Receiver/Transmitters are required
elements in the signal circuit to convert the parallel transmission to
serial (in the serial port--or COM port).  For systems using external
modems, the terminal UART is employed.  Internal modems, however, make
use of their own internal UART.  Most commonly supplied in these cases
is the 8250 or (in AT's) the 16450.  For the majority of users, these
chips will suffice for 9600+ bps DCE rates.  Some systems, however,
that are experiencing interrupt latency (IL) are liable to experience
data overruns and lost bits when using the stock UARTs.  If you fit
into that category, you will likely require an NS16550AFN (now known
also as the PC16550CN), a direct plug-in replacement for the above
named UARTs.  The 16550 has onboard 16 byte FIFO buffers (transmit &
receive) to retain the otherwise lost bits, during IL occurrences. The
factors that increase the IL risk are: slow hardware, fast modems,
multitasking, and other CPU intensive software. Because you fit the IL
profile does NOT mean you require a 16550, just a higher risk.  If you
ARE losing bits during xfers, however, you should definitely consider
the 16550.  Sometimes this means switching to a separate plug-in
serial card (and external modem) in order to incorporate the new chip.

