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From: Gord Hannah
To: All
Date: 2010-07-15 01:00:06
Subject: [1 of 12] Comm Primer

                          Fidonet COMM Echo Primer
                          Revision 1.3.6 12/1/2000

                | = Revised Entry        + = New Entry
                      (1)                     (2)


For newcomers to this, the FidoNet International echo COMM, there follows a
discussion of terms which will be encountered frequently in the messages
herein.  A firm grounding in these will add considerable to understanding
the messages in this echo.

 +========+                                               +========+
 |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.

DEFINITIONS:

56Kbps Modems [Pre-V.90] - Rockwell, USR, Lucent Technologies, and Motorola
marketed incompatible chipsets/modems that operated in a server/client
format at up to 56Kbps over standard telephone lines prior to the adoption
of ITU-T V.90. USR implemented a protocol dubbed X2, and the remainder
combined efforts to implement a protocol dubbed K56Flex (a combination of
Rockwell's K56Plus and Lucent's VFlex/2 protocols). The X2 and K56Flex
protocols do not interoperate.

ADSL (Asymmetric Digital Subscriber Line) -  a modem technology that
converts existing twisted-pair telephone lines into access paths for
multimedia and high speed data communications. ADSL transmits more than
6Mbps to a subscriber, and as much as 640 kbps more in both directions.

An ADSL circuit connects an ADSL modem on each end of a twisted-pair phone
line, creating three information channels; a high speed downstream channel,
a medium speed duplex channel, and a POTS (Plain Old Telephone Service)
channel. The POTS channel is split off from the digital modem by filters,
thus guaranteeing uninterrupted POTS, even if ADSL fails. The high speed
channel ranges from 1.5 to 6.1 Mbps, while duplex rates range from 16 to 640
kbps. Each channel can be sub-multiplexed to form multiple, lower rate
channels.

ARQ - (A)utomatic (R)epeat Re(Q)uest - a general term which describes
detection and retransmission of defective blocks of data. When appended to a
CONNECT string (eg. CONNECT 28800/ARQ) it indicates that the modems have
negotiated some manner of error control for the link.

ASCII - (A)merican (S)tandard (C)ode for (I)nformation (I)nterchange. A
standard for defining codes for information exchange between equipment
produced by different manufacturers.

ASYNCHRONOUS - Describes data transmission technique in which the length of
time between transmitted characters may vary. Because the time lapses
between transmitted characters may vary, a 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 serves this purpose.

ATM - An international ISDN high-speed, high-volume, packet-switching
transmission protocol standard. ATM uses short, uniform, 53-byte cells to
divide data into efficient, manageable packets for ultrafast switching
through a high-performance communications network. The 53-byte cells
contain 5-byte destination address headers and 48 data bytes. ATM is the
first packet-switched technology designed from the ground up to support
integrated voice, video, and data communication applications. It is
well-suited to high-speed WAN transmission bursts. ATM currently
accommodates transmission speeds from 64 Kbps to 622 Mbps. ATM may support
gigabit speeds in the future.

BANDWIDTH - The frequency range available for use by modems on an ordinary
two-wire dial-up telephone line. This corresponds to the frequency range
required to reproduce the human voice, or approximately 3500Hz
(200-3700hZ).

BAUD - Perhaps the most mis-used term in all of the discussions posted in
this forum. It actually refers to the unit of measure for the number of
discrete changes of state which occur in a communication channel per second
(ie. the number of times per second that carrier frequencies are
modulated). It is an old term from the days of Frequency Shift Keyed
modems. The name honors Jean Maurice Emile Baudot, who invented a bit
encoding scheme for characters (it is/was not the same as that presently
used for encoding ASCII characters however).

Relative to FSK modems, the use of Baud referred to the rate that you could
shift from one FSK Tone to another. The tones directly represented the ones
and zeros of data being transmitted. In the early days they were generally
referred to as the Mark Frequency and the Space Frequency. Accordingly,
with this direct correlation of tones to 1s and 0s, the Baud Rate was the
same as the Bit Rate. [Note: The FSK transmission schemes referenced above
are to bi-frequency implementations such as V.21 and the Bell 103 protocol.
Multi-frequency FSK schemes also exist, but they have not been widely
implemented over the PSTN].

As more complex ways of transmission were devised it was natural to try to
extrapolate this concise definition to define their operation. An early
extrapolation was to Phase Shift Keyed (PSK) modems such as the V.26 Series
of modems. This was unfortunate, but it did actually occur. The
extrapolation went like this:  The PSK modem  generated a signal with 4
possible phase states and thus 4 possible phase changes. The states were 0,
90, 180 and 270 degrees of the carrier. The possible changes were the same.


--- MPost/2 v2.0a
 * Origin: Marsh BBS (c) Dawson Creek BC Canada (1:17/23)
SEEN-BY: 3/0 633/267 640/954 712/0 313 550 620 848
@PATH: 17/23 140/1 261/38 712/848 633/267


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