@MSGID: 1:17/23.1 004d2063
@SPLIT: 06 Jan 03 11:38:25 @17/23 18631 01/12 +++++++++++
* Copied (from: COMM) by Gord Hannah using timEd/2 1.10.y2k+.
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
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