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From: Gord Hannah
To: All
Date: 2010-06-01 01:00:06
Subject: [9 of 12] Comm Primer

two-stage process for detection and negotiation of LAP-M error control.
V.42 also incorporates support for MNP levels 1-4.

V.42bis - a complementary extension of V.42 which defines a specific data
compression scheme for use in conjunction with error control. [BTLZ -
(British Telecom Lempel-Ziv) the compression technology used in V.42bis].

+ V.44  - ITU-T data compression scheme for use in conjunction with error
+ control (uses LZJH compression algorithm).

| V.90 - ITU-T recommendation for asymmetric data signalling rates of up
| tp 56Kbps in the direction of a digitally connected server to a capable
| client, and up to 33.6Kbps in the direction of the client to the server.

+ V.92 - ITU-T recommendation for asymmetric data signalling rates of up
+ to 56Kbps in the direction of a digitally connected server to a capable
+ client, and up to 48Kbps in the direction of the client to the server.

V.110 - Support of data terminal equipments with V-Series type interfaces
by an integrated services digital network

V.120 - Support by an ISDN of data terminal equipment with V-Series type
interfaces with provision for statistical multiplexing

V.FC - V."Fast Class" is the name given to a proprietary modem technology
jointly developed by Rockwell and Hayes that is based on the basic features
of the proposed V.34 recommendation during the March-April 1993 time frame.
While under development by ITU-T, V.34 was given a number of "working"
names, the most common of which was "V.FAST". V.FC is not an ITU-T
recommendation, despite the "V." prefix. V.FC is incompatible with V.34,
but many modem vendors may offer products which support both protocols.

+ V.PCM - the "working" name of the V.90 protocol while it was under
+ development by ITU-T SG16.

V.Mach - A proposed 43.2Kbps duplex modulation scheme, designed to operate
over POTS.

X.25 - An ITU-T narrow-band (under 56Kbps) packet switching standard. A
forerunner to frame relay and ATM.

XMODEM - Xmodem is a file transfer protocol.  It was the first file
transfer protocol to come into widespread use in the microcomputer world.
It was developed by Ward Christensen in the mid-1970's to transfer files
between machines running the CP/M operating system. Until the late 1980's,
because of its simplicity and public domain status, Xmodem remained the
most widely used microcomputer file transfer protocol.

The file to be transmitted is divided into 128-byte blocks.  Each block is
sent in sequence, together with a special starting character, an
identifying number, and a checksum.  The checksum is a number created by
adding all of the bytes in the block together.

1K-Xmodem is an extension to Xmodem which increases the block size from 128
to 1,024 bytes, thereby increasing transmission speeds.

YMODEM - As a file transfer protocol, the Ymodem protocol was first
released as part of Chuck Forsberg's program YAM  ("Yet Another Modem").
Ymodem is actually an extension to Xmodem, to overcome some of the
limitations of Xmodem.

Ymodem allows information pertaining to the transmitted file to be sent
along with the data, such as filename and length.  Ymodem also increases
the size of the Xmodem block from 128 to 1,024 bytes and adds the
capability to transmit "batches" of files.  In fact, Ymodem is sometimes
referred to as Xmodem-1K, and vice versa.

Ymodem-G is a streaming protocol built around the philosophy that no news
is good news.  It sends the entire file before waiting for an
acknowledgment. If the receiving end detects an error in mid-stream, it
aborts the transfer.  YmodemG is not an error correcting protocol.

ZMODEM - As a file transfer protocol, the Zmodem protocol was commissioned
by Telenet and placed in the public domain.  Like Ymodem, it was also
designed by Chuck Forsberg and was developed as an extension to Xmodem, to
overcome some of the limitations of Xmodem.

Zmodem implements many of the features of the Kermit protocol, as well as
the capability to "step down" to the Xmodem or Ymodem protocols on demand.

Among the key features of Zmodem are a 32-bit CRC offering a degree of
error detection 100,000 times greater than Xmodem CRC, a server facility,
batch transfers, and fast error recovery.

Zmodem was also specifically engineered to avoid sending certain sequences,
such as ESCape-carriage return-ESCape, that the Telenet network uses to
control the connection.

DATA TRANSFER:

    Microsoft/IBM/Digital Research-DOS is a single threaded, non-
re-entrant, non-preemptive, interrupt driven system. Relative to serial
communications, when a byte of data is accumulated in the UART, it signals
the CPU via an IRQ to process the byte. IRQs are hierarchical in nature
With the most essential system functions being assigned the highest
priority IRQs. DOS does not place a particularly high priority on the
serial port IRQs, and data transfer errors can occur with high data flow
rates, or when interrupt servicing is retarded.

   The transmission of each byte (or character) requires that 10 actual
bits be moved; a start bit, the eight data bits, and a stop bit. The
theoretical maximum throughput in CPS for a given DCE is thus derived by
the formula DCE/10=CPS. Error correction and data compression complicate
this somewhat. Error-correcting modems do not transmit the framing bits,
and are thus able to deliver 25% more data throughput versus modems which
do not implement error correction. For an error-correcting link the
equation becomes {DCE/10} + {DCE/10}x.25=CPS (note that the numbers derived
by these equations represent maximums, and that insertion of protocol

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