Tutorial sobre Correcion de Errores y Compresion de Datos

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From rcp.pe!test!guille Fri Oct  9 00:28:25 1992
To: rcp!rcp.pe!todos
From: guille@test.pe (Guillermo GARRO GALVEZ)
Subject: Tutorial sobre Correcion de Errores y Compresion de Datos.
Date: Tue, 01 Jan 80 03:18:16 EST
Cc: rcp!rcp.pe!todos

Hola a todos:

En el manual de los nuevos modems "GVC fax 14400 V.42 bis /MNP5" que
estamos "testeando" con Enzo encontre este articulo que me parece bueno
copiarlo y difundirlo... alguien se anima a traducirlo??....


Telephone lines are prone to noise, distortion, and other disruptions that
can play havoc with data transmissions between two modems.  These
dirsuptions can cause the receiving modem to receive garbled data or
sometimes no data at all.

To make sure your data communications with remote modems remain
error-free, even over the most error-prone communications lines, modems
suport either the MNP error-correction/data-compression protocols or
both MNP and V.42/V.42bis error-correction/data-compression protocols.

- MNP is a protocol developed by Microcom Inc. Modems supports MNP Classes
  2 through 5.  If the remote modem supports MNP Classes 2-5, you can use
  this protocol to achieve error-free data communications between the two

- V.42/V.42bis are international protocols recommended to the industry by
  CCITT, a European-based standards organization.  The CCITT V.42
  Recommendation ensures error-free data for point-to-point communications
  using LAP-M, as well as MNP.  The V.42bis Recommedation provides data
  compression to increase throughput.

By supporting these error detection/correction protocols, modems allows
you to perform 100% error-free data communications with the installed base
of modems that support MNP and/or CCITT V.42/V.42bis.  When you make a
data connection with a remote modem, the CONNECT response you receive will
indicate whether the connection is a normal one, an MNP connection, or a
V.42/V.42bis connection.

Moreover, MNP Class 5 and V.42bis enhance the throughput of data by
providing data compression, albeit using different methods.  Data
compression reduces the number of bytes and reduces the amount of time it
takes to transfer information.  By compacting the number of bytes the
modem passes over the communications line, the effective throughput is
increased by the ratio of the compression (2:1 for MNP Class 5, 4:1 for

This article describes how these protocols achieve the exchange of
error-free data and provide data compression. 


The Microcom Networking Protocol is an asynchronous error
detection/correction protocol designed to work in a variety of
environments.  The Modem incorporates MNP at the link layer of the ISO
(International Organization for Standardization) model.  The error
controller or Link Layer is defined as the layer that separates the modem
(physical layer) from de DTE.  Because a number of different types of
physical layers exist, there have been several Classes of MNP that have
been established.  These Classes are:

1.- Asynchronous half-duplex
2.- Asynchronous full-duplex
3.- Synchronous between modems, full-duplex
4.- Adaptive packet assembly
5.- Data compression

Error Correction:

MNP Classes 1-4 are error-correction protocols.  When a connection is
established with MNP Classes 1-4 protocols, the data being transmitted
between the two modems is formatted into packets and transmitted with a
CRC (checksum).  The remote modem calculates its own CRC while receiving
the packet and then compares that value to the received CRC.  If the two
values match, then the data was transmitted without error.  Otherwise, a
negative acknowledgement is sent back to the transmitting modem to signal
that the packet should be retransmitted.  In addition, MNP Classes 3 and 4
improve your modem's throughput by deleting the transmission of start and
stop bits as well as automatically adjust the block length of transmitted
data according to the line condition.

Data Compression:

Combined with the ability to perform error correction, modems also add
to your effective system throughput by providing data compresssion.
During MNP Class 5 operation, a special algorithm is used to allow the
modem to send data using fewer number of bits.  By using a memory-lookup
table, the modem learns how to adjust to the type of data being
transmitted, and then eliminates unnecessary bits of information before
transmission.  The receiving modem then reinserts the full amount of
information before sending the data to the DTE device connected to that modem.

Speed Conversion

Some Modems provide a DTE speed conversion feature.  This feature allows
your computer or terminal to operate at faster speed than the connection
between your modem and the remote modem.  For example, your computer can
send data to your modem at 38400 bps while the modems communicate at
1200, 2400, 4800 or 9600 bps.

To compensate for this difference in speed, your modem provides flow
control.  Flow control ensures that the Modem will properly buffer or pace
the flow of data between the modem and DTE.


Error Correction:

The CCITT V.42 Recommendation details the international standard for
point-to-point error-control communications.  The primary protocol is
referred to as Link Access Procedure for Modems (LAP-M).  The standard
also includes Annex A, an alternative protocol that is compatible with MNP
Class 2, 3, and 4.

Data Compression:

CCITT V.42bis provides data compression, which can significantly reduce
the time required to transmit and receive data.  This on-the fly hardware
compression technique utilizes the LEMPEL-ZIV compression algorithm.

If your modem negotiated a V.42 LAP-M connection with a remote modem that
supports V.42bis, the CCITT V.42 bis compression protocol can be enabled
to increase throughput.  This international data compression standard
provides compression capabilities of 4 to 1.

If a CCITT V.42 alternative protocol connection is negotiated and the
remote modem supports MNP Class 5 compression, MNP Class 5 can be enabled
to increase throughput.  This compression method provides a 2 to 1 compression.

Although compression can be enabled to increasee throughput, you may need
to disable this feature for some connections.  For example, MNP Class 5
connections may expand an already-compressed file, increasing (rather than
reducing) the time to transfer data.  Therefore, if you will be using MNP
to transfer a file already compacted in ARC format, turn off data compression.

You do not need to turn off compression when a V.42 bis connection is
established.  The V.42 bis protocol includes an anti-expansion algorithm
to elimnate undesirable expansion.

Compression and Throughput:

When using V.42 bis, the computer-to-modem speed should be four times
faster than the speed between the two modems.  For example, if the two
modems are communicating at 9600 bps, the computer-to-modem speed should
be 38400 bps.

How to Use Error Correction and Data Compression:

You must configure both the modem and communications software to use error
correction and data compression.

Configuring Your Modem:

To activate the modem's MNP Classes 2-4 / V.42 error-correction protocls
(with auto-fallback), issue the following commands:


To activate the modem's MNP 2-4 / V.42 error correction with MNP 5 / V.42
bis data compression (with auto-fallback), issue the following commads:


You can issue AT&W&W1 to store this configuration into non-volatile memory.

Configuring Your Communications Software:

To configure the communicaction software, refer to the manual that came
with the software.

To take advantage of increased throughput, set your computer or terminal
serial port speed to at least 38,400 bps.  You must also configure your
communications software to enable hardware flow control (RTS/CTS) to avoid
data loss.

                                             Guillermo GARRO GALVEZ