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: firstname.lastname@example.org (Guillermo GARRO GALVEZ) Subject: Tutorial sobre Correcion de Errores y Compresion de Datos. Message-Id: 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??.... Saludos, Guillermo =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- ERROR CORRECTION AND DATA COMPRESSION ------------------------------------- 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 modems. - 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 V.42bis). This article describes how these protocols achieve the exchange of error-free data and provide data compression. UNDERSTANDING THE MNP PROTOCOL 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. UNDERSTANDING THE CCITT V.42/V.42bis PROTOCOL 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: AT\N6\Q3\J\V1 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: AT\N6\Q3\J\V1%C1 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 email@example.com