V.32 ---- V.32 is also a data transmission standard at 9600 bps, but V.32 defines a full-d

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V.32 ---- V.32 is also a data transmission standard at 9600 bps, but V.32 defines a full-duplex (two-way) modulation technique. It is a full modem standard, and also includes forward error correcting and negotiation standards as well. Many modem manufacturers already have or will be introducing V.32-compatible modems. This is generally considered "the" standard for high-speed modems today. V.32 is expensive to implement, since the technology required for it is complex. As this standard becomes more common and manufacturing techniques are refined, the pricing for V.32 modems should go steadily downward. At this writing, V.32 capable modems are selling for between $500 and $1000 each. Some manufacturers have created modems that can use both their own proprietary high speed standard and the V.32 standard, for compatibility with their older non-V.32 modems. The new Hayes Ultra and U. S. Robotics HST Dual Standard are examples of the new "dual personality" modems that are now on the market. V.32bis ------- This is a developing high speed standard. When fully defined (likely by early 1991), V.32bis will operate at 14,400 bps and, like V.32, will be a full-duplex method. The CCITT has not yet defined this standard, so no modems currently use it (although some new modems have implemented what is expected to be the standard and may claim V.32bis compatibility). Error Correcting and Data Compression ------------------------------------- The CCITT also has adopted formal standards for the higher layers of Error Correction and Data compression (See Part 1 of this series for a full description of these layers). In order for any error correction or data compression protocol to work, modems on BOTH ends of the connection must support it. Once two modems are connected, they automatically negotiate between themselves to determine the best mutual protocols they both support. V.42 ---- V.42 is a CCITT error-correction standard that's similar to MNP Class 4 (See "What is MNP" below). In fact, because the V.42 standard includes MNP compatibility through Class 4, all MNP 4- compatible modems can establish error-controlled connections with V.42 modems. This standard, however, prefers to use its own better performing protocol -- LAPM (Link Access Procedure for Modems). LAPM, like MNP, copes with phone line impairments by automatically re-transmitting data that is corrupted during transmission assuring that only error free data passes through the modems. Many modem manufacturers make MNP Class 4-compatible modems, and some offer V.42-compatible modems as well. V.42bis ------- V.42bis is a CCITT data compression standard similar to MNP Class 5, but providing about 35% better compression. Of course, this also means it provides better throughput. V.42bis only compresses data that needs compression. Each block of data is analyzed, and if it can benefit from compression, compression is enabled. Files on bulletin board systems are often compressed already (using ARC, PKZIP, and similar programs). While MNP Class 5 can actually decrease throughput on this type of data, V.42bis will not -- compression is only added when a benefit will be realized. To negotiate a standard connection using V.42bis, V.42 must also be present. Thus, a modem with V.42bis data compression is assumed to include V.42 error correction. Some modem manufacturers already make V.42bis compatible modems, and more are on the way. V.42bis is NOT compatible with MNP Class 5. A V.42bis modem will establish an error-free connection with MNP-capable modems (since V.42bis includes V.42), but only up to MNP Class 4. What is MNP? ------------ MNP stands for "Microcom Networking Protocol" and was created by Microcom, Inc., a modem manufacturer. MNP offers end-to-end error correction, meaning that the modems are capable of detecting transmission errors and requesting re-transmission of corrupted data. Some levels of MNP also provide data compression. As MNP evolved over time, different classes of the standard were defined, describing the extent that a given MNP implementation supports the protocol. Most current implementations support Classes 1 through 5. There are higher classes, but are usually unique to modems manufactured by Microcom, Inc. since they are still proprietary. MNP is generally used for its error correction capabilities, but MNP Classes 4 and 5 also provide performance increases, with Class 5 offering real-time data compression. The lower classes of MNP are not usually important to you as a modem user, but they are included here for completeness.


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