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    Bell Modem Standards

Bell 103, 202T, 202S, 201 B,  201 C, 208 A,  208 B
Tutorial &Technical information for your perusal

Bell Modem Standards (jump to CCITT Recommendations)

In the 1960s and 1970s, Bell Labs created carrier standards for use with Bell equipment and lines, to accommodate customers' fledgling networks. Today, a vast installed base of Bell Standard customers relies heavily on these modems. Bell Standards include the following:

Bell 103 - Asynchronous data transmission, full-duplex operation over 2-wire dialup or leased lines; 300-bps data rate. Ideal for the "low demand" user who exchanges files infrequently with another PC user or an on-line bulletin board. Comparable to ITU V.21. This modulation is well suited for bad phone lines – as the communication guys use to say "103 modems will work on barb wire".

Bell 201 B, Bell 201 C - 201 B: Synchronous data transmission, full-duplex operation over 4-wire leased lines and half-duplex operation over 2-wire leased lines; 2400-bps data rate. 201 C: synchronous data transmission, half-duplex operation over 2-wire dialup lines; 2400-bps data rate. Both are comparable to ITU V.26. Bell 201 B compatible modems are the first of the fast modems. Many businesses still use them in typical terminal-to-host, multidrop applications. Bell 201 C modems are for use with dialup lines and are basically a complement to the 201 B.

If you need to transmit synchronous data (for example, IBM' 3780/2780 applications) at 2400 bps, but you don't need the on-demand performance and cost of a leased line, then consider buying a Bell 201 C.

Bell 208 A, Bell 208 B - 208 A: Synchronous data transmission, full-duplex operation over 4-wire leased lines, half-duplex operation over 2-wire leased lines; 4800-bps data rate. 208 B: same as 208 A, but over 2-wire dialup lines. Comparable to ITU V.27. The first standards to enable higher-speed data transmission (4800 bps) over leased lines for multipoint networks. For example, these standards enable us connect IBM mainframes at a central office to terminals in branch office, with fewer modems and more A efficient in-house cable runs.

Bell 212 A - Synchronous/ asynchronous data transmission, full-duplex operation over 2-wire leased or dialup lines; 1200-bps data rate. This versatile standard provides for efficient full-duplex operation over 2-wire dialup lines. If you add a leased line, however, you can still use Bell 212A. Comparable to ITU V.22

Protocols for Error Correction and Data Compression in modems

Error correction and data compression ensure accurate, swift data transfers. The protocols are employed during the data exchange between two modems. The following protocols are the most widely used by today's modems:

MNP Levels 1-4 - Microcom Networking Protocol' (MNP ), developed by Microcom Systems, Inc., enables error-free async data transmission. Although MNP is proprietary, it became an industry standard in the '80s because users demanded it from manufacturers. Both modems in a connection must use the same MNP protocols.

MNP Level 5 - Incorporates the first four levels and also applies a data-compression algorithm. It "compresses" data 2-1. So you can double the amount of data you can send at the modem's top transmission speed. For example, modems that transmit data at 4800 bps would send, with MNP Level 5, an amount of data equal to an uncompressed 9600 bps.

V.42, V.42 bis -These "V Series" protocols are internationally recognized standards for error control and data compression. V.42 is the recommendation of the ITU for error control it contains two algorithms (LAPM, or Link Access Protocol, and MNP 1-4). When two V.42-compliant modems establish a connection, they use LAPM to control data errors and retransmit "bad" data blocks. If one modem supports V.42 and the other supports only MNP, then the two negotiate to use MNP protocol. In both cases, the error-control process is automatic and requires no special user actions or software programs. V.42 bis roughly corresponds to MNP Level 5. The difference is the amount of data compressed: V.42 can usually generate a 4:1 ratio of data compression, depending on the type of file transmitted


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