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Chapter 53
Portable Network Access

Gilbert Held

The steady increase in portable computer use has had a positive effect on productivity. It has also, however, challenged users to become educated on the vast array of networks and access methods that can facilitate the sharing of information. This chapter introduces users to several important modem terms, as well as a step-by-step guide to accessing today’s popular packet-switching networks.

MODEM TERMS

Until the incorporation of microprocessors into modems, the selection and operation of a modem was relatively easy. At that time, the major concern was the type of modulation performed by the modem expressed in terms of Bell System or TSS standards compatibility.

Until the divestiture of AT&T and its operating companies, Bell System equipment was a de facto standard in the US, and third-party modem manufacturers built most of their products to ensure compatibility with Bell System modems. Popular Bell System-compatible modems used on the PSTN included the 103, which operates at 300 b/s, and the 212A, which operates at 1,200 b/s and can also operate as a 300-b/s Bell System 103-compatible modem.

The TSS, which is a standards-recommending body, is part of the ITU, a United Nations organization headquartered in Geneva. Until the divestiture of AT&T, TSS modem standards were primarily followed in Europe and their modulation methods were incompatible with Bell System modems. With the AT&T, several TSS modem standards have been adopted worldwide, including the V.22bis and V.32 modem recommendations. V.22bis governs the operation of a 2,400-b/s, FDX modem for use on the PSTN; the V.32 recommendation governs the operation of a 9,600-b/s, FDX modem for use on the PSTN.

The incorporation of microprocessor technology into modems resulted in their use for a range of functions added to the basic data modulation function of modems. Among features added to modems are error detection and correction, data compression, and command set recognition.

Error Detection and Correction

Among the first modems to offer an error detection and correction feature were products manufactured by Microcom, Inc. This company created a revolution in modem technology by developing a modem protocol known as MNP.

The MNP Protocol. MNP is a communications protocol built into MNP-compatible modems that supports interactive and file transfer applications. In developing MNP, Microcom recognized that the first implementation of the protocol would not necessarily be the last, and structured it to accommodate changes in its implementation. To accomplish this, the major functions of the protocol are divided into classes. When an MNP modem communicates with another MNP modem, the two devices negotiate with each other to operate at the highest mutually supported class of MNP service.

Exhibit 1 summarizes the features associated with available classes. Until 1990, Microcom only licensed MNP through class 5to other modem manufacturers. In that year, they began to offer a full MNP license. Therefore, an MNP-compatible modem, though compatible with all other MNP modems, may be compatible only with a subset of available MNP classes unless a third-party vendor obtained a full license and incorporated all MNP classes into its product.

Exhibit 1 includes such terms as V.29 and V.32. Both reference TSS modulation standards, with V.29 originally developed as a half-duplex 9,600-b/s modulation technique for use on leased lines. Microcom, as well as other vendors, modified that technology to work on the PSTN. In addition, using the intelligence of a microprocessor to monitor the direction of transmission, it became possible to quickly turn off the transmitter of one modem and turn on its receiver, enabling the half-duplex transmission. In comparison, a V.32 modem uses echo cancellation technology to enable transmission and reception of data to occur simultaneously on the PSTN and also provides an inherent F-DX transmission capability.

V.42 Recommendation. Although MNP error detection and correction is included in more than 1 million modems manufactured by Microcom and more than 100 third-party vendors, it is not the primary method of error detection and correction recommended for use in modems by the TSS. In 1990, the TSS promulgated its V.42. Unlike other TSS V-series recommendations that govern modern modulation techniques, the V.42Recommendation defines a protocol in which modems block data for transmission and generate and add a CRC to each block for error detection. Under the V.42 Recommendation, the flow of data blocks occurs according to a LAP, which differs from data flow under the MNP protocol. In recognition of the large installed base of MNP modems, however, MNP error detection and correction is supported as a secondary standard (i.e., a V.42-compatible modem will first attempt to communicate in its error-free mode using LAP). If the distant modem does not support the V.42 protocol, the V.42 modem will next attempt to communicate using MNP error control.

Exhibit 1. MNP Classes
Class Description of Functions Performed

Class 1 Asynchronous, byte-oriented, half-duplex transmission that provides an efficiency of approximately 70%. A 2,400-b/s modem using MNP class 1 obtains a throughput of 1,690 b/s.
Class 2 Asynchronous byte-oriented full-duplex data transmission that provides an efficiency of approximately 84%. A 2,400-b/s modem using MNP class 2 obtains a throughput of approximately 2,000 b/s.
Class 3 Asynchronous start and stop bits are stripped, enabling synchronous, bit-oriented, full-duplex transmission between modems. This provides an efficiency of approximately 108%, enabling a 2,400-b/s modem to obtain a throughput of approximately 2,600 b/s.
Class 4 This class adds an adaptive packet assembly to previous classes, in which packet sizes are dynamically adjusted based on the number of retransmission requests. Data phase optimization, which provides a mechanism to reduce protocol overhead, is also included. The efficiency of class 4 is approximately 120%, enabling a 2,400-b/s modem to obtain a throughput of 2,900 b/s.
Class 5 This class adds data compression to class 4 service, which provides an average compression ratio of 1.6:1, meaning that every 16 characters are compressed into 10 characters for transmission. This increases the protocol efficiency of class 5 to approximately 200%, enabling a 2,400-b/s modem to obtain a throughput of about 4,800 b/s.
Class 6 This class adds universal link negotiation and statistical duplexing to class 5. Universal link negotiation enables MNP modems to begin operation at a common low-speed modulation method and negotiate the use of an alternative higher-speed modulation method. At the end of a successful link negotiation for class 6 operation, the pair of modems operate at 9,600 b/s using V.29 technology. Statistical duplexing results in the monitoring of user traffic patterns to enable the dynamic allocation of V.29 half-duplex transmission to resemble full-duplex transmission. Under class 6, 9,600-b/s operations MNP provide an average throughput approaching 19,200 b/s.
Class 7 This class adds an enhanced data compression capability to P, based on a Huffman statistical encoding technique. Under class 7, a compression ratio of between 2.0 and 3.0 is achievable and increases throughput from 2 to 3 times the modems operating rate.
Class 8 This class is no longer marketed.
Class 9 This class adds support of V.32 modulation to class 7, providing a throughput of three times the 9600-b/s full-duplex operating rate of a V.32 modem.


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