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INTRODUCTION TO V.42 As modem technology continued to make ever increasing advancements in data transfer speeds, interactive communications applications and PC-to-host asynchronous links, the requirement for modem-to-modem error-control came to the forefront as an issue needing a standardized procedure. Hayes announced the implementation of V.42 in V-series system products in November 1988. By offering V.42 compliant products the company underscored its support for international standards and helped establish this new standard by making it available in the market. Study Group XVII of the International Telegraph and Telephone Consultative Committee (CCITT) began work on an error-control recommendation in 1984. The result of the group's efforts is CCITT Recommendation V.42, Error-Correcting Procedures for DCEs Using Asynchronous-to-Synchronous Conversion. The primary V.42 protocol, Link Access Procedure for Modems (LAPM), is based on High-level Data Link Control (HDLC) procedures specified by the International Standards Organization (ISO). Basing the new V.42 standard on such a widely tested, accepted and utilized technique will provide great benefits for LAPM implementation. This new standard is important to the present, but it is equally important for the future. V.42 provides an international error-control standard for point-to-point communications which also provides the foundation for developing advanced modem capabilities. Also provided is a means by which manufacturers can provide proprietary enhancements without interfering with the future development and evolution of the functions covered by the standard. BACKGROUND CCITT is a United Nations agency whose voting members are countries, i.e. each country has only one vote which is cast by a designated representative. Most countries select CCITT representatives from their Postal, Telephone and Telegraph Administration (PTT), but the United States, lacking such a body, is represented by the U.S. CCITT National Committee, a Department of State body. The U.S. committee is comprised of five study groups, of which one, Study Group D, is in charge of positions related to modems and provides input to CCITT Study Group XVII. The International Standards Organization is made up of the national standards-making bodies of each country, ANSI in the case of the U.S. ISO, in cooperation with the International Electrotechnical Commission (IEC), develops information processing standards. Of particular interest to modem manufacturers is the ISO's activities concerning the bottom four layers of the Open Systems Interconnection (OSI) reference model. CCITT, ISO and a variety of governmental and data communications industry experts began work in 1984 to sort out the variety of concerns, issues, features and related matters that needed to be addressed by a point-to-point modem error-control protocol. Following more than three years of analysis and discussion, CCITT Study Group XVII met in Geneva, Switzerland in April 1988 to finalize Recommendation V.42 for error-control in asynchronous modems. The final recommendation specifies LAPM as the primary error-control protocol and includes an alternative protocol in Annex A for backward compatibility with MNP class 2-4 modems. The CCITT specifies an alternate procedure to recommendations in either an annex or an appendix. Procedures in annexes are mandatory for full compliance with the recommendation while implementation of procedures defined in appendices is optional. Recommendation V.42 states that "Compliance with this Recommendation requires implementation of both protocols. However, unless otherwise specified by user options, two V.42 DCEs will commicate using LAPM." As proposed in January 1988, all future enhancements for error-control in Recommendation V.42 will be directed toward LAPM and not the annex protocol. At the request of several European PTTs, the Annex A protocol was made mandatory for full compliance, but this could be dropped in four years. Study Group XVII completed its work in the spring of 1988 and passed the V.42 recommendation to the full CCITT for approval at its Plenary Meeting in Melbourne, Australia in November 1988. V.42 - ITS FUNCTION AND THE NEEDS IT FULFILLS V.42 provides the process by which data communications equipment (DCE) handles error-control during an exchange of data. That is, V.42 specifies an error-control protocol for V.22, V.22bis, V.26ter and V.32 modems to implement. These modems are full-duplex, two wire, dial-up products used on the Public Switched Telephone Network (PSTN) that use asynchronous-to-synchronous conversion. When two V-series system products implement LAPM to exchange data, the receiving modem uses a Frame Check Sequence (FCS) to verify the accuracy of the data it has received in a data frame. Based on the FCS, the receiving modem acknowledges receiving accurate data or tells the sending modem to retransmit the data frame if FCS indicates an error has occurred. The protocol defines the link establishment process, error-control procedures and negotiation parameters for establishing, maintaining and conducting data transfer. The V.42 standard does not explicitly apply to half-duplex ping pong modems. Hayes has enhanced the V-series Smartmodem 9600 to use LAPM over the high-speed link between two of these modems and allows data compression to operate in this mode. LAPM features include: % Benign detection phase - V.42 modems must have the capability to detect the presence or non-presence of another V.42 modem at the other end of a connection. This detection phase must not interfere with a V.42 modem's capability to establish a connection with a non-V.42 modem. This detection phase may be enabled or disabled with no impact on the V.42 modem's performance. % Extension of LAPB and LAPD - LAPM is an extension of LAPB and LAPD. It uses basically the same connection establishment and termination procedures, as well as similar data transfer procedures. Implementors familiar with either of these protocols should have no difficulty with a LAPM implementation. % Poll/Final bit procedure - The P/F bit procedures allow one modem to force the other to transmit a response. This LAPM feature improves error recovery capabilities by bypassing timer expiration recovery mechanisms.* % Separate primitives for ACK, NAK and BUSY - LAPM provides seperate frame types for these functions which improves protocol reliability and eliminates the chance of lockups due to misinterpretation of frame contents.* V.42 includes an enhanced Reject capability to improve error recovery performance in the presence of line noise. % 8-bit address field default - The address field allows for differentiation of commands and responses, and in the future will also allow for multiple simultaneous virtual data paths between the modems for remote configuration, network management or user data such as multiplexing multiple terminals or devices.* % Address extension bit used - The address extension bit may be used to provide for multi-octet addresses. % Modulo-128 I-frame sequence numbers - The large numbering base for information frames permits a larger "window size" (number of outstanding frames) than would be permitted under modulo-8 sequence numbering. This improves performance on connections with long propagation delays, such as satellite links. % XID frame exchange for negotiation - LAPM uses the internationally standardized procedures for negotiation defined by ISO and CCITT. Using this standard mechanism, V.42 modems can negotiate standard parameters as well as manufacturer-specific enhancements. % Private parameter negotiation - Enhancements provided by individual modem manufacturers may be negotiated through a mechanism defined in V.42. % Parameter renegotiation - V.42 permits the renegotiation of link parameters between the stations any time during the connection. This may be useful if line or user data flow conditions change and the modem determines that different data link parameters would improve performance. % UI frame exchange for break signalling - Unnumbered Information (UI) frames are used for break signalling out-of-band with the user. V.42 supports three types of breaks: in-sequence, expedited and destructive. % Break length preserved (10 msec to 2.54 seconds) - In some environments the length of the break sent is important. LAPM preserves the break length up to a maximum of 2540 milliseconds in 10 millisecond increments.* The rich functionality of V.42's LAPM satisfies a variety of needs for personal computer modem data communications. First among the needs met is the establishment of the point-to-point error-control international standard. Much had been said in recent years about how best to meet the error-control needs for PC communications, but the rhetoric confused potential users and delayed the progress needed for continuing the evolution of data communications. LAPM provides an expandable standard for error-control. This enables manufacturers to provide products based on an internationally recognized standard and provides users with both a reliable protocol and peace of mind because it is an approved international standard. Users' comfort level needs are especially addressed by V.42 since the previous environment featured multiple, incompatible techniques. Knowing that future products implementing V.42's LAPM protocol will interact with other V.42 modems will be of great benefit to users. Planning for future point-to-point communications systems can proceed with the knowledge that error-control concerns have been fully and adequately addressed by LAPM. Another future benefit provided by V.42 is a well defined platform for advanced functions. These advanced functions will be applicable to LAPM only and not the alternative protocol in Annex A of V.42. Enhancements to V.42 will furnish the next-generation communications features required by the high-speed, sophisticated PCs of the future. Future V.42 plans include addressing issues as they relate to LAPM such as: % Data compression - Improving data throughput by means of data compression is one of the most widely discussed error-control topics in the data communications industry. A standardized technique is likely to be approved through accelerated procedures early in the next CCITT study period. Contributions relating to existing techniques have already been made to the CCITT and others are expected in the future. The resulting data compression standard will likely be based on the best attributes of the existing methods. % Network management and remote configuration - In large networks there is a great need to receive status reporting and diagnostic information from widely dispersed, often unmanned equipment. Additionally, the capability to set parameters and run tests remotely is desirable. Error-control modems are currently among the different types of equipment being studied in the area of network management by ISO and CCITT. % Data encryption - Currently, this topic is still being approached at the feasibility level. Issues which need resolution include implementation at the data link layer versus higher layers such as presentation layer and key management. Significant progress on this feature will most likely come in the 1990s provided other regulatory issues such as export controls can be overcome. % Forward error correction (cellular radio) - Cellular radio applications present monumental challenges to modem designers. Not only do drop-outs occur during cell transitions, but even normal traffic (i.e. a large truck driving by) can interfere with the signal and produce significant fading and other impairments. Error rates can be very high, causing any normal error-control protocol to break down and not be able to transfer even a single frame (the human ear masks the resulting noise, but a modem cannot). Forward error correction, such as used in compact discs, could be applied to V.42 modems. % Transport of interface state information - In addition to prerving user data, it is sometimes desirable to have end-to-end carriage of interface state information. For example, this may occur if the remote device is a printer with a paper-out signal that needs to be received by the host. V.120 has this capability today and a similar scheme could be added to V.42. % Statistical multiplexing (multi-port) - As mentioned in the address field section of LAPM features, the capability exists in V.42 for multiple simultaneous virtual circuits between the modems. High speed modems are currently used to connect multiple terminals or remote terminals. This capability would also be desirable in an error-control modem. % ISDN compatibility (terminal adapter interworking) - The similarity between the V.42 LAPM protocol and the LAPD-like protocol used in the V.120 terminal adaption standard will permit the development of rules for interworking between these devices. This allows devices on the ISDN to easily interwork with devices on the PSTN without significant protocol conversion resources. % Asymmetrical and half-duplex operation - Many existing error-control modems, such as Hayes V-series Smartmodem 9600, use half-duplex ping-pong or asymmetrical transmission techniques to achieve high throughput at reduced cost. Most of these modems use proprietary techniques (since there was no standard) resulting in one manufacturer's modems not being able to communicate with another manufacturer's at 9600 bps. Work is in progress in the CCITT to develop standard techniques for such lower-cost, high-speed solutions, and error-control using V.42 will be an essential part. Changes in the timers and acknowledgment rules may be necessary. % Modem rate negotiation (multi-speed modems) - Although significant degradation of circuit quality during a single call is quite rare, there may be some benefit gained by the ability for modems, based on error rates or other objective factors, to request a change to alternative (slower) modulation methods with improved performance (and to switch back if conditions improve). % Character format indication and negotiation - Some confusion currently exists in error-control connections due to the fact that the character format (parity, stop bits) is independently set on each DTE-modem interface, with an 8-bit format used between the modems. Rules are provided in V.42 for encoding 5,6,7 and 8-bit data into protocol frames, but no method is provided to coordinate this setting between the two modems. Establishment of a method to coordinate data frame formats settings between the modems and a method for warning the user of possible problems need to be addressed. % Preservation of framing and parity errors - There are some cases where it is desirable for modems to pass along data with improper parity rather than adjusting it as currently performed by existing error-control modems. Tandem modem links where part of the connection has error-control and part does not would benefit from preserving irregular parity formats. % Multi-frame selective reject - This will allow several individual frames to be requested in one SREJ frame, thereby reducing substantially the overhead on asymmetrical links. There is no guarantee that any of these will become part of the V.42 standard. As work continues in the CCITT study groups, the fact that all of these are on the agenda gives an idea of the intensity of work focused on this standard by the international telecommunications community. While some of these may have little or no market value, the study groups will continue to evaluate and establish the enhancements which are practical, based on the technical merit arguments of the participating members. V.42 COMPLIANCE V.42 specifies that a modem claiming full "compliance" to the standard must implement all parts of the standard, including both the primary and alternative protocols. The standard is written to be compatible even with modems having no error-control capabilities. As a result, modems implementing a portion of the standard or other modems (such as a non-error-control V.22 or V.32 modem) which can communicate with a V.42 modem may claim "compatibility". In short, a claim of V.42 compatibility does not necessarily mean a modem provides error control. While these semantic differences may seem insignificant, users must understand the product capabilities they will receive with a compliant versus compatible modem. A V.42 compatible modem may only implement selected portions of the standard, rendering it less useful in some applications where it must interact with a V.42 compliant modem which offers users the full range of V.42 error-control capabilities. HAYES COMMITMENT TO V.42 Hayes believes strongly that V.42 is the point-to-point modem error-control technique of the present and the future, consistent with existing standardized techniques and independent of proprietary control. The achievement of the goal of a standardized error-control technique will eliminate uncertainty in the marketplace, greatly increasing the demand for modems with LAPM error-control capability. Additionally, the establishment of an international standard will promote the evolution of a more homogeneous communications environment. In keeping with its history of implementing international standards, Hayes is offering V.42 compliant data communications products which will also be compatible with the installed base of the company's current V-series system products. Where applicable, the company will also offer upgrades to add V.42 capabilities to existing V-series system products. The CCITT's recommendation of an HDLC-based procedure for modem error-control affirms the Hayes decision to base error-control in its V-series system products on an extension of the LAPB protocol. Knowledge concerning HDLC-based protocols has enabled Hayes to be an active, early supporter of V.42's LAPM. The new standard includes a number of error-control features (XID frames and benign recognition sequence) which Hayes has already successfully used in its V-series system products. Hayes engineers have taken a very active role in the development of the new error-control standard and will continue to work within the CCITT and ANSI to develop new standards and enhance the existing standards.