2400 White Paper EXPANDING THE 2400 BPS OPTION FOR PERSONAL COMPUTERS Hayes Microcomputer Products, Inc. 705 Westech Drive Norcross, Georgia 30092 INTRODUCTION From its earliest days, personal computer data communications has constantly changed and moved forward with ever increasing emphasis on higher speed and improved features. New standards have been developed or have evolved and new products have been developed implementing these standards. Nothing has remained still. The standards themselves, while constant in the early stages of implementation, have even undergone change or have been abandoned in favor of more applicable standards for current needs. The dynamic nature of data communications compels the developer of technology to constantly seek to refine and expand products and techniques. And, now, the options for 2400 bps communications are continuing to expand to meet the ever growing requirements of more sophisticated, demanding users. ACCEPTANCE OF THE V.22BIS STANDARD The International Telegraph and Telephone Consultative Committee (CCITT) V.22bis standard employed for 2400 bps, full-duplex modem communications over the Public Switched Telephone Network (PSTN) was recommended by the international standards in 1984. Since that time it has become widely employed by modem manufacturers around the world. The precursor to today's CCITT V.22bis standard for 2400 bps communications was Bell 201C. Bell 201C suffered from several limitations, the most prominent being it was a half-duplex standard in a world that was going steadily toward full-duplex transmission for personal computer communications. Bell 201C modems implemented Phase Shift Keying (DPSK) which is more ideally suited to 1200 bps full-duplex communications. These modems also required expensive leased lines for operation. In order for communications to move forward, a standard to facilitate full-duplex 2400 bps communications over the PSTN was needed. V.22BIS AND QUADRATURE AMPLITUDE MODULATION The solutions to the Bell 201C problems arrived with the publication of the CCITT V.22bis recommendation. DPSK only provided recognition of phase shifts of the analog signal and in order to step up to 2400 bps full-duplex transmission, a method for recognizing amplitude shifts in combination with phase shifts was necessary. CCITT V.22bis provided recognition of both amplitude modulation and phase shifts through Quadrature Amplitude Modulation (QAM). QAM was actually an enhancement or addition to DPSK that enabled personal computer communications to evolve and migrate to its next, feature enhanced level. Now, by implementing CCITT V.22bis standards, modem manufacturers could increase the capabilities of their products to include full-duplex 2400 bps communications. A bonus to users which helped increase the popularity of these new 2400 bps modems was their ability to communicate accurately over the PSTN. Costly leased lines were no longer necessary and the one-time price of the modems could be easily justified from a business standpoint when compared to a repetitive monthly leased line charge. And, since the new modems communicated at twice the speed of 1200 bps products, connect time was cut in half for considerable reduction of long distance charges. The trend toward implementing changes to enhance the performance of communications was continuing and, as with its 300 bps and 1200 bps products, Hayes helped promote and support standards by introducing the Smartmodem 2400 in 1984 and Smartmodem 2400B in 1985. Both modems are CCITT V.22, V.22bis and Bell 103/212A compatible. TECHNOLOGICAL ADVANCES BRING MORE FEATURES Advances in technology have allowed the introduction of new features into modems. New 2400 bps products offered adaptive equalization, improved self testing, and call progress monitoring to further enhance their popularity. As the users became more sophisticated and grew accustomed to the features they had, they demanded more. The most requested new features are error-control, data compression, and automatic feature negotiation capabilities. HAYES V-SERIES SYSTEM PRODUCTS Hayes has developed a series of four high speed dial-up modems and an enhancement product for use with existing Hayes external modems that implement the following advanced features: - Error-Control - Implementation of the LAPB link level portion of X.25 and Asynchronous Framing Technique (AFT) allows point-to-point error-control, supports international standards, and provides a migration path to X.25 or other synchronous communications. Adaptive Data Compression - Increased throughput typically provides twice the data transfer and lowers both transmission costs and connect time. Automatic Feature Negotiation - Analyzes all options available for modem link and selects combination that maximizes data transmission and efficiency. V-series Smartmodem 2400, V-series Smartmodem 2400B, V-series Smartmodem 9600, V-series Smartmodem 9600B, and V-series Modem Enhancer provide all of these advanced features in addition to standard modem features available in current Hayes modems. ERROR-CONTROL In order to have a high speed feature, there must be some form of error-control to instruct the modem to re-transmit a block of data if the data has errors in it. Rather than use a proprietary error-control method, Hayes selected the LAPB link level portion of X.25 for a variety of reasons. First, it allows point-to-point error-control. Second, it represents support for an existing, widely used international standard that is versatile. Finally, it provides a logical migration path for future products with full implementation of X.25. The CCITT X.25 protocol uses bit-oriented synchronous communications to provide error-control, bidirectional communications service to communicating applications. The CCITT has adopted a policy for maintaining the dynamic nature of X.25 by enhancements to the protocol in 1980 and 1984. The V-series system products can also coordinate link setup using synchronous LAPB with the Asynchronous Framing Technique (AFT). AFT is an asynchronous augmentation for any bit-synchronous protocol such as LAPB, LAPD, or IBM's SDLC. It part of the link level of the synchronous protocol and brings error-control operation to asynchronous transmission links. The migration of communications that is enhanced by the V-series system products and their features is taking communications more and more toward the higher speed, synchronous, and eventually, Integrated Services Digital Network (ISDN) environments. ADAPTIVE COMPRESSION Other features evolved as modem manufacturers looked for ways to "squeeze" more performance over the PSTN out of modem technology. One such feature was data compression. By using algorithms to compress the data, 2400 bps modems can improve their actual data throughput and potentially provide data transfer benefits as high as 4800 bps transmission. Similarly, a 1200 bps modem using data compression may get up to 2400 bps of effective throughput and provide its user with a high speed benefit at a lower speed price. Greater throughput is not just a function of a modem's speed. The combination of adaptive data compression with any modem increases the effective throughput typically two-to-one. This is particularly useful when dealing with high volume data transmissions. All V-series system products can adapt dynamically to match the type of data being compressed and sent. The modems build and constantly update compression tables to typically accomplish a two-to-one compression for normal text, data, and spreadsheet files. This method also ensures that an expansion of files never occurs if other compression algorithms are being used. This Hayes-developed algorithm allows the V-series system products to typically achieve throughput at 4800 bps or 19,200 bps respectively for 2400 bps and 9600 bps Hayes modems. AUTOMATIC NEGOTIATION The ability to automate communications was one of the driving forces behind the success of the Hayes Smartmodem 300 when it was introduced in 1981. As features in modems have increased in complexity, the need to continue to provide the user with a product that is easy to use is still vitally important. By providing automatic negotiation capability, modems conduct and coordinate activities among themselves. By negotiating certain features among themselves (transmission speed, asynchronous versus synchronous, error-control, etc.) modems can improve the quality of communications and lower the involvement level required of their users to eliminate potential frustrations. And, since 2400 bps modems can communicate with each other so rapidly, they can independently handle this function more swiftly than it could be handled by a user. By providing automatic feature negotiation, the V-series system products automate a significant portion of the communications session. Intelligent features resident in the V-series system products maximize the obtainable transmission speed from the receiving modem. This is done automatically during their "handshake" when the V-series system products originate a call. If one of the modems is not a V-series system modem, the connecting V-series system modem will recognize this fact and shift to a compatible mode of operation. This type of adaptability is evident in all Hayes modems. For example, the Smartmodem 2400 upon originating a call to any 1200 bps modem will "recognize" that the remote modem operates at 1200 bps and fall back to a compatible speed for data transmission. Once the "handshake" has been completed, the negotiation of link parameters such as transmission mode (asynchronous, AutoSync, or synchronous), asynchronous transparency mode, compression, and method of error-control is conducted. If a non-V-series system modem is either the originating or receiving modem, the V-series system modem will recognize this and make the necessary adjustments. If the receiving modem is a V-series system modem, the connection will be completed enabling the maximum available features including highest speed, synchronous, adaptive data compression, and error-control. The inclusion of the V-series Modem Enhancer in this segment of the Hayes product line is extremely important to the installed base of Hayes external modem users. By providing the V-series system features to existing Hayes external modems, this enhancement product upgrades modems already in use and helps businesses add features to current products, extend the useful life of those products, and take advantage of emerging technology without purchasing new modems. THE HAYES COMMITMENT Hayes recognizes the necessity of change and will endeavor to adapt its products to the needs of users in the future. At the same time, the company is committed to supporting the large installed base of data communications equipment in use It is our intention to promote the advancement of personal computer communications through new products and adherence to open, international standards. We will continue our progress with a commitment to move forward without abandoning established, viable communications options that exist and we will innovate to provide users requested features where standards do not exist.