                          

                              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.

