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ͻ
 Modem Standards 
ͼ

You've probably wondered what all those modem standards mean. There are
currently several active standards. They involve more than the modem's
operating speed.  Without standards, modems from one manufacturer most likely
couldn't communicate with modems made by another manufacturer.  

At least a basic understanding of modem standards is necessary if you want to
make the right choices when selecting modems for use with your system.

Generally, 300, 1200 and 2400 bps modems each use a standard followed by all
modems and modem makers. Standards for 300 and 1200 bps modem transmission are
different in the United States than they are in Europe.

Standards for 9600 bps transmission have been established for some time.
However, the technology to fulfill those standards has been expensive. To get
around the high cost of using the existing standard, modem manufacturers have
created their own. This is why so many high-speed modems will only talk to
another high-speed modem of the same brand.









Data transmission speeds are not the only type of modem standard. 

            Ŀ
                   Ŀ                                       
                 Ĵ - Data Compression              (IV)  
               Ĵ - Error Correction              (III) 
             Ĵ - Negotiuation                  (II)  
              - MOdulation/DEModulation       (I)   
            Ĵ
                          DESCRIPTION                LAYER   
            

Actually, modem standards are grouped into four distinct areas which can be
thought of a a sort of stairstep, or the layers of a pyramid.









ͻ
 I.  Modulation 
ͼ

Modulation is the starting (or bottom) layer for all modems  ("modem" means
MOdulator - DEModulator). Each layer builds upon the next like a pyramid.

Modulation refers to the signaling method used by the modem. Two modems must
use the same modulation method in order to understand each other. Each data
rate uses a different modulation method. Sometimes there is more than one
method for a particular rate. An example of this is the Bell 212A and V.22
modulation standards; they both specify 1200 bps modulation. However, they work
differently and are not directly compatible.












ͻ
 II.  Negotiation 
ͼ

Negotiation refers to the manner in which two modems establish which modulation
method will be used during a connection. Modems listen to the tones sent by
another unit to determine what modulation method will be used. Different
methods often use different answer tones and can be used by the calling modem
to determine which method to use. Negotiation standards have been created to
make the process easier. These standards govern the sequence of events that
occur when a modem answers the phone. This eliminates the guesswork associated
with the "listen to the tones" method. Negotiation is part of many modem
standards.












ͻ
 III.  Error Correction 
ͼ

Error correction refers to an ability some modems have to identify errors
during a transmission, and to automatically re-send data damaged in transit. If
error correction is used, both modems must use the same standard to make it
work. Fortunately, there are error correction standards which are followed by
most modem manufacturers.









ͻ
 IV.  Data Compression 
ͼ

Data compression refers to a built-in ability in some modems to compress the
data they're sending. This saves time and can result in considerable money
saved by long-distance modem users. A Data file can be compressed by as much as
50%. This effectively doubles the speed of the modem.

A 2400 bps modem with data compression is capable of sending some files as
quickly as a 4800 bps modem WITHOUT data compression. Not all types of data can
be compressed by 50%, but gains are almost always realized.

Ŀ
 Standards for 300 and 1200 Bps 


Most 300 bps modems follow the standard created initially by AT&T, called Bell
103 (common in the United States). Most modems manufactured for use outside the
United States support the CCITT V.21 standard instead. They are not compatible
with Bell 103 modems. Some modems can be set to follow either standard.


AT&T also created the Bell 212A standard for 1200 bps modems.  It's become the
common standard in the United States. Most modems manufactured for use outside
the United States support the CCITT V.22 standard and are not compatible with
the Bell 212A modems. Some modems can be set to follow either standard.  Most
modems manufactured since 1985 are capable of distinguishing between the two,
and can effectively handle either one. 

Ŀ
 2400 Bps Standards 


The international standard for 2400 bps communications is CCITT V.22bis. This
is used by modems manufactured for use both inside and outside the United
States. Most 2400 bps modems include automatic detection of the data rate fall
back. If a data rate lower than 2400 bps is detected at the other end of the
connection, the modem automatically drops to 1200 bps.

Ŀ
 9600 Bps Modem Standards 


Standards for high speed data transmission have been in place for some time.
Acknowledged standards came in two forms: 

1.   A half duplex standard (commonly used in FAX machines and called
     V.29).

2.   A full duplex standard called V.32. The technology required to
     implement the V.32 standard remained financially restrictive for many
     years. This forced most manufacturers to create their own less
     expensive proprietary transmission methods.

U.S. Robotics, for example, created the Courier HST (High Speed Technology).
This design is not full duplex . . .  meaning it does not support high speed
transmission in BOTH directions.  Current HST modems send data at 14,400 bps in
one direction, and 450 bps in the other direction. The high speed channel
changes direction depending on which side of the transmission has the most data
to send. HST modems can only talk at high speed with other HST modems. They
also adhere to existing standards for 300, 1200 and 2400 bps operation.

Telebit, another modem manufacturer, created PEP (Packetized Ensemble
Protocol), which is used in their Trailblazer modem series. Like the HST, PEP
modems will only connect at high speed with other PEP modems.

Hayes also developed their own technology for high speed transmission. Like the
others, Hayes high speed modems only talk high speed to other Hayes modems. 











The cost of V.32 high speed transmission technology has come down drastically
in recent years. It is displacing other high speed proprietary protocols in
popularity. This means high speed modems are finally starting to communicate
with a common standard. U.S. Robotics' new Courier HST Dual Standard is one
example of a new high speed modem utilizing both U.S. Robotics' own HST
transmission standard and the V.32 high speed standard. The new Hayes V-series
Ultra Smartmodem 9600 is another multiple-standard high speed modem utilizing
the V.32 standard.












                                ͻ 
                                Ŀ
                                ͻ
                                  CCITT  
                                ͼ
                                ٺ
                                ͼ

The international body of technical experts responsible for developing data
communications standards for the world is the Consultative Committee on
International Telephone and Telegraph (CCITT). This group falls under the
organizational umbrella of the United Nations and its members include
representatives from major modem manufacturers, common carriers (AT&T, etc.),
and governmental bodies.







Ŀ
 CCITT Modulation Standards 


The CCITT establishes standards for modulation (actual modem signaling
methods). It also determines standards for error correction and data
compression It is possible one modem might adhere to several CCITT standards,
depending on the various features andcapabilities the modem offers.

All modems signal one another at a variety of speeds. CCITT standards for
modulation are used by almost every modem manufacturer. Some of the standards
(primarily modulation) do include some of the higher layers (such as
negotiation) as well. 

Multi-speed modems may use several of these standards, including:

V.21:          V.21 is a data transmission standard at 300 bps. This standard
               is used primarily outside of the United States. (300 bps
               transmissions in the United States primarily use the BELL 103
               standard).





V.22:          V.22 is a data transmission standard at 1200 bps. This standard
               is also used primarily outside of the United States. (1200 bps
               transmissions in the United States primarily use the BELL 212A
               standard).

V.22bis:       V.22bis is a data transmission standard at 2400 bps. This is the
               international standard for 2400 bps. It is used both inside and
               outside the United States.

V.23:          V.23 is a split data transmission standard, operating at 1200
               bps in one direction and 75 bps in the reverse direction.
               Therefore, the modem is only pseudo - full duplex, meaning it is
               capable of transmitting data in both directions simultaneously
               (but not at the maximum data rate). This standard was developed
               to lower the cost of 1200 bps modem technology. The technology
               was still very costly in the early 1980s when these modems were
               designed. This standard is still in use, mostly in Europe.







V.29:          V.29 is a data transmission standard at 9600 bps which defines a
               half duplex (one-way) modulation technique. Although modems do
               exist which implement this standard, it has generally only seen
               extensive use in Group III facsimile (FAX) transmissions. 

               Since it is a half-duplex method, it is easier to implement this
               high speed standard than it would be to fulfill a high speed
               full-duplex standard. V.29 is not a complete standard for
               modems, so V.29-capable modems from different manufacturers will
               not always communicate with one another. 

V.32:          V.32 is also a data transmission standard at 9600 bps, but V.32
               defines a full-duplex (two-way) modulation technique. It is a
               full modem standard, and also includes forward error correcting
               and negotiation standards as well. 

               Many modem manufacturers already have or will be introducing
               V.32-compatible modems. This is generally considered THE
               standard for high-speed modems today.





V.32           V.32 is expensive to implement. The technology required for it
cont'd...      is complex. As this standard becomes more common and refined 
               manufacturing techniques become available, the pricing for V.32
               modems should go steadily downward. 

               Some manufacturers have created modems using both their own
               proprietary high speed standard and the V.32 standard (for
               compatibility with their older non-V.32 modems). The new Hayes
               Ultra and U. S. Robotics HST Dual Standard are examples of the
               new dual personality modems now on the market.

     V.32bis:  This is a developing high speed standard. When fully defined
               (possibly by early 1991), V.32bis will operate at 14,400 bps
               and, like V.32, will be a full-duplex method. 

               The CCITT has not yet defined this standard, so no modems
               currently use it (although some new modems have implemented what
               is expected to be the standard and may claim V.32bis
               compatibility).



Ŀ
 CCITT Error Correcting and Data Compression Standards 


The CCITT also has adopted formal standards for the higher layers of Error
Correction and Data compression. In order for any error correction or data
compression protocol to work, modems on BOTH ends of the connection must
support it. Once two modems are connected, they automatically negotiate between
themselves to determine the best mutual protocols they both support.

V.42:     V.42 is a CCITT error-correction standard similar to MNP Class 4. In
          fact, because the V.42 standard includes MNP compatibility through
          Class 4, all MNP 4- compatible modems can establish error-controlled
          connections with V.42 modems. This standard, however, prefers to use
          its own better performing protocol -- LAPM (Link Access Procedure for
          Modems). LAPM, like MNP, copes with phone line impairments by
          automatically re-transmitting data corrupted during transmission
          assuring only error free data passes through the modems. Many modem
          manufacturers make MNP Class 4-compatible modems, and some offer
          V.42-compatible modems as well.







V.42bis:  V.42bis is a CCITT data compression standard similar to MNP Class 5,
          but providing about 35% better compression. Of course, this also
          means it provides better throughput. V.42bis only compresses data
          needing compression. Each block of data is analyzed, and if it can
          benefit from compression, compression is enabled. Files on bulletin
          board systems are often compressed already (using ARC, PKZIP, and
          similar programs). While MNP Class 5 can actually decrease throughput
          on this type of data, V.42bis will not -- compression is only added
          when a benefit will be realized.

          To negotiate a standard connection using V.42bis, V.42 must also be
          present. Thus, a modem with V.42bis data compression is assumed to
          include V.42 error correction. Some modem manufacturers already make
          V.42bis compatible modems, and more are on the way.

          V.42bis is NOT compatible with MNP Class 5. A V.42bis modem will
          establish an error-free connection with MNP-capable modems (since
          V.42bis includes V.42), but only up to MNP Class 4. 


ͻ
 What is MNP? 
ͼ

MNP stands for Microcom Networking Protocol and was created by Microcom, Inc.,
a modem manufacturer. MNP offers end-to-end error correction, meaning the
modems are capable of detecting transmission errors and requesting corrupted
data to be re-sent. Some levels of MNP also provide data compression.

Over time, different classes of the MNP standard were defined. Each described
the extent a given MNP implementation supports the protocol. Most current
implementations support Classes 1 through 5. There are higher classes, but
these usually apply only to modems manufactured by Microcom, Inc.

MNP is generally used for its error correction capabilities, but MNP Classes 4
and 5 also provide performance increases. Class 5 offers real-time data
compression. The lower classes of MNP are not usually important to you as a
modem user, but they are included here for your information.

MNP Class 1:   MNP Class 1 ( Block Mode) uses asynchronous, byte oriented,
               half-duplex (one way) transmission. This method provides only
               about 70% efficiency. It provides error correction only, and is
               rarely used today.



MNP Class 2:   MNP Class 2 (Stream Mode) uses asynchronous, byte oriented, full
               duplex (two way) transmission. This class also provides error
               correction only. Because of protocol overhead (the time it takes
               to establish the protocol and operate it), throughput at Class 2
               is actually only about 84% of that for a connection without MNP,
               delivering about 202 cps (characters per second) at 2400 bps
               (240 cps is the theoretical maximum). Class 2 is rarely used
               today.

MNP Class 3:   MNP Class 3 incorporates Class 2, and is more efficient. It uses
               a synchronous, bit-oriented, full-duplex method. The improved
               procedure yields throughput about 108% of that of a modem
               without MNP, delivering about 254 cps at 2400 bps.

MNP Class 4:   MNP Class 4 is a performance enhancement class using  Adaptive
               Packet Assembly(tm) and Optimized Data Phase(tm) techniques.
               Class 4 improves throughput and performance by about 5%,
               although actual increases depend on the type of call (local or
               long distance, noisy or clean connection), and can be as high as
               25% to 50% on some links.



MNP Class 5:   MNP Class 5 is a Data Compression protocol which uses a real 
               time adaptive algorithm. It can give an increase of up to 50% in
               throughput, but the actual performance of Class 5 is very
               dependent on the type of data being sent. 

               Raw text files will allow the highest increase, while program
               files cannot be compressed as much and the increase will be
               less. On precompressed data (files already compressed with ARC,
               PKZIP, etc.), MNP 5 can actually EXPAND the data and performance
               can actually decrease. For this reason, MNP 5 is often disabled
               on BBS systems.

MNP Class 7:   MNP Class 7 is the other major MNP protocol you are likely to
               encounter. MNP 7 provides Enhanced Data Compression. When
               combined with Class 4, it can obtain about a 300% improvement in
               performance. It is designed primarily for use with V.22bis (2400
               bps) modem. 

               This class is currently unique to Microcom modems. Since it
               requires much more hardware and is usually inferior to V.42bis,
               it is not likely to proliferate.






Ŀ
 Okay, but what does it all mean? 


Despite the fact they might seem confusing, all of these standards exist to
benefit you the modem user. You want to be able to compare modems on price,
reliability, performance, and support. You also want to be able to know that
different manufacturer's modems will communicate with each other.

The past couple of years in the high speed modem arena has shown what happens
when market demand occurs faster than associated standards. You are forced to
pick a single manufacturer and become locked in to gain the capabilities you
want. The purpose of standards is to prevent this situation.

When standards are widely adopted, you get the best of technology and the
effect of competition. However, you need to know what the standards mean to be
able to be an informed consumer.











                         ͻ
                          Of Bits and Parity... <PARITY> 
                         ͼ

We have taken a close look at the most common and often least understood terms
and standards in the world of the modems we use. There are, however, several
other telecommunications terms that can be confusing. 

Though they don't necessarily relate to modem-buying decisions specifically,
understanding these terms can add important additional power to your
communications dealings. They also will help you understand how to set up the
terminal programs your users will have to configure to call your BBS system.
Among the most commonly faced (and least understood) are the concepts of Data
Bits, Parity, and Stop Bits.




Ŀ
 Data Bits 


The American Standard Code for Information Interchange - ASCII - is a standard
defining 128 different characters used for data transmission. These include
control characters, letters of the alphabet (in both upper and lower case),
numbers, and a full set of punctuation characters. Because there are only 128
ASCII characters, only 7 binary digits (bits) are required to form each of the
128 possibilities.

Many computer makers have extended the ASCII character set by adding 128 more
characters. This was accomplished by simply adding one more binary digit,
resulting in a total of 256 transmittable data characters. Each manufacturer,
however, created their own set of 128 additional characters. All extended
character sets are NOT the same.

In the case of the IBM PC and compatibles, the extended characters include
international alphabet, graphics and mathematics characters. These are commonly
known as IBM Graphics characters.





In communications, common settings are either for 7-bit or 8-bit data.
Generally, both ends of the connection must be set the same way. If one end is
set to 7-bit data and the other end is set to 8-bit data, reliable
communication cannot usually be established. This is because one end interprets
the 8th data bit as a parity bit (explained in a moment), and the other end
tries to interpret it as a part of the current character. On a connection like
this, some characters will display properly, while others will appear as
garbage, depending on which direction the data is traveling.

If the communications link is set to transmit only 7-bit data, the sendable
characters are limited to the 128 defined ASCII characters. The extended
character set, such as the PC's single- and double-line boxes and foreign
characters, CANNOT be sent unless the link is first set to allow the
transmission of 8-bit data.

Some systems have even 5-bit and 6-bit data, and use character sets such as
Baudot and Selectric, but these systems are uncommon today.



Ŀ
 Parity Bit 


When you establish communications with another computer, parity is set to
"even," "odd," "mark," "space" or "none." These are terms for the manner in
which the parity bit is interpreted by the receiver.

Parity is a primitive form of error-checking. The state of the parity bit, when
set to be even or odd, is based on a simple mathematical formula. Depending on
the data bits, the parity bit will either be on or off. Normally, the limited
error checking capabilities are not utilized. This explains why the setting of
parity to NONE is so common in communications today. This allows the parity bit
to be used as a normal data bit instead.

Ŀ
 Start and Stop Bits 


Start and stop bits allow each character sent to be set in a frame. The
beginning of the character, the first part sent, is the start bit, and the end
of the character, the last part sent, is the stop bit. Each character sent is
thus framed with a distinct beginning and ending bit and this allows the
receiving system to know when each complete character has been sent.






There is always just one start bit. However, there may be one, one and a half
or two stop bits.

Stop bit length used to be critical when serial communication was primarily
handled with electro-mechanical equipment, such as an old-fashioned Teletype
machine. The print head in this type of equipment took a fixed amount of time
to return to its home position, and this was accomplished during the sending of
the stop bits. A longer stop bit length gave the print head more time to return
to its home position.

In modern all-electronic serial communication, the stop bit is still necessary,
but only to mark the end of a character. A delay isn't necessary as there isn't
usually anything mechanical involved.








                       ͸
                        Asynchronous Communications <RS232> 
                       ;

Framing the character with start and stop bits forms the basis for asynchronous
communications. In asynchronous transmission, characters do not have to flow
constantly - there can be gaps, or spaces, between each character. The receiver
knows when a character is sent by the framed nature of asynchronous
transmission - the start and stop points can easily be determined.

                           ͸
                            Synchronous Communications 
                           ;

An alternate serial transmission method exists known as synchronous
communications. It occurs when there are no start or stop bits, and is possible
only if data characters flow constantly at a fixed bit rate with no
interruptions. When there is no data to send, idle or padding characters are
sent at the fixed rate (to keep data bits flowing constantly), but they are
discarded by the receiver.







Because there are no start or stop bits, it is possible to remove 2 of every 10
bits used in Asynchronous communications. This results in a 20% faster data
speed with the same serial bit rate.  However, because of the requirement for
constant data flow, Synchronous transmission requires additional protocol and
is primarily used in mainframe computer or specialized applications.

One place it is used with a BBS system is hidden inside of high speed modems.
When these modems use MNP or V.42 protocols, they have the needed protocol to
use synchronous communications between the modems themselves. However, you
still use asynchronous communications between the computer and the modem so
this instance of hidden synchronous communications is primarily of interest as
trivia.






                                  ͻ
                                   Duplex 
                                  ͼ

Duplex is a term which refers to whether a data communications path is one- way
or two-way. Full duplex means data flows in both directions at the same time.
Half duplex means data flows in only one direction at one time. Most modems are
full duplex, but communications software can most often still be set to take
advantage of half duplex connections.

Some less expensive high speed (9600+ bps) modems are pseudo- full-duplex. This
means they cannot transmit data at high speed in both directions at the same
time because they are really operating in a fast turn-around half duplex mode
internally.

Ŀ
 Flow Control 


The term, Flow Control, refers to a method of controlling the flow of
transmitted data, so it doesn't overrun the data receiver's ability to receive
the incoming signals. Flow control allows the receiver to signal the
transmitter to pause, while recently received data is properly assimilated,
then signal it to restart the data flow when it's ready to receive more.





There are generally two forms of flow control, software and hardware:

RTS/CTS:  Hardware flow control is not always required. It is generally needed
          only with modems capable of buffering out-going data, or with high
          speed modems. Hardware flow control, called RTS/CTS flow control,
          uses two of the RS-232 (serial) pins to start and stop the data flow.
          Its advantage is that it is data independent and thus can be used for
          reliable flow control with any type of data stream.

X-ON/X-OFF:    Software flow control, called XON/XOFF flow control, starts and
               stops the data flow based on the reception of certain control
               characters. Although this type of flow control can be used by
               hardware devices, software flow control is usually used with a
               bulletin board to allow the BBS user to start and stop data
               transmission by using control keys. This allows the user to
               press Ctrl-S at any time to temporarily halt data flow, and then
               press Ctrl-Q at any time to restart data flow. 








                               ͻ
                                What is ANSI? 
                               ͼ

"ANSI" is a common term in the bulletin board community today, but it's also
usually misused.

ANSI stands for the American National Standards Institute, a standards
development organization (sort of like the CCITT). ANSI develops and documents
standards for thousands of different areas, from architectural specifications
for the handicapped to computer programming languages.

Within the bulletin board community, the term, ANSI, generally refers to an
ANSI standard called X3.64 as implemented by IBM in ANSI.SYS. The ANSI X3.64
standard specifies a series of codes a host system can send to a remote data
terminal to control color attributes, cursor positioning, inverse video and
screen clearing on the terminal display. 


ANSI Graphics" are terms often used in the bulletin board community, but this
actually refers to two separate elements. ANSI controls color and cursor
positioning, while Graphics usually refers to characters in the IBM PC extended
character set, such single- and double-line boxes, shading characters, and so
on. ANSI Graphics are common terms, since normally only an IBM PC is capable of
handling both ANSI and Graphics. In reality, many data terminals and software
packages for various computers are capable of handling ANSI codes, although
they may not always handle the IBM extended characters.

Actually, ANSI Graphics does NOT refer to a standard for displaying pictures or
graphic images on the remote terminal.

The VT-100 terminal (a data terminal from Digital Electronics Corporation) and
software capable of emulating a VT-100 terminal can also be used with ANSI
escape codes, since the codes for both ANSI and VT-100 are very similar. 


ANSI works by sending a series of characters to the remote terminal. The codes
all begin with an escape character and a left bracket, and are followed by a
variable quantity of numbers and letters. The terminal understands the meaning
of these codes, and acts accordingly by setting screen colors or moving the
cursor.







                                   ͻ
                                    Graphics 
                                   ͼ

These are the characters in the IBM PC extended character set. They are
characters beyond the original 127 possible ASCII characters as defined by IBM
in all of their display adapters. These include single and double line boxes,
shading characters, international characters and mathematical symbols.

IBM Graphics characters have become enough of a de-facto standard. Many other
computers now emulate them. Many terminal programs on the Apple Macintosh
computer will allow proper display of the IBM graphics character set, as will
many of the true display terminals on the market today.
