


                        Modem Standards, Simplified...

A lot of you who have seen v.32, v.32bis, v.42, etc. mentioned wonder what it 
all means. Well, hopefully this short text will help.  

There is one committee which sets the standards for modem communication 
today.  This is the CCITT (International Consultive Committe for Telegraph & 
Telephone).  Bell Laboratories used to set standards and a number of their 
standards are still in use, but most modems today conform with CCITT 
standards.

There are three categories of standards:

    1. Modulation - v.32, v.32bis, HST
    2. Error Correction - MNP 1-4, v.42, Hayes V-series
    3. Data Compression - MNP 5, v.42bis, CSP

Modulation standards represent the actual mechanism of sending data with the 
modem over the telephone.  What all sending modems do is convert data into 
sound.  What all receiving modems do is convert the sound back into data.






Error correction standards work on top of the modulation standards to ensure 
that the data being sent and received is accurate.  The sending modem 
surrounds the data by various checksum or CRC bytes (bits of data).  The 
receiving modem then decodes the checksum or CRC bytes to see if the data 
arrived intact.  If the data doesn't check out, the receiving modem will ask 
the sending modem to resend the last block of data.  This is just like the 
various file transfer protocols such as Xmodem, Kermit, Zmodem, etc.

The third category, data compression standards, is very similar to the error 
correction standards.  And, in fact, they work on top of the error correction 
standards.  The data is first compressed before the modem sends it.  This 
compression will not be useful when already compressed data files are 
transmitted.  For instance, files archived with compression programs having 
extensions like ZIP, PAK, ARJ, ARC, and LHA.  With normal text files, 
compression can help dramatically.









                          Half or Full Duplex?

Half duplex is when data is transmited in one direction at a time.  Full 
duplex is both directions at once.  2400 baud modems are always full duplex. 
HST modems (USRobotics) use an improved form of half duplex, in which one 
direction is at high speed while the other direction is sending 
acknowledgement signals back at a slower speed.

v.32 and v.32bis modems are full duplex, they transfer data in both 
directions.  However, this does not help much because most file transfer 
protocols such as Ymodem and Zmodem only work one direction at a time. There 
is a new transfer protocol coming called HS/Link which will allow full speed 
transfers in both directions.








                         Common Modem Standards

Now . . . let's look at some of the most common modem standards in use today 
on high speed modems.  There are many more standards out there, but these are 
the ones most often seen with BBSs.

CCITT v.32 - Full duplex 9600 baud modulation protocol.

CCITT v.32bis - Full duplex 14400 baud modulation protocol that can
    fallback to v.32.  It is not as common as v.32 due to high price.

HST - High Speed Technology - a proprietary 9600/14400 baud modulation
    protocol used by USRobotics.  The 14400 was added in 1989, all
    HSTs sold today include the 14400 baud speed as well as the original
    9600 mode.  HST has become a "standard" for BBSs because more BBSs
    use HST than any other type of high speed modems.  File transfers
    with HSTs at 14400 baud is as fast as v.32bis.

MNP levels 1-4 - Microcom Networking Protocol - an error correction
    protocol developed by Microcom.  At MNP level 4, it can increase
    throughput by 20% by stripping start & stop bits.

CCITT v.42 - an error correction protocol that can fallback to MNP
    levels 1-4.  The new method used is called Link Access Procedure,
    known as LAP-M.  Most v.32 modems are fully v.42 compliant, but they
    don't have to be.

Hayes V-series - a proprietary error correction protocol by Hayes.

MNP level 5 - a data compression protocol which can double throughput on
    non-compressed text.  Note that a higher MNP level protocol also
    includes all lower protocols.  This means a MNP level 5 modem contains 
    MNP levels 1-4 as well.

CCITT v.42bis - a data compression protocol that can fallback to MNP
    level 5.  It can sometimes quadruple throughput.  v.42bis is superior to 
    MNP because it analyzes the data first, and then determines whether 
    compression should be used.  Be careful because a 2400 baud v.42bis 
    modem might be advertised as "9600 bps", but it can only achieve this 
    speed on non-compressed material.

CSP - CompuCom Speed Protocol - an error correction/data compression
    protocol available on CompuCom modems.  It can sometimes quadruple
    throughput like the v.42bis modems.  One problem with this protocol
    is that it will only connect with HST or v.32 modems at 2400 baud and 
    achieve the 9600 baud rate when connected to other CompuCom (CSP) modems.
