Modem Protocols Explained


Modems communicate with each other using a MODULATION method, which translates between digital computer data and analog telephone signals. Two modems must support the SAME modulation method or they can't communicate at all. The common modulation methods are:
  Bell 103              110, 150, 300.
  CCITT V.21            110, 150, 300.
  Bell 212A             1200.
  CCITT V.22            1200, 600.
  CCITT V.22 bis        2400.
  CCITT V.32            9600, 4800, 2400.
  CCITT V.32 bis        14400, 12000, 9600, 7200.
  Telebit PEP           High speeds, proprietary to Telebit.
  US Robotics HST       High speeds, proprietary to US Robotics.
  "V.Terbo"             20000 bps, not a standard, never will be.
  "V.Fast"              Preliminary implementations of V.34.
  "V.FC"                "V.Fast-Class".
  ITU-T V.34            28800, 26400, 24000, 21600, 19200, 16800, 14400


Bits Per Second. 10 bps = 1 cps (character per second); One character = 8 data bits plus 1 start bit and 1 stop bit.
International Telegraph and Telephone Consultative Committee (Note: CCITT changed its name to ITU-T on March 1, 1993).
International Telecommunications Union, Telecommunication Standardization Sector.
ITU-T (CCITT) series of "recommendations" (standards) relating to telecommunications.
"the second".
"the third".
Packet Ensemble Protocol (Telebit proprietary).
High Speed Technology (US Robotics proprietary).

For consistency we use "ITU-T" to refer to CCITT standards.

Modulation Fallback

When you make a modem call, the calling (originating) modem sends out a tone using the type of modulation that you have selected. Usually the selection is made automatically by the modem, based on the interface speed between your PC and the modem, which is sensed automatically by most Hayes-compatible modems when they are given an AT command. In many cases, there are also modem-specific commands to choose a particular modulation.

If the answering modem supports the type of modulation you have selected, the connection is made immediately. Otherwise the modems attempt to "fall back" to a lower type of modulation, the highest one they have in common. For example, if a 2400-bps V.22bis modem calls a 14400-bps V.32bis modem, the two modems will communicate at 2400 bps, the highest speed they have have in common. Fallback is generally reliable among the ITU-T standard modulations:

  V.34 ->. V.32bis ->. V.32 ->. V.22bis ->. V.22
In the USA, most modems fall back from V.22 to Bell 103, but different (incompatible) low-speed modulation methods are used in other countries, such as ITU-T V.21 or V.23. Most modern high-speed modems have a command to choose the low-speed fallback standard; in the USA, you should set your modem to fall back to the Bell 103 standard.

Within a particular modulation (V.32 and up) it is also possible to fall back to lower speeds when a higher-speed connection can't be made, for example because of excessive noise on the telephone connection. For example, a V.32bis connection can be made at 14400 bps, 12000 bps, 9600 bps, or 7200 bps. A V.34 connection can be made at 28800, 26400, 24000, 21600, 19200, 16800, or 14400 bps.

Mixing proprietary modulations like PEP or HST into the fallback scheme generally requires special configuration settings on your modem.

It is also possible for the two modems to change speeds in the middle of a connection, perhaps because they have detected an increase in telephone-line noise; this is called "retraining" -- the connection goes "on hold" for a couple seconds, but is not broken. Retraining occurs only if both modems have this capability and have agreed with each other to use it.

Interface Speed and Connection Speed

When you make a dialup connection, there are (at least) three separate components to the connection. Assuming you are dialing up from a PC, there is the connection between your PC and your (originating) modem, the connection between the two modems, and the connection between the "other" (answering) modem and whatever device it is attached to. Each of part of the connection can be running at a different speed:
  1. LOCAL INTERFACE SPEED: The speed used on the connection between your PC (or terminal or workstation) and your modem.
  2. CONNECTION SPEED: The speed of the connection between the two modems, based on the modulation technique that they negotiate with each other.
  3. REMOTE INTERFACE SPEED: The speed of the connection between the remote (answering) modem and the terminal server.

Speed Buffering

When originating a call, some modems change their interface speed to match the negotiated connection speed automatically. When that happens, your communications software must also change its speed at the same time. For example, if you dial at 9600 bps, but the remote modem answers at 1200 bps, your modem will print a message like:
  CONNECT 1200
which your communication software takes as a signal to change its interface speed to 1200 bps before attempting to go "online" with the remote computer or service.

Most modern modems can be configured to fix their interface speed at a given value, rather than change it according to the connection speed. This is desirable when using data compression. In this case, the CONNECTION SPEED (or MODULATION SPEED) between the two modems is different from the INTERFACE SPEED between the modem and the computer. The modem performs the speed conversion between its telephone side and its data side, and your communications software must be configured to IGNORE the speed given in the CONNECT message.

Error Correction

After the modems have agreed on a modulation technique, they might also try to negotiate an error-detection and -correction method:
MNP Level 1, 2, 3, or 4
Telebit PEP (proprietary)
US Robotics HST (proprietary)

When modems' initial error-control methods do not agree, automatic fallback is usually as follows:

  V.42 ->. MNP 4 ->. MNP 3 ->. MNP 2 ->. MNP 1 ->. none
When PEP, HST, or other proprietary methods are involved, special configuration settings are needed on the modems to specify the fallback sequence.

Please note that no connection can ever be free of errors. The error correction technique used between the modems might be extremely effective, but it is not foolproof. More to the point, the connections between the modems and the computers are not error-corrected, nor or the data paths within the computers. Thus it is still quite common to experience data loss or corruption, even on an error-corrected modem connection. Common causes include: buffer overflows (often due to a lack of adequate flow control between the modem and the computer -- see below), interrupt conflicts, loose connectors, and malfunctioning devices.

Beware of RPI modems. They do NOT perform error correction themselves, but rely on external software to do it. Most software does not. These modems are NOT SUPPORTED at Columbia University.


Link Access Protocol for Modems
Microcom Networking Protocol, proprietary, but licensed by most modem makers.
Rockwell Protocol Interface

Data Compression

Modems may incorporate data compression methods to increase the effective throughput of your data beyond the actual connection speed. Compression is possible only if (a) error correction is also being done, and (b) the interface speed between the computer and the modem is higher than the connection speed between the two modems.
MNP Level 3
108% efficiency by removing start & stop bits (synchronous)
MNP Level 4
120% efficiency by optimizing modem-to-modem protocol
MNP Level 5
True compression on top of Level 4, efficiency depends on data
True compression, efficiency depends on data
Telebit PEP
Proprietary, characteristics unknown
US Robotics HST
proprietary, characteristics unknown

Effectiveness of MNP 5 and V.42bis compression vary between 0% and 400% or higher, depending on the nature of the data. Compression fallback:

  V.42bis ->. MNP 5 ->. none
When PEP or HST is involved, special configuration settings are needed on the modem to specify how these fit into the fallback sequence. Again, beware of RPI modems. They do not do compression themselves, but rely on external software to do it.

Flow Control

"Flow control" is the method by which one device can control the rate at which another device sends data to it. There are various methods of flow control. The two most commonly used in dialup data communication are:

When using error correction or compression, or modems that are capable of retraining, it is essential to enable an effective form of flow control between each modem and the computer (or terminal, or other device) it is immediately connected to. Without effective flow control, data will be lost when one device sends data faster than the other one can receive it.

Flow control between the two modems is handled by the underlying error modem-to-modem correction protocol: MNP or V.42. If there is no underlying error-correction protocol, then there can be no flow control between the modems, and therefore no protection against data loss EVEN IF there is flow control between the modem and the computer. This applies, in particular, to RPI modems.

Further Reading

  1. Gianone, C., Using MS-DOS Kermit, Second Edition, Digital Press (1992), Chapter 4, "Cables, Connectors, and Modems", pp.27-36.

  2. da Cruz, F., and C. Gianone,
  3. Kermit FAQ (Frequently Asked Questions).

  4. Your modem manual.

  5. The International Telecommunications Union V series of standards (V.32bis, V.34, V.42, V.42bis, etc). It is not obvious that these are ZIP'd PostScript files, so you must transfer them, Unzip them, and then look at them with a PostScript viewer or printer. E(nglish), F(rench), or S(panish), your choice. These documents are not light reading.

The following additional sources of information may or may not be useful. Since they are outside Columbia University, we have no control over their content (accuracy, timeliness, maturity, political correctness). Use at your own risk.

  1. A V.34 Modems FAQ, primarily Microsoft Windows oriented, with links to many modem vendors.
  2. The comp.dcom.modems newsgroup.

Last update: 4 May 1995

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