ARCHIVED: General information about modems and modem standards

This content has been archived, and is no longer maintained by Indiana University. Information here may no longer be accurate, and links may no longer be available or reliable.

Note: This document deals with largely outdated modem standards and terminology. For more recent information about modem standards and terminology, see the Knowledge Base document ARCHIVED: What are some common modem terms?

Following is a list of the major transmission, data compression, error correction, and file transfer protocols and what they mean.

Transmission protocols refer to the way in which the modem talks on the telephone line with another modem. When two modems start communicating, the first thing performed is handshaking. In this process, the modems agree on the highest speed they both support, and then agree upon data compression and error correction protocols, if any. Data compression is the ability of the modem to take data in from the computer, reduce it in volume, and then send it out via the modem. Error correction protocols are the ways that the modems have to protect against errors, even in the face of a poor quality phone line.

2400bps (bits per second) will usually be sufficient for local terminal connections. 2400bps with MNP level 5 data compression would be better, since the compression would give you 4800bps actual data speed. If you are doing a great deal of file transfers, you should use a 9600bps or higher modem. The standard services modem pool at Indiana University Bloomington supports connection speeds up to 14,400bps with MNP 5 and v.42bis compress, a maximum throughput of 57.6Kbps (about 28.8Kbps average). The extended services modem pool supports connection speeds of up to 28.8Kbps with similar support for data compression protocols, giving a maximum throughput of 115.2Kbps (about 57.6Kbps average). When you are using a modem which does compression, you should set your communications software to the highest speed and enable data compression. This will maximize the speed with which your communications programs can talk to another modem. When you know you will be transferring files that are in an archived format (e.g., .zip, .arj, .zoo), it is often best not to use data compression, since the files are already compressed. The modem's compression algorithm cannot do any further compression, but it would spend time trying. This would cut your transmission rate by up to 10%.

In 1994-5, vendors established two unofficial 28.8Kbps protocols (often referred to as v.FC and v.TERBO) before the CCITT established the official 28.8Kbps protocol, v.36. These modems are approximately twice as fast as 14.4Kbps modems.

Modem protocols and standards

Name Function Other information
Bell 103 Async transmission 300bps modulation scheme
Bell 201A Sync transmission 2400bps modulation scheme
Bell 212A Async transmission 1200bps modulation scheme
CCITT V.22bis Async transmission 2400bps modulation scheme
CCITT V.32 Async transmission 9600bps modulation scheme
CCITT V.32bis Async transmission 14.4Kbps modulation scheme
CCITT V.36 Async transmission 28.8Kbps modulation scheme
CCITT V.42 Error correction Includes LAPM and MNP4
CCITT V.42bis Data compression
Yields a maximum of 4:1 data compression
MNP 4 Error correction
Yield of 120% of normal throughput
MNP 5 Data compression
Yields a maximum of 2:1 data compression
MNP 9 Data compression
Piggybacking & Optimized Retransmission
MNP 10 Error correction
Designed for cellular and poor phone lines
USR HST Async transmission
9600bps modulation scheme (proprietary)
Hayes Express Async transmission
9600bps modulation scheme (proprietary)
Telebit PEP Async transmission
9600bps modulation scheme (proprietary)
LAPM Error correction
Modem version of ISDN LAP-D protocol
Kermit File transfer protocol
Approximately 75% efficiency, 128 byte packet*
X-Modem File transfer protocol
Approximately 78% efficiency, 128 packet size
Y-Modem File transfer protocol
Approximately 80% efficiency, 1024 byte packet
Z-Modem File transfer protocol
Approximately 95% efficiency, variable packet
Sea-Link File transfer protocol
Approximately 80% efficiency, 1024 byte packet
CompuServe B File transfer protocol Approximately 80% efficiency

Notes to table:

  • Bell: AT&T Bell Laboratories
  • CCITT: Consultative Committee for International Telephony and Telegraph
  • USR: U.S. Robotics Corporation
  • HST: High Speed Technology (up to 14.4Kbps one direction, 400bps the other)
  • PEP: Packet Ensemble protocol (scheme optimized for cellular phones)
  • MNP: Microcom Network protocol
  • File transfer protocol (FTP) efficiencies refer to the percentage of the actual throughput that contains parts of the file.

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Last modified on 2018-01-18 08:42:36.