The earlier V.xx Standards
Modem Standards
ITU Modem Standards
The International Telegraphic Union (ITU)
- formerly CCITT - is an international association that establishes worldwide
communication standards. Its standards are prefaced by the letter “V” and include the following:
The earlier V.xx Standards
V.13—Provides for simulated half-duplex (switched-carrier) control. V.32- and V.33-compliant modems that
support V.13 can be used in sync IBM® RJE environments, so these networks can also take advantage of
V.32/V.33 technology.
V.22, V.22 bis—Synchronous/asynchronous data transmission, full-duplex operation over 2-wire leased or dialup
lines; 1200-bps data rate (V.22 bis, 2400 and 1200 bps). Small businesses can beat the expense of leased lines
by using V.22 modems with dialup lines. If they add a 2-wire leased line later, they can still use this modem. V.22
accommodates the equipment found in today's typical “hybrid” network: synchronous mainframes and terminals
and asynchronous PCs. V.22 bis doubles data throughput (to 2400 bps) for quick service of large file transfers.
V.25, V.25 bis—Provides for automatic calling and answering circuitry for use on dialup lines. V.25 defines a dialup
parallel interface; V.25 bis defines a dialup serial interface. These standards enable any computer, sync or async,
to perform autodialing functions with a V.25-compliant modem.
V.32—Synchronous/asynchronous data transmission, full-duplex operation over 2-wire dialup or 2-wire leased
lines; 9600-bps data rate (fallback to 4800 bps). The V.32 recommendation is the first universal standard for
9600-bps modems on dialup or leased phone lines. V.32-compliant modems are the industry standard for
high-speed networks. Trellis-encoding modulation enables high data speeds and reduces errors. Data can be sent
over standard dialup lines, which are a lot less expensive to use than leased lines. And V.32-compliant modems
will work anywhere in the world. Any business, large or small, that handles large quantities of data transfers or huge
data files will benefit from the fast, accurate data transfers of V.32 technology.
V.32 bis—Synchronous/asynchronous data transmission, full-duplex operation over 2-wire dialup or leased lines;
14.4-, 12-kbps, and 9600-, 7200-, 4800-bps data rates. It offers two advantages over V.32. A V.32 bis modem
transmits data up to 14.4 kbps. And V.32 bis redefines modem-connection negotiations (called training and
retraining). Training is the procedure two modems use to make a connection; they “discuss” and “agree upon” a
data rate. Retraining is a subsequent negotiation after data exchange is under way—both modems “agree” to
reduce transmission to a slower speed to overcome ambient line noise. Retraining is triggered by the line noise.
After modems retrain, data transfer resumes. V.32 bis also provides a procedure called “fastrain,” which enables
the modem to fall either backward to a slower speed or forward to a faster speed. When two V.32 bis modems do a
fastrain, they stop, determine that they can run faster, and then switch speed in a few milliseconds. One advantage
of V.32 bis over V.33 (see below): With V.33 you can send sync data at 14.4 kbps over a 4-wire leased line, but
with V.32 bis, you can do the same over a 2-wire dialup line—much less expensive to use than a leased line.
V.33—Synchronous data transmission, full-duplex operation over 4-wire leased lines, 14.4- or 12-kbps data rate.
A V.33-compliant modem uses the same signal-modulation techniques that are used by V.32 modems, but
restricts operation to 4-wire leased lines. If your network requires sync data transmission up to 14.4 or 12 kbps
(as do many IBM and all supercomputer environments), choose a V.33 modem.
V.34—Synchronous/asynchronous data transmission, full-duplex operation over 2-wire dialup, and 2- or 4-wire
leased line; up to 28.8-kbps data rate with automatic fallback to compatible lower modems such as V.32 bis, V.32,
V.22 bis, and V.22. V.34 supports speeds from 1200 bps to 28.8 kbps.
V.34+—Synchronous or asynchronous data transmission, full-duplex over 2-wire dialup and 2- or 4-wire leased
lines; up to 33.6-kbps data rate with automatic fallback to compatible lower modems such as V.34, V.32, V.22 bis
and V.22. V.34+ supports speeds from 1200 bps to 33.6 kbps.
V.34bis: Capable of 33,600 bps or fallback to 31,200.
V.42: Same transfer rate as V.34 but is more reliable because of error correction (see Error Correction and Data Compression below).
V.42bis: A data compression protocol that can enable modems to achieve a data transfer rate of 34,000 bps (see Error Correction and Data Compression below).
V.44: Allows for compression of Web pages at the ISP end and decompression by the V.44-compliant modem, so transmitting the same information requires fewer data packets.
V.90: The fastest transmissions standard available for analog transmission, it is capable of 56,000 bps. U.S. Robotics® 56K V.90 Data Faxmodems use pulse-code modulation technology to boost the channel rate to 56 kbps.
V.92: Transmits at the same speed as V.90 but offers a reduced handshake time and an on-hold feature.
Error Correction and Data compression.
Error correction and data compression ensure accurate, swift data transfers. The protocols are used during
the data exchange between two modems. The following protocols are the most widely used by today's
modems:
MNP Levels 1–4—Microcom Networking Protocol® (MNP®), developed by Microcom Systems, Inc., enables
error-free async data transmission. Although MNP is proprietary, it became an industry standard in the
1980s because users demanded it from manufacturers. Both modems in a connection must use the same
MNP protocols.
MNP Level
5—Incorporates the first four levels and also applies a data-compression algorithm. It “compresses” data 2:1, so you can double the amount of data you send at the modem's top transmission speed. For example, with MNP Level 5, modems that transmit data at 4800 bps send an amount of data equal to an uncompressed 9600 bps.
V.42, V.42 bis—These “V Series”
protocols are internationally recognized standards for error control and data
compression. V.42 is the exception - it is the recommendation of the ITU for error
control. V.42 contains two algorithms (LAPM, or Link Access
Protocol, and MNP 1—4). When two V.42-compliant modems establish a connection,
they use LAPM to control data errors and retransmit “bad” data blocks. If
one modem supports V.42 and the other supports only MNP, then the two negotiate
to use MNP protocol. In both cases, the error-control process is
automatic and requires no special user actions or software programs. V.42 bis
corresponds roughly to MNP
Level 5. The difference is the amount of data compressed: V.42 can usually
generate a 4:1 ratio of data
compression, depending on the type of file transmitted.