Specifications
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Appendix A Glossary
to answer V.21 calls from overseas. The actual trans-
mission rate is 300 baud and employs frequency
shift keying (FSK) modulation, which encodes a
single bit per baud.
V.22 An ITU standard for modem communica-
tions at 1,200bps, with an optional fallback to
600bps. V.22 is partially compatible with the Bell
212A standard observed in the U.S. and Canada.
The actual transmission rate is 600 baud, using dif-
ferential-phase shift keying (DPSK) to encode as
much as 2 bits per baud.
V.22bis An ITU standard for modem communi-
cations at 2,400bps. Includes an automatic link-
negotiation fallback to 1,200bps and compatibility
with Bell 212A/V.22 modems. The actual transmis-
sion rate is 600 baud, using quadrature amplitude
modulation (QAM) to encode as much as 4 bits
per baud.
V.23 An ITU standard for modem communica-
tions at 1,200bps or 600bps with a 75bps back
channel. Used in the United Kingdom for some
videotext systems.
V.25 An ITU standard for modem communica-
tions that specifies an answer tone different from
the Bell answer tone used in the U.S. and Canada.
Most intelligent modems can be set with an ATB0
command so they use the V.25 2,100Hz tone when
answering overseas calls.
V.32 An ITU standard for modem communica-
tions at 9,600bps and 4,800bps. V.32 modems fall
back to 4,800bps when line quality is impaired and
fall forward again to 9,600bps when line quality
improves. The actual transmission rate is 2,400
baud using quadrature amplitude modulation
(QAM) and optional trellis-coded modulation
(TCM) to encode as much as 4 data bits per baud.
V.32bis An ITU standard that extends the stan-
dard V.32 connection range and supports 4,800bps,
7,200bps, 9,600bps, 12,000bps, and 14,400bps
transmission rates. V.32bis modems fall back to the
next lower speed when line quality is impaired, fall
back further as necessary, and fall forward to the
next higher speed when line quality improves. The
actual transmission rate is 2,400 baud using quadra-
ture amplitude modulation (QAM) and trellis-coded
modulation (TCM) to encode as much as 6 data
bits per baud.
V.32terbo A proprietary standard proposed by
several modem manufacturers supporting transmis-
sion speeds of up to 18,800bps. Because it was not
an ITU industry standard, it did not achieve wide-
spread industry support.
V.34 An ITU standard that extends the standard
V.32bis connection range, supporting 28,800bps
transmission rates as well as all the functions and
rates of V.32bis. This was called V.32fast or V.fast
while under development.
V.34+ An ITU standard that extends the standard
V.34 connection range, supporting 33,600bps trans-
mission rates as well as all the functions and rates
of V.34.
V.42 An ITU standard for modem communica-
tions that defines a two-stage process of detection
and negotiation for LAPM error control. Also sup-
ports MNP error-control protocol, Levels 1–4.
V.42bis An extension of CCITT V.42 that defines
a specific data-compression scheme for use with
V.42 and MNP error control.
V.44 ITU-T designation for a faster data-
compression scheme than V.42bis. V.44 can com-
press data up to 6:1. V.44 is included on most
V.92-compliant modems. See also V.92.
V.90 ITU-T designation for defining the standard
for 56Kbps communication. Supersedes the propri-
etary X2 schemes from U.S. Robotics (3Com) and
K56flex from Rockwell.
V.92 ITU-T designation for an improved version
of the V.90 protocol. V.92 allows faster uploading
(up to 48Kbps), faster connections, and optional
modem-on-hold (enabling you to take calls while
online). Most V.92 modems also support V.44 com-
pression. See also V.44.
V-Link A VIA Technologies high-speed
(266MBps) bus between the North Bridge and
South Bridge chips in VIA chipsets, such as the
P4X266 (for Pentium 4) and KT266/266A (for
Athlon/Duron). V-Link is twice as fast as the PCI
bus and provides a dedicated pathway for data
transfer.
vaccine A type of program used to locate and
eradicate virus code from infected programs or
systems.
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