Contents of the WHYV34.TXT file
ASYNCHRONOUS COMMUNICATION AT 28.8K BPS 17 Dec 94
By Paul Munoz-Colman
What are these terms V.FC and V.34?
V.FC ("FC" stands for "Fast Class") is a proprietary implementation
of an early version of the 28.8kbps asynchronous international
communications standard, designated V.34. For the past two years due
to marketplace pressure, V.FC implementations have been rushed to the
public, while the international standards body ITU-T (formerly known
as the CCITT) was slowly and carefully working on the design and
development of V.34. Those who released the early V.FC modems were
participants of the ITU-T Study Group who cooperatively developed
The current state of the technology.
V.34 is now a reality.
It was ratified late this past Summer, and is now the international
standard for asynchronous communication at modulations up to 28.8k
bits per second (bps). V.34 operates at a top speed which is twice
that of the previous generation of high speed modems (which were
called V.32-bis and operate at 14,400bps), and three times the speed
of the generation before that (called V.32, which operates at
At this writing, the marketplace is very volatile. Today, some
vendors have V.34 modems on the market, and several more are in
various stages of testing. Within six months or more, nearly all
vendors will have V.34 modems readily available.
The high-speed chaos which this has created.
In the haste to get modems to the marketplace and supply chipsets to
other modem manufacturers, there have been many releases of V.FC,
even within the same manufacturer of modems. This has caused
terribly confusing difficulty in interoperability between modems, in
establishing connections, maintaining them properly, and in
transferring data across them. To a much lesser degree, the
implementations of V.34 also suffer from some compatibility problems,
due to some difference in interpretation by modem vendors of the high
complex specification for this transmission rate.
Why is it such an issue?
Because of the design limits of 28.8k.
It is not only perfectly normal, but even typical in a V.34 or V.FC
connection to see a less than 28.8kbps connection. V.34 and V.FC are
not fixed-speed standards, and make/change their connections based on
phone line quality.
Very few people can get consistent 28.8kbps connections. Speeds of
28.8kbps require pristine phone line quality along the entire length
of the connection. But VFC and V.34 modems are capable of pushing
the limits of analog phone lines, commonly offering connection speeds
of 21.6k, 24k, and even 26.4kbps.
The bandwidth (or "bandpass") of a voice-grade phone line is about
3,000Hz to 4,000Hz (3-4KHz). Because the mathematics of compressing
28.8kbps pushes the phone line to near its theoretical limits, V.34
was designed to accommodate a variety of phone line conditions. V.FC
and V.34 are both smart enough to do what is called a "channel
probe", which is a frequency response and signal-to-noise ratio test
of frequencies at various points across the bandpass. During the
modem handshake, the modems send a series of tones to each other, at
known signal levels and specific frequencies. The modem calculates
the level of the received signal at each frequency, and therefore can
determine the maximum bandwidth available for use.
So, just how good does a line have to be?!
In reality, it takes line clarity at about -44dB or better (about the
sound level of a clearly whispered conversation across a medium size
room) at the top of the phone line's "bandpass" to obtain and
maintain a 28.8kbps connection. At about -46dB and below, modem
receivers tend to "go deaf". The typical long distance connection
can be much worse than this at that frequency; it is not unusual to
see -55dB to -70dB (closer to the background hiss level of a
factory-fresh medium-grade audio tape).
Standard transmit levels for domestic (US/Canada) modems are
approximately -10 dB, although V.34 and V.FC negotiate these levels
during the initial connection attempt. Receiving levels can vary
widely, depending on the conditions on your local phone line, the
line at the remote modem, and any long-distance or inter-office
Typical receiving levels range from -40 dB at the low end, to -15 dB
at the high end, with figures in the -20dB to -35dB range being most
common. Extreme values in either direction probably indicate a
problem in the connection from your modem to your local phone
company, which in some cases the phone company may be able to adjust.
However, be aware that Ma Bell and the long distance carriers are not
required by law, statute, or tariff to "fix" this "problem" on
unconditioned voice grade lines, because it is not really a
Why does it get bad?
Simple line impairment.
Variations in line quality are typically the culprit for low connect
rates. Line impairments can result in link timeouts (when the error
control protocol does not receive a block of data within its expected
timeframe), link naks (when the error control protocol requests
retransmission of data), blers (block errors; errors in received
error control protocol or data blocks), and resent data blocks.
Everyone occasionally gets "a bad line" and has to hang up and call
again to get a better connection. However, if you find that you
never or rarely connect at rates above 19.2kbps, you will want to
investigate the line quality of your connections.
All right, so how is V.34 more robust?
Recovery from adverse line conditions.
The goal of 28.8 modem protocols is not only to have a high top
speed, but to spend as much of that time as possible operating at the
highest possible speed under inevitably changing conditions. The
V.34 protocol has advanced procedures for training and for recovery
from transient disturbances during training. There are several
retrain and speed switching procedures to insure link integrity under
The line (channel) probe.
Both V.FC and V.34 "probe" the phone line for quality. The line (or
channel) probe quickly examines line conditions and selects the best
transmission strategy to optimize data transmission (there are a
variety of such strategies available). This technique can detect
certain unusual non-linear distortion mechanisms present on some
phone circuits, particularly international ones. The modems can then
select the operational modes that better combat distortion.
V.FC's weak implementation of probing.
The Channel Probe determines proper connection speed. V.34 measures
signal levels every 150 Hz across the entire channel, whereas V.FC
measures only 6 points, concentrated at the upper end of the
frequency range. This provides V.34 with a much more accurate sample
of the channel bandwidth, and greater accuracy in selecting the
appropriate symbol rate.
Thus, in V.FC, the weak implementation of the probe can generally
result in a "retrain" (when the two modems lose synchronization with
each other), which usually ends up lowering the speed to where it
should have been in the first place!!
The retrain is a Killer!!
A retrain is where the two modems suspend operations and renegotiate
the best possible connection all over again. V.FC retrains are
extremely slow, and can take 5 to 60 seconds, during which time the
modems appear "dead" to the network, host, or PC to which they are
connected. With V.FC, a retrain is generally required to change the
speed. This might be tolerated by some PC-to-PC connections, but it
is rarely tolerated in a network environment, particularly a
packet-switched one. The "timeouts" which will be sensed by a
variety of network software packages simply won't tolerate them, will
perceive them as disconnects, and will act accordingly, interrupting
end user service.
V.34's improvement of the probe and rate renegotiation.
A first major factor is that V.34 probes 25 frequencies across the
channel (vice 6 concentrated at the high end for V.FC). Because the
frequencies are spaced closer together, the frequency response
profile (ie the channel probe) is more accurate. That is a main
reason why V.34 connections are more reliable than V.FC connections
(more accurate line problem detection). The channel probe occurs
during initial modem negotiation, and during training and retraining.
Additionally, line noise and the line's signal-to-noise ratio is
remeasured continually during the connection.
Besides a better probe, rather than retrain, V.34 does a cooperative
and nearly instantaneous speed shift (also called a "fallback"),
which hosts can better tolerate. This rate renegotiation procedure
allow rapid switching ranging from 4.8kbps up to 28.8kbps, as line
V.34 speeds will usually be slightly lower, more truthful, and more
reliable than V.FC.
Other reasons why V.34 is a more robust standard.
V.34 has a number of features which may be implemented to a lesser
degree, a poorer degree, or may not available at all in V.FC:
precoding (changing the transmitted signal to reduce the effects of
noise multiplication in adaptive equalization, which compensates for
severe amplitude distortions); powerful multidimensional trellis
coding; constellation shaping and other innovations that give V.34 a
greater immunity to noise; and nonlinear coding (changing the
transmitted signal to improve operation in the receiver, which
addresses the problem of signal peaks being distorted due to
nonlinear circuit elements).
A key improvement in V.34 is independent receive and transmit channel
speeds (and their associated "symbol rates"). This allows the
receive and transmit channels of the modem to be adjusted
independently and operate at different speeds, thus making maximum
use of available bandwidth in the face of channel impairments. V.FC
forces both the receive and transmit channels to operate and the
lowest of the two speeds (and thus symbol rates), so a channel
impairment in either direction drops both speeds to that tolerated by
V.34 has more robust Trellis Coding in use by the modem's receiver
and transmitter. Trellis coding is a mathematical operation
performed on the transmitted data which improves the system's noise
immunity. The type of coding may vary significantly when connecting
modems from different manufacturers. V.34 supports a 64 state 4
dimensional coding scheme for greater noise immunity than the V.FC
All right, you convinced me! I just bought a V.34 modem and am
still having problems! What can I do to get a better connection.
*Try calling a different location. Line quality differs from region
to region, and it may be a problem with the lines or modem at the
other end of a particular call.
*Try connecting with a local call. Sometimes the connections within
a long distance call can cause impairments. (If this isolates the
problem, you can try switching long distance companies.)
*Try plugging the modem to a different phone line or wall jack.
*Try eliminating all telephone extensions, phone line surge
suppressors, line switches, utility monitoring devices connect to the
phone line, and anything else on the line with the modem.
*If you know someone else in your area with a high speed modem, ask
what type of connections they make. Try making the connection from
their location. If you encounter the same low connection rates, the
problem may be resulting from impairments along the lines running to
the local telephone company or within your home or office. Your
telephone company or a private consultant may be able to help.
Dropped V.FC Connections and V.FC Rate Switching.
VFC connections can only switch rates down to 14,400 bps. If you
connect using VFC and line quality drops below that allowable for a
14,400 connection, the modems will disconnect. If this occurs
frequently for a particular call, you will want to disable VFC before
calling that modem again. A slower modulation, (V.32-bis at
14,400bps, for example) will be established and will allow the modems
to switch to lower bit rates as line quality warrants. If the
problem is severe, use the modem's command set to disable V.FC, so
that V.34 (or a lower speed modulation on those modems which don't
have V.34) is forced. Some VFC modems from some manufactures do not
support rate switching (it's a tossup as to who does and in what
version they do). These connections are more likely to drop. For
these calls, you can force a lower connect speed by locking the modem
to a lower link rate.
Dropped V.34 Connections and V.34 Rate Switching.
Dropped connections can occur when there is a sharp decrease in line
quality during a call. V.34 modems will switch to rates as low as
4,800 bps to compensate for these changes. If the loss of quality is
extremely severe, even V.34 will drop the connection.
Technical phone line bandwidth requirements, and how a connection's
bandwidth and symbol rates are determined.
As already stated, V.34 and VFC connection rates are based on the
available bandwidth over the phone line.
The modems use the channel probe to test the phone lines before
establishing a connection rate, and will select the highest "symbol
rate" allowable. V.34 and V.FC modulations allow adjusting the
symbol rate to any of six possible values, to obtain the best match
with the available bandwidth. Other protocols only allow a single,
fixed value for the symbol rate, regardless of the bandwidth of the
A "symbol" is a waveform transmitted by the modem, which contains a
certain number of encoded bits of data to be moved across the link.
The receiving modem decodes this waveform, recovers the package of
bits, and re-assembles it. The noise levels in the channel determine
how many bits are encoded in each symbol; lower noise levels allow a
greater number of bits per symbol. The bandwidth of the channel
limits how many of these symbols may be sent each second.
Symbol rate is directly related to overall connection speed. In
general, a higher "symbol rate" allows greater data transfer speeds,
but requires greater bandwidth. Once a symbol rate is determined
through negotiation, it remains constant. The bit rate then is
adjusted on-the-fly to maintain low error rates, based on the modem's
tracking of noise and the signal-to-noise ratio.
The approximate bandwidth requirements for each symbol rate are shown
in the chart below. Thus, based on the connections you make, and/or
by diagnostics contained in the better brands of modems, you can
determine the approximate bandwidth detected by the modem. The
connection can be made at any of the frequency ranges for any of the
given symbol rates. This allows it to select the frequency range of
best quality for that call.
Symbol Carrier Bandwidth Maximum
Rate Protocol Frequency Requirements Bit Rate
2400 V.34 1600Hz 400-2800 Hz 21600
V.34/VFC 1800Hz 600-3000 Hz 21600
2743 V.34 1646 Hz 274-3018 Hz 24000
VFC/V.34 1829 Hz 457-3200 Hz 24000
2800 V.34 1680 Hz 280-3080 Hz 24000
VFC/V.34 1867 Hz 467-3267 Hz 24000
3000 V.34 1800 Hz 300-3300 Hz 26400
V.34/VFC 2000 Hz 500-3500 Hz 26400
VFC 1875 Hz 375-3376 Hz 26400
3200 V.34 1829 Hz 229-3429 Hz 28800
VFC 1920 Hz 320-3520 Hz 28800
3429 V.34 1959 Hz 244-3674 Hz 28800
NOTE: These are maximum bit rates. V.34 will connect at speeds as
low as 4,800 bps with any of the above symbol rates. VFC will only
connect down to 14,400 bps. If the bit rate is much lower than the
maximum bit rate supported by the symbol rate, the phone line has
lots of noise or other impairments on it.
Permission is granted to reprint and redistribute this information only
in its entirety.
Acknowledgement for selected source materials to:
- Paul Gebert, Joe Frankiewicz, and Dale Walsh of US Robotics, Inc.