ASYNCHRONOUS COMMUNICATION AT 28.8K BPS 17 Dec 94 By Paul Munoz-Colman Background. ~~~~~~~~~~ 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 V.34. 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 9,600bps). 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 carrier facilities. 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 "problem"! 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 adverse conditions. 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 conditions vary. 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 the impairment. 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 protocol. 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 link. 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.