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A BERLITZ COURSE IN HIGH SPEED DATA COMMUNICATIONS or How to Speak Modem by Theodore M. Rosenberg (301) 467-8988 2541 St. Paul St. Baltimore, MD 21218 Prepared for: The Columbia-Baltimore Users Group and Users of the PC_Tech Echo on EchoNet Available on: Columbia Online and The Writers Block Copyright 1990, Theodore M. Rosenberg, all rights reserved. Non-commercial distribution permitted without charge as long as this article is distributed unaltered and with full attribution. This restriction is not intended to interfere with either the fair rights to quote and abstract, or a SYSOP's ability to compress, reformat, or otherwise change the electronic form, rather than the content. The latest update of this is available from the Author in either print or media for a $5.00 handling charge, or may be FREQ'ed from The Writers Block BBS (EchoNet or FidoNet) or downloaded from either there, Columbia Online (CBUG), or GENIE (It is either on the WP or Laptop roundtable). Please send corrections, additions, comments, etc. to the Author at the above address or phone, by Netmail c/o The Writers Block, or, if of public interest, on the PC_TECH or ECHO_MOD areas of EchoNet. Rev 2.0 - July 14, 1991What is a Modem? A modem is a Modulator - Demodulator, it is a device that allows the transfer of digital data as an analog signal - such as over a phone line. Who Makes them? About 70% of all modems available in the US are Rockwell International chipsets or boards, with the named manufacturer providing some firmware and the packaging. Most of the rest are made by Motorola, with a few brave companies actually making their own from scratch. This could change with AT&T entering the market. Who Sets The Standards? CCITT: The Consultative Committee on International Telephone and Telegraph is an international body of technical experts responsible for developing data communications standards for the world. This is a United Nations sponsored group, and its members include representatives from major modem manufacturers, common carriers (such as AT&T), and governmental bodies. The CCITT establishes standards for modulation -- actual modem signaling methods. It also determines standards for error correction and data compression. It is possible (and likely) that one modem might follow several CCITT standards, depending on the various features and capabilities the modem offers. All modems signal one another at a variety of speeds, so CCITT standards for modulation are utilized by virtually every modem manufacturer. Some of the standards that are primarily modulation do include some of the higher layers (such as negotiation) as well. Multi-speed modems may use several of these standards. CCITT standards for analog modems begin with "V.", digital standards begin "X." pronounced "Vee dot" or "Ex Dot." Standards also may end with a "bis" or "ter," meaning second generation or third generation respectively. A common confusion arises out of connector standards with similar numbers. The CCITT does NOT set connector standards. ANSI: The American National Standards Institute, is a US standards development organization. The ANSI develops standards for thousands of different areas, from architectural specifications for the handicapped to computer programming languages. With regard to data communication, the term "ANSI" generally refers to ANSI standard X.64 as implemented by IBM in ANSI.SYS. The ANSI X.64 standard specifies a series of codes that a host system can send to a remote data terminal to control color attributes, cursor positioning, inverse video and screen clearing on the terminal display. ASCII: The American Standard Code for Information Interchange is a standard that defines 128 different characters that can be page 1used for data transmission. These include control characters, letters of the alphabet, both upper and lower case, numbers, and a full set of punctuation characters. Because there are only 128 ASCII characters, only 7 bits are required to form each of the 128 possibilities. There are also versions of ASCII for less than 7 bits, more commonly called BAUDOT, Correspondence, or "Navy." Many computer makers have extended the ASCII character set by adding 128 more characters. This was accomplished by simply adding one more binary digit, resulting in a total of 256 transmittable data characters. Each manufacturer, however, created their own set of 128 additional characters. All extended character sets are NOT the same. In the case of the IBM PC and compatibles, the extended characters include international alphabet, graphics and mathematics characters. These are commonly known as "IBM Graphics" characters, or "ANSI graphics". In addition, some languages, (such as APL) also define the extended characters for special purposes. What do the various standards mean? Modulation Modulation refers to the signaling method that is used by the modem. Two modems must use the same modulation method in order to understand each other. Each data rate uses a different modulation method, and sometimes there is more than one method for a particular rate. For example the Bell 212A and V.22 modulation standards that both specify 1200 bps modulation, but they work differently, and are not directly compatible. Common Modulation Standards V.21: a data transmission standard at 300 bps. This standard is used primarily outside the United States. (300 bps transmissions in the United States primarily use the Bell 103 standard). It is Full-Duplex and is Frequency Shift Keyed. Bell 103: a 300 bps transmission standard. This standard is used primarily inside the United States. (300 bps transmissions outside the United States primarily use the V.21). It is Full- Duplex and is Frequency Shift Keyed (FSK). Bell 202: 1800 bps Asynchronous 4 wire leased line, or 1200 bps Synchronous 2 wire leased line. Bell 208: 4800 bps Synchronous leased line. 208A is 4 wire and 208B is switched. Bell 212A: another 1200 bps transmission standard. This standard is also used primarily inside the United States. (1200 bps transmissions outside the United States primarily use the V.22 standard). It is Full-Duplex and is Phase Shift Keyed (PSK or DPSK). page 2V.17:a Half-duplex implementation of V.33, 14000 bps transmission, now used by 14400 bps Group III fax. V.22: a data transmission standard at 1200 bps. This standard is also used primarily outside the United States. (1200 bps transmissions in the United States primarily use the Bell 212A standard). It is Full-Duplex and is PSK. V.22bis: the international data transmission standard at 2400 bps.It is used both inside and outside the United States, and is Full-Duplex. QAM, Quadrature Amplitude Modulation allows modems to increase speed from 1200 bps to 2400 bps.In QAM, each signal represents four data bits.Both 1200 bps and 2400 bps modems use the same 600 baud rate, but each 1200 bps signal carries two data bits, while each 2400 bps signal carries four data bits: 600 signals per second X 4 bits per signal = 2400 bps. A technique known as Automatic Adaptive Equalization enables 2400 bps modems to adapt to phone line impairments call-by- call. Essentially, if the modem is experiencing problems with a noisy line, it looks for a "sweet spot" in the bandwidth and attempts to avoid troublesome frequencies. This technique makes 2400 bps modems more tolerant of line noise than their 1200 bps counterparts that use compromise equalization. V.23: is a split data transmission standard, operating at 1200 bps in one direction and 75 bps in the reverse direction. Therefore, the modem is only "semi-full-duplex," meaning that it transmitting data in both directions simultaneously, but not at the maximum data rate. This standard was developed to lower the cost of 1200 bps modem technology, which was still very costly in the early 1980s, when such modems were designed. This standard is still in use, but primarily in Europe. It is PSK. V.26: 2400 bps Synchronous 4 wire leased line. V.26bis: 1200 bps/2400 bps Asynchronous 2 wire leased line. V.27bis: 4800 bps/2400 bps Synchronous 2/4 wire leased line. V.27ter: 4800 bps/2400 bps Asynchronous 2 wire leased line. Also used by Group III Fax 4800 bps/2400 bps V.29: is a 9600 bps data transmission standard that defines a Half-duplex (one-way) modulation technique. Although modems do exist which implement this standard, it has generally only seen extensive use in Group III facsimile (FAX) transmissions at 9600 bps/7200 bps. Since it is a Half-duplex method, it is substantially easier to implement this high speed standard than it would be to implement a high speed Full-duplex standard. V.29 is not a complete standard for modems, so V.29-capable MODEMS (not faxes) from different manufacturers will not necessarily communicate with one another. Prior to the development of Group III Fax, V.29 was used primarily for 4 wire leased line, or short haul transmission. page 3V.32:is also a data transmission standard at 9600 bps, but V.32 defines a Full-duplex (two-way) modulation technique. It is a full modem standard, and also includes forward error correcting and negotiation standards as well. This is generally considered "the" standard for high-speed modems today. It is Full-Duplex and uses Quadrature Amplitude Modulation and Trellis Coded Modulation. ECHO-CANCELLATION is the method V.32 uses to solves the problem of overlapping transmit and receive channels. The transmit and receive bands overlap almost completely, each occupying 90 percent of the available bandwidth. Measured by computations per second and bits of resolution, a V.32 modem is roughly 64 times more complex than a 2400 bps modem. V.32bis: is the new high speed standard. V.32bis operates at 14400 bps and, like V.32, will be a Full-duplex method. V.32bis will be generally available by the end of 1991 and will rapidly replace V.32 in general use. V.33: A version of V.32 without echo cancellation, uses QAM and TCM at 14400 bps, normally Synchronous 4 wire leased line, but some two wire implementations exist. V.35: Even though the number is higher, this is a supposedly obsolete standard for high-speed (19200 bps) communication originally used on leased lines. As I talk to manufacturers, I am finding that this is still in use. Hayes "V": Also called Hayes Ping Pong modulation. Hayes also developed its own technology for high speed transmission. Like the others Proprietary modems, Hayes "V" series high speed modems only talk high speed to other Hayes modems. HST: High Speed Technology is a proprietary method of U.S. Robotics, it is not Full-duplex, and it does not support high speed transmission in BOTH directions. Current HST modems send data at 14400 bps in one direction, and 450 bps in the other direction. The high speed channel changes direction depending on which side of the transmission has the most data to send. HST modems can only talk at high speed with other HST modems, although they also adhere to existing standards for 300 bps, 1200 bps and 2400 bps operation. PEP: Packetized Ensemble Protocol is a proprietary method used by Telebit in their Trailblazer modem series. Like the HST, PEP modems will only connect at high speed with other PEP modems. PEP communicates at 20600 bps., the highest speed in general use. PEP is based on a multi-carrier technique, the transmission channel is divided into 512 independent, very narrow channels. The main advantage is that no receiver adaptive equalizer is needed because each channel is very narrow compared to the overall channel bandwidth. The modulation rate in each narrow channel can be changed somewhat independently. Trailblazer is different from many other modems in that the decision to fall back to lower speeds is built into the modem protocol, rather than controlled by the user's computer port. Traditional modulation systems would have to fall back in larger steps. But there are three problems: page 41. The turn-around delay is very long compared to conventional modulation techniques because data must be sent in large blocks. A typed character may take as much as a half of a second to be echoed back to the system that sent it. As a result, the system is not the best for interactive online sessions. 2. The Trailblazer receiver cannot track carrier phase jitter, Instead of canceling out phase jitter, PEP can only respond by lowering throughput. 3. The ability to transmit at the maximum rate when subject to some types of channel impairment is considerably less than for conventional modems, HOWEVER, The multiple channel technique offers extremely good immunity to impulse noise (the most common) because the impulse energy is distributed over narrow channels. Due to the better overall performance of PEP, and the better turnaround time of HST, US Robotics had captured a lot of the general high speed traffic in the PC world, and Telebit captured the majority of similar high speed traffic in the Unix world prior to V.42bis. Both US Robotics and Telebit both offer modems that have both the proprietary and standard transmission options. DIS : A relatively inexpensive, non-standard method of Full- duplex 9600 bps communication is Dynamic Impedance Stabilization, which improves the signal-to-noise ratio of the telephone line by increasing the clarity and power of the signal, and automatically compensating for impedance variations on the phone line. It has a fallback rate of 7200 bps if too much line noise exists for 9600 bps communications. DIS is a proprietary method owned by CompuCom, and is a companion to CSP data compression,(see below). Error Correction Most error correction would better be called error detection, it refers to the ability to identify errors during a transmission, and to automatically resend data that appears to have been damaged in transit. If error correction is to be used, both modems must adhere to the same error correction standard to make it work. Most error correction is Automatic request for retransmission (ARQ), in that the modem sends a block and a cyclical redundancy checksum (CRC). The receiving modem recalculates the CRC, and if it doesn't match, asks for a re- transmit. TCM: Trellis-Coded Modulation (with Viterbi coding) is an optional error-correction method included in the V.32 standard. TCM allows modems to check for transmission errors with a redundancy bit, which results in fewer errors on noisy lines. Trellis Encoding works WITH other error correction methods such as V.42. Unlike other methods, TCM is true error correction, in that for small errors, it does not force a retransmit, it actually fixes them. MNP - Microcom Network Protocol is a set of standards developed by Microcom, and made available by them to other manufacturers. page 5MNP Class 1 is referred to as Block Mode. It uses asynchronous, byte-oriented, Half-duplex transmission. This method provides only about 70% efficiency, and is rarely used today. MNP Class 2 is called Stream Mode, and uses asynchronous, byte- oriented, Full-duplex transmission. Because of protocol overhead (the time it takes to establish the protocol and operate it), throughput at Class 2 is actually only about 84% of that for a connection without MNP, delivering about 202 cps (characters per second) at 2400 bps and is rarely used today. MNP Class 3 incorporates Class 2, and is more efficient. It uses a synchronous, bit-oriented, Full-duplex method. This procedure yields throughput about 108% of that of a modem without MNP, delivering about 254 cps at 2400 bps. MNP Class 4 uses Adaptive Packet Assembly and Optimized Data Phase techniques, it improves throughput and performance by about 5%, although actual increases depend on the type of call (noisy or clean), and can be as high as 25% to 50% on some links. V.42: is a CCITT error-correction standard that's similar to MNP Class 4, In fact, because the V.42 standard includes MNP compatibility through Class 4, all MNP 4- compatible modems can establish error-controlled connections with V.42 modems. This standard, however, prefers to use its own better performing protocol -- LAPM LAPM : Link Access Procedure for Modems, like MNP, copes with phone line impairments by automatically re-transmitting data that is corrupted during transmission assuring that only error free data passes through the modems. LAPB : Link Access Procedure Binary, like MNP or LAPM, copes with phone line impairments by automatically re-transmitting data that is corrupted during transmission assuring that only error free data passes through the modems. Used with X.25. LAPD : Link Access Procedure Direct, like MNP or LAPM & B, but for internal or leased lines. Data Compression Data compression refers to the ability in some modems to compress the data they're sending, squeezing data to a smaller size as it is sent. This saves time and can result in considerable money saved by long-distance modem users. Not all types of data can be compressed by the same amount, but gains can nearly always be realized. Raw text files will allow the highest increase, while program files cannot be compressed as much and the increase in transfer speed will be less, graphic files even less, and compressed files such as ARC and ZIP, cannot be compressed at all. Current compression methods are still improving, with Bell's application of B-Trees and Brents application of Huffman encoding to LZ methods. The latest methods (Fiala-Greene TRIE data structures) are not yet in commercial form, but promise faster and greater compression. MNP Class 5 is a Data Compression protocol which uses a real-time adaptive algorithm. It can provide up to 2-1 compression, On page 6pre-compressed data MNP 5 can actually EXPAND the data and performance can actually decrease. For this reason, MNP 5 is often disabled on BBS systems. MNP 5 uses a form of Huffman Coding and a dynamic dictionary algorithm. Huffman Coding replaces ASCII with a variable length "minimum redundancy code" with the length of the character based on the frequency of occurrence. For example the code for the most common character "E" is 00, and the code for the much less common character "D" is 11000. MNP Class 7 provides Enhanced Data Compression. When combined with Class 4, it can obtain about a 3:1 improvement in performance. It is designed primarily for use with a V.22bis (2400 bps) modem. This class is currently unique to Microcom modems. Since it requires much more hardware and is usually inferior to V.42bis, it is not likely to be seen in the future. V.42bis: a CCITT data compression standard similar to MNP Class 5, but providing about 4-1 compression. Of course, this also means it provides better throughput. V.42bis only compresses data that needs compression. Each block of data is analyzed, and if it can benefit from compression, compression is enabled. Files on bulletin board systems are often compressed already (using ARC, PKZIP, and similar programs). While MNP Class 5 can actually decrease throughput on this type of data, V.42bis will not -- compression is only added when a benefit will be realized. V.42bis uses Lempel-Ziv encoding (like the PKZIP and LHARC programs) which is a dynamic dictionary algorithm which assigns codes to repeated strings in the actual file being compressed, rather than theoretical forms. To negotiate a standard connection using V.42bis, V.42 must also be present. Thus, a modem with V.42bis data compression is assumed to include V.42 error correction. Most V.42bis modems also support MNP5, however it is not required by the standard. CSP: CompuCom Speed Protocol offers compression up to 4:1. CSP is a proprietary method owned by CompuCom, and requires DIS. Fractal: I am unable to locate the material at present, however a firm has developed a unique data compression algorithm specifically for graphics files, using fractal analysis methods. This method is currently only available in software (like zip, arc, etc. for data) but, when I spoke to the developers a few months ago, they said that they were in negotiation with a modem manufacturer to provide a hardware solution for people who need to transmit large amounts of graphics data regularly. They were also hoping to provide a better alternative to V.29 Group III Fax. A Fax is just a graphics file. This is especially appropriate, as all modern data transmission is based on Benoit Mandelbrot's early work on the nature of telephone line noise. His discovery that line noise could be described as a Cantor set was one of the first steps in the development of fractal mathematics. Fallback Negotiation page 7If the line noise is high enough, more data will get through with a lower speed than with the high speed and retransmission. V.32(bis): includes an optional fallback to 4800 bps under conditions of extreme line noise. MNP Class 10: is a fallback protocol developed by Microcom, which provides for small movements down (and back up) in speed under poor line conditions. It has been recently licensed to Rockwell International. Since Rockwell makes at least 70% of all modem chipsets sold in the US, it will rapidly become a de-facto standard in all US Modems. Connection Negotiation Connection Negotiation refers to the manner in which two modems establish which modulation method will be used during a connection. Modems listen to the tones sent by a remote modem to determine what modulation method will be used. Since different modulation methods often use different answer tones, these can be used by the calling modem to determine which method to use. Negotiation standards have been created to make the process easier. These standards dictate the sequence of events that will occur when a modem answers the phone, eliminating the guesswork associated with the listen to the tones method. Negotiation is part of each modem standard. It should be noted here that the tones sent to originate a call are usually different than those sent to answer a call. Some modems can originate a call forcing an answer set of tones, or vice-versa. Most negotiation standards are included in the base standard, the only separately named one that I have seen is Annex A, the method used by V.42 modems to fall back from V.42 to MNP if the other modem is not a V.42, and then to non-corrected if the responding modem is not MNP. The fancier the modem, the longer it can take to negotiate a connection. If you stop and think about this, it is obvious, with more choices, the two modems will spend more time finding the best combination. WARNING: With most high speed modems you must "lock" your port speed at a set speed ( as high as your system will allow). The modems will negotiate the best speed that they actually use, and may change it as line conditions dictate. If your communications software is set to detect baud rates, the software will also try to change, but the modems will ignore it and you will lose contact with your own modem. Duplex Duplex refers to whether a data communications path is one-way or two-way. "Full-duplex" means that data can flow in both directions at the same time. "Half-duplex" means that data can flow in only one direction at one time. Most modems are Full- duplex, but communications software can most often still be set to take advantage of Half-duplex connections. Some modems are pseudo Full-duplex. This means they cannot transmit data at high speed in both directions at the same time page 8because they are really operating in a fast turn-around Half- duplex mode internally. Bits and Parity Data Bits: In communications, common settings are either for 7- bit or 8-bit data. Generally, both ends of the connection must be set the same way. If one end is set to 7-bit data and the other end is set to 8-bit data, reliable communication cannot usually be established. This is because one end interprets the 8th data bit as a parity bit, and the other end tries to interpret it as a part of the current character. On a connection like this, some characters will display properly, while others will appear as "garbage," depending on which direction the data is traveling. If the communications link is set to transmit only 7-bit data, the sendable characters are limited to the 128 defined ASCII characters. The extended character set, such as the PC's single and double line boxes and foreign characters, CANNOT be sent unless the link is first set to allow the transmission of 8-bit data. Some systems have even 5-bit and 6-bit data, and use character sets such as BAUDOT and Selectric, but these systems are uncommon today. For some reason, TTY's for the deaf use the extremely obsolete BAUDOT codes, making them out of step with the rest of the communications world. Parity Bit: When you establish communications with another computer, parity is set to "even," "odd," "mark," "space" or "none." These are terms for the manner in which the parity bit is interpreted by the receiver. Parity is a primitive form of error-checking. The state of the parity bit, when set to be even or odd, is based on a simple mathematical formula. Depending on the data bits, the parity bit will either be on or off. Normally, the limited error checking capabilities are not utilized. This explains why the setting of parity to "none" is so common in communications today. This allows the parity bit to be used as a normal data bit instead. Start and Stop Bits: Start and stop bits allow each character sent to be set in a "frame." The beginning of the character, the first part sent, is the start bit, and the end of the character, the last part sent, is the stop bit. Each character sent is thus framed with a distinct beginning and ending bit and this allows the receiving system to know when each complete character has been sent. There is always just one start bit. However, there may be one, one and a half or two stop bits. Stop bit length used to be critical when serial communication was primarily handled with electromechanical equipment, such as an old-fashioned Teletype machine. The print head in this type of equipment took a fixed amount of time to return to its "home" position, and this was accomplished during the sending of the stop bits. A longer stop bit length gave the print head more time to return to its home position. page 9In modern all-electronic serial communication, the stop bit is still necessary, but only to mark the end of a character. A delay isn't necessary as there isn't usually anything mechanical involved. Asynchronous:Framing the character with start and stop bits forms the basis for "asynchronous" communications. In asynchronous transmission, characters do not have to flow constantly - there can be gaps or spaces between each character. The receiver knows when a character is sent, by the framed nature of asynchronous transmission - the start and stop bits. Synchronous (or bisynchronous): An alternate serial transmission method exists known as synchronous communications. It occurs when there are no start or stop bits, and is possible only if data characters flow constantly at a fixed bit rate with no interruptions. When there is no data to send, null characters are sent at the fixed rate to keep data bits flowing constantly, but they are discarded by the receiver. Because there are no start or stop bits, it is possible to remove 2 of every 10 bits used in Asynchronous communications. This results in a 20% faster data speed with the same serial bit rate. However, because of the requirement for constant data flow, Synchronous transmission requires additional protocol, it is used is with high speed modems. When these modems use MNP or V.42 protocols they use synchronous communications between the modems themselves. However, you still use asynchronous communications between the computer and the modem. There are software emulations of Synchronous communications, such as Flashlink from Cardinal, or SynchUp from Motorola that will increase thruput on lower speed modems. Flow Control Flow control refers to the method of controlling the flow of transmitted data, so it doesn't "overrun" the data receiver's ability to receive the incoming signals. Flow control allows the receiver to signal the transmitter to pause, while recently received data is properly assimilated, then signal it to restart the data flow when it's ready to receive more. There are generally two forms of flow control - software and hardware. Hardware flow control is not always required. It is generally needed only with high speed modems. Hardware flow control uses two of the RS-232 (serial) pins to start and stop the data flow. Its advantage is that it is data independent and thus can be used for reliable flow control with any type of data stream. CTS/RTS: Clear To Send - Ready To Send is one form of Hardware flow control. DSR/DTR: Data Set Ready - Data Terminal Ready is another form of Hardware flow control. page 10 V.24: is the CCIIT standard for, not only flow control, but all of the communications between the modem and the serial interface. X-ON/X-OFF: Software flow control, called XON/XOFF flow control, starts and stops the data flow based on the reception of certain control characters. Although this type of flow control can be used by hardware devices, software flow control can be used by either the user or the application program to start and stop data transmission by using control keys. <Ctrl>S to temporarily halt data flow, and <Ctrl>Q at any time to restart data flow. Software flow control has two problems; 1) Due to the time it can take for an X-off to reach the processor, data can overrun the buffers. To prevent this, X-off is normally sent at 80% of capacity. At high speeds, this may not be enough. 2) It is easy to have an accidental X-off triggered, either by a code in a binary file, or by operator error. On many types of keyboards it is surprisingly easy to accidently hit a <ctrl>S. Anyone who has worked on a large system will have run into the mysterious intermittent terminal lock up problem. Commands To tell the modem what you want it to do, you have to be able to communicate to the modem itself. There are two common ways to do this: AT: The common US command set was developed by Hayes, and is called the "AT" command set because commands begin with the prefix "AT". If a modem uses this set, it is usually advertised as "Hayes compatible". If it doesn't use this set, it probably is not sold in the US. V.25bis: This is approximately the European equivalent of the Hayes set. It includes auto-dial from NVRAM. V.54: Actually not a command set, this is the specification for a "loop-back" test to test the modems performance and status. X.25: This command set is a special set, used to communicate with a "PAD" or Packet Assembler Disassembler. A PAD allows you to communicate simultaneously with more than one system. Put simply, it puts data going to each destination into a "packet", tags it with an address, and ships it off. Incoming packets are sorted and sent to each actual and "virtual" destination. When you are on-line to any major network (such as Timenet,Telenet, Sprint, PDN, etc) you are actually hooked up to a PAD. If you have X.25, and the right communications software, you can do more than one thing simultaneously,such as download, read mail, and upload at the same time.As far as I know, Hayes and TL Systems are the only general market modem companies to offer X.25 at the present time, more WILL follow in coming years. page 11 All RBOC's, and most, if not all, long distance carriers and independent telephone companies offer X.25 dial up service. Short Haul Some modems are sold as "short haul" modems, these differ from regular modems in that they are really a type of repeater used to boost the signal on long runs of serial cable. Short haul modems need not use any common protocol as they are always Full-duplex, used on dedicated cable, and in pairs. In addition, they must provide the power to send the signal along the line to the other end. On the other hand, they don't need error correction, compression, connection negotiation, fallback, or any of the other stuff I've been discussing, they just squirt the data down the line, and demodulate what is coming back. Point To Point / Switched You may sometimes see modems called "point to point" or "switched". These modems are for use on leased or dedicated lines. A pure point-to-point modem may actually be a short-haul, or it may be too dumb to survive a switching network. A switched line modem is very similar to a regular dial-up modem, but is intended for use on leased or internal lines. More Buzzwords PSTN : Public Switched Telephone Network - regular voice dial-up service. ISDN: Integrated Services Digital Network - a new switched digital high-speed system expected to eventually replace the current analog PSTN. RS-469 (A or B): A specification for testing modems, adopted by the Telecommunications Industry Association and the Electronics Industries Association. NVRAM: Non Volatile Random Access Memory, on board memory which does not require power to keep data. It is used to store phone numbers and setup information. What's New New products starting to arrive are: The Pocket Modem: a very small external modem (currently only available at 2400 bps) which takes its power off of the serial line. It can be put in your pocket and shifted from system to system. It is mainly used with laptops or smaller systems. The Cellular Modem and/or FAX: a combination Cellular Telephone and modem. These have two uses: The most obvious is for travel, but some are being made to function as backups for WAN's, so that even if the phone lines go down, the system stays up. The Gateway modem: These operate directly off of LAN's, and do not require a communications server - just plug in the ethernet cable. page 12What's Coming A new high-speed protocol will probably be released in 1991 (V.32ter ??) the main disagreement now seems to be whether it will be 19200 bps, 24000 bps, or 25600 bps. It will take at least a year after that for any of these modems to reach the market. V.17, 14400 bps Group III fax is out, but few manufacturers are offering it now, within the next year, it will replace 9600 bps fax on most new purchases - at lower cost. 2400 bps modems are now where 1200 bps modems were two years ago - within the next one to two years, no one will be making them, and 14400 (or higher) will be the standard. page 13 Litigation As far as I can ascertain, there is only one current major suit in the data communications area (unlike the rest of the computer field). A few years ago, Hayes sued some modem manufacturers over the use of one element of the Hayes AT command set, the only part that they had not released into the public domain. That was the programmable escape code, i.e. the ability to CHANGE the escape codes with software. I was not able to get any information from Hayes, but others in the industry said that the suit seemed to be slowly dragging on. Recent news reports, however, seem to indicate that the action on this suit is heating up. Licenses The V.42bis is an unusual standard in that, while it is an international standard, the Zemple-Lev-Welsh (LZ84) Compression algorithm that it uses, infringes on patents held by IBM, British Telcom, and UNISYS. To produce a legal V.42bis modem, one must have licenses from all three. I was unable to get details of licensing arrangements from either Unisys or BT, but IBM is probably rather typical in their approach. Unlike their usual closely protected licenses, IBM will make its patent rights under patent #4814746 available to ANY modem manufacturer for either. 1) A one-time payment of $20,000, or 2) A royalty of 1% of V.42bis sales, or 3) If you have anything IBM is interested in, or are doing other business with them, as part of a negotiated license or cross-license agreement. MNP-1 to MNP-4 have been put in the public domain by Microcom, as has most of the Hayes AT command set. MNP-5 and MNP-10 require licenses from Microcom, and, to actually build a saleable modem, licenses MAY be necessary from AT&T, Motorola, and/or Rockwell International. The result of this is to make business easier for Rockwell and Motorola, aside from their continuing lead in technology, they have the legal staff to keep everything straight, a rough job for a start-up or small company. Pricing I remember RENTING 100 bps modems for $45/month from AT&T, later I bought a high speed (300 bps) modem for $1,200. Now, 1200 bps modems are dead, almost no-one makes them, and they cost MORE to make than 2400 bps modems. A 2400 bps internal modem should cost under $70, street price, with MNP-5, maybe $95. A 9.6 kbs V.42bis modem can be bought for $300-500. External modems cost a little more due to their case and power supply. page 14 Modem prices should continue to drop, with many observers calling for V.32bis\V.42bis internal modems available as low as $100 by the end of 1992. Fax capability $10-$25 extra. This is due in part to: 1) The cost of making modems has plummeted. If you look at an old 300 bps modem, you can see that it is MUCH larger than a new high-speed modem, it has a LOT more chips, and lots of circuit traces. A new modem uses LSI low power chips. 2) There are a lot more modems sold today, and with new services such as Prodigy, and IBM's decision to make modems standard on the PS/1, more people are becoming aware of modems than ever before. You can spread the costs of ramp- up over millions of units, rather than thousands. 3) The promised entry of AT&T into the market as a competitor to Rockwell creates more competition. Tables: Signal Baud BITS/ Constell Type Modulation Bits/Sec Rate Symbol Points v.32bis TCM 14,400 2400 6+TCM 126 v.32bis TCM 12,000 2400 5+TCM 64 v.32 TCM 9,600 2400 4+TCM 32 v.32 QAM 9,600 2400 4 16 v.22bis QAM 2,400 600 4 16 Bell 212A QAM 1,200 600 2 4 Bell 103 FSK 300 300 1 - page 15 Thanks to: Dick Checket and Richard McCarty of A T & T Paradyne Ed Prentiss of Microcom - Racal Vadic Steve Mills of UDS - Motorola Peter Theune of Johns Hopkins Applied Physics Lab MICC technical support (probably Casey Garrigan) Rich Blatt - Octocom Systems Special Thanks to: Mitch Baker, Senior Engineer, Rockwell International Gary Sanderson, Chief Engineer, Cardinal Industries John Lowe, Intellectual Property Attorney, and Commercial Relations program manager for V.42bis, IBM CHQ No Thanks to: Hayes, who could only send me the regular handouts, no useful information, and couldn't put me in touch with anyone but untrained marketing reps. Unisys, who after three weeks of passing me through their system, never located anyone who knew anything about either modems or patents. Bibliography: Sales materials, users manuals, technical manuals, etc. for about 50 different modems, and lots of columns and articles, particularly in PC Week (Ziff Davis). A Comparison of High Speed Modems, Mike Ehlert SysOp: PACIFIC COAST MICRO BBS Taking The "Buzz" Out of Buzz Words, Alan D. Applegate eSoft Possibilities Newsletter, June, July, and August 1990 issues. A monthly customer support publication of eSoft, Inc., Aurora, Co While I did not receive it until after Rev 1.4, recommended reading includes: Racal-Vadic's 1991 High Speed Dial-Up Modem Handbook Racal-Vadic Communications Group, Milpitas California page 16Sources Manufacturer Toll Free Charge Call Support BBS ================================================================= Anchor Automation 1 (818) 998-6100 American Mitec 2 (800) 648-2287 (408) 432-1160 Anderson Jacobson (408) 435-8520 ATI Technologies 2 (416) 756-0718 Best Data Prod.1 (800) 632-2378 (818) 773-9600 Black Box Corp 2 (412) 746-5500 Cardinal Technologies 2 (717) 293-3800 CMS Enhancements 1 (714) 222-6000 Codex - Motorola 3 (800) 426-1212 (508) 261-4000 Compucom 4 (800) 228-6648 (408) 732-4500 (714) 946-9337 also Educational Progr. (918) 224-0065 (918) 224-0005 Computer Perip. 2 (800) 854-7600 (805) 499-5751 (805) 499-9646 Data Systems, Inc.1 (708) 459-8881 Digicom 3 1 (800) 574-2730 (408) 262-1277 Dove Computer 1 (800) 622-7627 (919) 763-7918 Dowty Comm. 2 (800) 227-3134 (301) 317-7710 Everex Systems 1 (415) 498-1111 E-Tech Research 2 (800) 328-5538 (408) 730-1388 Farallon (415) 596-9100 Fastcomm Comm 2 (800) 521-2496 (703) 620-3900 Forval America 3 (800) FORVAL-1 (801) 561-8080 General Datacom 2 (203) 574-1118 GVC Technologies2 1 (800) 289-4821 (201) 579-2702 Hayes Micro 5 1 (800) 241-9625 (404) 441-1617 (800) 874-2937 Image Comm. 2 (800) 666-2496 (201) 935-8800 Inmac 2 1 (800) 547-5444 (408) 727-1970 Intel Corp 2 (800) 538-3373 Logicode 2 (818) 879-0533 Magic Modems (800) 622-3475 (512) 343-3421 Mastercom (213) 834-6666 Memotec DATA 3 (800) 423-6144 (508) 681-0600 MICC 2 (800) 289-6422 (408) 980-9565 Microcom Inc. 2 1 (800) 822-8224 (617) 551-1000 Micro Electronic 1 (508) 435-9057 Multi-Tech Syst. 2 (800) 328-9717 (612) 785-3500 NEC America 2 (800) 222-4632 (408) 433-1250 Octocom Systems 3 (508) 658-6050 OmniTel Inc. 3 (800) 666-4835 (415) 490-2202 Outbound Systems 1 (800) 444-4607 (303) 786-9200 Paradyne-A T & T (800) 482-3333 Patton Electron 1 (301) 975-1000 Penril DataComm 3 (301) 921-8600 Practical Perip.2 1 (800) 442-4774 (818) 706-0333 Prometheus Prod.2 1 (800) 477-3473 (503) 624-0571 Racal Data Comm.2 (800) 722-2555 (305) 846-4942 Racal-Vadic 2 (800) 482-3427 (408) 432-8008 Racal Milgo (800) 327-7909 (305) 846-1601 Shiva 6 (800) 458-3550 (617) 252-6400 Spectron Celluar 1 (214) 630-9825 Telcor Systems 2 (800) 826-2938 (508) 651-0065 Telebit Corp 7 1 (800) 835-3248 (408) 734-4333 Telenetics 1 (800) 826-6336 (714) 779-2766 Toshiba Inc. 2 (212) 682-2595 Touchbase Sys. 1 (800) 541-0345 (516) 261-0423 TL Systems 8 (508) 970-1295 UDS - Motorola 2 (800) 451-2369 (205) 430-8000 page 17 Manufacturer Toll Free Charge Call Support BBS ============ =============================================== US Robotics 9 1 (800) DIAL-USR (708) 982-5001 (708) 982-5092 Ven-Tel 2 (800) 538-5121 (408) 463-7400 Vocal Tech. 1 (203) 356-1837 Western DataCom 2 (800) 262-3311 (216) 835-1510 1. Pocket or Cellular Modem. 2. V.32/V.42/V.42bis 3. V.32/V.32bis/V.42/V.42bis 4. CSP/DIS 5. V.32/V.42/V.42bis, X.25, and "Hayes Ping Pong" 6. V.32/V.42/V.42bus - Running under AppleTalk and Novel IPX. Ethernet port only, not serial. 7. V.32/V.42/V.42bis and PEP. 8. V.32/V.42/V.42bis and X.25 9. V.32/V.32bis/V.42/V.42bis and HST. Rev List. 1.1 Added leased line material 1.2 Updated Mfgrs names & numbers 1.3 Added CTS/RTS and DSR/DTR specifics. Changes in V.35, V.29, V.27ter definition. Added Point-Point/Switched and Short Haul, Port locking. 1.4 Added V.17, V.24. 1.5 More info on AT&T. 2.0 A lot of small corrections and additions; Baud/BPS/signal table, more data on LZ & Huffman coding, Fiala Greene, Brent, & Timothy Bell, additional buzzwords, more sources, whats coming, whats new. page 18