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Text File | 1991-02-19 | 13.7 KB | 297 lines

High Speed Modem Info for PC users Written by Mike Benna, January 1991. Because of the confusion people seem to have related to all the new advances in modem technology I thought it was about time that somebody compiled the relevant information into a single document. I have now done so with this article in the hopes of clearing up many of the mysteries and misconceptions about modems which operate using MNP and V.42 style protocols. Within this article, please note that some of the table values have been derived from actual testing whereas others have been calculated based on the theory behind the operation of all the protocols involved. All table values assume clean (i.e. noise free) telephone connections and a computer which is fast enough to feed the modem all the data it wants and can accept all the data the modem gives to it. First of all, a short glossary might be in order to clear up the most common terms: cps : Characters per second (usually used to measure the effective throughput from the source computer, through the modems, and into the receiving computer). In the end, this is the only number which you should really care about. bps : Bits per second (usually used to describe the raw data link between two modems or between the modem and the computer). Note that this is _not_ the baud rate. baud : The single most often misused word in telecommunications. Most people really mean "bits per second" when they use this term. For example 2400bps modems only run at 600 baud and 9600bps V.32 modems only run at 2400 baud. If you don't know what a baud is then you should probably be using 'bits per second' instead. Next, you can probably forget most of what any salesman has told you; most PC salesmen do not fully understand this stuff either. Don't blame them however, they usually have the disadvantage of having read sales literature from the manufacturers which was designed to be misleading in the first place. Your best bet is to read this article and then go to the store and test the hardware you think you want to purchase. Link Protocols -------------- In trying to explain how the various modems communicate I will start at the lowest layer you care about: the link protocol. These protocols describe the standard methods which the modems use to talk to each other; they have nothing to do with error correction or data compression. Note that there are many other protocols in use but it is unlikely that you will encounter them in day-to-day PC use. The rates described in the link protocol descriptions are often called the 'link rates'; they are not necessarily the same as the rate between the modem and the computer. V.22bis: 2400bps (this applies to _most_ 2400bps modems). Normal V.22bis (i.e. without MNP or V.42) is capable of a maximum sending rate of approx. 240cps in both directions at the same time. V.32: One of the 9600bps standards. It is not compatible with the HST protocol. V.32 modems can send at full speed in both directions at the same time. Most V.32 modems come with MNP and/or V.42. V.32bis: The upgraded V.32 standard which runs at 14400bps in both directions at the same time. In all other ways it is similar to V.32. V.32bis is newly emerging (I don't think the standard has even been totally finalized yet). When V.32bis modems become available you can expect all of them to offer V.32 compatibility. HST9600: The U.S. Robotics 'High Speed Transfer' protocol. It offers 9600bps in one direction and 300bps in the opposite direction. It is only available from USR in their Courier HST and Courier Dual Standard modems. The USR Courier HST9600 modems are no longer for sale from USR (they only sell the 14400 models). HST14400: The upgraded HST9600 standard which runs at 14400bps in one direction and 450bps in the other. Again, it is only available from USR in their Courier HST and Courier Dual Standard modems. These modems are still compatible with HST9600 modems but not with any others in HST mode. note: None of the above standards are compatible with FAX machines. Error Correction ---------------- Once two of these modems get talking to one another they may try to establish an error free connection using either MNP4 or V.42 (V.42 is also known as LAP-M for Link Access Protocol for Modems). There are other protocols as well but these two are by far the most common. Both of these protocols perform an asynchronous to synchronous conversion which allows them to avoid sending start and stop bits. In general this increases throughput for all data (even compressed files) by about 20% (25% increase due to not sending the start/stop bits and approximately 5% decrease due to error correction and sync data). There is only a slight difference in throughput between these two protocols with MNP4 coming out just barely ahead of V.42. It should also be noted that these two protocols are not compatible with each other and therefore many of the newer modems on the market support both standards. By getting two MNP4 or V.42 modems talking together you can expect to get throughputs such as these: Link Rate Without MNP4/V.42 MNP4 V.42 --------- ----------------- ------- ------- 2400bps 240cps 287cps 285cps 9600bps 960cps 1138cps 1124cps 14400bps 1440cps 1707cps 1686cps Data Compression ---------------- The next layer of standards which can be added is data compression. The two common data compression protocols are MNP5 and V.42bis (not to be confused with V.42 which is an error correction protocol). Data compression works similarly to programs like PKZIP except that they compress 'on-the-fly' as you send the data to the modem. In general, if you are sending files which are already compressed with a program such as PKZIP then there is no advantage to turning on data compression in your modems. In fact, if you are using MNP5 then you should disable data compression (go back to MNP4) before sending compressed files because it will actually take longer to send with MNP5 than it will with MNP4. V.42bis on the other hand is smart enough to realize that it can't compress the data any further and it turns itself off until it decides that it will be useful again. Data compression has its biggest advantage when you are reading text which may repeat itself frequently (e.g. ANSI codes and menu boxes have a lot of redundancy and consequently they compress very well). Because different types of data have different amount of redundancy, I've broken the throughput table into several types of data: A) Compressed data (e.g. .ZIP, .ARC, .SIT, etc. files). B) Regular text (e.g. this article). C) Typical BBS Menus. Typical throughput table for MNP5 and V.42bis (in cps): Protocol: MNP5 | V.42bis Link Rate Data Type: A B C | A B C --------- ---- ---- ---- | ---- ---- ---- 2400bps 254 489 609 | 285 768 928 9600bps 1013 1956 2440 | 1124 3072 3718 14400bps 1520 2934 3658 | 1686 4608 5574 As you can see V.42bis does a better job than MNP5 for all types of data and has the advantage that you can always leave it on (even if you are going to be doing file transfers of compressed data). Software MNP5 ------------- Some 2400bps modems for sale today offer MNP5 compatibility in software, not in hardware (read the box carefully). If the modem manufacturer is offering software MNP5, he is really selling you a regular 2400bps modem (without any MNP capability) and including a terminal program for your PC which allows any modem to perform some of the functions of the MNP protocols. In fact, if you were to buy any old 2400bps modem you could then go out and purchase a terminal program which had software MNP support. Confused? I'll try to clear this up further... The MNP protocol cannot be implemented fully from the computer side of things. In order to run at full speed it must be able to do the asynchronous to synchronous conversion and this cannot be done from the computer, it must be done inside the modem. At 2400bps these are some of the typical throughput speeds you might expect to encounter for software MNP5: No MNP Hardware MNP5 Software MNP5 ------- ------------- ------------- Compressed data 240 cps 254 cps 193 cps Regular text 240 cps 489 cps 371 cps Typical BBS Menus 240 cps 609 cps 487 cps As you can see from the table, software MNP5 is not nearly as efficient as hardware MNP5 and it also means that you cannot choose your terminal program - you must use the one which supplies the software MNP5 support. Since software MNP4 does not benefit from the async to sync conversion it will offer you an error free line but it will only run at about 228 cps (instead of the regular 240 cps you will get with no MNP support). Not Getting the Throughputs I claim? ------------------------------------ The throughput numbers I've provided in this document are the raw total throughput numbers. Please note that this is not the same as what you would measure using a typical file transfer protocol. For example, Zmodem normally gets about 234cps on a 240cps link, or to put it another way, Zmodem runs at 97.5% efficiency. Therefore to calculate your expected throughput using Zmodem you simply need to multiple the numbers I've provided by 0.975. Of course in this complicated world of communications no single number is enough: Zmodem with the Moby-Turbo option usually offers about 99.2% efficiency (238cps on a 240cps link). Also note that non-streaming protocols (such as Xmodem and Ymodem) do not fair as well as streaming protocols (such as SEAlink, Ymodem-G, and Zmodem) on higher speed links because propogation delays and response times do not necessarily decrease when the link rate increases. Which to buy: an HST or V.32? ----------------------------- Many people in the PC world who want to upgrade to a modem which goes faster than 2400bps are faced with the question of which standard to go with. As of this writing (Dec '90) the only two standards which are popular in the PC world are the USR HST standard and the V.32 (and soon to come V.32bis) standard. In this section I won't tell you which to buy but I will give you some information which may help you to make your choice: - Only USR is currently building HST modems. If you wish to get an HST modem it must come from USR. The advantage of this is that you aren't likely to run into compatibility problems when using the HST standard. The disadvantage is the price: USR modems aren't cheap (but they are of good quality). - Many other manufacturers are supporting the V.32 standard. Competition seems to be driving the price of V.32 modems below that of the HST modems (this was not always the case). In the future we can expect a big difference between the two standards; V.32 modems will likely be much cheaper in the long run than HST modems (even though V.32 modems are more complicated to build). - USR makes a modem called the 'Courier Dual Standard' which supports both HST and V.32 protocols in the same modem. It's disadvantage is cost. - Most public BBS's which support speeds higher than 2400bps only support the HST standard. This is because USR used to offer Courier HST modems to BBS operators at a reduced cost. - HST modems are only high speed in one direction at a time. This causes severe speed degradation problems during some kinds of file transfers. In all cases it is best to try before you buy. - One very popular type of modem in the Unix world is the Telebit. The older Telebit modems are not compatible with either HST or V.32 modems but many of the newer ones (if not all) have V.32 support added to them (in addition to their native modes). This is further evidence that V.32 has more long term potential than HST. - Compuserve recently purchased a bunch of rack-mount USR Dual Standard modems but is unwilling to enable the HST mode on them because once they start supporting HST mode they feel they must continue to do so for years into the future and they do not want to be locked into purchasing more modems from only one supplier. It seems they also feel V.32 is going to be the high speed modem choice of the future. (They apparently purchased Dual Standard modems because they are the only rack mount V.32 modems currently available.) My feelings are that V.32 modems are technically superior to HST modems in many ways and are likely to become common in the next few years. The problem with this is that very few public BBS's support V.32 making a V.32 modem almost useless at anything over 2400bps if the only places you call are BBS's (you should check your favorite BBS yourself). Disclaimer ---------- The information provided in this document is for the convenience of the BBS and uunet community. It may be freely distributed but may not be modified. There are no warranties as to the accuracy of anything which has been written here. In all cases if you are buying any computer equipment (including modems) it is best to test the setup you wish to purchase before purchasing anything. Remember: if it won't work in the store then why would you expect it to work at home? This document is Copyright (C) 1991 Mike Benna. -- ---> Mike Benna, Vancouver, B.C., Canada MindSpan Technologies Corp - Video Game Design and Development UUCP: Mike_Benna@mindlink.UUCP or uunet!van-bc!rsoft!mindlink!Mike_Benna