Maximizing USR Courier Performance ================================== By Alex Boge ============ The contents of this file are Copyright 1991-1992 by Alex Boge. All rights reserved. You may distribute this freely, without charge, so long as no portion of this document is modified. While nothing I describe or suggest in this document should be harmful to any part of your computer system or modem, I will not be responsible for anything (my parents told me that a long time ago). US Robotics Courier Modems and Max Speed ======================================== Congratulations! You have a US Robotics Courier modem. You own what I consider to be the best modem available today and the best value too. An impossible combination to resist. This document deals with newer genera- tions of the Courier modem. Your modem should be capable of using the v.42bis protocol. Check your manual (or try entering AT&K3, if it works, you've got v.42bis). This text file will tell you, in no uncertain terms, how to get THE maximum performance out of your modem. I'll lay it out in simple, layman terms. I'll give you the easy setup information first. If you don't want to learn more, that's up to you. If you really want to know the nuts and bolts of it all, just read further. It's up to you. We live in America, land of freedom and unlike our school systems, here in computer land you still can decide when enough is enough and leave the details in the hands of professionals like myself. Instant proof. Here is the quickest way to get the best performance out of your HST in three easy steps: Step 1) Set your DIP switches. These are the default settings: 1-UP 2-UP 3-DOWN 4-UP 5-DOWN 6-UP 7-UP 8-DOWN 9-UP 10-UP Setting DIP switch 9 DOWN will allow you to examine performance while still connected and I recommend changing it. This will NOT affect performance in any way. Older Courier modems may have different default settings, check your settings. Steps 2 & 3 should be entered from your communications program. Set the COM port speed to 38400 bps (or 19200 if that's as fast as you can go). Your communications program must use hardware (or CTS/RTS) flow control. Step 2) AT&F This restores factory default settings Step 3) ATX4&B1&C1&D2&H1&K3&R2&S1&W This sets & stores max performance. That's it folks. You will now have the fastest (about 100 Kb per minute) possible download or upload speed you can get out of your Courier modem. I know it seems simple but that's the beauty of a well designed product. There are some other things to consider though, like which transfer protocol you should use (Ymodem-g versus Zmodem-90 (MobyTurbo) versus HS/Link versus the rest), which link protocol to use when both are available (v.32 versus HST) and whether or not to use compression and which one (MNP 5 versus v.42bis). Since we covered the setting up of your modem so briefly, I will do the same for these other considerations. If you want details, read further in this document. If you want solid answers, look below. v.32 versus HST Link Protocol ============================= If you have a HST only or v.32 only modem then this discussion doesn't apply to you, you have no choice in what you use (I sure hope you picked the right one). You can skip ahead. (If you are a modem expert I recom- mend that you do read the detailed explanation on this decision. It's interesting that it's not really the link protocol but the error control that provides for the slow down in unidirectional transfers with v.32bis.) Actually, the decision here is quite easy. If you spend the majority of your time downloading files (or upload but rarely have the need to do both at the same time), choose HST. HST is faster. You can reach 1740 cps in either direction using HST whereas you can only get about 1700 cps using v.32bis and 1100 cps with v.32. If you plan to use HS/Link to Up and Download files simultaneously then there is no choice, v.32 is the only way to go. HST is not designed to do this. You can reach 1680+ (averaging about 1650 though) cps in both directions simultaneously when using HS/Link and a pair of v.32bis modems. (More on HS/Link later. Suffice it to say that, obviously, HS/Link allows you to simultaneously upload and download files). If you don't transfer files then it really doesn't matter which you use. v.32 provides a quicker, crisper feel to your typing and on-line games may respond noticeably quicker but not significantly. Both HST and v.32 provide good control over noisy line conditions. You say you have a Dual Standard and are calling another Dual Standard and want to use HST instead of v.32bis? There is an easy solution. If you would like to disable v.32 then simply use "ATS27=4". This will disable v.32 and v.32bis until you enter a "ATS27=0" to re-enable it. The B1/B0 setting has no effect when you are calling, only answering. When answer- ing, B0 will allow a v.32 connection while B1 will not. Using Compression: v.42bis versus MNP 5 ======================================= Here you have two decisions, do you use compression and if so, which one. In the past I've recommended against using any data compression. After LONG and detailed reexamination of my own download and upload experi- ences I am ready to suggest differently. This is not a full reversal. If you want to get THE FASTEST transfer times (on compressed files) out of your modem, do not use compression. It's as simple as that. If you don't mind slowing down by approximately 1-5% (but an average of 2%) then use v.42bis. Unless you are only transferring non-compressed files (and cannot use v.42bis) or do not perform file transfers, do not use MNP 5, the slowdown is up to 45% O.K. Alex, you say to me, why is all this so? Doesn't it make sense that compressing data before it's sent will speed up my transmissions? You are, of course, right (the customer is always right). I say to you, THAT is why BBS's (among other reasons) have all their files in .ZIP format (or .ARJ, .ARC, .LHZ and others). A ZIP file is already compressed and far better than v.42bis could EVER hope to do. When you try to compress an already better compressed file you just end up incurring (and passing along) extra overhead. This slows down transfers. v.42bis is designed to detect a compressed data packet and minimize the overhead sent with such a packet, MNP 5 cannot do this. This means that if you transfer a compressed file with MNP 5 your speed will drop dramatically. How much? Transferring a typical ZIPped file using HST and Ymodem-g. Without data compression this file will transfer at about 1730 cps. Using v.42bis it will go at about 1700 cps or less and with MNP 5 this same file will move at less than 1550 cps (in some cases as slow as 1000 cps). (Reduce these figures by about 2.5% for a connection using v.32bis.) If you decide to use compression, which should you use? v.42bis is the better compression, simple as that. MNP 5 can provide up to a 2 to 1 compression ratio on average. v.42bis can provide up to 4 to 1 (but usually closer to 3 to 1). You may be saying to yourself, hey, 1700 versus 1730 isn't so bad. Why don't I use v.42bis and just live with the 2% loss of speed. That is a perfectly valid argument and is up to you how important 2% is. If it's a long distance call it may be worth considering, if it's a local call don't lose sleep over it. Also remember, if you are using v.32bis, reduce this by an additional 2.5% to accommodate in reduced efficiency of the LAPM error control protocol over HST error control. Now you are nearly 5% slower. How fast is compression, really? Well, I have transferred several CAD files, each over 4 megs in size, using HST & v.42bis at a speed of just under 3835 cps. I have transferred (as par to testing) the PC Board BBS documentation file, PCBOARD.DOC, at a steady 3800 cps. The same files under MNP 5 scored about 2850 and 2550 cps respectively. Without compres- sion, 1730 cps average. Now you are saying, Hey! I want to get 3835 cps downloading .ZIP files, why can't I do that. Well, instead of telling you to reread the above paragraphs and quite dreaming I'm going to make you feel better and tell you that in fact, you are actually going even faster! If you were to transfer the PCBOARD.DOC file after ZIPping it you would be getting about 6400 cps!! How? Well, as I said, a ZIPped file is better compressed then transferring an non-compressed file with compression. This means your real throughput is higher than the simple ZIP file transfer throughput. Look at the detailed explanations for the math. You will be surprised. WHICH TRANSFER PROTOCOL? Ymodem-g vs Zmodem-90 vs HS/Link vs others ==================================================================== Again, I'll give you the easy answers and if you want details, look in the detailed explanation area. The 'others' category includes such golden oldies as Xmodem, Kermit and hacks of other protocols as well as others that claim to be faster or are around to satisfy special applications. Ok, let's jump right to it. The fastest transfer protocol is Ymodem- g. The fastest implementation of this protocol I've ever used is part of the DSZ (and GSZ) driver from Omen Technology. To use this protocol requires registration of DSZ or GSZ. This is such a good performer (al- though the documentation and user interface sucks) that I can recommend it worth your while to register this shareware product. Zmodem is also from Omen Technology. Unlike Ymodem-g, Zmodem was written by Omen. No one writes a better Zmodem then Omen and the only true implementation worth talking about comes from the DSZ and GSZ drivers. The internal versions and hack version of Zmodem I've tried have never per- formed as well as the Omen version. Now there is a version of Zmodem called Zmodem-90 (nicknamed MobyTurbo by it's author). This version is even faster and comes closest to the times reported by Ymodem-g. Zmodem is worth considering even if it isn't as fast as Ymodem-g for several reasons. Like Ymodem-g it is a batch protocol, which means you can transfer several files in a row and both do not require you to enter the filenames, they are detected during transfer. Zmodem, however, adds a whole barrel of additional features. Most notably, it has the ability to recover from an error (which Ymodem-g cannot and is one of the reasons it is so much faster) during transfer. Also worth noting is that it has a feature called Crash-Recovery. This lets you resume downloading a file that got cut-off prematurely without having to start back at the beginning of the file. The other protocol to consider is HS/Link. This is a brand new protocol and as of this writing is only at version 1.00. I followed this protocol through beta testing and knew that it help promise as it addresses a feature of modems that has long been ignored. You will only want to consider HS/Link if you have a Courier capable of v.32 (the Courier Dual Standard and Courier v.32 both do, the HST does not) and you perform both uploading and download of files. While this protocol can transfer in one direction only (and does so nicely), you'd be much better off using Ymodem- g or Zmodem-90. HS/Link allows you to upload multiple files and download multiple files in both directions at the same time at speeds that are very close to what Zmodem can do in just a single direction. Think of the benefit on a BBS. You are going to be spending the next 20 minutes downloading so why not upload something at the same time. You get upload credits, the BBS gets new files (which are always needed) and you are pushing the envelope of modem communications technology (the air sure is clear up here). Unlike the only other bidirectional protocol I've found out there, BiModem, HS/Link is very easy to use and quite fast. HS/Link has error correction and crash recovery like Zmodem. It is available as shareware and has only it's display slightly crippled, not it's performance. That said, I must also comment that it is well worth the registration fee and I suggest that you do register and help this protocol improve. To summarize: Fastest -> Ymodem-g Fault tolerant -> Zmodem-90 (MobyTurbo) Bidirectional -> HS/Link Choose depending on your requirements. A NOTE ON SERIAL PORTS AND THE 16550 UART ========================================= The UART is the chip that handles data between your computer and your modem. If you have an internal Courier modem, you have the best UART in the market already installed, pat yourself on the back. If you are using an external modem, you have to plug into a serial port via a cable. This serial port is either on a plug-in card in your computer or on the motherboard itself. Most serial ports use a quite inferior UART chip. This chip doesn't handle high speeds very well. Even if you have a very fast computer, sometimes the UART still can't keep up and loses some of the data coming in your serial port. This will cause errors that will abort your Ymodem-g transfer or severely slow down your Zmodem and HS/Link transfers (other protocols suffer as well). If you replace the UART in your system with a NS16550AFN UART chip (which, by some miracle, is a straight forward, pin-for-pin replacement) you will find this situation much improved. Both DSZ (GSZ) and HS/Link, discussed above, support this chip. Your transfer speeds will improve slightly. More importantly, your computer should be able to handle the high speeds correctly, even if you multitask or have a slower computer. You can get this chip from electronics mail order shops like Arrow Electronics. You can also order the chip (by itself or already installed in boards) from Rusty 'n' Edie's BBS (call (216) 726-2620 data). Just look around, it is well worth the search. If your serial port is part of the motherboard, it may actually be worth it to disable that port and use an add-on board. More on this in the detailed explanations section that follows. The chip has "16550" right on it so you an identify it easily. DETAILED (SOMETIMES TECHNICAL) EXPLANATIONS ============================================ Who is Alex Boge? Well, I'm a 14 year computer veteran. I've worked on a large variety of computer but mostly on PC's based on MS-DOS (uh, that's 'IBM Compatible' to you uneducated lugs out there). I have been using modems for all 14 of those years. My first experience was at 300 baud on a teletype and now I'm at 14,400 bps on a 80486. I've come a long way baby! I don't claim to know everything (not publicly at least) but I do know enough that I can write this document. THIS DOCUMENT ============= In producing this document I draw upon years of experience in the PC BBS scene, all around the USA and outside. To produce the figures and reach the conclusions presented here I downloaded over 500 megabytes of files and uploaded over 200 megabytes. Much of that in the last few months alone. This is the third revision of this document, it is a complete rewrite. This document is UP-TO-DATE. I am talking about v.32bis and v.42bis technologies. The modem I'm using right now is the latest generation US Robotics Dual Standard. It's got all the buzzwords: HST, v.32bis, v.32, MNP 2-5, LAPM, v.42bis and all the other lower speed protocols which I won't even bother to mention. If you are running under 9600 bps then I don't have time to wait for you...get real, go 14.4! SETTINGS ======== Here is the result from an ATI4 command on my Dual Standard. It should be the same (except the title, and perhaps the B0 setting). If you have an HST only then instead of a B0 you will have a B1. On my settings you will see just a couple of differences from those I suggest at the beginning of the document. M3 keeps the speaker quiet during dialing and only on during the phone call and link negotiation (you can hear the difference between a quick HST connection and the much longer v.32 negotiation). &A3 reports all the protocols in use on this connection on the CONNECT line. I use &K0 because I want the fastest possible transfer speeds, you may select to use &K3 which will allow v.42bis compression but never MNP 5. ati4 USRobotics Courier 14400 HST Dual Standard Settings... B0 C1 E1 F1 M3 Q0 V1 X4 BAUD=38400 PARITY=N WORDLEN=8 DIAL=HUNT ON HOOK TIMER &A3 &B1 &C1 &D2 &G0 &H1 &I0 &K0 &L0 &M4 &N0 &P0 &R2 &S1 &T5 &X0 &Y1 %R0 S00=000 S01=000 S02=043 S03=013 S04=010 S05=008 S06=002 S07=060 S08=002 S09=006 S10=007 S11=070 S12=050 S13=000 S14=000 S15=000 S16=000 S17=000 S18=000 S19=000 S20=000 S21=010 S22=017 S23=019 S24=150 S25=000 S26=001 S27=000 S28=008 S29=020 S30=000 S31=000 S32=001 S33=000 S34=000 S35=000 S36=000 S37=000 S38=000 LAST DIALED #: OK While we're talking about AT commands, try ATUSR. I'll explain some of these settings here. I don't cover them all because most settings don't affect performance. Several may be different depending on your phone system, computer system and other local conditions. B0 or B1 ======== This setting is B1 on an HST only modem and B0 on a v.32 only modem. The Dual Standard can use either. This setting only affects what a Dual Standard will do when answering a call from another modem. If you use B1 then only a HST link will be negotiated. If you use B0, then your Dual Standard can answer calls in v.32(bis) mode from v.32 modems. &B1 === Here is an important setting. Your Courier modem has the ability to use two different speeds at the same time during communications. There is the speed at which the modem talks to the other modem (this is the DCE/DCE rate or modem speed) and the speed at which the modem talks to your computer (this is the DCE/DTE rate or terminal speed). Both are measured in Bits Per Second, or BPS. Do not confuse this with baud, they are different. You will be using the fixed DTE mode. This means that regardless of what speed the modems are talking to each other, your computer and modem will always communicate at the same, higher, speed. We use a high speed so that the modem doesn't have to wait for the computer. Your computer is almost always faster than the modem (except in the case of really slow computers) so this is desirable. When you use compression, sometimes more data comes in then you might think. Two modems communicating at 14,400 bps can transfer at a rate of up to 3840 cps, that's 38,400 bps. You'll need every bit of speed your serial port can provide. That is why you want to have your communications program port speed set to 38400 bps. This way your more will never overrun your computer. By default (&B0), when the modem connects with another modem, it changes the speed that it communicates with your computer to match that of the modem link. If you connect at 2400 bps, then the serial port speed changes to 2400 bps. Using &B1 overrides this and forces the DCE/DTE speed to stay at 38400. Newer Courier modems will override the &B0 setting and force a speed of 19200 bps when making a 12000 or 14400 bps connection. &C1 and &D2 =========== These settings don't affect performance but make the Courier to accurately report a connection to another modem and allow you to hang up your connection quickly. &H1 and &I0 =========== These two settings define flow control. The &H1 is most important. It tells the modem to use hardware flow control. At the speeds we are working, hardware flow control is absolutely necessary to keep each modem from overrunning the other (sending data too quickly in one direction or the other, or both). While you can use software (XON/XOFF) flow control, it is no where near as quick and will probably not be up to the task. Also, it will lower performance. You must set your communications program to use hardware flow control as well. It may be called CTS/RTS (or simply CTS) flow control. &I0 turns off software (XON/XOFF) flow control on received data, this sort of control is unnecessary and adds overhead. See &R2) &K3 is discussed in the section about compression. === &M4 === This setting controls error correction. The &M4 setting is most flexible. If it is possible to use error correction, the modem will. If not, it will connect anyway and continue. Note, it is required to have an error correcting connection in order to communicate over 2400 bps. If you turn off error correction with &M0 then you will never connect over 2400 bps. The only time I ever turn this off (&M0) is when I'm calling some lame board that has lamer modems that choke during error correction negotiation and fail to connect. Turning this off will make for a quicker connection if you know the modem you are calling doesn't use error correc- tion. Error correction is negotiated depending on the link protocol. If the protocol is HST, the HST error correction is used. If it is v.32 then LAPM is used. Otherwise, LAPM is tried first then MNP 4, 3, and 2. Courier modems do not use MNP 1. Compression will not be used unless error correction is also used. &N0 === This setting permits the modem to negotiate for highest speed. If you set this to any other value, then only the speed for which you set it will be permitted. If the other modem doesn't support that speed then the connection will not be made. Using &N0 also activates the fallback and fall forward features of the Courier modem. Fall forward is not found in other modems. Most modems will fallback the connect speed if line condi- tions are poor. Courier modems use Adaptive Speed Leveling (ASL) which continue to monitor line conditions and when they improve, bring the speed back up. &R2 === This activates hardware flow control for use on received data. This is as important as the &H1 setting. Data coming in from the other modem can sometimes overrun the receiving modem and/or computer. This signal is lowered when the modem receive buffer is nearly full and is only raised when the buffer is near empty again. This signal is controlled by both your computer's serial port (in turn controlled by your communications program) and the modem itself. There can be a situation where your modem's buffer is not yet full but your computer is not ready to receive data. For example, if you are multitasking and the computer is busy accessing the disk for some other program. It may spend so much time doing the disk function that it can't keep up with the steady flow of data from the modem so it tells the modem to wait for a moment. Flow control is necessary at the speeds we are dealing with. If this setting is ignored then you will almost certainly fail during downloads. DIP SWITCH 9 - DOWN =================== I recommend changing your DIP switch number 9 to the down setting. The reason for does not effect performance, but gives you an opportunity to examine the condition of your connection while still connected. With switch 9 in it's default position, up, when you type the modem escape command, three plus signs with at least half a second of nothing before and after, the connection is hung up and you are returned to command mode. With it in the DOWN position, when you do this, you are returned to command mode but still connected to the other modem. While in command mode there is little you can do but one very useful command. Use ATI6 while in command mode to view the link diagnostics screen. Here you can see exactly how many characters you've sent and received since connecting. You can also see what speed you are currently communicating at (which could be lower because of bad line conditions and automatic fallback). You can see how many errors error correction has corrected. By looking at this screen you can see if the sudden slow down you had in the last file was because of line noise. This will also tell you how the connection was broken after it is broken. Was it a burst of line noise or did the sysop hit "Logoff". Look the ATI6 command up for an explanation of the various fields. It's too much to repeat here. BLINKY LIGHTS ============= What do all those lights on my external HST mean anyway. Well, mostly I'd tell you to look in your manual but there is something things you should know without having to dig this up. When the ARQ light is lite you have a connection with Error Correction (either LAPM or MNP). If the HS light is lite then you have a connection at over 2400 bps. The OH light is on as soon as the modem starts to dial and goes off when it hangs up the phone line. The CD light goes on when the modem is connected to another modem. This pretty obvious stuff. When the ARQ light is flashing, error control is functioning. I mean, that an error has been detected and is being corrected. ARQ stands for Automatic Repeat Request (got me why they didn't call it ARR) which is generic for error correction. When the MR light is flashing, the modem is retraining. That means that it is either falling backwards in speed due to excessive line noise or errors or it is falling forwards because conditions have improved. This is the time to use your +++ then ATI6 to see what speed you are at. Perhaps you've fallen so far down that it would simply be better to hang up and call back with a cleaner connection. If you are not connected to another modem then the AA light is lit if you are in auto answer mode. When you connect, the modem that has the high speed channel (when in HST mode) has the AA light lit. If you are down- loading, then it will be off. If you are uploading, after a brief moment (the MR light will flash while the HST reverses the line) the AA light will lite up telling you that you now have the full speed channel. v.32 modems do not exhibit this behavior as they never need to. TERMS/GLOSSARY ============== Here is a rambling guide to all the different terms that I used in this document. I'm putting it here because it's about time I explained some of these things and you'll need to know what they are before you can tackle the stuff that follows. PROTOCOL - This is a fancy term to use when you want to say, how something talks to something else. We use it here when we want to describe how one modem talks to another. Does it use v.32 or HST. A protocol is just a predetermined, ordered way of communicating that all parties agree on ahead of time. It is fixed. The specific protocol used can be negoti- ated prior to it's use, all modems do this. LINK NEGOTIATION - This is what goes on first when two modems begin talking to each other. The squeal and squawks you hear when a modem first begins to connect is the series of tones that the modems emit and detect when trying to find out what the other side has. In the simplest terms it's something like this: "Hi, I'm a modem, are you a modem too?" "Yes, I'm a modem. Are you a fax?" "No, I'm not a fax, are you?" "No, but I can talk using v.22, can you?" "Yes, I can, can you use v.22bis?" "Yes, I can, can you use HST?" "Sorry, never heard of it, how about v.32?" "Sure, I can do v.32, in fact, I can do v.32bis, can you?" "Wow! So can I! But first, tell me, can you handle LAPM? "I sure can." "How about v.42bis?" "My master won't let me the creep, wanna do it anyway?" "Naw, they'll catch us, let's just link and report back." "O.K." All you see, of course, is CONNECT 14400/ARQ/v32bis/LAPM. Had the devious modems misbehaved, there would have been a /v42bis on the end of that string. Why do some modems croak when you use your mighty HST to connect with a puny 2400 non-mnp modem? Because their negotiations goes something like this: "Hi, I'm a modem, are you a modem too?" "Yes, I'm a modem." Hmm, quiet type, "Oh, ok. Can you talk using v.22?" "Yes, I can, can you use v.22bis?" Maybe this guy isn't too bad, "Yes, I can, can you use HST?" "Huh? What?" oh-oh, "Can you use MNP 4" "Duh...wha?" "How about MNP 3 or 2?" "I'm so confused." "NO CARRIER" How rude. LINK PROTOCOL - This is what is first negotiated during Link Negotia- tion. This protocol is the means by which the two modem are actually talking to each other. It is the basis on which everything else is built. Mainly, it defines the speed of the two channels of communications. I'll mention the ones we're most interested in. HST - this has 14400 bps in one direction (the one most heavily used) and 450 bps in the other. (Old HSTs may only have 9600). v.32 - this has 9600 bps in both directions. v.32bis - this has 14400 bps in both directions. (v.22bis - 2400 bps in both directions.) Note that v.42 is not listed here. You may sometimes see BBSes advertising 19.2K or 38.4K HST, call here, we have the best Warez! These are signs of stupid/ignorant people. Their I.Q.'s are reflected in their insistence on spelling words that end in "es" with an "ez". Those speeds are the DCE/DTE port speed. There is no such thing as a 19.2K HST modem. Ignore these claims as you should claims that Elvis lives in Kalamazoo (he lives in Akron, of course). ERROR CONTROL PROTOCOL - This is the means by which the modems will correct errors during transmission. When one modem detects an error, usually caused by static on a phone line, it will send a request to the other modem to repeat the bad packet of data. When you see the ARQ light lit and get a /ARQ on the CONNECT line, you have an error controlled connection. The Courier line of modems has three, not two, types of error control available for it to use. Error control is not available for connections under 1200 bps. HST - Yes, there IS an error control protocol called HST as well as a link protocol. This protocol is a slightly more efficient version of MNP 4 also modified for the asymmetrical modulation used by HST modems (that's the high speed/low speed channels difference). This is a very low overhead protocol. The performance of this protocol is similar to MNP 4 (see below) and actually yields higher than normally expected throughput for a given data rate. For example, although the link is at 14400 bps, which would yield, at 10 bits per character (8 bits per byte, plus 1 start bit and 1 stop bit), an expected maximum rate of 1440 cps, it in fact returns throughput as high as 1740 cps. And the 1740 figure includes overhead for the transfer protocol (Ymodem-g). LAPM (Link Access Procedure for Modems) - This is part of the v.42 recommendation. It is used when an HST link is not made and is picked over MNP. It is NOT as efficient as the HST error control protocol. THIS is why a HST download at 14,400 bps is faster than the exact same download using v.32bis. v.32bis cannot use the HST error control protocol, only a HST connection can. This is why an HST connection is preferred over a v.32bis when doing transfers in one direction only. LAPM provides a throughput of only about 118% of expected. It is argued that LAPM is more robust than MNP and would fair better during a cellular phone call. I have no evidence to support this (and ask, what about MNP 10?). MNP 2-4 - The modems will attempt to use the highest version of MNP they can, as each level is better than the previous. At level 3, performance is break even with a non-error control connection. At level 4, performance is actually better, yielding about 120% of the normal expected throughput (i.e., a 2400 bps connection yields about 280+ cps transfer versus the expected 240 cps maximum at MNP 3 or non- error control). Fortunately, there is little choice here. You either have error control on or off. You cannot select one over the other. The Link Protocol will determine which is present on the other end and respond in kind. The good part is that by having all the options in one box you are sure to get the best possible connection with all other modems. v.42 - v.42 by itself is not a protocol, instead it is a (quoting the HST manual now): "standard for modem communications that defines a two- stage process of detection and negotiation for LAPM error control." I'm not sure why they didn't call LAPM v.42 or v.42 LAPM. It's too bloody confusing. Basically, forget v.42. There is LAPM (the error control) and v.42bis (the compression), just remember those two. Note also, v.42 supports the use of MNP levels 1-4 when a LAPM error control cannot be negotiated. This is what allows v.42 compliant modems to connect to older, MNP only modems. This is the wisest part of the v.42 standard. COMPRESSION PROTOCOL - The Courier modem supports two types of compression, v.42bis and MNP level 5. v.42bis is the newer and more efficient protocol. MNP 5 was first but is not nearly as good. If the other side has both protocols (and you are using compression) then v.42bis will be picked over MNP 5. In cases where the answering side has MNP 5 but not v.42bis you can have your Courier not use compression for this session but be ready to make a v.42bis link whenever next possible by using the &K3 option. This option only permits v.42bis compression and denies the less efficient MNP 5. If you are going to use compression, this is a very desirable feature and are, as far as I know, exclusive to the Courier line of modems. TO USE COMPRESSION OR NOT TO USE COMPRESSION plus WHICH COMPRESSION TO USE IF I DO ================================================= This is one of the "fun" parts of this document. Readers familiar with my first two revisions of this document (this is a complete rewrite, by the way) will recall that in those I recommended flatly against using any data compression. I still maintain that if you want the very fastest transfers possible you should not use data compression when transferring compressed files. Of course, if you were transferring non-compressed files I would easily suggest using v.42bis. But, read on, there are many things to consider. First, what do you really want? Do you want to go fast, really fast. Do you intend on performing transfers in one direction at a time? Are you transferring compressed (ZIP, ARC, LZH, ARJ, GIF) files? If you answer yes to all of these, do not use any compression and use HST whenever possible. This will result in the fastest possible transfers, bar none. I'll stake my reputation on it (and have). If you use Ymodem-g (as provided by DSZ or GSZ from Omen Technology and use a 16550A UART) for your transfer protocol then you can except your transfers to reach as high as 1740 cps. Higher than that has been recorded but I attribute to the relatively coarse resolution of the PC timer (approximately 1/18th of a second) used in the calculations. Some people have reported faster times with other protocols but after I timed them using external timers and higher resolution hardware clocking methods I've discovered that the authors of these protocols, shall we say, enhanced their calculations just a tiny bit... If you don't mind loosing just a little speed, we're talking about 2% here, then go ahead and leave v.42bis on all the time. Use the &K3 I recommend at the beginning of this file. It will screen out MNP 5 which is deadly slow but let v.42bis calls connect. Instead of 1740 cps as a maximum, expect 1710 as the maximum with the average being just under 1700. On GIF files you may even see just a bit higher, but there is a lot that could effect that. For example, non-interlaced GIFs transfer slightly slower than interlaced ones. Dithered images transfer slightly slower than non-dithered. Don't worry about it too much, it's only by a few cps. Do you enjoy waiting for your computer to grind through a download? Got nothing better to do then watch your modem lights flicker? Go ahead, use MNP 5. It's your time, not mine. But don't do it on boards I call please, I actually dislike busy signals! Lemme draw ya a little chart that outta help a lot: HST v.32bis v.32 14,400bps 14,400bps 9600 bps ÕÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍÍÍÍÍÑÍÍÍÍÍÍÍÍÍÍÍÍÍ» None ³ 1710-1740 ³ 1670-1700 ³ 900-1120 º ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄĶ v.42bis ³ 1650-1710 ³ 1620-1690 ³ 880-1110 º ÃÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄÄÅÄÄÄÄÄÄÄÄÄÄÄÄĶ MNP 5 ³ 1450-1575 ³ 1400-1520 ³ 600-750 º ÔÍÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍÍÍÏÍÍÍÍÍÍÍÍÍÍÍÍͼ Transfer Speeds Using Ymodem-g Reduce these figures by about 3% if you are using Zmodem-90 (Moby- Turbo) or HS/Link. If you are using HS/Link and a v.32 protocol, then you can expect to get these figures, minus 3-5%, in both directions simulta- neously. v.32bis can expect a combined transfer rate of about 3250-3350 cps, which is quite spectacular. Remember to try to keep your upload and download bytes about the same or one channel goes unused eventually. Performance only picks up by 1-2% in unidirectional transfer mode with this protocol. TRANSFER PROTOCOLS - WHICH TO USE AND WHY ========================================= If you want to move a file from A to B in as short a time as possible, use Ymodem-g. It's as simple as that. Ymodem-g doesn't wait for an acknowledgment from the receiver that data is getting there correctly, it just keeps shoving it down the line, only subject to flow control (hardware flow control is very important at this point). Also, Ymodem-g uses only a 16-bit CRC, this is both faster and smaller than the 32-bit CRCs used by Zmodem and HS/Link. Ymodem-g is also very plain. It has no special features, it's very simple and works very fast. Fortunately, it is also a batch protocol and doesn't require user entry of file names to receive. All these features combine to make it a very good choice for your default high speed protocol. Omen Technologies has a shareware protocol driver called DSZ (and a slightly more "graphical" version called GSZ which contains the same protocol's but with a nicer display and, fortunately, the same command line interface) which provides the fastest version of Ymodem-g I've been able to find. This driver also provides the only source for Zmodem-90. This makes this driver very valuable to have. In order to use the Ymodem-g portion, you have to register this program. It is shareware well worth the invest- ment, I suggest it for anyone into modeming. The Zmodem-90 is free. Make sure to get the latest version. Once you register, you get all updates free via BBSes around the world. Zmodem-90, or MobyTurbo, is a very powerful protocol. It has many, many features with terrible documentation and an equally annoying command line interface (for example, everything must be typed in lower case. Give me a break, what is this, UNIX? ugh). If it wasn't so good, no one would use it but credit is given where credit is due. It works and it works damn good. It is about 2-4% slower than Ymodem-g. This is mostly due to the fact that a lot more control information (which is all considered overhead) is being transferred along with actual file data. Zmodem uses 32-bit CRC values, twice the size of Ymodem. Also, it passes error control informa- tion twice the size or more than that of Ymodem. When an error occurs, this is very useful to have around. If you have an error free, reliable link then this is all extra baggage that no one wants. On my own system, I use Ymodem-g for all my unidirectional transfers. On the rare occasion that an error occurs (9 times out of 10 this is because I did something in another Desqview window that caused the computer to pause for an unusually long period of time and therefore drop a character at the serial port that the 16 byte FIFO buffer in my 16550A couldn't hold) I resume the download at where it was aborted using Zmodem's Crash-recovery feature. If you've ever crashed just 5 Kb away from finishing a 5 megabyte download you'll immediately appreciate the benefit of this. The second most popular reason for using Zmodem over Ymodem-g is that Zmodem is free, Ymodem-g isn't. When using DSZ (instead of GSZ), try to use the .EXE version. It is slightly faster at calculating the CRC but best of all, allows you to specify a larger receive buffer, 16 Kb versus only 4 Kb with the .COM version. This alone can generate a few cps speed improvement. GSZ.EXE has these features and only comes in an .EXE form. I would suggest to Omen to drop the confusing double format (released in separate files too!) and continue only on with the .EXE forms. To use this larger buffer, you should specify "-pB16384" on the command line. The "p" must be in lower case and the "B" in upper case. Use this for both Ymodem-g and Zmodem. Also, to use Hardware flow control you must specify "handshake on" on the command line too. This can be abbreviated to "ha on" in lower case only. If you have a v.32 modem, especially a v.32bis, and you are planning on both uploading and downloading from the same place then bless the gentleman who wrote HS/Link. The new protocol is exactly what Dual Standard owners around the globe needed. It lets you perform simultaneous uploading and downloading at full speed ahead (about the same or slightly slower than Zmodem-90) with a very easy, friendly user interface. If you have a Dual Standard and HS/Link, there is no reason not to upload to your favorite board. Support the BBS ideal, spread good files everywhere. You're going to be on-line download anyway, why not stop being a leech and contribute. You'll gain respect from your peers (and more importantly, the Sysop). HS/Link basically has all the features of Zmodem-90. It is almost as fast (but if you were doing unidirectional transfers, use Ymodem-g or Zmodem-90 instead for max speed) but a whole lot friendlier to setup. Once setup, all three protocols are easy to use. HS/Link also uses 32-bit CRC for error control but that is redundant in a error controlled connection. It's there for 2400 bps users (they have 2400 in both directions so can benefit from HS/Link as well, and at 2400, you need all the help you can get!). It can also perform crash recovery and it has the most efficient error recovery schemes I've ever seen. Instead of bringing the entire transfer to a halt, rewinding past good data and resending from the bad block forward, back over what was otherwise good data. HS/Link requests only the block that was bad, receives it whenever the sender has a pause to do so, inserts it where it belongs and all in the meanwhile is speeding ahead with the rest of the file. Fantastic! Let's hope that in later versions the author finds some way to speed things up (perhaps with a 16- bit CRC option and larger block sizes). HS/Link also is shareware and I would recommend you register it to promote further development. However, unlike DSZ, it is not performance crippled. The shareware version provides a full features set, only the display as somewhat affected. When using HS/Link, remember to increase the default block size to 4 Kb by using "-B4096" on the command line. Additional performance MAY be gained by using either -A or -W16. I haven't finished testing these options fully to recommend one over the other yet. The -A turns off all acknowledgments between blocks and -W16 increases the gap between acknowl- edgments from the default 8 to 16 (the maximum). We do this because assuming error free conditions, we should not require acknowledgments. This should bring us closer to Ymodem-g performance by eliminating unneces- sary overhead. This file was typed February 9-10 during a Diet Pepsi and Domino's Pizza binge. It was updated because of new data collected after updating to a v.32bis Dual Standard modem. The data presented in earlier versions is still accurate except that now I have clarified my recommendations regarding compression and have changed the default (easy) suggestion to using v.42bis. This file was initially distributed by posting it on Rusty & Edie's BBS in Youngstown, Ohio. (216) 726-2620 - 120 lines/18+ Gigs. This is the friendliest, finest BBS in the world. No doubt. Drop in and say hi to Rusty, Edie, Carl and the rest of the gang there. You can e-mail me there under the name Drestin Black (I check mail daily). You can contact me on CompuServe at ID 72470,402 (I check mail there weekly). If you read this far, you must have liked something so I like you. You can call me voice most anytime at (313) 781-2142 (until May 16, 1992).