The following is borrowed from the Ascend homepage for your use.
Checking Your Line Quality
So you think your noisy phone line is causing you modem problems but how do you check? What do you look for?
Well the first thing is quite obvious if you stop and think about it. When you pick your phone up and listen, what do you hear? Dialtone I hear you say! No, what I meant was, what do you hear in the background. OK, first we need to get rid of that annoying dialtone so we can really listen to the line. Dial a number, any number but obviously one which is valid for your area so that you don't get the dialtone replaced by that equally annoying (for our purposes!) busy tone. The easiest and most common across all areas is a "1". So dial this digit. OK, dialtone gone yes? Now listen and really concentrate while you are doing so cause you are only going to get 30 seconds before it times out and gets replaced by busy tone again....
This is the first major step to checking if you have a good line for a modem or not. Is it quiet? No, I mean really quiet, like almost silent. No pops, bangs, crackles, creaking (yes, really! I've heard it myself!) or faint voices in the background from other peoples conversations. All or any of these noises will cause you a problem on a high speed modem link. Heck, they would have caused you a problem on an old 2400 link as well some years back but in this case even more so. Before in the days of slower baud rates and speeds, they might have caused your transfers to slow down. Now they just plain won't let you connect!
Just what are these noises? Well they can be caused by many things but generally it in can be something as simple as a loose screw on a connector block somewhere between you and the exchange or water getting into one of Telcos cable ducts and the waterproofing (around the cable joints) has started to break down. In these days of it being much easier to "build your own extensions" at home, you could have a bad connection in the house somewhere and not necessarily on the line extension that's feeding your modem. Other people's conversations (called "crosstalk" in the trade) are not so easy to pin down and are usually more likely to be further "into the system" before the source becomes apparent.
So, let's presume you have one of these noises to some greater or lesser degree. Next we need to eliminate the phone instrument itself. After all, if it's just the instrument causing the noise, it won't affect the quality we get from the modem now, will it? Got a spare phone? You have? OK, just take it and plug it into the same socket and then repeat paragraphs 2 & 3 again. You don't have a spare phone? It might be worth heading to the local Radio Shack or other electronics store and picking up a cheap phone. Make sure you test the phone in the shop before bringing it home.
Let's assume now that you are happy with the perceived "quietness" of the line and that nothing seems untoward with it that you can tell with your own ear. Now we move to the modem but we are still concentrating on physical things really. How can we check if we have a dodgy plug or socket associated with the modem? Well, surprisingly, it's reasonably simple but you'll need to be fairly quick in carrying out the next "test". What we are going to do is to get the modem to go on line but not actually dial out a full number (similar to what we did just now but using the modem instead). To do this you'll also need to have a phone plugged in too that's fairly near to where your PC/Modem are. Don't pick up the phone yet.
You're about to enter a command string to the modem so obviously we'll need to have the PC powered up and talking to the modem with whatever piece of communication software you use. In the case of Procomm, Smartcom, SmartTerm, Zterm, etc., it's simply a case of just firing up the software and getting to a terminal screen with the "OK" prompt from the modem staring at you. If you normally use 1st Reader or Robocomm (say) then you'll need to fire up the Terminal Session for those packages and again just get to an OK prompt. Don't dial out a number at all.
OK now type this "ATX0D1;" (without the quotes) and what you should hear (if you have the modem's speaker on) is that the modem will pick up the line (dialtone heard), then dial a 1 and finally drop back to command mode (OK prompt on your screen) whilst holding the line open. The next bit is easy or more difficult depending on the accessibility of your modems leads. i.e. internal or external modem. Now pick up the phone and listen. At this point I should mention that THIS time, it's going to be your modem that may "time out" before your phone line does so be a bit sprightly! Check the modem's lead, plugs and sockets by physically getting hold of them and just gently pushing and pulling on the lead at both ends. You'll find on these type of plugs that there is possibly a couple of millimeters of "play" even when the plug is fully seated in it's socket. This is enough for our purposes. Again, be gentle. If there is a dodgy connection, it won't take a lot of movement before it becomes apparent in the phone you are listening to. OK, that was good? No noise? Right, if your modem hasn't already timed out, drop the line by typing "ATH". If you did here some crackling from that, then you'll need to get that lead replaced. It could be just the contacts of the plugs that have become a little tarnished. A good ink eraser with a fairly sharp edge should clean these up nicely.
OK, now we're down to the nitty gritty. You've carried out all the above and you are still getting poor high speed connects with your modem? Now it's going to matter exactly what type of modem you have. Whether it has a Rockwell chipset in it or not. How do you find out? Well possibly the easiest way (and most standard) is either AT&V or ATI4. If you have a Rockwell chipset, from the former you'll see a complete configuration dump starting with "Active Profile" then "Stored Profile 0" followed by "Stored Profile 1" and finally "Telephone Number Stores". The latter should return a couple of rows of checksum digits (up to 4 rows) which are the capabilities of the modem in "hex speak!
We'll consider that you have a Rockwell chipset in this first instance. Now follow this procedure as you have two very useful commands at your disposal...
1. Dial into another modem [a BBS or other service provider] from
a terminal screen (not automatic with a script - manually)
2. Do whatever you need to get your Login prompt [if your service
does immediate PPP or immediate TELNET, use a different service].
3. Pause (do nothing) for at least 1 second.
4. Type "+++" (without the quote marks - just the pluses)
5. The modem should now have dropped back into command mode and you
should be looking at an "OK" prompt *but* you will still be online...
(You'll need to be fairly sprightly at this point as you may lose the
connection if you aren't quick enough...)
6. NOW you can type AT%L%Q[return]. The modem should return 2
figures on successive lines like 20 8 or something.
These are the Line Level (%L) in -dbm and the Eye Quality Monitor (%Q) index. In the above example that would be -20dbm (reasonable) and 8 (also reasonable). Higher figures mean worse connections. Anything worse than about -24 with the first and you'll be seeing bad connects or possibly none at all. Worse than 15 on the second and you've got real line problems on your circuit that your Telco should be able to sort out...
Oh, and when you've finished, you might like to go back online with ATO (not zero) and then log off normally. You *may* find you get logged off anyway after you've got the response to %L%Q...
If you do get bad figures from these tests, you'll need to contact your Telco to try and get your line improved. We'll go over "What to ask for" in a minute...
So, what do those who don't have Rockwell chipsets do? Well there are a number of options. If you have a USR based modem (and there are some "badge engineered" models about) then chances are you will be able to use ATI6 for your checking. This can be done either online (see above for how to stay online but get back to command mode...) or offline as the USR chipset "remembers" the figures it got from the last connect you made. I won't go into the full detail of what all the figures mean (ATI6 produ ces nearly a whole screen full on a USR!) but the bits you are interested in are BLERS. These are "Bit Link Error RateS". Ideally, this figure should be 0. If it 1 or 2 after you have been online for a while, then you probably have nothing to worry about. If it climbs much higher over say a 3-4 minute connect then you are probably suffering from noise or low level or both; as in the Rockwell situation above. If you want to know what the rest of the figures mean, consult your modem manual. Unfortunatel y, the USRs do not give an indication of the actual received carrier level in decibels/milliwatt (dbm).
If you have neither of these types of modem, then I'm afraid you are going to have to consult your manual. At least now you should have a fair idea of what you are looking for though...
So, we've got noise and a low line level. What do we do? Your Telco provider is the next port of call. Explain to them that you are using a modem on your line and that you are getting poor cct quality figures from your modem and that the line level is low. Try and be polite and they will usually be helpful. You are entitled to have this problem sorted. Make sure that you say there is nothing wrong with your normal voice communications (if that is the case...) else they will just do a normal line check. If you are on a digital exchange, ask them if you can have the AGC (automatic gain control) turned OFF and your line setting at the exchange, set to position "5". In most cases this should give you a good cct and level and cure your connect problems at a stroke.
If you are experiencing these funny little noises I described earlier, be warned that it may take several visits and tests by the telephone company before the cause of the problem gets rooted out.
Happy modeming and I hope this has been of help to you...
The above is borrowed from the Ascend homepage for your use.