Datacomm Troubleshooting

The online world brings along its own unique problems. In this article, I address some of the more common ones.

Last month, I had promised to look at more Linux connectivity options. However, a massive bombardment from readers with all sorts of datacomm problems forces me to change this month's topic - this month, I will answer some of the more common questions I get bombarded with on a daily basis via E-Mail.

Modem troubleshooting

Q: My modem keeps returning "No Dialtone" when dialling, even though I can hear the dialtone.

A: A very common problem. Most modems are designed to detect American-style dial tones. Unfortunately, here in India there is no real standard, and most modems simply do not detect the dial tone at all! To fix this, change your modem's initialisation string so that it includes X1 (X-one) instead of the X2 or X4 you may find there. It would also be a good idea to add the X1 command to the dialling prefix , so that says "ATX1DT" (if you use tone dialling) or "ATX1DP" (for pulse dialling) instead of the more usual "ATDT" or "ATDP".

Q: My modem dials fine, but as soon as it hears the first ring at the other side, it disconnects with a "BUSY" message! What's going on?

A: Probably the next most common problem I come across. The answer is that your modem recognises the Indian ring-back signal as an American BUSY tome, and disconnects. If you have set your modem as shown above, this will not happen, however, if your modem does recognise Indian dialtones, change the Initialisation and dialling strings to include X2 instead of X1 or X4. This problem is also the scourge of most online services (such as BBSs or Internet Service Providers) - someone dials in and immediately disconnects, while the service's modem tries for upto 90 seconds to handshake with a non-existing modem! This can easily be fixed by the service provider - set your modem to answer after two rings instead of one (ATS0=2).

Q: I have a high speed modem, but it keeps disconnecting or freezing every now and then.

A: The better the modem quality, the less likely it is that this will happen. Cheap "Glue 'n' Go" modems tend to exhibit this on a regular basis. The answer is naturally to buy a good modem (where "good" is not defined as "cheap/economical/affordable"). However, if you have already bought the modem, try reducing the speed of the modem from its default 28800/14400 bps to 9600 or even 4800 bps. If the modem still cannot hold onto the line or keeps retraining (that's the "freezing" part), then I am afraid all you can do is buy another modem. This is why PC Quest keeps insisting that you buy a modem only after trying it out in real-life conditions from your site.

Q: My modem disconnects every 5 minutes - on the dot!

A: The DoT beeps you on the line once every 5 minutes. This causes the modem to disconnect. Try setting your modem to ATS10=200&W - this may fix it. Good modems will retrain at that moment and resume the connection, bad modems will disconnect no matter what you do. By the way - be careful not to set the S10 register to 255 - this causes the modem never to hangup! You may think that this is exactly what you want, but remember that the remote modem will have given up after a max of 20 seconds anyway, so you are uselessly keeping the line open. Also note that your modem must have re-training enabled - some modems have this turned off by default (usually through the AT%E0 command - it should be AT%E1).

Q: How do I know whether my internal modem has disconnected the line properly?

A: The only way you can be sure is to try to dial again immediately after disconnecting, or, even better, pull the phone line plug out of the modem. Internal modems are notorious for this - some don't hang up even though the connection has been terminated. This is why we keep stressing that an external modem is better than an internal modem.

Q: My modem dials fine, connects to the remote service, gives me a "CONNECT" message and immediately disconnects.

A: Your modem is a non-error correcting type - it does not have MNP or V.42 error correction - and the remote service has set its modems not to allow non-error correcting connects. This is very common with most service providers. The only solution you have is to buy a modem with error-correction in hardware.

Q: My modem is rated for 57600 bps, but when I transfer a file, I get throughputs of only 1500-1600 bytes per second! Why am I not getting the rated 57600 bytes per second?

A: First of all - "bps" stands for "bits per second", not "bytes per second". In asynchronous communication, every 8-bit byte requires two additional bits to be transferred, so you end up with 10 bits per byte. Therefore, 14400 bps actually means (14400/10)=1440 bytes per second. Next, there is no dialup modem rated at 57600 bps - this figure refers not to the modem-to-modem connect speed but to the modem-to-PC connect speed. Modems are rated at 14400 or 28800 bps. This is known as the "DCE" (Data communication Equipment) speed. The "57600 bps" is the "DTE" (Data Terminal Equipment) speed, which is typically set to 2-4 times higher than the DCE speed to derive the benefits of modem data compression, if and when applicable. The transfer speeds you report (1500-1600 bytes per second) indicate that your modem is a 14400 bps modem, and the transfer speed is quite acceptable.

Q: My modem power supply keeps blowing up, and the vendor blames it on my electrical environment, and demands a service fee everytime he has to fix the power supply.

A: There have been instances where certain modem manufacturer's power supplies have blown out consistently. One thing you must understand is that in virtually all cases, the modem power supply is just a transformer block - any rectification is done inside the modem. If the power supply blows up, it is almost certainly a design failure, for which you cannot be blamed. If your modem vendor attempts to charge you for such repairs, you are being taken for a ride, and can take the vendor to court. The modem power supply is covered by the modem warranty, and you are not liable for repair/replacement charges in case the unit malfunctions.

Communication Software Troubleshooting

Q: My communication software does not work with my modem.

A: Modems are "black boxes" that do not care what communication software you are using. Communication software cannot control the quality of the modem connection - that is a hardware and phone line related issue. If you say that your software does not work with your modem, it is probably only a case of setting the software to suit the modem. If you cannot achieve this, simply change the software. No modem vendor can dictate what kind of software you use with his modem.

Q: I get a lot of CRC errors while transferring files

A: If you are using a highspeed modem, you are almost certainly seeing a problem with flow control. Modern communication no longer uses software XON/XOFF (^S/^Q) kind of flow control, but uses something known as "Hardware Flow Control" (RTS/CTS). You must set your software to use hardware flow control. Another issue is that of your serial port - if you do not have a 16550A based serial port, you cannot set your terminal (DTE) speed greater than 19200, no matter what your modem vendor or other "experts" may say. Set your terminal speed to 19200 bps and see if that improves things. If you are working with a slow machine (say a 386 or a slow 486) and are working under Windows, it is likely that you will see problems even with terminal speeds of 19200. Get out of Windows and use a DOS based program such as Telix or Procomm Plus instead.

Q: I cannot transfer files using Procomm

A: Procomm version 2.4.x or earlier do not have Zmodem - the default transfer protocol used by virtually all services these days. Get a communication program that supports Zmodem.

Q: I use Hyperterm under Windows 95, but I cannot resume crashed transfer even though I use Zmodem to download the file.

Unfortunately, the Hyperterm program supplied with Windows 95 does not suppport crash recovery. An update is available over the internet at http://www.hilgraeve.com/htpe.html. This update also fixes a whole lot of other problems.

Q: When I start a download, the status panel comes up and the transfer immediately crashes.

A: Check your default download path. If this points to a non-existent directory, you will observe this problem. The easiest way to fix it is to blank out the default download path completely and try again.

Q: I am unable to download/upload files from VSNL's GIAS using Zmodem

A: VSNL, in its infinite wisdom, uses damaged versions of the sz (send Zmodem) and rz (receive Zmodem) program. There is very little you can do about it. A method know to work sometimes is to invoke the sz program with the following parameters : sz -e -l 1024 -L 1024 filename. Similarly, you can invoke the rz command like this : rz -e -b

Q: When I connect to an online service, I get all sorts of junk characters on my screen.

A: You have almost certainly not enabled the VT100 terminal emulation mode required to connect to VSNL, or the ANSI-BBS mode required for most BBSs. Enable it and the problem should go away.

Q: When I connect in TCP/IP mode to the Internet using Windows 95, I am able to work for a long time, but then everything stops working, even though the connection is alive and there are no retrains.

A: Microsoft recently admitted that there is a memory leak problem in the Windows 95 kernel that would cause TCP/IP connections to stop responding if you have been having very dense TCP/IP traffic that opens and closes a lot of sockets. A fix is available from www.microsoft.com.

Q: I have a TCP/IP account, and I also have Netscape, but I cannot 'surf the Net".

A: You also need to install TCP/IP software before you can use Netscape. Examples are the Windows 95 TCP/IP support, Trumpet Winsock, etc.

Q: When I connect in TCP/IP mode, Netscape keeps coming back with "Host not found".

A: Your DNS settings are wrong. If you are connected to VSNL, the settings should be 202.54.1.30 and 202.54.1.18. Make sure that your DNS is set to these addresses and not to 144.16.1.1 and 144.16.1.100 that VSNL used to use earlier.

Whew!

That's it for this month. Space is at a premium for columns like mine. Before I sign off, here is a WWW address that you should find useful in determining what your connection problems are:

http://web.aimnet.com/~jnavas/modem/faq.html

This is John Navas' Modem FAQ - an excellent resource for people needing to know more about modems, especially high speed ones. He also debunks a lot of modem myths, and has a lot of useful links to other communication-related resources. Check it out!

By the way, here is a little "in-your-face" for all those people who kept saying "why buy a V.34 28.8K modem - there will never be 28.8K services in India". Well, almost every BBS in the country now sports a 28,800 bps modem, and as we go to press, VSNL has set up 28,800 bps modems in Delhi and Calcutta, with the other cities to follow in quick succession.

With that, the era of those troublesome 14,400 bps modems finally comes to an end. And as for those old 2400 bps modems . . . the less said the better!

Cheerio!