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This guide is intended to assist clients with problems related to modems and dialup networking. It is not intended to cover the basic troubleshooting you learn in training, although some basic steps will be covered in the introductory paragraphs. In addition, one must consider that each situation is unique and may not always be solvable based on the following instructions. This guide is based on the information compiled in the black binder here in the office. The binder contains more in depth explanations on mtu, tcp/ip, modems, AT commands, and troubleshooting. Please email comments and suggestions to aamjohns@iupui.edu. |
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The
following symptoms require installing Dialup Networking
version 1.3. Dialup Networking
version 1.3 (win95) is available here.
The above two errors may also be resolved by uninstalling Dial-Up Networking
using the Add/Remove Programs tool in Control
Panel, and then install Dial-Up Networking using a retail version of Windows 95.
Installing Dialup Networking version 1.3 is the recommended method of
resolution.
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UPDATE: the instructions below
are specific to Windows 95 and Dialup Networking
with Indiana University. If you are getting this error with your ISP, you may want to follow the instructions in the
bullet list only.
Below is the long way, but almost always works. First,
check to see that 'bring up terminal window after dialing' has a checkmark
next to it (My computer>dialup networking>properties of connection>configure>options.
Make sure client has TCP/IP installed (Control Panel>Network). Multiple
copies of TCP/IP should not exist unless each is separately bound to an
Adapter. Also, the client needs to be navigating the Post Dial Terminal
Menus correctly (11, 1, F7). These instructions will work if the client
has or had AOL (AOL adapter)
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If problem persists after performing the
above operation, assume the modem
is not functioning properly.
More specifically, I have found that some X2 modems (Sportser 56k) have a problem with
v.42 error correction. Often, many problems with X2 modems can be
resolved by adding this init string to extra settings: ATS15=128
(disable v.42 which forces the modem to use MNP). Do not use this extra setting with
Rockwell or Lucent Chipsets. Here are some possible indicators of this problem:
To add this setting: My Computer>Dialup Networking>Properties
of connection>Configure>Connection>Advanced>Extra Settings Box - add ATS15=128.
Symptoms
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Diagnosing problems with the Dialup Tool tends to be more difficult
because the error messages are less specific than those produced by Windows
Dialup Networking. Troubleshoot the Dialup Tool by first trying the
most basic elements. Next, create a Connectoid in Dialup Networking.
Use this without scripting. Finally, you may want to
recopy
dialup networking and consider the modem. I will attempt to outline
a troubleshooting path.
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This usually occurs with older, slower modems.
If the computer hangs when Windows
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This error occurs when the modem did not disconnect
properly or is being used by software. It can also be indicative
of a port setting conflict with another device. Check for fax software
or another communications software running. So far I have found this to
be the most common problem. By closing all programs and trying a
connection, one can determine that software needs to be disabled to use
the modem. Also, make sure the modem is installed properly. Here are some
things to try:
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| This error correlates closely with two particular issues. One, the phone cord is not
plugged in, or not plugged in to the proper (line-in) jack. Two, the modem is damaged. See below for details.
After making sure your phone cord is inserted into the line in on the modem, try turning off wait for dialtone before dialing
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| Win95/98: My Computer>Dial Up Networking>Properties of Connectoid>Configure>Connection>remove
checkmark under call preferences
Win2000: Right click My Computer>Properties>Hardware Tab>Device Manager>Browse to the Modem> Right click the modem>Properties |
Windows modem diagnostics sends various ATi commands
to the modem, which should then report the results. If diagnostics
responds, this should tell you that the modem and computer have open
communication. It doesn't necessarily mean your modem will function
properly. Here are examples of report identification codes and their
meaning (Diamond SupraExpress 56k).
The following instructions will allow you to run modem diagnostics in Windows 95/98. Click on Start>Settings>Control Panel>Modems>click the Diagnostics tab>highlight the COM port that has the modem listed>click the More Info...button. This will bring up a More Info...window that shows the replies to the various ati commands. At the top, information is given including: Port, Interrupt, Address, UART, and highest speed. |
| The cord connecting the modem to the wall jack should be kept as short as possible. It is recommended that the cord be 6 feet or less. The phone cord should not be wrapped around or routed close to any cord carrying electrical current. Avoid running the cord through a line splitter. A cord that has been chewed or has sustained other types of physical damage may not function properly and should be replaced. |
Unwanted disconnections may occur for a variety of
reasons. Don’t forget the most basic reason – the client has exceeded
the time limit for the particular dialin line. Diagnosing the disconnection
may require some background questioning. Have they always had this
problem? Recent development? What has changed on the system?
New modem? New devices on phone loop? Call waiting? Caller
I.D.? Develop some direction for your troubleshooting pursuit.
Determine if the problem is the modem, dialup networking, phone line, etc.
Below is a listing of things known to cause or contribute to disconnections:
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This usually occurs after the modem has been used previously to a cold
boot. Drivers are usually responsible. Have them cold boot, point the to
them drivers section. They can download the
latest drivers and update.
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| Modem users may encounter PBX or centrex
digital phone lines while Traveling or
at the office. Analog modems will not work with these types of high current phone lines. In fact,
attempting to use an analog modem on these lines may result in permanent damage to the modem. Some modems
have a built in protection against the accidental interface with a digital line in the form of a digital line
protector (smart circuit).
It is possible to use a modem on a digital line. This requires a analog to digital interface. These devices are currently going for over $100. For examples, check out Konexx Mobile Konnector or Ositech’s Digital Phone Interface Since it may not be obvious what type of phone line is in a hotel or office, those who travel a great deal may want to invest in a line identifier. These small tools are inexpensive and effective. An example is the B+K Precision Model 201 Telephone Line Checker |
In order to use an analog modem outside of the United States, the traveler may need
various adapters to meet the standards of the foreign country.
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This script requires Dialup Scripting Tool to be installed on Windows95 build 0
and A. This tool is available for download. Windows95
build
B, C,
and Windows98 support Dialup Scripting. To use this script, copy and paste into
notepad. Save as
dialup.scp (whatever.scp). Next, use the Scripting Tool or for builds B and C, go
to the properties of the connectoid and select scripting. Browse to the script.
Next, deselect 'bring up terminal window after dialing' in properties of
connectoid>configure>remove checkmark. Username and Password must be supplied in
the 'Connect To' window.
procmain delay 2 waitfor "name:" until 20 delay 1 transmit $USERID transmit "^M" delay 1 waitfor "word:" until 20 delay 1 transmit $PASSWORD transmit "^M" delay 1 waitfor "selection:" until 20 transmit "11^M" delay 1 waitfor "selection:" until 20 transmit "1^M" endproc |
Error 745: An Essential File Is Missing
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This usually occurs with X2 modems. It seems to
be related to V.42 error correction protocol. The S-register setting
below will change the error correction protocol
to MNP. See X2 Modem.
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Before attempting this change to your registry you must first agree that: 1) you know that a goof in the registry can entirely disable your system, 2) you are smart enough to backup your registry beforehand, 3) I am not responsible for the repercussions of attempting this change. If you're still up for it... Why does it help? The data you send over the internet may encounter a router with MaxMTU of 576. If your frame size is larger than this value the data must be fragmented to pass. This disassembly and re-assembly will slow transfer rates (~8% performance loss). It is best to avoid fragmentation. Also, if one uses the –f modifier (don’t fragment) and attempts to pass a 576 router without ICMP (internet control message protocol) sending the appropriate response (destination unreachable), data will be dropped. Microsoft calls this a black hole router. If you would like to test for fragmentation use ping utility. Do this: ping <destination address> -f –l xxxx. You’re testing fragmentation so use a frame size greater than 576. The size you choose can be the MTU minus the IP header and ICMP header which is MTU-28 bytes. Here’s an example of an ethernet MTU: ping 202.176.15.5 –f –l 1472. If you receive an error message you know the packet passed a router with lower MTU value. I tested from home and found that my data passed a 576 router (only tested once: ip address 204.91.68.3) Windows sets the default value for MaxMTU as if your system will be using an ethernet connection to the network. The default value is 1500 bytes. This breaks down as 1500 – Maximum Segment Size = 40 (20 byte tcp header + 20 byte ip header). The actual size of data in each packet is 1460 bytes. One may want to alter the Receive Window (RWIN) when changing MTU. The RWIN is the amount of data received before ACKnowledge is sent to the remote host. The headers are stripped before data reaches RWIN so use a multiple of the MSS. Recommended values range from 4*MSS to 8*MSS. I’ll let you be the judge. We are concerned with data transmission through dialup
networking. Other network protocols use TCP/IP but lowering the MTU
for these may inhibit performance. To make the correct change we
want to find the NetTrans folder associated with Dialup. Here’s how:
Start>Run>in the Open field type: Regedit (I know you already knew this right?) Expand Hkey_local_machine/Enum/Root/Net The net subkey will have numbered subkeys. Click on the numbered subkeys and look for the one with DeviceDesc “Dialup Adapter” in the data column. Expand the relevant subkey and look for string with MSTCP/0000 or some other number. Write down this number Next, expand Network key under Hkey_Local_Machine/Enum and then expand MSTCP. Click on the four digit key with the same number as you wrote down. In that key there’s a string on the right called Driver with a value resembling NetTrans/0002. Write down this number. Now, expand Hkey_Local_Machine/System/CurrentControlSet/Services/Class/NetTrans.
Click on the four digit key (do not expand, highlight) with the same number
as the second one you noted. You will see string values like DriverDesc.
From the edit menu select New>String Value. Name the new value MaxMTU.
It should appear in the right window. Double click and enter the
value 576 hit enter to finish. Reboot. Your done. If
you want to know how to change RWIN, email me. |
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