Line Activation Problems
- Ensure that Layer 1 is active - see Level 1 problems
- Ensure that Layer 2 is active - see Level 2 problems
- Ensure that Layer 3 is active - see Level 3 problems
Level 1 (Physical Layer) Problems
Generally speaking, Level 1 problems are the most common cause of line activation failure. Unfortunately they are also the most difficult to diagnose.
To determine whether the problem is at layer 1 (as opposed to layer 2 or layer 3), use the X25STATS program, and look at the statistics counters for the number of frames received. If all of these counters are zero, then it's most likely to be a layer 1 problem.
| Possible Cause | Diagnosis Action | Corrective Action |
| Cable not connected properly (or at all). | Have a look at the cables. | Make sure that the cable is connected both to the X.PORT card itself, and to the network, and that the cable is making good electrical contacts. |
| Cable not connected to the correct port. | Have a look at which port the cable is connected to, and then check the configuration: run the X.PORT Configuration program, click on the line definition and press the Properties button, and then look at the Port setting (on the General page). | Connect the cable to the correct port, or reconfigure the port setting. |
| Incorrect interface configured. | Check the interface configuration: run the X.PORT Configuration program, click on the line definition and press the Properties button, click the Level 1 tab, and then look at the Line Interface setting. | Check that the cable is connected to the correct port, and that the interface type i.e. V.24 (RS232C), V.35 or X.21 has been configured to match. |
| Line clocks not operating correctly. Note: one side and only one side of the link must generate clocks. |
(Other than checking the configuration, there's not much you can easily do to diagnose a clocking problem. If you have an oscilloscope, you could try checking the signal on the clock pins.) | Find out whether clocks should be generated internally or externally, and make sure that the configuration matches. (Clocks are almost always generated externally when connecting to a network. Internal clocking is typically required for back-to-back testing.) Check that crossover cable is the correct specification. |
| The card may be connected to an asynchronous modem instead of a synchronous modem (e.g. connected to a PAD port of the X.25 switch or TA rather than a proper X.25 port). Change or reconfigure the modem or line connection so that clocks are produced.
Also ensure that the remote end you are connecting to is configured for X.25 operation |
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| Incorrect crossover (Null-Modem) cable. |
Find out whether you need to use a straight-through or a crossover cable; if using a crossover cable, make sure that it is suitable. | The specification of a suitable crossover cable can be found in the help file of the X.PORT Configuration Program. Run the X.PORT Configuration Program, and click on the Help button, and then double click on Cable Specifications. |
| Modem Signals | Run the RpSigs.exe diagnostic program. Check that the CTS & DCD signals (for V.24), or the Indication signal (for X.21), are active. |
If the DCE is supposed to be providing these signals, then you probably have a cabling or network fault. Alternatively, the X.PORT software can be configured to ignore the state of the signals: Run the X.PORT Configuration Program, select the line, and click on the Properties button, then click on the Level 1 tab, and check the Ignore Signals box. |
| The connector on the card has been damaged or the card itself is faulty. | Contact FarSite customer support (email: ) | |
| The network is down. | Contact your network provider |
Level 2 (Link Layer) Problems
The best way to diagnose a Layer 2 problem is to monitor the line. The X.PORT software comes with a monitor program, X25MON. Note that X25MON only monitors in real time, and doesn't display any historical information.
The other tool that may provide useful information is the X.25 Statistics program, X25STATS. Run X25STATS, and then click on the Frame button to get Layer 2 statistics. Note: you will need to make sure that you have selected the correct line - press the Options button.
| Problem Symptom | Possible Cause(s) | Action |
| Both sides of the link are sending unnumbered command frames (SABMs and DISCs), but no response frames can be seen. | DTE/DCE configuration is wrong - one side of the link must be a DCE, the other a DTE. If connected to a network, then the network will be the DCE. | Change the link station configuration: Run the X.PORT Configuration Program, select the line, and click on the Properties button, then click on the Level 2 tab, and change the Link Station Type parameter. |
| One side of the link is transmitting SABM frames, and the other side is transmitting SABME frames. | Sequence number modulus configuration error - both sides must be configured to use the same size (either modulo 8 or modulo 128). | Change the configuration of the L2 sequence number modulus: Run the X.PORT Configuration Program, select the line, and click on the Properties button, then click on the Level 2 tab, and change the Modulo parameter. Note: modulo 8 is used much more commonly than modulo 128. |
| Command frames are being transmitted rapidly, and although response frames are being received, the link keeps resetting. | T1 is too short. | Reconfigure the value of the T1 timeout period so that it is longer: Run the X.PORT Configuration Program, select the line, and click on the Properties button, then click on the Level 2 tab, and increase the size of the Retransmission Timeout (T1) parameter. |
| The Line Speed is too fast. | Reconfigure the line speed to a lower setting: Run the X.PORT Configuration Program, select the line, and click on the Properties button, then click on the Level 1 tab, and decrease the Baud Rate parameter. |
Level 3 (Network Layer) Problems
The best way to diagnose a Layer 3 problem is to monitor the line. The X.PORT software comes with a monitor program, X25MON. Note that X25MON only monitors in real time, and doesn't display any historical information.
The other tool that may provide useful information is the X.25 Statistics program, X25STATS. Run X25STATS, and then click on the Packet button to get Layer 3 statistics. Note: you will need to make sure that you have selected the correct line - press the Options button.
| Problem Symptom | Possible Cause(s) | Action |
| Restart Request packets get sent out repeatedly, but no Restart Confirmation packets are received. If the network is configured for 1984 operation, it may transmit a Diagnostic Packet in response to the Restarts. |
Sequence number modulus configuration error - both sides must be configured to use the same size (either modulo 8 or modulo 128). | Change the configuration of the L3 sequence number modulus: Run the X.PORT Configuration Program, select the line, and click on the Properties button, then click on the Basic Level 3 tab, and change the Modulo parameter (in the Window Size section). Note: modulo 8 is used much more commonly than modulo 128. |
Last updated: 22-Jul-2005
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