Line Activation Problems

Level 1 (Physical Layer) Problems

Generally speaking, Level 1 problems are the most common cause of line activation failure, and they are also often the most difficult to diagnose.

To determine whether the problem is at layer 1 (as opposed to layer 2 or layer 3), use the X.25 Line Admin 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 FarSync PCI 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: refer to the BOARD CHANNEL command in the setup file. (Note: the BOARD CHANNEL command must come after the LINE PROTOCOL command). Connect the cable to the correct port, or reconfigure the port setting.
Incorrect interface configured. Check the interface configuration: refer to the LINE PROTOCOL command in the setup.x25 file. 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.
Check the state of the LEDs on the back of the card, determine which LED applies to the line with the problem - if the LED for the port in question is flashing then a clock is present.
If you have an oscilloscope, you could try checking the signal on the clock pins, this would allow you to check the clock speed.
Find out whether clocks should be generated internally or externally, and make sure that the configuration matches - refer to the GENERATE CLOCKS command in the setup.x25 file.

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 correct.

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 (the LED will flash).

Also ensure that the remote end you are connecting to is configured for X.25 operation

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 (Null-Modem) cable can be found at cable specs.
The connector on the card has been damaged or the card itself is faulty.   Contact FarSite customer support.
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 with a Protocol Analyser or the Line Monitor program supplied with the FarSync product.

Another tool that provides useful information is the X.25 Line Admin program.

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: refer to the LOGICAL DCE command in the setup.x25 file.
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: refer to the EXTENDED LEVEL2 SEQUENCES command in the setup.x25 file.

Note: modulo 8 is used very 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:
refer to the TIMERS T1 command in the setup.x25 file.

Level 3 (Network Layer) Problems

The best way to diagnose a Layer 3 problem is to monitor the line with a Protocol Analyser or the Line Monitor program supplied with the FarSync product.

Another tool that may provide useful information is the X.25 Line Admin program.

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:
refer to the EXTENDED LEVEL3 SEQUENCES command in the setup.x25 file.

Note: modulo 8 is used very much more commonly than modulo 128.

05-Oct-2011