How to Debug Trunk Interfaces

Connecting a trunk interface should be easy - right? Well it is more complicated than it seems because many conditions have some effect on the final installation.

Here are some examples:

1. Cables
   
2. Port used on Tmedia units
   
3. Port used on remote device
   
4. Framing
   
5. Line encoding
   

In general, cabling makes all the difference when connecting a trunk interface. In most cases, after the local and remote trunk Ports are carefully selected and default configuration is used, cabling is the last thing that can cause a problem.

To check the status of the trunk : Toolpack v2.5:Status Menus:TDM Lines

On the web portal :

Status -> TDM lines -> click on the trunk -> Click on extended status -> Refresh now

Most of these parameters are defined in RFC2495 (www.faqs.org/rfcs/rfc2495.html), but here is a guideline for different errors encountered:

1. If you have LOS (Loss of Signal) -> Check cabling

2. If you have LOF/OOF (Loss of Frame) -> Check framing and type of trunk (E1 vs T1)

3. If you have RAI/RRA (Remote Alarm Indication) -> Check cabling

4. If you have RSP/RSN (Receive Slip) -> Clocking is not set correctly

5. Other errors, like FEC (Forward Error Correction)  and CVC (Code violations) are usually not critical, but should be verified -> Check cabling

You can also reset the status to get fresh information:

Click on Reset status -> Refresh now -> Unclick on Reset status  -> Refresh now (again)

You can also test with Trunk Loopbacks. Here is the procedure how to use Loopback.

Possibles causes of bad cabling:

1. Straight vs Crossover

Depending on the installation, the cable required can be a straight one or a crossover. Make sure the right one is used in the installation. This is detected with a LOS signal on the line.

• Straight E1 cable (same for T1 and J1): pairs 1-2 and 4-5 must be the twisted pairs. This is different than a ethernet cable. It does not affect the system over short distances.
  
• Crossover E1 cable: pairs 1-2 and 4-5 must be crossed.
  

• Note: In the field, for ISDN configuration on T1/E, we found it clearly expects a specific pinout for ports configured for USER side (i.e. straight cables) and NETWORK side (i.e. cross-over).

2. Only half way is working

This is easy to detect as the RAI/RRA alarm will be set on one end (1) and LOS on the other end (2). This means the path from (1) to (2) is down and path from (2) to (1) is up.

3. Quality of the cable

If the cable is not perfect, it may causes various errors like Code Violations. In general this will not affect the status of the line, but will generate static on the line and may prevent data communication, like HDLC processing for ISDN and SS7. Reference to other status parameters can be found in RFC 2495

3. 75 ohms E1 connections

E1 75 ohms/120 ohms conversion requires an external Balun module

Default parameters:

E1

• Line encoding is HDB3
  
• Framing is AUTO, which supports both MFRAME (multiframe, or with CRC) and STD (doubleframe without CRC)
  
• Line length is SHORT
  

T1/J1

• Line encoding is B8ZS
  
• Framing is ESF (extended Super Frame)
• Line length is SHORT

Note: Trunk ports on Tmedia 16 E1 and Tmedia 64E1 with patch panels are not in the same order of connection. Please verify in the respective installation guides.

References

T1 Alarm Troubleshooting