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| 4.4 |  | Troubleshooting ISDN |
| 4.4.1 |  | T1 framing errors |

When an interface receives a frame, the receiving party must be able to locate several parts. They are the start of a frame, the components within the frame and the end of the frame. These low level activities rely on the receiver being able to extract clocking information from the signal and recognize the difference between a 1 and a 0. Where these essential elements break down framing errors will occur.

Framing errors are most likely to occur where different frame and encoding formats are used and where equipment from different vendors is expected to work together. This situation is likely to occur where corporate networks link to those provided by telcos. Although this topic concentrates on T1 connections such as the ISDN Primary Rate services, the principles are broadly applicable to other Layer 2 technologies.

In Figure , the show controllers t1 exec command provides the following information that can be used to determine whether or not framing errors are occurring:

- Statistics about the T1 link. If a slot and a port number are specified, statistics for each 15 minute period will be displayed.
- Information to troubleshoot physical layer and data-link layer problems.
- Local or remote alarm information, if any, on the T1 line.

Use the show controllers command to verify if there are alarms or errors displayed by the controller. To determine if the framing, line coding, and slip seconds error counters are increasing, use the show controllers t1 command repeatedly. Note the values of the counters for the current interval.

As a first step when troubleshooting a suspected framing problem, ask the telco or service provider to provide details of the required framing and line coding settings. It is common to use Binary 8-Zero Substitution (B8ZS) line coding with Extended Super Frame (ESF), and Alternate Mark Inversion (AMI) line coding with Super Frame (SF).

If Slips Secs are present on the T1 line, there is a clocking problem. The Customer Premises Equipment (CPE) will need to synchronize to the clocking from the T1 provider. Complete the following steps to correct this problem.

Ensure that the clocking source is provided by the telco. Review the output from the show controllers t1 command to ensure that Clock Source is Line.

> Note: If there are multiple T1s coming into an access server, only one can be the primary source. The other T1s derive the clock from the primary source. If there are multiple T1s, ensure the T1 line designated as the primary clock source is configured correctly. A second T1 line may also be configured that provides clocking in case the primary source goes down. To do this, use the clock source line secondary command from controller configuration mode.

Set both the primary and secondary T1 clock source from controller configuration mode.

> Router(config-controller)#clock source line primary
> Router(config-controller)#clock source line secondary 1

Ensure that the T1s that were specified as the primary and secondary are both active and stable.

Ensure the framing format configured on the port matches the framing format of the line. Look for the statement Framing is {ESF|SF} in the show controllers t1 output.

To change the framing format, use the framing {sf | esf} command in the controller configuration mode. For example:

> Router(config-controller)#framing esf

Change the line build-out using the cablelength long or cablelength short command.

Contact the Service Provider and consult the Cisco web site documentation for details on build-out settings. Basically, this command will change the signal levels used by the T1 interface to allow for losses experienced due to the distance between the WAN connection and the telco office.

Line codes are a method of encoding data such that there are regular transitions between ones and zeros. This is necessary in a digital environment because the ones and zeros are represented by the presence or absence of a signal. Long sequences of ones or zeros make it difficult for the controller to know just how many ones or zeros are occurring. Figure shows an incoming signal with regular transitions that is easy to interpret. However, the long sequence of zero’s makes it more difficult. In fact, the longer the sequence, the more accurate the clocks must be and the more susceptible to errors the controller will be.

To avoid the difficulties associated with long sequences of one’s and zero’s line coding algorithms are used to ensure that regular one to zero and zero to one transitions occur.

Ensure that the line coding configured on the port matches the line coding of the line. Look for the statement Line Code is {B8ZS|AMI} in the show controllers t1 output.

BZ8S stands for Bipolar with 8 Zero Substitution. This means that continuous sequences of zeros are replaced with a special BZ8S code that includes transitions. BZ8S is used with Alternate Mark Inversion (AMI) which encodes ones alternately as a positive and negative voltages which is more compatible with transformers and older cabling technologies used by telcos in the local loop.

To change the line coding, use the linecode {ami | b8zs} command in controller configuration mode. For example:

> Router(config-controller)#linecode b8zs

Aside for the necessity of correct encoding, the actual line voltage levels also need to be within certain limits. The signal level diminishes as the length of the local loop increases. This variation can be compensated for with the cablelength long or cablelength short commands.

Path code violations are frame synchronization errors when Super Frames are in use, and cyclic redundancy check (CRC) errors for Extended Super Frames. Path code violations and line code violations are typically present simultaneously.