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.
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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.
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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.
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.
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.