Information Technology Reference
In-Depth Information
Clocking Problem Solutions
Table 15-8 outlines suggested remedies for clocking problems, based on the source of the problem.
Table15-8 Serial Lines: Clocking Problems and Solutions
Possible Problem
Solution
Incorrect CSU
configuration
Determine whether the CSUs at both ends agree on the clock
source (local or line).
1.
If the CSUs do not agree, configure them so that they do
agree (usually the line is the source).
2.
Check the LBO 1 setting on the CSU to ensure that the
impedance matches that of the physical line. For information
on configuring your CSU, consult your CSU hardware
documentation.
3.
Incorrect DSU
configuration
1. Determine whether the DSUs at both ends have SCTE mode
enabled.
2. If SCTE is not enabled on both ends of the connection,
enable it.
(For any interface that is connected to a line of 128 kbps or faster,
SCTE must be enabled. If your DSU does not support SCTE, see
the section “Inverting the Transmit Clock,” later in this chapter.)
3. Make sure that ones density is maintained. This requires that
the DSU use the same framing and coding schemes (for
example, ESF and B8ZS) used by the leased-line or other
carrier service.
Check with your leased-line provider for information on its
framing and coding schemes.
4.
If your carrier service uses AMI coding, either invert the
transmit clock on both sides of the link, or run the DSU in
bit-stuff mode. For information on configuring your DSU,
consult your DSU hardware documentation.
Cable to router
out of
specification
If the cable is longer than 50 feet (15.24 meters), use a shorter
cable.
If the cable is unshielded, replace it with shielded cable.
1.
LBO = line build out
Inverting the Transmit Clock
If you are attempting serial connections at speeds greater than 64 kbps with a CSU/DSU that does not
support SCTE, you might have to invert the transmit clock on the router. Inverting the transmit clock
compensates for phase shifts between the data and clock signals.
The specific command used to invert the transmit clock varies between platforms. On a Cisco 7000 series
router, enter the invert-transmit-clock interface configuration command. For Cisco 4000 series routers,
use the dte-invert-txc interface configuration command.
To ensure that you are using the correct command syntax for your router, refer to the user guide for your
router or access server and to the Cisco IOS configuration guides and command references.
Search WWH ::




Custom Search