Troubleshooting Guide

A trunk that is clocked by the carrier and the carrier's clock is not used as the clock source for
the node. This configuration prevents uncontrolled frame slips (and the corresponding data
loss) in the carrier's facilities.
♦
Is it OK to Have Multiple Clock Sources in Use in the Network?
In some cases, it is unavoidable for some nodes and trunks in a network to synchronize to one clock
source and to synchronize other nodes and trunks in the network to another clock source. This is
especially common in international networks or in networks in which trunks are obtained from a
variety of service providers. Such a network is said to be synchronized in a plesiochronous fashion.
If two pieces of equipment that are synchronized to different clock sources are joined by an unclocked
trunk, input buffers on the interfaces at each node periodically overflow (at one end) or underflow (at
the other end). This overflow or underflow condition is commonly known as a frame slip because an
overflow condition usually causes one (or more) frame of data to be discarded.
In a TDM−based network, almost every frame slip causes data to be lost, since there is likely to be
data contained in at least one timeslot of every frame.
On a trunk in a FastPacket or ATM network, many idle packets or cells transmit every second. All
IGX and BPX trunk cards discard idle cells from the network before they are buffered and processed.
This prevents the error condition of input buffer overflow from occurring.
Because of the fundamental characteristic of cell−based networking, a network with a plesiochronous
synchronization plan can usually operate free of error.
6.
What Errors Do Clock Problems Cause?
Clocking problems typically cause frame slips on circuit−line interfaces, especially circuit lines to
TDM devices such as a PBX. Frame slips can occur on either or both ends of the line. Both the PBX
and the switch can record frame slips. In order to help resolve frame slips, configure the external
equipment to receive clock from the network. If the external equipment cannot accept network clock,
configure the circuit line interface for loop clock. If the configuration of one end of the circuit line for
loop clock does not eliminate frame slips, evaluate the clocking architecture of the network and
external equipment.
Clocking problems typically cause Packet Errors, HEC errors, PLCP errors, or frame−sync errors.
The error depends upon the type of trunk interface used. Errors result from a difference in frequency
between adjacent nodes or Telco clocking of trunks. Clock errors on trunks typically occur on one
end. This is because the BPX or IGX trunk−card suppresses the input buffer overflow by deleting idle
cells. The errors indicate underflow slips. Configure the end of the trunk that does not experience
errors for loop clock to minimize errors.
A trunk clocked by the carrier can exhibit errors on both ends. Configure either or both ends of the
trunk for loop clock to minimize the error condition.
7.
Related Information
Cisco WAN Switching Solutions − Cisco Documentation•
Guide to New Names and Colors for WAN Switching Products•
Downloads − WAN Switching Software•
Technical Support & Documentation − Cisco Systems•
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