Troubleshooting Guide

If, for any reason, the network administrator wishes to prevent the trunk from being included

in the clock path between any nodes in the network.

This is sometimes done for trunks that are prone to frequent outages.

Note: Subrate trunks by definition cannot pass clocks and are therefore blocked from being

configured as Pass Sync. Virtual trunks are physically unable to pass clocking information but

are not restricted from being configured as "Pass Sync: Yes." Ensure that you do not

configure the network to pass clocking information across virtual trunks.

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A trunk configured as "Pass Sync: Yes" cannot be configured as a network clock source.

A trunk configured as "Pass Sync: No" is not used in the clock path for any node.

Note: An IGX node cannot be included anywhere in the clock path of a BPX node. The reason for

this is that the clock recovery circuitry and internal oscillator of the IGX is stratum 4 whereas the

internal oscillator on the BPX is stratum 3.

How Can I Tell if a Trunk is Clocked or Unclocked?

The simple answer is that only the service provider who provides the trunk can determine this. The

reason is that a particular trunk can be either clocked or unclocked based on what equipment the trunk

traverses inside the infrastructure of the service provider.

Some reasonable rules of thumb are:

A cable is unclocked.a.

A fractional T1 trunk is usually clocked because it goes through the Digital Access and

Crossconnect System (DACS) of a carrier somewhere.

A full T1 is usually not clocked unless it is provided by Sprint. However, some short−haul

trunks provided by other carriers may be clocked.

A T3 trunk is rarely clocked because broadband framing structures are specifically designed

to support a large number of DS3 data streams. Each is clocked independently with the

performance of dynamic bit stuffing.

Loop Clock on a Trunk or a Line: Yes or No? What Does it Mean?

In the configuration command for each trunk and each line (the cnftrk and cnfln commands,

respectively), there is a parameter that allows the network administrator to specify "Loop Clock: Yes"

or "Loop Clock: No." This parameter specifies the source of the transmit clock (used to send bits from

the node out onto the trunk or line).

If "Loop Clock: No" (the default) is chosen, then the transmit clock on the trunk or line is derived

from the master clock of the node. (This is not necessarily the internal oscillator of the node. If the

node is frequency−locked to a remote clock source or the internal oscillator on a remote node, then

the master clock of the node is not its internal oscillator.)

If "Loop Clock: Yes" is chosen, then the transmit clock on the trunk or line is frequency−locked to the

receive clock (derived from the incoming bit stream) on the trunk or line. This is commonly done on:

A time−division multiplexing (TDM)−based line (such as one that connects to a PBX) when

the device at the other end of the line cannot be synchronized to the node. This allows the

device to transmit and receive bits at its own frequency (which can be different than the

frequency of the node). This prevents the data loss associated with uncontrolled frame slips.

In such a case, the line and the attached CPE have no problem using a frequency that is

independent of the master clock of the node.

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