Synchronizing Network Clocks

This chapter describes how to synchronize a network and the commands related to synchronization.

Understanding Network Synchronization

Available clock sources are defined within the network as primary (p), secondary (s), or tertiary (t). This hierarchy is based on clock source stability. Each trunk that can pass clock synchronization is defined. Each network node's clock is based on the most stable clock source. If multiple, equal clock sources are available, each node chooses the closest one (measured in number of hops).

Clock sources can be changed by you or automatically by a node. If there are no primary, secondary, or tertiary clock sources defined or working in a network, then the internal oscillator of one node is automatically selected as the active network clock source.

Whenever a clock source changes—because of a line repair or an operator's command, for example—the node ensures that the clock path remains hierarchical. Also, whenever a subnetwork is merged with another subnetwork, each node in the new network verifies that it has the nearest, most stable clock that is available.

A continuous clock test compares the frequency of the node clock source to a reference on the control card. If it detects a clock source outside preset frequency limits, the controller declares the source defective and selects another source.

Defining Clocks and Lines

Ordinarily, a network's clock sources and line characteristics are configured as part of the node installation process. Thereafter, clock sources are redefined when a network is reconfigured or a line status is changed.

Clock sources are manually defined as primary, secondary, or tertiary. The definition depends on the stability of the clock source. Considerations for assessing and defining clock sources include:

•

Reliability of each clock source ( Figure 11-1 illustrates clock source reliability)

•

Availability of multiple clock sources in a plesiochronous network (see Figure 11-2)

A plesiochronous network is a network in which there are two or more independent, active clock sources. For example, a network in which multiple vendors provide multiple lines that require clock mastership can be a plesiochronous network. Figure 11-1 depicts clock source reliability.

Refer to Figure 11-2. One trunk parameter is the ability to pass a clock. A line passes a clock if the clock information transmitted from one end arrives as the identical clock at the other end. Normal T1 or E1 lines pass clock. Lines that do not normally pass clock include:

A long-distance line that passes through another provider's network may or may not pass clock. The default ability for an IGX trunk is to pass clock. The following applies to clocks and lines:

•

Defining a trunk as a clock source is incompatible with defining it as passing clock.

•

In an IGX/BPX network, a clock source functions as a source for the entire network.

•

A trunk should be defined as a clock source only if a DACS-type device connects to the trunk.

Synchronizing the Network

Network synchronization includes specification of primary, secondary, and tertiary clock sources. The latter two sources serve as backups in case of clock failures. The cnfclksrc command specifies the source of a clock and can remove a previously specified clock source. Multiple primary sources, multiple secondary sources, and multiple tertiary sources are allowed.

The designation of the clock source depends on the stratum (or stability) of the clock source. Each node in the network synchronizes to the nearest (fewest number of hops) primary clock source. If no primary source is available, the nearest secondary clock source is used, and so on. If no other source is available, the network synchronizes to the internal oscillator of one of the nodes in the network. The following commands manage the network clocks.


•cnfclksrc	Specifies a primary, secondary, or tertiary clock source in a network.
•dspclksrcs	Displays all the currently defined clock sources.
•dspcurclk	Displays the current clock source that the network is currently using.
•clrclkalm	Clears an alarm associated with a clock source. The cause of an alarm usually clock source that fails or is outside of the frequency limits. You must clear a clock alarm before the corresponding clock source is usable.

Summary of Commands

Table 11-1 shows the name and starting page for the description of synchronization commands.

Table 11-1 Commands for Clock Synchronization
Command	Description	Page
clrclkalm	Clear clock alarm	11-5
cnfclksrc	Configure clock source	11-6
dspclksrcs	Display clock source	11-8
dspcurclk	Display current clock	11-10

clrclkalm

Clears the alarm status of a clock source after a problem is cleared. (The alarm can be a "Bad Clock Source" or "Bad Clock Path.") Before the node can use the original clock source, you must clear the alarm with clrclkalm. The system displays no messages after execution.

Full Name

Syntax

Related Commands

Attributes


Privilege	Jobs	Log	Node	Lock
1-5	No	Yes	IGX, BPX	Yes

Example 1

Description

Example 2

Description

Table 11-2 clrclkalm—Parameters
Parameter	Description
c/p	Specifies the type of line. A "c" indicates a circuit line. A "p" indicates a trunk.
line number	Specifies the number of the circuit line or trunk.

cnfclksrc

Specifies a network-wide clock source. The clocking scheme ensures that all nodes in the network automatically synchronize to the nearest, most stable clock available. After you specify a clock source, the location and type of the network clock source goes out to all nodes in the network. This synchronization remains in effect despite line failures, power outages, controller card switchovers, line repairs, and the joining of subnetworks and all other network topology changes. Each node in the network maintains a list of the available clock sources for the network.

A clock source can be either circuit line (C), a trunk (P), or an external source (E). The clock type can be primary (P), secondary (S), or tertiary (T). To remove a clock source, enter its type as "r" at the end of the cnfclksrc command line.

Designation of the clock type depends on the stratum (or stability) of the clock source. In a large network, for example, you could designate all stratum 2 clocks as "primary," all stratum 3 clocks as "secondary," and all stratum 4 clocks as "tertiary." The network regards all primary clocks as equal in the network clocking hierarchy, regards all secondary clocks as equal, and regards all tertiary clocks as equal. Each node synchronizes to the highest stratum clock source that is available. If multiple, equal clock sources are available, the node synchronizes to the source that is physically the closest. If none of the sources is available, the network synchronizes to the internal oscillator of one of the nodes in the network. When you are planning clock sources, consider the following:

•

The dspclksrcs command displays all clock sources in a network. The dspcurclk command displays the clock source that a specific node is currently using and the path between the source and the local node.

•

To avoid unnecessary clock disruptions, configure all primary clock sources for the network first.

•

A line must be upped and not in an alarm before you can configure it as a network clock source.

•

Before you define a trunk as a clock source, use cnftrk to specify that the trunk does not pass synchronization.

Full Name

Syntax

Related Commands

Attributes


Privilege	Jobs	Log	Node	Lock
1	No	Yes	IGX, BPX	Yes

Example 1

Description

Configure circuit line 15 as a primary clock. The network clock sources screen shows that circuit line 15 has been configured as a primary clock source for the network.

System Response

Table 11-3 cnfclksrc—Parameters
Parameter	Description
line type	Specifies whether the clock source is a trunk (p), circuit line (c), or external (e).
line number	For a network clock source of either a circuit (c) or trunk (p), this specifies the back slot location of the source. For external clock sources (e), enter either front card slot 1 or 2 as long as either slot has a card. This external source designation applies to IGX, and BPX nodes.
source type	Specifies where the clock fits in the hierarchy: p = primary; s = secondary; and t = tertiary. To remove the clock source configuration for the current type and line, enter an "r."

Table 11-4 cnfclksrc—Optional Parameters
Parameter	Description
freq	Specifies the frequency of the clock source. An entry is necessary only if the line type is an external line. The supported frequencies are 1.544 MHz and 2.048 MHz. Enter a "1" for 1.544 MHz or a "2" for 2.048 MHz.

dspclksrcs

Displays all clock sources for the network. The display for unreachable or failed clock sources flashes on and off.

Full Name

Syntax

Related Commands

Attributes


Privilege	Jobs	Log	Node	Lock
1-6	No	No	IGX, BPX	No

Example 1

Description

System Response

dspcurclk

Displays the current clock source. The display for dspcurclk contains the following information:

•

Source Line: the type of line used as the clock source and its back slot number (for example, "CLN 15", TRK 22, "EXTERNAL 2", or "INTERNAL").

•

Clock Type: the clock type configured for the source clock (primary, secondary, or tertiary). If the source clock for the node is an internal oscillator, no clock type is given.

•

Clock Frequency: the received clock frequency as measured by the local NPC/BCC.

•

Path to Source: the path from the current node to the node of the originating clock source. This includes all intermediate nodes and trunks.

Full Name

Syntax

Related Commands

Attributes


Privilege	Jobs	Log	Node	Lock
1-6	No	No	IGX, BPX	No

Table Of Contents