BITS Network Clocking

Feature Overview

Benefits

BITS Network Clocking

This document describes the Building Integrated Timing Supply (BITS) Network Clocking feature. It includes information on the benefits of the feature, supported platforms, related documents, and configuration.

This document includes the following sections:

Feature Overview

The BITS Network Clocking feature enables the Cisco 6400 universal access concentrator (UAC) to derive network timing from the central office (CO) BITS as well as from a clock recovered from a specified node line card (NLC) interface. The Cisco 6400 UAC can also distribute the BITS network timing with stratum level 3 accuracy to other network devices.

Benefits

Global Clocking Synchronization

For some telecommunications deployments, BITS clocking is required to provide global clocking synchronization of network equipment in the end-to-end data path.

Improved Physical Layer Synchronization and Transmission Quality

Stratum level 3 accuracy reduces clock drifting, which reduces frame loss.

Supported Platforms

BITS Network Clocking is supported on the Cisco 6400 UAC.

Supported Standards, MIBs, and RFCs

Standards

Telecommunications - Synchronization Interface Standard---ANSI T1.101-1994

MIBs

CISCO-6400-CHASSIS-MIB

For descriptions of supported MIBs and how to use MIBs, see the Cisco MIB web site on CCO at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.

RFCs

None

Prerequisites

NSP-S3B Module

The BITS Network Clocking feature requires the node switch processor with stratum 3/BITS (NSP-S3B) module. Figure 1 shows the NSP-S3B module faceplate.

Figure 1: NSP-S3B Module Faceplate

In addition to enabling the BITS Network Clocking feature, the NSP-S3B allows the Cisco 6400 to serve as a stratum 3 network clock source for other network devices. When no external clock source is available, the NSP-S3B provides stratum level 3 internal timing on the Cisco 6400. Otherwise, the NSP-S3B is identical to the default NSP. For information about installing the NSP-S3B, see the Cisco 6400 UAC Hardware Installation and Maintenance Guide .

Valid BITS Clock Source

In order to derive the network clocking from the central office (CO) BITS, the BITS input must be less than 9.2 parts per million (ppm) off center. Otherwise, the NSP-S3B declares the clock source invalid.

Configuration Tasks

See the following sections for configuration tasks that enable the BITS Network Clocking feature. Each task in the list is optional.

Configuring the Network Clock to Use the BITS Clock Source

Use the following commands on the NSP-S3B to derive network clocking from the BITS signal, beginning in global configuration mode:

	Command	Purpose
Step 1	`Switch(config)# network-clock-select 1 BITS`	Set BITS as highest priority clock source.
Step 2	`Switch(config)# network-clock-select BITS {T1\|E1}`	Specify BITS port as either T1 or E1.

Configuring Lower Priority Clocking Sources

Use the following command to configure additional but lower priority clock sources for the Cisco 6400 NSP-S3B network clock:

Command	Purpose
`Switch(config)# network-clock-select priority {ATM{slot/subslot/port}\|BITS\|system}`	Choose a clock source and assign a priority value between 2 and 4.

Configuring Revertive Clocking

Use the following command to configure automatic switchover to the highest priority clock source available:

Command	Purpose
`Switch(config)# network-clock-select revertive`	Configure revertive behavior on the network clock.

Verifying the Network Clock

To check the current clock source and the status of all configured network clock sources, use the privileged EXEC command show network-clocks.

Switch#show network-clocks
clock configuration is Revertive
Priority 1 clock source: bits up
Priority 2 clock source: ATM1/0/0 up
Priority 3 clock source: ATM5/0/0 up
Priority 4 clock source: ATM7/0/0 up
Priority 5 clock source: system
 
Current clock source: bits, priority: 1

Troubleshooting Tips

To troubleshoot BITS network clocking:

Make sure the show network-clocks command output includes the following lines:
Make sure the BITS T1 or E1 cable is securely attached to the BITS terminal block on the rear of the Cisco 6400 UAC chassis.

Configuration Examples

In the following example, the network clock is set to automatically switch to the highest priority clock source available. BITS is assigned the highest priority value of 1, and the clock input is an E1 line. ATM interfaces are assigned to all available lower priority values:

!
network-clock-select revertive
network-clock-select bits e1
network-clock-select 1 bits
network-clock-select 2 ATM1/0/0
network-clock-select 3 ATM5/0/0
network-clock-select 4 ATM7/0/0
!

Command Reference

This section documents one modified command. All other commands used with this feature are documented in the ATM Switch Router Command Reference .

network-clock-select

network-clock-select

Use the network-clock-select global configuration command to specify selection priority for the clock sources. Use the no form of this command to cancel the network clock selection.

network-clock-select {priority {ATM slot/subslot/port | BITS | system} | BITS {E1 | T1} | revertive}

no network-clock-select {priority {ATM slot/subslot/port | BITS | system} | BITS {E1 | T1} | revertive}

Syntax Description


priority	Specifies the priority, between 1 (highest) and 4.
ATM	ATM interface on NSP.
slot/subslot/port	Specifies the slot, subslot, and port number of the ATM interface.
BITS	Selects the BITS input as the network clock source.
system	The free running local clock oscillator on the NSP-S3B.
E1	Configures the BITS port as E1.
T1	Configures the BITS port as T1.
revertive	Automatic switchover to highest priority clock source when available.

Defaults

System clock is the priority 5 clock source.

Command Modes

Global configuration

Command History


Release	Modification
11.1 CA	This command was introduced for ATM-CES interfaces.
11.2	This command was introduced for the LightStream 1010.
12.0	Command modified for Catalyst 8510 MSR, Catalyst 8540 MSR, and LightStream 1010.
12.0(7) DB	Command modified for the NSP-S3B of the Cisco 6400 UAC.

Usage Guidelines

You can specify up to four clock priorities. If the network-clock-select revertive command is used, the highest priority active interface in the router supplies the primary reference source to all other interfaces that require network clock synchronization services.

The fifth priority is always the system, or local clock oscillator on the NSP. You cannot configure the priority 5 clock source.

Examples

The following example shows how to configure the network clock as revertive and assign clock sources to each available priority:

Switch(config)# network-clock-select revertive
Switch(config)# network-clock-select bits e1
Switch(config)# network-clock-select 1 bits
Switch(config)# network-clock-select 2 ATM1/0/0
Switch(config)# network-clock-select 3 ATM5/0/0
Switch(config)# network-clock-select 4 ATM7/0/0

Related Commands


Command	Description
show network-clocks	Displays network clock status.

Glossary

BITS---Building Integrated Timing Supply. A single building master timing supply that supplies DS1 and DS0 level timing throughout an office.

DS0---Digital signal level 0. Framing specification used in transmitting digital signals at 64 kbps. Twenty-four DS0s equal one DS1.

DS1---Digital signal level 1. Framing specification used in transmitting digital signals at 1.544 Mbps on a T1 facility.

Stratum 3---A precision timing reference that provides a free-run accuracy of +/- 4.6 PPM (parts per million), pull-in capability of 4.6 PPM, and holdover stability of less than 255 slips during first day. Thorough descriptions can be found in ANSI T1.101-1994 and the Bellcore document GR-1244-CORE.

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