Overview

This chapter describes the Cisco Cable Clock Card and contains the following sections:

The Cisco Cable Clock Card (see Figure 1-1) is a dual-port field-replaceable component that is designed to provide a reliable national clock signal to your Cisco uBR7246 VXR universal broadband router. The primary and secondary DB-15 interfaces on the front of the clock card connect to external T1 clock signal sources.

When installed in the Cisco uBR7246 VXR chassis, the clock card can propagate a national clock signal throughout the router's midplane by locking onto an external T1 signal originating over the PSTN, locking onto a T1 clock signal originating from a port adapter installed in the same chassis, or by connecting to a Global Positioning System (GPS) receiver generating a T1 clock signal.

Figure 1-1: Clock Card---Faceplate View

Caution Do not attempt to remove or install the clock card unless you have first powered off the Cisco uBR7246 VXR.

Clock Source Overview

There are three methods used to acquire and distribute both the primary and secondary national clock sources on the Cisco uBR7246 VXR. Using one of these three methods, the clock card can propagate a national clock signal throughout the chassis midplane to any installed MC16S and MC16E cable modem cards, thus synchronizing communications over all cable interfaces in the Cisco uBR7246 VXR to other routers. The three methods used to acquire a national clock signal are:

Connecting to external T1 source signals from the Public Switched Telephone Network (PSTN)
Propagating the clock signal from an active T1 connection originating on a port adapter installed in the same Cisco uBR7246 VXR
Connecting to an external GPS receiver generating the appropriate T1 clock signal

External clock sources must conform to the following minimum specifications:

Traceable to Stratum 1 clock signal

Note A Stratum 1 clock has a long-term accuracy of 10^-11. Stratum 1 clocks are generally used for synchronizing a few master sites in a digital telecommunications network. The synchronized signals propagate the time standard throughout the network.

Wander on the reference input must not exceed ANSI T1.101 section 7.2.2, time deviation for Type III DS1 reference signals
Jitter must not exceed the maximum allowable specified in GR-499-CORE section 7.2 for DS1 rates

Note When installed in the Cisco uBR7246 VXR chassis, the clock card will only operate in conjunction with Cisco MC16S and MC16E cable modem cards. The "C" series cable modem cards---MC11C, MC12C, MC14C, and MC16C---do not support the clock card.

Modes of Operation

The clock card can provide timing reference to the Cisco uBR7246 VXR in a number of ways:

Locked to either primary or secondary signal---The clock card remains in continuous contact with the primary and/or secondary external Stratum 1 clock source until a loss of service occurs, at which time the clock card assumes either holdover or freerun operation. This is the normal mode of operation for the clock card.
Holdover mode (crystal stabilized)---The clock card enters holdover mode when a loss of service to the current primary and/or secondary external Stratum 1 clock source has occurred, providing that a connection to the external clock source was active and stable for at least 25 seconds. Holdover mode provides a reliable Stratum 3 clock to the Cisco uBR7246 VXR for up to 72 hours after a loss of external service.

Note Holdover mode features 1 ppm maximum drift per day accuracy.

A Stratum 3 clock has a long-term accuracy of 10^-9.

Free running crystal source---The clock card enters freerun mode when it is unable to establish reliable service with either the primary or secondary external clock source when the Cisco uBR7246 VXR is powered on. This is the least stable of all operating modes.

Note The clock card features a minimum freerun frequency accuracy of 4.6 ppm.

Locked to midplane TDM bus clock---If no connection to an external clock source can be established, the clock card can lock on to a T1 clock signal originating from a T1 port adapter installed in the same Cisco uBR7246 VXR and drive the TDM bus, which can then be used by cable modem cards in the Cisco uBR7246 VXR transporting traffic using the TDM bus.

If the primary external clock source experiences a loss of service, the clock card enters holdover mode. After a few seconds of running in holdover mode, the clock card will switch to the secondary external clock source, if one has been connected. The clock card will switch back to the primary external clock source at its first opportunity to restore a reliable connection.

Interface Specifications and Pinouts

The interfaces on the front of the clock card are receive-only DB-15 connectors that enable you to connect your Cisco uBR7246 VXR to external T1 national clock sources originating over the PSTN or a GPS receiver. After you have successfully connected to a primary and or secondary external clock source, you can synchronize data, voice over IP (VoIP) and video communications to other routers around the corner, around the country, and around the world.

The primary and secondary DB-15 interfaces on the front of the clock card connect to external clock signal sources via standard DB-15 T1 interface cables.

Warning To reduce the risk of fire, use only No. 26 AWG or larger telecommunication line cord.

Figure 1-2: DB-15 T1 Clock Card Interface

Table 1-1 lists connector pinouts for the DB-15 T1 clock card interface.

Table 1-1: DB-15 T1 Clock Card Interface

Clock Card End External Clock Source End

Pin¹ Signal Pin Signal

1

Transmit Tip

3

Receive Tip

3

Receive Tip

1

Transmit Tip

9

Transmit Ring

11

Receive Ring

11

Receive Ring

9

Transmit Ring

¹Any pins not described in this table are not connected.

Clock Card End	External Clock Source End
Pin¹	Signal	Pin	Signal
1	Transmit Tip	3	Receive Tip
3	Receive Tip	1	Transmit Tip
9	Transmit Ring	11	Receive Ring
11	Receive Ring	9	Transmit Ring

LEDs

The clock card features "ACTIVE" and Loss of Service ("LOS") status indicators for both the primary and secondary clock source connectors. In addition, the clock card features single "ENABLE," "HOLDOVER," "FREERUN," and "FAULT" LEDs. (These LEDs are shown in Figure 1-3.)

Figure 1-3: LEDs on the Clock Card---Faceplate View

Table 1-2 describes the purpose and behavior of each LED on the clock card.

Table 1-2: Clock Card LED Descriptions

LED Color Description

LOS (Primary)

Yellow

Indicates that the clock card has lost contact with the primary external clock source.

LOS (Secondary)

Yellow

Indicates that the clock card has lost contact with the secondary external clock source.

ACTIVE (Primary)

Green

Indicates that the clock card is currently locked to the primary external clock source signal.

ACTIVE (Secondary)

Green

Indicates that the clock card is currently locked to the secondary external clock source signal.

ENABLE

Green

Indicates that the clock card is correctly installed in and recognized by the Cisco uBR7246 VXR chassis.

HOLDOVER

Yellow

Indicates that the clock card has lost contact with the current primary and/or secondary external clock source(s) and has entered holdover mode. This means that the clock card was locked to an external clock signal for at least 25 seconds before losing contact and has temporarily become the clock source for the Cisco uBR7246 VXR.

FREERUN

Yellow

Indicates that the clock card has lost contact with both the primary and secondary external clock sources and has inadequate storage from prior connection to enter holdover mode. Unlike holdover mode, entering freerun mode means that the clock card was initially unable to lock on to either a primary or secondary external clock signal.

Note Freerun mode is most commonly encountered at startup when the clock card is unable to recognize an external clock source.

FAULT

Amber

Indicates a hardware failure with the clock card circuitry.

Clock Card Location in the Cisco uBR7246 VXR Chassis

The chassis slot of the Cisco uBR7246 VXR chassis into which the clock card can be installed is on the left side of the chassis immediately below port adapter slot 1. Figure 1-4 shows a Cisco uBR7246 VXR with the clock card installed.

Figure 1-4: Clock Card Location in the Cisco uBR7246 VXR

Table of Contents

Overview

Overview

Clock Card Overview

Clock Source Overview

Modes of Operation

Interface Specifications and Pinouts

LEDs

Clock Card Location in the Cisco uBR7246 VXR Chassis