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Cisco Cable Clock Card Hardware Installation Guide for the Cisco uBR7246VXR Universal Broadband Router
OL-26705-01
Chapter 1 Overview
Clock Source Overview
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 a supported cable interface line card, thus
synchronizing communications over all cable interfaces in the Cisco uBR7246 VXR to other routers.
To use a national clock source, the cable interface line card must be one of the following models:
•
Cisco uBR-MC16S
•
Cisco uBR-MC16E
•
Cisco uBR-MC28C
•
Cisco uBR-MC28U/X
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 a Cisco uBR7246 VXR chassis, the clock card operates with a Cisco uBR-MC16S or
Cisco uBR-MC16E cable modem card using Cisco IOS Release 12.1(1a)T1 or later, as well as Cisco IOS
Release 12.1(2)EC1 or later. The clock card also operates with a Cisco uBR-MC28C cable modem card
using Cisco IOS Release 12.1(3a)EC1 or later. You can use other cable interface cards, such as the
Cisco uBR-MC16C, with the clock card, but these other cable interfaces will not synchronize their
downstream SYNC messages with the external clock source.
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.
