Cisco UBR7246VXR Installationshandbuch - Seite 9
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Chapter 1
Overview
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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.
OL-26705-01
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.
Holdover mode features 1 ppm maximum drift per day accuracy.
Note
A Stratum 3 clock has a long-term accuracy of 10
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.
The clock card features a minimum freerun frequency accuracy of 4.6 ppm.
Note
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.
Cisco Cable Clock Card Hardware Installation Guide for the Cisco uBR7246VXR Universal Broadband Router
Interface Specifications and Pinouts
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