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T640 SONET Clock Generators (SCGs) Description


The SONET Clock Generators (SCGs) provide 19.44-MHz Stratum 3 clock signal for the SONET/SDH interfaces on the router. One SCG is shipped as part of the standard router configuration, but up to two SCGs can be installed to provide redundancy. SCGs are installed into the upper rear of the chassis in the slots labeled SCG0 and SCG1.

Backup SCGs are hot-removable and hot-insertable. Master and nonredundant SCGs are hot-pluggable.

The router supports the SCGs in Table 1.


Redundant SCGs must be the same model number, except during upgrade.

Table 1: Supported Power Supplies


Model Number

First Supported Junos OS Release

SCG with DB-9 ports (Figure 1)



SCG with RJ-48 ports (Figure 2)



Figure 1: SCG with DB-9 Ports
SCG with DB-9 Ports
Figure 2: SCG with RJ-48 ports
SCG with RJ-48 ports

Each SCG consists of the following components:

  • 19.44-MHz Stratum 3 clock.

  • Field-programmable gate array (FPGA) that performs multiplexing of clock sources.

These components are located on the SCG faceplate:

  • Three LEDs—OK, FAIL, and MASTER, that display the status of the SCG.

  • SCG online/offline button.

  • Two external clock inputs.


    Junos OS Release 10.4 and later supports the external clock inputs on the SCG with RJ-48 ports. The external clock inputs are not supported on the SONET Clock Generator (SCG) with DB-9 ports.

    For information about configuring external clock synchronization for T Series routers, see Junos OS Administration Library

  • Two LEDs for each RJ-48 external clock input that display the status of the links.