Specifies the T1 clock source

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Use the framing controller configuration command to select the frame type used by the PRI service provider. The table shows framing commands that you can use.

framing Command




Super Frame. Use for some older T1 configurations.


Extended Super Frame. Use for T1 PRI configurations.

crc4 or no-crc4

Cyclic redundancy check 4. Use for E1 PRI configurations.

Without a sufficient number of ones in the digital bit stream, the switches and multiplexers in a WAN can lose their synchronization for transmitting signals. Use the linecode command to identify the physical layer signaling method to satisfy the "ones" density requirement on the digital facility of the provider.

The table shows line code commands that you can use.

linecode Command




Alternate mark inversion. Use for T1 configurations.


Binary 8-zero substitution. Use for T1 PRI configurations.


High density binary 3. Use for E1 PRI configurations.

Binary 8-zero substitution (B8ZS) accommodates the ones density requirements for T1 carrier facilities using special binary signals that are encoded over the digital transmission link. It allows 64 kbps (clear channel) for ISDN channels.

Settings for these two Cisco IOS software controller commands on the router must match the framing and line-code types used at the T1/E1 WAN CO switch of the provider.

Use the clock source {line | internal} command to configure the T1 and E1 clock source on Cisco routers. T1 configurations typically require framing esf and linecode b8zs. E1 configurations typically require framing crc4 and linecode hdb3.

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