5.8 ISDN Primary Rate Interface
5.8.4
Configuring the framing, linecoding, and clocking of the controller

The framing controller configuration command is used to select the frame type used by the PRI service provider:

Router(config-controller)#framing {sf | esf | crc4 | no-crc4}

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

Router(config-controller)#linecode {ami | b8zs| hdb3}

B8ZS accommodates the ones density requirements for T1 carrier facilities, using special bipolar signals 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 linecode types used at the T1/E1 WAN provider’s CO switch.

Use the clock source command line and internal options to configure the T1 clock source for the Cisco 7000 family, and the Cisco 3600 and 4000 series modules. The AS5000 access server series uses the line primary and secondary version of the command to select either the primary or secondary time-division multiplexing (TDM) as the clock source. With two controllers in an AS5000 series, one will be primary and one will be secondary. With four controllers, one will still be primary with multiple secondaries.

Router(config-controller)#clock source {line [primary | secondary] | internal}

The following are the usual configurations:
For T1 -- framing esf and linecode b8zs
For E1 -- framing crc4 and linecode hdb3

One instance in which you would use clock source internal is when two routers are connected back to back in a test environment.