The serial links connect into either a private data network or a service provider’s network. Both
the line encoding and the framing must match the service provider’s equipment. To configure
a PRI on a serial link, you must supply the following information:
Channel type Either T-1 or E-1.
Frame type When using a T-1, this can be either D4, sometimes referred to as Super Frame,
or Extended Super Frame (ESF). D4 is the original T-1 frame format and comprises one framing
bit and a DS0 time slot for each channel on the line. ESF comprises 24 D4 frames. As each D4
frame contains a framing bit, an ESF has 24 framing bits that it uses for synchronization (6 bits),
error checking (6-bit cyclic redundancy check), and diagnostic data channel (12 bits).
Linecode This will be either alternate mark inversion (AMI) or binary 8-zero substitution
(B8ZS). B8ZS is typically used in the U.S.; however, most legacy phone systems still use AMI.
Dynamic Multiple Encapsulation Back in the old days, prior to Cisco IOS 12.1, the interface
encapsulation that we used in the previous example—PPP and others such as Frame Relay,
High-Level Data Link Control (HDLC), Link Access Procedure (LAP), and X.25—could support
only one ISDN B channel connection over the entire link, or as in the case of HDLC and
PPP, the entire link needed to use the same encapsulation method. With the Dynamic Multiple
Encapsulation feature, the ISDN B channel becomes a forwarding device, and the D channel is
ignored, thereby allowing different encapsulation types and per-user configuration.
Which T-1 time slots to use By using the pri-group command on your PRI interface, you
can define which time slots will be controlled by the D channel (subchannel 23). You can also
specify dedicated time slots on the same interface with the channel-group number time slot
range command. This will assign the time slots in the range specified, to the subchannel group
of number.
In the following example, we chose to configure slot 1, port 0 of the MIP card in a 7000 router,
and we opted for ESF framing, with B8ZS line coding. Remember not to get confused with the
channel group and time slot numbering; the channel group numbers range from 0 to 23, whereas
the time slot values range from 1 to 24. Also remember that channel 15 on the E-1 and channel 23
on the T-1 are for the D channels. The command pri-group timeslots 12-24 indicates that
the D channel will control time slots 11 through 23 on the PRI circuit. Channel group 1 has six
time slots running at 64Kbps. We could choose up to 24 DS0s but purchased only six from our
provider, with 12 through 24 being controlled with the PRI D channel. Here’s the output:
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#controller T1 1/0
Router(config-if)#framing esf
Router(config-if)#linecode b8zs
Router(config-if)#pri-group timeslots 12-24
Router(config-if)#channel-group 1 timeslots 1-6 speed 64
Router(config-if)#^Z
An IP address and the serial encapsulation method (HDLC is the default) then needs to be
assigned to each interface, as shown in the following example:
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#interface serial1/0:23
Router(config-if)#encapsulation ppp
Router(config-if)#ip address 172.16.30.5 255.255.255.252
Router(config)#interface serial1/0:1
Router(config-if)#encapsulation hdlc
Router(config-if)#ip address 172.16.30.5 255.255.255.252
Output for the five other B channels (serial/0-2-6) has been omitted to save space.
Router(config-if)#^Z
Router#
When connecting two MIP cards back-to-back, you must specify the clocking on
one controller. This is done with the clock source internal command.