Hello,
Backhaul means transporting signaling (all Q.931 messages) from an ISDN PRI/BRI interface on an MGCP gateway to CUCM for processing. That information is backhauled to CUCM through a TCP connection.
ISDN Q.931 is the L3 specification that is responsible for call control and management (call setup, tear down, etc).
Hope to help, please remember to rate the post if it does.
Kind regards,
- Adrian.