The packet gateway and the PGC communicate through virtual interfaces. JUNOS interface names are not known or communicated to the PGC. You configure a virtual interface on the packet gateway, and this virtual interface is provided to the PGC. The virtual interface configuration includes the media service for the virtual interface, which contains the name of the NAT pool.
Included in the H.248 message exchange between the PGC and the VPG is a virtual interface identifier. This identifier instructs the packet gateway which media resources to use.