Figure 15: Unidirectional Gate
The voice feature uses gates to control voice flows in the transport plane. Gates are created through signaling instructions that the PGC provides to the PG. Using the signaling instructions, the PG defines gates to allow, drop, or manipulate voice flows as they traverse the router.
Each gate provides a unidirectional voice flow. A pair of gates provides a bidirectional voice flow. Figure 15 shows a unidirectional gate.
Figure 15: Unidirectional Gate
Gates are defined by their local source and destination addresses and their remote source and destination addresses.
Figure 16 shows a gate pair, which represents a bidirectional voice flow. The local destination address of Gate 1 is equal to the local source address of Gate 2, and the local source address of Gate 1 is equal to the local destination address of Gate 2.
Figure 16: Addressing of Gate Pairs
Based on information acquired through VoIP signaling, the PGC instructs the packet gateway through PGCP commands which gates to create and which actions to associate with them. Each gate can have many actions associated with it; for example, NAT, DSCP marking, and latching. The pgcpd process decodes PGCP commands that it receives from the PGC and uses IPC messages to instruct the PIC to create, delete, or modify gates and apply required actions to each gate.
The following IPC messages are exchanged between the pgcpd process and the PIC:
When a gate is created, it is assigned an identifier. You can use this identifier with the PGCP show commands to monitor specific gates.
