General:
MGCP stands for Media Gateway Control Protocol. It’s a client-server protocol which means that it has to depend on Media Controller (Call agent) for call routing. In case of Cisco this MC is a CUCM. The Call agent controls the ports of a MGCP gateway.
UDP Port 2427 is used for communication between Call manager and Gateway. TCP port 2428 is used for PRI Backhauling and Keepalives from Gateway to Call manager. PRI backhauling is process where layer 3 information is passed back to Call manager while layer 2 stays at the gateway. Prior to Call manager 3.1, gateway use to pass all the L2 (Q921) information back to Call manager. Call manager use to serve Q921 and Q931 messages. So loss of communication with that call manager would bring the L2 and L3. Losing L2 means losing D-channel effectively dropping the call. In new method of Call preservation an MGCP gateway preserves a call by terminating L2 at the gateway itself. We will discuss Call Preservation in another section.
MGCP messages between CCM and Gateway:
AUEP -> Audit Endpoint -> CCM to GW
AUCX -> Audit Connection-> CCM to GW
CRCX-> Create Connection-> CCM to GW
DLCX-> Delete Connection-> CCM to GW or GW to CCM
EPCF-> Endpoint Configuration-> CCM to GW
MDCX-> Modify Connection-> CCM to GW
RQNT-> Request Notification-> CCM to GW
NTFY-> Notify-> GW to CCM
RSIP-> Restart in progress-> GW to CCM
Call Flow:
- Phone goes off-hook
- CCM asks Calling Gateway to create connection to that endpoint
- Gateway creates a connection and send an SDP message to CCM
- CCM asks receiving gateway to create connection on one of its endpoints
- Receiving gateway creates a connection and sends its own SDP
- CCM sends the Session Description information from receiving gateway to Calling gateway with MDCX message
- Communication is now bi-directional and call can proceed
- When an endpoint hangs up, CCM sends DLCX to both gateways
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