Media Gateway Control Protocol (MGCP) controls VoIP through external call-control elements. The first version of MGCP was based on the fusion of SGCP and IPDC. Therefore, this section concentrates on the differences between MGCP and SGCP, which are largely due to functionality inspired by IPDC.
MGCP enables telephony gateways to be controlled by external call-control elements (MGCs; referred to as call agents in SGCP). Telephony gateways include the following:
• Trunk Gateways—The interface between the telephone network and VoIP network
• Voice over ATM Gateways—The interface between the telephone network and Asynchronous Transfer Mode (ATM) network
• Residential Gateways—Enable traditional analog telephone access to inter-work across the VoIP network
• Business and Access Gateways—Provide an analog or digital Private Branch eXchange (PBX) and soft-switch interface to a VoIP network
• Network Access Servers—The interface that provides access to the Internet through the Public Switched Telephone Network (PSTN) and modems
• Circuit or Packet Switches—Offer call-control access to external call-control elements
MGCP utilizes the same connection model as SGCP, where the basic constructs are endpoints and connections. Endpoints can be physical or logical, and connections can be point-to-point or multipoint. MGCP, however, enables connections to be established over several types of bearer networks, as follows:
• IP Networks—Transmission of audio over Transmission Control Protocol/Internet Protocol (TCP/IP) networks using RTP and UDP
• ATM Networks—Audio transmission over an ATM network using ATM adaptation Layer 2 (AAL2) or another adaptation layer
• Internal Connections—Transmission of packets across the TDM backplane or bus of the gateway (such as hairpinning, which occurs when a call is not sent out into the packet network but is sent back to the PSTN)
The remainder of this section covers some simple differences between SGCP and MGCP. This section also provides a primer for the two larger sections covering MGCP event packages and control functions.
MGCP uses SDP to provision gateways with IP addresses and UDP/RTP profiles identical to SGCP. MGCP utilizes SDP for two media types, however: audio circuits and data access circuits. Also, MGCP messages are transmitted across the packet network over UDP, but they can piggyback messages. MGCP enables several messages to be sent to the same gateway in one UDP packet. These piggybacked messages should be processed as if they were received as several simultaneous messages.
A formal wildcard structure, inspired from IPDC, is introduced in MGCP. MGCs or call agents can use the wildcard convention when sending commands to gateways. The wildcard enables the call agent to identify any or all the arguments in the command.
If a multipoint call is completed and a number of connections need to be disconnected, for example, the call agent can send one DeleteConnection request using the all of argument to specify all connections related to the specified endpoint.
Additional call-flows are not provided in this section, given that MGCP has the same call-control functions, messages, and sequencing features as SGCP.
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