| 摘要 |
Node A represents an originating signalling point to which a calling party is connected whilst node C represents a terminating signalling point to which a called party is connected. Each of the signalling points comprises a call control level and a bearer control level. The bearer control level, comprising a bearer network such as an Internet Protocol (IP) network or an ATM network. When a calling party initiates a call, the originating signalling point generates a list of possible options relating to the capabilities of the originating signalling point which must be negotiated with the terminating signalling point node C. Node A sends a message to node C specifying the list of options in order of preference. Node C selects the preferred option and signals its choice to node A. The call control level comprises a number of Media Gateway Controllers (figure 3, MGC_A, MGC_B, MGC_C ) whilst the bearer control level comprises a number of Media Gateways (figure 3, MG_1 to MG_6). When a Media Gateway controller receives a connection set-up message containing a list of possible options relating to a specific capability from a peer Media Gateway Controller, the receiving Controller reads the list and removes those options which are not supported by the associated Media Gateway. The controller then sends the modified list to the Media Gateway. The receiving Media Gateway choses the option having the highest priority which the Gateway is currently able to support. The Media Gateway then informs the controller of the selected option which is signalled to the peer Media Gateway controller. Capabilities which may be negotiated in this way include the choice of a suitable speech codec for the call or security capabilities such as voice ciphering or data encryption.
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