Network Components (part 4)

This section describes the function of the network components listed in Figure 1. Depending upon the particular network architecture some of these network components may be combined into a single solution, for example a combined signaling and trunking gateway.

Call Agent/SIP Server/SIP Client 

The Call Agent/SIP Server/SIP Client is located in the service provider’s network and provides call logic and call control functions, typically maintaining call state for every call in the network. Many call agents include service logic for supplementary services, e.g. Caller ID, Call Waiting, and

also interact with application servers to supply services that are not directly hosted on call agent. The Call Agent will participate in signaling and device control flows originating, terminating or forwarding messages. There are numerous relevant protocols depending upon the network architecture including SIP, SIP-T, H.323, BICC, H.248, MGCP/NCS, SS7, AIN, ISDN, etc. Call Agents also produce details of each call to support billing and reconciliation.  A SIP Server provides equivalent function to a Call Agent in a SIP signaling network, its primary roles are to route and forward SIP requests, enforce policy (for example call admission control) and maintain call details records. For example the SIP Server in Service Provider 1’s network will route and forward SIP requests from SIP Phones belonging to customers.

 

A SIP Client provides similar function to a SIP Server, but originates or terminates SIP signaling rather than forwarding it to a SIP Phone or other CPE device. For example a SIP call is shown in figure 1 between a SIP Phone in Service Provider 1’s network and the SIP server in Service Provider 3’s network. The Call Agent/SIP Server terminates the SIP signaling and converts it to H.248 or MGCP to set up a call to the correct subscriber.  Call Agents are also known as Media Gateway Controllers, Softswitches and Call Controllers. All these terms convey a slightly different emphasis but maintaining call state is the common function.

Call Waiting, and also interact with application servers to supply services that are not directly hosted on call agent. The Call Agent will participate in signaling and device control flows originating, terminating or forwarding messages. There are numerous relevant protocols depending upon the network architecture including SIP, SIP-T, H.323, BICC, H.248, MGCP/NCS, SS7, AIN, ISDN, etc. Call Agents also produce details of each call to support billing and reconciliation.  A SIP Server provides equivalent function to a Call Agent in a SIP signaling network, its primary roles are to route and forward SIP requests, enforce policy (for example call admission control) and maintain call details records. For example the SIP Server in Service Provider 1’s network will route and forward SIP requests from SIP Phones belonging to customers.