Practical VoIP Using VOCAL

SIP Messages and Feature Servers

The VOCAL system routes calls to Feature servers by using SIP messages . When the servers first come online, they download a register from the Provisioning server but do not download the file that controls the feature. It is not until the first time that the server runs a script that it downloads the controlling file from the Provisioning server.

SIP Messages to the Feature Servers

Figure 13-1 shows a Feature server receiving a message from a Marshal and then requesting routing information from the Redirect server. It is possible that a call signal may be routed to several Feature servers before leaving the VOCAL system. Some calls may not be routed to any Feature servers before going to the outbound Marshal.

The call flow shown in Figure 13-1 is continued in Figure 13-2.

Tip

We left the 100 Trying messages out of these figures to make them less cluttered.

Figure 13-1. SIP message flow to the Feature servers

Table 13-1 describes the messages illustrated in Figure 13-1.

Table 13-1. SIP message flow to the Feature servers

Interaction	Step	Description
SIP phone to UAMS	1	A call is initiated at one SIP phone to call a party attached to the PSTN. The SIP phone sends an INVITE message to the User Agent Marshal server (UAMS).
	2	The UAMS authenticates the user and forwards the INVITE message to the Redirect server (RS).
	3-4	The RS generates a new ...