Call Bureaucracy Using a Alter Server
A alter server is programmed to ascertain a aisle to the destination. Instead of forwarding
the INVITE to the destination, the alter server letters aback to a UA with the destination
coordinates the UA should try next, as adumbrated in Figure 5-26.
Chapter 5: Examining VoIP Gateways and Gateway Control Protocols 301
Invite
ACK
OK
Trying
Ringing
Moved
Invite
SIP
Gateway
Redirect
Server
Calling Party Called Party
IP
BYE
RTP Stream
SIP Signaling and SDP
(UDP or TCP)
Bearer or Media
(UDP)
200 OK
V
SIP
Gateway
IP V
Figure 5-26 Alarm Bureaucracy Using a Alter Server
A alter server offers abounding of the advantages of the proxy server. However, the number
of letters complex in redirection is beneath than with the proxy server procedure.
The UA has a added workload because it charge admit the consecutive invitation.
When a alter server is used, the alarm bureaucracy procedure, as illustrated in Figure 5-26,
uses the afterward steps:
1. The basic UAC sends an allurement (INVITE) to the alter server.
2. The alter server, if required, consults the area server to actuate the aisle to
the almsman and its IP address.
3. The alter server allotment a “moved” acknowledgment to the basic UAC with the IP
address acquired from the area server.
4. The basic UAC acknowledges the redirection.
5. The basic UAC sends an INVITE to the alien UAS.
6. If the UAS of the almsman determines that the alarm ambit are acceptable, it
responds absolutely to the UAC.
7. The basic UAC issues an ACK.
The UAC and UAS now accept all the advice that is appropriate to authorize RTP sessions
between themselves.