Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

Figure 16.3 TDM-IP preemption event

The scenario in Figure 16.4 starts with UA1 initiating a call to UA2 using an
INVITEmessage with resource priority R-P:3at level 3. In the success sce-
nario shown here, an RTP media stream is established.
During the conversation, an emergency caller, UA3, initiates a new INVITE,
but with priority level 2 (R-P:2), which is higher than for the existing call.

Figure 16.4 SIP access preemption with reason header

INVITE (R-P : 3)

UA1 UA2 UA3

200 OK

BYE (Reason: Preemption

INVITE (R-P : 2)

ACK

ACK

200 OK

200 OK

RTP

Preemption

SS7

Location

Server

TDM-IP

Gateway

IP

Domain

Preemption

SIP

Emergency and Preemption Communication Services 283