Linksys SPA9000 Administration Guide

Linksys Voice System Administration Guide 131
Technology Background
Figure A-1 SIP Requests and Responses
Note In this manual, the term client station is used to describe any SIP UA
(including IP phones) that registers with the SPA9000.
In a SIP VoIP network, when the SIP proxy receives a request from a client station (UAS) for a
connection and it does not know the location of the UAC, it forwards the message to another
SIP proxy in the network. Once the UAC is located and the response is routed back to the UAS, a
direct peer-to-peer session is established between the two UAs. The actual voice traffic is
transmitted between UAs over dynamically assigned ports using the Real-time Protocol (RTP).
In Figure A-2, UserA and UserB are client stations (UAs) that register over the local area network
to which the SPA9000 PBX is connected. When UserA calls UserB, the SPA9000 acts as a SIP
proxy and establishes a session between the two UAs. After the session is established, RTP
traffic flows directly between the two client stations.
SIP UA
SIP UA
SIP Proxy
SIP Proxy
RTP
1
2
3
4
SIP Proxy

Linksys SPA9000 Specifications

Physical Interfaces

Ethernet ports	2 10/100BaseT RJ-45
Phone ports	2 RJ-11 FXS
Ring voltage	40-55 VRMS
Ring frequency	10 Hz - 40 Hz
Maximum ringer load	3 REN

Power

On-hook voltage	-50 V NOMINAL
Off-hook current	25 mA min
Terminating impedance	Configurable, including 600 ohms