Cisco Linksys SPA9000 - Manual (Page 21)

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Linksys SPA9000 Administrator Guide
Document Version 3.01
Chapter 1 Using the Linksys Voice System
Technology Background
In Figure 1-3 , 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.
Figure 1-3 SPA9000 as a SIP Proxy
When a user picks up the handset in an LVS system, the SPA9000 collects DTMF digits from a touchtone
analog telephone or the locally connected SPA900 Series IP phones. Unless the call is for a local client
station, the SPA9000 system sends the full number in a SIP INVITE message to another SIP proxy server
for further call processing.
To minimize dialing delay, a dial plan is maintained that is matched against the cumulative number
entered by the user. Invalid phone numbers that are not compatible with the dial plan are detected and
the user is alerted using a configurable tone (reorder) or announcement.
Figure 1-3 also illustrates connectivity between the SPA9000 and the ITSP over the Internet. When
UserA calls UserC, the SPA9000 directs the request to the SIP proxy at the ITSP, which is then
responsible for routing the request to UserC. Again, once the session is established, RTP packets are
exchanged directly between UserA and UserC. However, this requires that the firewall on the Internet
routers allow UserA access to the Internet. Because the SIP UAs are generally assigned IP addresses
dynamically through DHCP, this makes implementing a secure firewall policy more difficult.
SPA9000 Media Proxy
To address this possible security issue, the SPA9000 can also function as a media (RTP) proxy. This
option forces RTP traffic destined for the Internet (or IP WAN) to be directed to the SPA9000, which
then directs it to the remote UA. This configuration may simplify firewall configuration because the
client stations do not require direct access to the Internet through the firewall.
To enable the media proxy, set the PBX Parameters:<Force Media Proxy> parameter to True. With the
media proxy enabled, when UserA calls User C, the SPA9000 still acts as the SIP proxy and forwards
the request to the SIP server on the ITSP. However, even after the SIP session is established, the
SPA9000 continues to direct RTP packets between UserA and the ITSP.
SIP Proxy with
media proxy enabled
Internet
IP Router (firewall)
Broadband modem
Hub/switch
SPA9000
UserA
UserB
UserC
Internet (WAN)
Interface
ITSP
ISP