Linksys SPA9000 Administration Guide

Linksys Voice System Administration Guide 40
Network Address Translation (NAT) and Voice over IP (VoIP)
Configuring the SPA9000 for ITSP Interoperability
Figure 3-2 SPA9000 as a SIP Proxy for Internet Calls
Network Address Translation (NAT) and Voice over IP (VoIP)
NAT is a function that allows multiple devices to share the same public, routable, IP address to
establish connections over the Internet. NAT is present in many broadband access devices to
translate public and private IP addresses. To enable VoIP to co-exist with NAT, some form of NAT
traversal is required.
Some ITSPs provide NAT traversal, but some do not. If your ITSP does not provide NAT traversal,
you have several options.
• ”NAT Mapping with SIP-ALG Router,” on page 40
• ”NAT Mapping with Session Border Controller,” on page 41
• ”Configuring NAT Mapping with a Static IP Address,” on page 41
• ”Configuring NAT Mapping with STUN,” on page 42
NAT Mapping with SIP-ALG Router
You can achieve NAT mapping by using a router that has a SIP ALG (Application Layer Gateway).
The Linksys WRV200 router is recommended for this purpose, although any router with a SIP-
ALG can be used. By using a SIP-ALG router, you have more choices in selecting an ITSP.
SIP Proxy with
media proxy enabled
Internet
IP Router (firewall)
Broadband modem
Hub/switch
SPA9000
UserA
UserB
UserC
Internet (WAN)
Interface
ITSP
ISP

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