Switching to VoIP

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

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Chapter 13: Network Infrastructure for VoIP

Placing your VoIP server on a DMZ solves the NAT problem for all

signaling protocols

—SIP, H.323, SCCP, and MGCP.

Using a DMZ requires that you have access to more than one IP address. You’d have

to obtain, at a minimum, three public IP addresses from your ISP—one for the soft-

PBX, one for the DMZ interface on the firewall, and one for the Internet-facing inter-

face on the firewall. If you are unable to get a block of IP addresses, then you’ll have

to consider another solution. But if you are able, placing VoIP server resources on a

DMZ is the way to go.

Setup B in Figure 13-4 is what you may be considering if you plan to have some road

warriors or Internet-based subscribers accessing voice services through a server on

your private network. DMZ solves the NAT problem here, too.

But there may be situations in which the Internet-based phone must be behind a

NAT firewall, and there’s nothing the user can do about it. Setup C in Figure 13-4

illustrates this idea. Residential broadband routers have built-in NAT firewalls, and

so do the firewalls at hotels and in some coffee shops and public access points. Since

you’re relying upon the policy of other organizations for your voice transport, you’re

at the mercy of what their infrastructure permits—or ...