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VoIP White Paper

300 Main Street • East Rochester, NY 14445 • Toll Free 1-866-ALLWORX • 585-421-3850 • www.allworx.com

trademarks or registered trademarks of their respective owners.

Revised: February 8, 2007

Page 21

• Remote phones off a single Allworx server at different sites should be able to call each other, but if both

connected phones are behind NAT/Firewalls, getting audio between them can be problematic. It is sort of

a chicken-and-egg type problem: neither phone can determine its public IP address or RTP port numbers

until after the opposite end receives the first audio packet. Since both ends are in the same situation,

neither end ever receives the first packet. To resolve this, at least one of the two phones must have a

static mapping through the firewalls for its RTP ports. See the troubleshooting section for more details.

• ITSP services configured on the Allworx server may not be accessible from remote phones that are

behind NAT devices. This is a limitation of the service providers and not under control of the Allworx

devices.

Caveats and details about remote phones operating over the Internet, especially when using third-party

firewalls:

• Normal ISP Internet access for regular residential and business customers is a best effort service.

Specific QoS metrics are not guaranteed and poor quality audio can result at times, depending on traffic

flow between providers and through Internet peering points.

• Most low-cost NAT/Firewall routers do not prioritize traffic, and even if they do, the Internet service that

they are being used with typically does not. Therefore, normal user data traffic activity can affect audio

quality. For example, downloading e-mail while talking on the phone may cause audio interruptions.

• Depending on the service provider and the quality and bandwidth of your ISP service, the above issues

may be rare or they may be occur regularly. See subsequent sections for more details.

• If none of the above are acceptable from time to time, then leased line and/or virtual private circuit-type

service with a specific Service Level Agreement (SLA) is required – along with proper router equipment –

to guarantee that the desired QoS metrics are always met.

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