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PSTN fallback

PSTN fallback
Tom Lancaster

Last week we discussed using VoIP over VPNs to connect remote offices to a central site. Using a separate, non-encrypted VPN, you avoid the delay associated with encryption, but the Internet itself is still an unreliable mechanism that is often congested or unstable and generally isn't very voice-friendly. For most businesses, the cost benefits of VoIP are attractive, but not worth the occasional loss of quality or total loss of service that is inevitable when using the Internet. Thus, the rate of adoption of VoIP over the Internet is still fairly low for business users. However, many manufacturers are addressing this need by creating solutions that offer the best of both worlds.

One example is PSTN Fallback, which is a newer feature, offered by Avaya, Cisco, Siemens and most other VoIP vendors. Fallback allows you to take advantage of cheap voice service over the Internet (or any data network) when available, but resort to the guaranteed service of the PSTN when your voice quality degrades below a configurable threshold.

In a typical fallback scenario, you will configure your branch office router with a connection to the Internet and the PSTN. It will monitor the devices in the VoIP path to the central office using ICPIF (Calculated Planning Impairment Factor), derived from ITU-T's G.113 recommendation. If this value, which predicts the quality of the connection by providing delay, jitter and packet loss information for each voice-destination IP address, drops below the threshold you determine, the router will set up any future H.323 calls across the PSTN instead of the data network. (Note that this only affects new calls and not existing calls. The quality of existing calls must be maintained by whatever QoS mechanisms you have in place.)

For more information, visit Cisco's PSTN Fallback;

Avaya's PSTN Fallback (formerly Lucent);

Siemens PSTN Fallback;

ICPIF and the ITU-T's G.113

Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.

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Related Book

Converged Networks and Services: Internetworking IP and the PSTN
Author : Igor Faynberg, Hui-Lan Lu and Lawrence Gabuzda
Publisher : John Wiley & Sons
Published : Jul 2000
Convergence of the Internet and the Public Switched Telephone Network (PSTN) makes possible a wide range of new telecommunications services. Written by the industry leaders at Bell Laboratories, this book puts the future of convergence in perspective and helps you make decisions that leverage its power.

This was last published in May 2001

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