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Archive    6 Dec 2007      #65



By William Flanagan, Publisher

If you are responsible for voice communications, you know that voice is the service that everyone expects to work all the time. That's one reason fax transmission on voice circuits is the backup method of last resort for transmitting certain kinds of information.

To maintain voice service, a common practice is to protect local loops against "backhoe fade" with a second local loop. That approach can be hard at branch sites--and expensive: diverse routings for cables, separate entry points into the building, perhaps two local exchange carriers. It's not always practical, and seldom is worth the expense of a second line if both are in the same cable.

The recent high-speed data services from cellular carriers (for example, EVDO) offer an alternative that integrates easily with existing PBXs. This ViewsLetter looks at how this works for a multi-site organization.

You don't want a branch office to be isolated, unreachable by customers. This approach provides voice and data connections from the branch to headquarters if the branch loses its local phone lines. The key components are

1) the branch router with voice interfaces and a high-speed wireless capability;

2) the larger headquarters router that complements the branch equipment to work with branch voice connections and link them to other phones.

Drawing:  thanks to NSGDatacom

Note that the cellular service used is high-speed data (often over 350 kbit/s) and not the normal voice connection. The traffic is all IP, between the radio voice router in the branch and the cellular network. From there the voice packets are treated as prioritized data and delivered to the IP address of the HQ voice router (which doesn't need a radio interface).

Because the central site relies on a land line, it is not protected by radio backup in this scenario. However, protecting only one site with a diversely routed second line is much more practical.

The branch router doesn't dial phone number, it uses the "always on" IP link for packetized compressed voice to the central voice router. Voice compression (to 8 kbit/s or less) allows the cellular IP link to carry a full T-1's worth of calls (24) and some data.

Staying Connected

When landlines are working, there are no voice packets sent by radio. If the landlines fail, the radio link keeps the branch connected.

-- Out-bound calls from the branch phones pass through the radio router, which detects when the local trunks have failed and routes calls over the radio link to the cellular service which passes packets to headquarters. There the router connects to the PBX--the caller can reach other extensions or the public network. In a different topology, a radio voice router as the last part of a trunk hunt group can provide outbound overflow capacity.

-- Branch land line trunks are set up in the central office to forward in-bound calls--whenever the caller gets a busy, ring/no-answer, or failed trunk--to the main number at headquarters. The PBX attendant transfers those incoming calls to the extension group assigned to the branch, which are connected to ports on the voice router. The call connection is passed over an IP line to the cellular network for completion over the radio data link. No lost business.

To reduce overhead and processing, the voice routers combine voice samples from multiple conversations in one packet (standard VoIP has one conversation per packet to permit routing each call independently, which is not needed here). That makes the format proprietary--you need matching boxes at both ends.

Many branch sites can share one larger voice router at headquarters, so the solution scales up fairly well. For details on the hardware, see the data sheets at http://www.nsgdata.com/solutions/autofailover.html

There are not as many concerns about Voice over IP (VoIP) in this context as you'll find on the Internet. The IP connection is point-to-point on the cellular and back-haul infrastructure, not over the Internet. The proprietary packet format with compression make interception very unlikely. The radio routers prioritize voice to allow a mix of voice and data on the link and preserve voice quality. The routers also handle the signaling, so there's no need for a call processor server, SIP configuration, etc.

Of course there's a catch: all protected sites must have a high speed cellular service. Not a lot to pay for highly available voice.


Previously, ViewsLetter has been composed in text only.  Recent comments urged an HTML format, with better graphics and more attractive presentation. We've taken steps to minimize the file size as the graphics were added. Let us know your opinion. If you really, really want a text-only version we'll probably offer it.

Email to publisher@ViewsLetter.com

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Flanagan Consulting
W. A. Flanagan, Inc.
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