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PBX clustering

Posted on 2010-11-19
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Last Modified: 2012-05-10
Hello

Our Telecoms guys are looking into PBX's and also high availability. So far, I can see two options:

1. Active/ Passive

2. Active/ Active

I'm not a clustering wiz....but one requirement we have is - in the event one of the PBX's go down - that calls don't drop on the failover. I assume we need active/active PBX clustering for that? How does that actually work, does anyone know?

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Question by:bruce_77
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by:feptias
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To have it so established calls don't drop when one PBX goes down - that would be very difficult, if not impossible or very expensive. With SIP you might find that calls continue but it depends on the route the media is taking. Even if the media connection isn't broken, it is likely that the call will still be terminated after a while if session timers are being used for dead call detection.

In my experience, Active/Passive makes most sense because you can use a virtual IP address that switches across to the standby unit when the primary fails.
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With most every PBX on the market today, the call processing is separate from the transport, and that transport is IP.  Since with IP the call packets traverse the network (data switches and routers) without running through the central call processing unit, all current calls will stay up if the CPU fails.  Only calls that are in the middle of dialing or ringing will drop, because the call control would be off line to respond to DTMF tones, answer supervision, etc.  To meet your requirements as stated -- calls don't drop -- you do not need any server backup.  The purpose of a secondard CPU is for the next call that needs to complete.  Thus if the true requirement is that the telephone service continues normally even with a failure, this is a more complex issue.
With most IP PBX units using some type of "normalized" data server architecture, there are several different ways to achieve high availability.  However, it is a layered process.  
The active core call control unit can be active / passive if the backup unit detects failure and then goes live - only calls that were in the middle of call setup will fail.  The more standard approach is the equivalent to Active/active, where the call processing continues without interruption.  This is referred to as "stateful failover" where the exact state of everything is maintained.  Some vendors uses a mirroring process (often proprietary), some use clustering, some use third party software such as Double Take."  
What is important to remember is that is not High Availability by itself.  Several other components need to be looked at as well.  The first one of course is dual power supplies, each with backup power,  The next is dual network interface cards, preferrably connected to two different switches in the network core (also on backup power).   Then you also need to look at the peripheral servers such as voice mail servers, call center servers, etc. -- if you need those capabilities to be HA then those may need the same level of duplication and backup.  Finally, you need to look at the carrier equipment - if you use ISDN-PRi then the NIU and CSUs needs to be on backup power.  If you use SIP trunks, then the Session Border Controller needs to be on backup power.  Since power problems are more common than any other source of outage, you can see the theme - everything that is part of the data network (including the PoE switches in the distribution closets) plus all core common equipment need to be on backup power.
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