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H323 Call Preservation

Hi All,


Can some one please explain the detailed mechanism of H323 Call Preservation or Survivability.






Hall of Fame Master

Re: H323 Call Preservation

The idea is that if you have an active call across H323 controlled GW using specific CUCM for signaling and that CUCM server fails during the call, the call does not fail and is preserved.


Re: H323 Call Preservation

I understand the idea of call survivability here; however, I am looking for a detailed mechanism for H323 call preservation. Need to understand how exactly it works.
Will really appreciate if someone could share a good doc for it.

Re: H323 Call Preservation

Hi -


The Communications Manager System Guide is a good reference for you on this.  This like is for Version 9 and I hope it gets you what you need.  Search for Call Preservation.





VIP Advisor

Re: H323 Call Preservation


This is the summary of my understanding and testing.

By default H323 will drop active calls on WAN failure when connectivity is
lost to the other H323 endpoint. In order to preserve active calls on WAN
failure, two methods are available:

1. For PSTN-IP Calls

voice service voip


no h225 timeout keepalive

1. For IP-IP Calls

In this type of calls, preservation can take place by IOS GW or CUCM. In
IOS it can be done as follow:

voice service voip


call preserve


voice-class h323 4

call preserve


dial-peer voice 1 voip

voice-class h323 4

On CUCM it can be configured as follow:

a. Choose Service > Service Parameters.

b. From the Service menu select Cisco Unified CallManager

c. Click Advanced

d. Scroll to the Clusterwide Parameter (Device — H.323) section

e. Set the “Allow Peer to Preserve H.323 Calls” parameter to True.

Do rdp

Once the call is preserved, media-inactivity-detection can be used to
identify the end of call (since there is no signaling any more).


RTR(config-gateway)#media-inactivity-criteria ?

all Both RTP and RTCP for silence detection

rtcp Use RTCP for silence detection

rtp Use RTP stream for silence detection, this is default

rtplib Use RTP ( comfort noise = activity) for silence detection


voice service voip


call preserve limit-media-detection

Here are the restrictions of media-inactivity-detection feature:

- This feature does not support MGCP call legs.
- The Media Inactive Call Detection feature works in IP only. This
feature does not include PSTN inactive call detection.


IP PHONE <----> H323/SIP GW <----> PSTN

In this example if silence is detected from IP PHONE side,
media-inactivity-detect will be triggered and terminate both call legs.
However, if silence is detected from PSTN side, media-inactivity-detection
won't be triggered.

- This feature supports RTP/RTCP media inactivity detection and
notification only on H.323 and SIP basic calls (RTCP will take precedence
over RTP).

How it work?

This feature is controlled by two commands which are ip rtcp report
interval global command and the timer receive-rtcp gateway-level command.
The multiplication of both commands will generate inactivity timer.

Once calls are preserved, after each RTP packet (in any direction)
inactivity timer will start. In case the timer expired before the next RTP
packet, the GW will drop both call legs.

Note: In case media-inactivity-detection is enabled, it will be applied to
all calls (preserved and non-preserved). This might cause interruption on
normal calls. Therefore you must limit it for preserved calls ONLY using "call
preserve limit-media-detection" and disable VAD as well.

Note: If the call (between GW-CUCM) is placed on hold and WAN interruption
took place, the call will be disconnected. MoH calls aren't preserved by
CUCM or GW. CUCM will indicate to GW in order not to preserve MoH calls.
This is because media-inactivity-detection won't work for MoH calls.

Note: media-inactivity-detection will drop calls if no RTP stream is
detected in any direction (not mandatory to have no RTP in both direction)

To verify call preservation,

RTR#show h323 calls preserved

CallID = 11EC , Calling Number = , Called Number = 3210000 ,

RemoteSignallingIPAddress= , RemoteSignallingPort=49760 ,

RemoteMediaIPAddress= , RemoteMediaPort=17910 , Preserved Duration
= 262 , Total

Duration = 562 , H225 FD = -1 , H245 FD = -1

In case any of those endpoints is part of the call, CUCM won't use call

- Annunciator
- H.323 endpoints such as NetMeeting or third-party H.323 endpoints
- CTI applications
- TAPI applications
- JTAPI applications

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