Subject: [Megaco] andisp/dwa and Hook State Change Race

From: "Stephen Mell" <[email-masked]>
Date: Fri, 3 Jun 2005 13:58:53 +0100

Hello Kevin,

We have experienced a situation which highlights a slight drawback with 
the current definition of andisp/dwa in H248.23, and thought it might be 
worth pointing it out now, should an "xandisp" ever materialise.

With most signals that my MG needs to apply, the nature of the signal 
implies whether or not it is sensible or not to apply that signal in the 
face of a given hook state. This is good as it helps greatly to mitigate 
the effect of network crossover situations. For example I can choose to 
fail ringing if the line is now off-hook and send the required Notify to 
indicate failure if g/sc is armed.

andisp/dwa is constructed in such an economical way that that the on and 
off hook signalling messages are syntactically the same, but are 
interpreted differently at the Gateway, dependant on the actual Hook state 
- However the MGC's anticipated hook state when it requested the signal is 
not conveyed to the Gateway.

When there is a crossover between an OFF/ON-HOOK notification from the MG 
and a anddisp/dwa Signal from the MGC, the MG may end up inadvertantly 
applying inappropraite signalling to the line. Where there is packet loss 
this Crossover window is naturally opened further.

Another optional signal parameter, identifying the MGC's anticipated hook 
state, which the MG is required to match before attempting to apply the 
signal, would help to shut this window.

Regards,

Steve 

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From: Christian Groves <[email-masked]>
Date: Thu, 16 Jun 2005 17:32:08 +1000

Hello Stephen,

I believe this issue as been discussed at a recent Tispan meeting. I expect to 
see a liaison come into the July SG16 meeting on enhancements to H.248.23. So 
this will be discussed then.

Regards, Christian

Stephen Mell wrote:

> interpreted differently at the Gateway, dependant on the actual Hook
> state - However the MGC's anticipated hook state when it requested the
> signal is not conveyed to the Gateway. 
> 
> When there is a crossover between an OFF/ON-HOOK notification from the
> MG and a anddisp/dwa Signal from the MGC, the MG may end up
> inadvertantly applying inappropraite signalling to the line. Where there
> is packet loss this Crossover window is naturally opened further. 
> 
> Another optional signal parameter, identifying the MGC's anticipated
> hook state, which the MG is required to match before attempting to apply
> the signal, would help to shut this window. 
> 
> 
> Regards, 
> 
> Steve 
> 
> 
> ------------------------------------------------------------------------
>