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

From: "Kevin Boyle" <[email-masked]>
Date: Thu, 16 Jun 2005 11:15:13 -0400

This race condition, in practice, occurs extremely rarely and does not seem
to cause confusion to the end-user.  In more than 5 years of active
deployment across literally millions of lines of service there has never
been a single compliant.

There are two possible scenarios:
1. User picks up and listens for dialtone, intending to place a call but
discovering that there is already a connection. They hear Call Wait tone and
potentially get the number delivered (if the Signals descriptor isn't
canceled fast enough).
2. User hangs up the phone from an existing call, and instantly gets ringing
and number delivery.

Scenario one is a weird case anyway.  People don't generally enjoy picking
up the phone and finding a connection present when they are expecting
dialtone.  This does happen on rare occasions, and the inclusion of a call
wait tone at that moment does not seem to create any additional discomfort
or problem over the already existing "twilight zone" connection. (The
creepiest of these is when the person on the other end of the connection
happens to be the person you were intending to call!)

Scenario 2 is OK, because the call was going to present anyway -- this
actually saves a NACK from the MG and a potential call loss.

So, I think the issue here is being made out to be much bigger than it
really is.  In fact, given that scenario 2 saves a NACK, there is some
beneficial economy to the methodology that exists.

Kevin 

-----Original Message-----
From: Christian Groves [mailto:[email-masked]] 
Sent: Thursday, June 16, 2005 3:32 AM
To: Stephen Mell
Cc: Boyle, Kevin [NCRTP:3Z40:EXCH]; [email-masked]
Subject: Re: [Megaco] andisp/dwa and Hook State Change Race

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:

> 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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> 
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---

From: "Jan Lindquist (AL/EAB)" <[email-masked]>
Date: Wed, 22 Jun 2005 11:31:01 +0200

Hello,

Even if the situation of a cross over is rare there are requirements in
legacy systems that dictate which end wins the race condition.  Some
countries require that the originating end wins the race condition so
the off-hook will give you a dialtone and not somebody calling you.
Other countries require the terminating end to win so if you lift up the
off-hook the terminating call should come through so no dialtone, etc.
There is a liaison from TISPAN requesting that dwa signal indicates
clearly the expected state of the line in order to apply the ringing or
call waiting.  If this change is not provided then the MG has to be
provisioned differently in different countries.  This is what we want to
avoid.

Regards,
JanL

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf
Of Kevin Boyle
Sent: den 16 juni 2005 17:15
To: Christian Groves (BR/EPA); Stephen Mell
Cc: [email-masked]
Subject: RE: [Megaco] andisp/dwa and Hook State Change Race

This race condition, in practice, occurs extremely rarely and does not
seem to cause confusion to the end-user.  In more than 5 years of active
deployment across literally millions of lines of service there has never
been a single compliant.

There are two possible scenarios:
1. User picks up and listens for dialtone, intending to place a call but
discovering that there is already a connection. They hear Call Wait tone
and potentially get the number delivered (if the Signals descriptor
isn't canceled fast enough). 2. User hangs up the phone from an existing
call, and instantly gets ringing and number delivery.

Scenario one is a weird case anyway.  People don't generally enjoy
picking up the phone and finding a connection present when they are
expecting dialtone.  This does happen on rare occasions, and the
inclusion of a call wait tone at that moment does not seem to create any
additional discomfort or problem over the already existing "twilight
zone" connection. (The creepiest of these is when the person on the
other end of the connection happens to be the person you were intending
to call!)

Kevin 

-----Original Message-----
From: Christian Groves [mailto:[email-masked]] 
Sent: Thursday, June 16, 2005 3:32 AM
To: Stephen Mell
Cc: Boyle, Kevin [NCRTP:3Z40:EXCH]; [email-masked]
Subject: Re: [Megaco] andisp/dwa and Hook State Change Race

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

---

From: "Kevin Boyle" <[email-masked]>
Date: Wed, 22 Jun 2005 11:03:04 -0400

Why wouldn't we want the MG to be provisioned differently in different
countries?  It has to be already for the toneset...

It seems to me that the problem you describe wouldn't exist anyway.  The
call will not be connected completely until the terminator's termination is
in the context with the ephemeral for the bearer and is set to SendRecv.
This would either happen as an Add Command to add the terminator to the
context, or as a Modify Command to set the Mode to SendRecv for the
terminator if the term is already in the context.  Either way, cut through
of the voice path will require MGC intervention.  So there is time for the
MGC to decide whether to connect the call or apply dialtone.

Again, I believe the overwhelming focus on this race condition is making a
mountain out of a molehill.

Kevin

-----Original Message-----
From: Jan Lindquist (AL/EAB) [mailto:[email-masked]] 
Sent: Wednesday, June 22, 2005 5:31 AM
To: Boyle, Kevin [NCRTP:3Z40:EXCH]; Christian Groves (BR/EPA); Stephen Mell
Cc: [email-masked]
Subject: RE: [Megaco] andisp/dwa and Hook State Change Race

Hello,

Even if the situation of a cross over is rare there are requirements in
legacy systems that dictate which end wins the race condition.  Some
countries require that the originating end wins the race condition so the
off-hook will give you a dialtone and not somebody calling you.
Other countries require the terminating end to win so if you lift up the
off-hook the terminating call should come through so no dialtone, etc.
There is a liaison from TISPAN requesting that dwa signal indicates clearly
the expected state of the line in order to apply the ringing or call
waiting.  If this change is not provided then the MG has to be provisioned
differently in different countries.  This is what we want to avoid.

Regards,
JanL

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf Of
Kevin Boyle
Sent: den 16 juni 2005 17:15
To: Christian Groves (BR/EPA); Stephen Mell
Cc: [email-masked]
Subject: RE: [Megaco] andisp/dwa and Hook State Change Race

There are two possible scenarios:
1. User picks up and listens for dialtone, intending to place a call but
discovering that there is already a connection. They hear Call Wait tone and
potentially get the number delivered (if the Signals descriptor isn't
canceled fast enough). 2. User hangs up the phone from an existing call, and
instantly gets ringing and number delivery.

Kevin 

-----Original Message-----
From: Christian Groves [mailto:[email-masked]]
Sent: Thursday, June 16, 2005 3:32 AM
To: Stephen Mell
Cc: Boyle, Kevin [NCRTP:3Z40:EXCH]; [email-masked]
Subject: Re: [Megaco] andisp/dwa and Hook State Change Race

Hello Stephen,

Regards, Christian

---

From: "Jan Lindquist (AL/EAB)" <[email-masked]>
Date: Wed, 22 Jun 2005 22:33:49 +0200

Hello Kevin,
 
It does not have to do with the ADD commands.  It has to do with the
Notify command with the off-hook and the Modify command with the
andisp/dwa.  Note that when the Modify command is sent the streammode is
set to sendreceive so any event will cause the connection to through
connect.  The question is what do with when they cross over and the
andisp/dwa is misinterpreted and through connection occurs.  If I make a
direct comparison with V5 this scenario may occur and the LE behaves
differently depending on the country.  It seems very logical to avoid
any misinterpretations of the andisp/dwa and through connection by
keeping the logic of accepting or not the off-hook in the MGC.  Then the
MG does not need to be provisioned and the MGC can be configured
accordingly.
 
Regards,
JanL
 
-----Original Message-----
From: Kevin Boyle [mailto:[email-masked]] 
Sent: den 22 juni 2005 17:03
To: Jan Lindquist (AL/EAB); Christian Groves (BR/EPA); Stephen Mell
Cc: [email-masked]
Subject: RE: [Megaco] andisp/dwa and Hook State Change Race

It seems to me that the problem you describe wouldn't exist anyway.  The
call will not be connected completely until the terminator's termination
is in the context with the ephemeral for the bearer and is set to
SendRecv.  This would either happen as an Add Command to add the
terminator to the context, or as a Modify Command to set the Mode to
SendRecv for the terminator if the term is already in the context.
Either way, cut through of the voice path will require MGC intervention.
So there is time for the MGC to decide whether to connect the call or
apply dialtone.

Again, I believe the overwhelming focus on this race condition is making
a mountain out of a molehill. 

Kevin 

-----Original Message----- 
From: Jan Lindquist (AL/EAB) [mailto:[email-masked]] 
Sent: Wednesday, June 22, 2005 5:31 AM 
To: Boyle, Kevin [NCRTP:3Z40:EXCH]; Christian Groves (BR/EPA); Stephen
Mell 
Cc: [email-masked] 

Hello, 

Regards, 
JanL 

-----Original Message----- 

Kevin 

-----Original Message----- 
From: Christian Groves [mailto:[email-masked]] 
Sent: Thursday, June 16, 2005 3:32 AM 
To: Stephen Mell 
Cc: Boyle, Kevin [NCRTP:3Z40:EXCH]; [email-masked] 
Subject: Re: [Megaco] andisp/dwa and Hook State Change Race 

Hello Stephen, 

Regards, Christian