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Its before setting up the traffic channel.

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Posted by HamidAliKhan

www.freesoft411.com/freeware/mask.html

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Posted by crouse

Before setting up the traffic channel.

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Posted by waqqas1

1.)VP mask is used by MS and BS to encrypt user information sent over the digital traffic channel during call
2.0VP mask is generated at the beginning of the call
3.0VP is only possible if the global challenge operation is successful
4.0MS may request VP at call setup or during the call
5.)VP is a subscriber feature that can be enabled and disabled using IS-95 signaling.

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Posted by sagitraz

Voice privacy can be activated before setting up the traffic channel and also once the traffic chanel is already established.Once it is activated after having proper negotiation with the Base station the mobile will transition to Private long code mask instead of using Public long code mask.It is generated with the algorithm called Key_VPM_Generation procedure where the 2 most significant bits that is M40 & M41 are set to 01 and Bits M39 through M0 shall be the 40 least significant bits of the Voice
Privacy Mask (VPM) generated by the Key_VPM_Generation procedure.Hope it clarifies.

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Posted by elexdeepak1