When a subscriber wishes to de-register from the network, the subscriber device
sends the REGISTER message again, with an EXPIRES header value of 0 seconds.
This indicates that the existing registrations for the subscriber should be canceled.
Subscribers de-register when they are disconnecting (powering down) from the network.
All sessions in progress are terminated, but any IP connections must be maintained
until the subscriber has successfully de-registered with the S-CSCF.
The S-CSCF will then send a PUT over the Cx interface to the HSS containing the
public user identity or private user identity, along with instructions to either clear the
S-CSCF or keep the S-CSCF. If told to keep the S-CSCF, the HSS will maintain an association
between the subscriber and the S-CSCF (for use for other services that may
not require registration).
There are a number of reasons why a subscriber may wish to de-register, but the operator
may also force de-registration (dependent on operator implementation). Reasons
for this include
?– Network maintenance
?– Network or traffic (for example, if two network operators change the parameters
of their roaming agreements, they would want to de-register all subscribers to
prevent multiple registrations)
?– Application layer issues (various applications may require de-registration of individual
subscribers or groups of subscribers)
?– Subscription management (delinquent accounts, for example)
Throughout the registration processes, the various devices will store specific information
during and after registration.
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