2.3.5.... Pre-release
information transport
The release procedures are based on a two-message (release, release complete) approach whereby the release message initiates release of the circuit switched connection.
The same procedures are used in the network irrespective of whether they are initiated by the calling party, the called party or the network.
To satisfy the need for rapid transfer of release across the network, it is required that the circuit is selectable from the subsequent exchange within the mean cross-office transfer time, Tcu, for simple messages as specified in Recommendation Q.766 [20].
a) Actions at the originating exchange
On receipt of a request to release the call from the calling party, the originating exchange immediately starts the release of the switched path. A release message is sent to the succeeding exchange and timers T1 and T5 are started to ensure that a release complete message is received from the succeeding exchange (expiration of timers T1 and T5 is covered in 2.9.6).
b) Actions at an intermediate exchange
On receipt of a release message from the preceding exchange, an intermediate exchange:
i) immediately starts the release of the switched path; when the circuit is re-selectable, a release complete message is returned to the preceding exchange;
ii) at the same time as the start of the release of the switched path, sends a release message to the succeeding exchange. Timers T1 and T5 are started to ensure that a release complete message is received from the succeeding exchange (expiration of timers T1 and T5 is covered in 2.9.6).
c) Actions at the destination exchange
On receipt of a release message from the preceding exchange, the destination exchange will start the release of the switched path. When the circuit is ready for re-selection, a release complete message is returned to the preceding exchange.
d) Charging (national use)
Charging is stopped upon receipt of the release message at the charging exchange or on receipt of a request to release the call from the calling party when the charging exchange is the originating exchange.
e) Collision of release messages
In the case when two points in the connection both initiate the release of a call, a release message may be received at an exchange from a succeeding or preceding exchange after the release of the switched path is initiated and after sending a release message to the adjacent exchange. In this case, the exchange will return a release complete message to the exchange from which the concerned release message was received. The release complete message will be sent only after the switch path has been released. The exchange will make the circuit available for new calls when both a release complete message is received (corresponding to the sent release message) and a release complete message is sent (corresponding to the received release message).
The procedures in 2.3.1 apply, except that the functions at the originating and destination exchanges are transposed.
The procedures in 2.3.1 apply, except that they can be initiated at any exchange (originating, destination or intermediate).
Each exchange of the connection shall store during the call set-up the initial address message information sent (the originating exchange) or received (intermediate or destination exchange). The information to be stored includes all parameters in the initial address message and, if the initial address message has been segmented, in the subsequent segmentation message. The contents of the initial address message information shall be updated, if the value of parameters change during the call set-up.
The initial address message information can be released from memory:
a) in the originating exchange when the address complete message or connect message has been received and the calling party does not subscribe to a supplementary service which would cause a new call set-up. The release of the information when the calling party does subscribe to a supplementary service that is covered in Q.730-Series Recommendations [17];
b) in the intermediate exchange when the address complete message or the connect message has been received;
c) in the destination exchange when the address complete message or connect message has been sent and the called party does not subscribe to a supplementary service which would cause a new call set-up. The release of the information when the called party does subscribe to a supplementary service that is covered in Q.730-Series Recommendations [17],
and when the call is released earlier and no automatic repeat attempt is to be attempted.
This capability allows
information to be transported at Release in a manner which is compatible with
the 1992 and subsequent versions of the ISUP protocol.
Since additional parameters
can not be carried in the Release (REL) message due to the possibility of their
loss at an intermediate exchange, an exchange wishing to send such parameters
at release time shall include them instead within a "Pre-release
Information" (PRI) message which shall be sent immediately prior to the
Release message. In the case that segmentation of the pre-release information
is necessary, the subsequent segments will be sent between the PRI and REL
messages.
An exchange receiving a Pre-release Information message shall determine whether to store the received information and process it upon release of the call or pass-on the PRI without awaiting REL, depending on the parameters received and the application present for the call at that exchange. In the case that an intermediate exchange receives a PRI containing one, (or more), parameters that should be passed on without awaiting REL, and one, (or more), parameters that should be processed upon receipt of REL This is for further study.