09.033GPPSignalling Requirements on Interworking between the Intergrated Services Digital Network (ISDN) or Public Switched Telephone Network (PSTN) and the Public Land Mobile Network (PLMN)TS
The use of OACSU in the PLMN has an impact on the interworking with the fixed network. Both outgoing and incoming calls have to be considered: the consequences are not the same.
The use of the OACSU procedure is optional and must be limited to national telephone calls only (see GSM 03.01).
5.1 Definition of OACSU
To save the radio resources, the radio traffic channel may be allocated to the communication only when both calling and called parties are present, i.e. at the answer instant. This method called "Off-Air Call Set-Up" (OACSU) has some implication on the interworking with the fixed network. But the consequences are not the same whether the mobile subscriber is the calling or the called party.
5.2 Outgoing call from a mobile station
Upon initiation of an outgoing call, a traffic channel is allocated to the communication when the called subscribers answer is received in the MSC. In some cases, no idle traffic channel may be available when necessary. Therefore an appropriate announcement must be given to the called party when no idle traffic channel is available within a certain interval upon receipt of the called party’s answer.
Whenever the announcement is used, it must always be played through in its entirety, even if a traffic channel becomes available before it is completed, except if the called party clears, in which case the call should be cleared forward.
If the ADDRESS COMPLETE message (ACM) indicates that there will possibly be no ANSWER message upon the connection of the called party (e.g. ACM without any information), the radio path must be established immediately upon receipt of the ACM. This applies in all cases on receipt of the ADDRESS COMPLETE message in TUP and ISUP (e.g. also in case the value "subscriber free" is received.
Due to interworking constraints coming from the characteristics of the different signalling systems used in the countries, the OACSU technique should only be used for national calls.
5.3 Incoming call to a mobile station
For incoming calls, the impact is not so important, but some rules must be applied in order to limit the influence on the service quality.
Concerning the sending instant of the answer message, the normal operating rules apply. If the call is successfully set-up to the mobile station, the answer message must be sent to the originating exchange only when the traffic channel is established upon recognition of the called party connection.