 there is no traffic channel assigned. Hence, signalling messages exchanged over air, Abis Radio Network Optimisation Paging Request assignment of a control channel Agreement about the type of service Authentication Ciphering Exchange of additional signalling information TCH assignment (air interface) TCH assignment (A interface) Ringing tone and connection is established Conversation phase release resources on air and A interfaces Figure 2.61 MTC and MOC establishment message flowchart and A interfaces are quite similar to a call establishment except for the TCH assignment, which is present only in the call establishment. Call Phases Each of the steps in an MOC or MTC is classified within a certain call phase number. Signalling scenarios are divided into 15 phases. By understanding properly call phases along with failure causes reported in the signalling trace, the origin can be determined of an abnormal channel release, for example. A failure cause gives a description of the error that generated the failure. The 15 phases cover the following items: Phase 1 covers the first part of signalling in an MOC or MTC over a dedicated channel, be it an SDCCH or a TCH. Phase 2 covers the MM signalling between the MS and MSC in a transparent way to the BSC. It may occur on the SDCCH or TCH. Phase 3 covers the assignment procedure step of the MS from the SDCCH to TCH by means of the basic call setup. Phase 4 englobes clear command and release steps on an SDCCH or a TCH. Phase 5 covers the FACCH call setup assignment signalling with channel mode modification (from Assignment request to Alert); it is the same as phase 3 where the FACCH call setup is active. The dedicated channel can only be a TCH. Radio Network Planning and Optimisation Network Network Paging Request Channel Request Channel Request Immediate Assignment Immediate Assignment Paging Response Service Request Authentication Reques Authentication Request Authentication Response Authentication Response Ciphering Mode Command Ciphering Mode Command Ciphering Mode Com