3g-2g Call Flow 3e5773

Here is how 3g to 2g ho takes place. 1. If the UE for some reason detects that the defined 3g ecno and/ or rs thres hold of the wcdma source cell is exceeded AND also , no wcdma neigh cell fulfill s the defined threshold , the UE sends event "3A". In other words , when u see e vent "3A" in ur trace , that's a request for possible 3g-2g irat ho. Event 3A ba sically indicates " the estimated quality of currently used UTRAN freq is below a certain threshold and the estimated quality for gsm system is above a certain thresold" 2. This event 3A causes the mobile to go to compressed mode . Basically this mea ns that the mobile , while monitoring the wcdma source cell (and neighbour cells ) will take a measuring break ( called gap ) and measure 2g frequencies. The en d result of this compressed mode is that the mobile will hopefully find a suitab le 2g cell ( identified by bsic - bcch and cellid ) that meets the required thre shold for irat ho. 3. On receiving event 3A the serving rnc sends ho request to MSC. This request i s sent as RANAP relocation preparation on IU-CS and is put in a container called oldbss-to-newbss. It is sent in bssmap format ( note if it was 3g to 3g ho the container will be oldrnc-to-newrnc) . The MSc uses this request to form the bssm ap ho request and forwards it to targeted bsc parenting the 2g cell.... The MSc knows the specific bsc because the compressed mode returns bcch, bsic and cgi. 4. Once the target bsc recieves this request , it will form a dtap ho command an d send back to the serving rnc via MSc .this is sent in ranap format . The srnc then forwards the dtap ho command to the UE embedded in the rrc message . Ho tak es place. 5. The rsl will send a ho detect message on the gsm abis interface To bsc indica ting that the cell has established connection with ue.