When UE in ECM /RRC-IDLE state, if it is getting new traffic in that case P-GW pump the UE traffic based on the IP address, And the UE is already EMM Registered so it is always connectivity provision of connection between the S-GW and P-GW.
So once Incoming downlink data arrives at S-GW trough S5 bearer, then S-GW triggers Downlink Data Notification to wards MME.now MME need to Page the UE in order to get the UE from ECM/RRC-IDLE to ECM/RRC-Connected. Once eNodeB got this message it just forward the paging on UE based on available UE identity. after that UE will get the resources and start sending and receiving the data.
Below IE are the main IE’s which used as a part of Downlink Data Notification.
Information Elements in a Downlink Data Notification
EPS Bearer ID:
If the Downlink Data Notification is triggered by the arrival of downlink data packets at the SGW, the SGW shall include the EPS Bearer ID stored in the EPS bearer context of the bearer on which the downlink data packet was received
Allocation/Retention Priority:
If the Downlink Data Notification is triggered by the arrival of downlink data packets at the SGW, the SGW shall include the ARP stored in the EPS bearer context of the bearer on which the downlink data packet was received;
IMSI :
This IE shall be included on the S11/S4 interface as part of the network triggered service restoration procedure if both the SGW and the MME/S4-SGSN support this optional feature
Reference: 29.274 rel10 clause 7.2.11.1
Once MME receives the Downlink Data Notification message it triggers paging to towards enodeb.
MME contain UE information given below.
Message type: paging
UE Identity Index value: IMSI NO(i.e:xxxxxxxxxx) 10bit string
UE Paging Identity: IMSI or S-TMSI(OCTET String size)
Paging DRX: Timing(i.e:32,64,128 etc)
CN Domain: CS,PS
TAI: PLMN ID+TAC
CSG ID: BIT STRING (SIZE (27))
Reference 36.413 rel 9
hope this info is helpful to you @ Srinivas Sambari