LTE: Why does "forward access context notification" send by source mme to target mme ?

Question

As part of S1-AP procedure with MME relocation, Handover Required message sent to source MME. Target eNodeBId is present within the handover required. How source MME figure out target cell is being served by XYZ MME and sends "Forward relocation Request" message to XYZ MME.

Please provide reference also if it is there ?

Answer 1

"Forward access context notification" and "Forward access context acknowledge" both messages sent over S10 interface from source MME to target MME and from target MME to source MME respectively. Forward access context notification message carry E-UTRAN transparent container information, which is nothing but PDCP SN receiver and transmitter status.

Please check the S1-handover call flow from 3GPP 23.401 spec.
source eNodeB sends "eNodeB status transfer" to source MME then MME sends this e-utran container to target MME in forward access context notification message. Once target mme receives this message, the received e-utran container information is sent to target eNodeB via mme status transfer message.

Hopefully above description will provide some more clarity to you.