Under what different situations does MME trigger OVERLOAD START towards eNodeB?

Question

From MME to eNodeB, UE context modification procedure is triggered I know but what makes trigger of UE Context modification procedure from eNodeB to MME ?

Answer 1

As per 36.413 below are causes when MME sends overload start to eNB .

The eNB receiving the OVERLOAD START message shall assume the MME from which it receives the message as
being in an overloaded state.
If the Overload Action IE in the Overload Response IE within the OVERLOAD START message is set to
- “reject RRC connection establishments for non-emergency mobile originated data transfer” (i.e., reject traffic
corresponding to RRC cause “mo-data” and “delayTolerantAccess” in TS 36.331 [16]), or
- “reject RRC connection establishments for signalling” (i.e., reject traffic corresponding to RRC cause “mo-data”,
“mo-signalling” and “delayTolerantAccess” in TS 36.331 [16]), or
- “only permit RRC connection establishments for emergency sessions and mobile terminated services” (i.e., only
permit traffic corresponding to RRC cause “emergency” and “mt-Access” in TS 36.331 [16]), or
- “only permit RRC connection establishments for high priority sessions and mobile terminated services” (i.e.,
only permit traffic corresponding to RRC cause “highPriorityAccess” and “mt-Access” in TS 36.331 [16]), or
- “reject only RRC connection establishment for delay tolerant access” (i.e., only reject traffic corresponding to
RRC cause “delayTolerantAccess” in TS 36.331 [16]).