We often see a UE in Cell_FACH/Cell_PCH/URA_PCH state sending Cell update with cause “Re-entering service Area”, In this blog we will try to understand in what all scenarios this can happen.
Before we go ahead it is required to understand below terminology
1. Out Of Service Area
2. In Service Area
Out Of Service
Area detection:
If the UE finds that the serving cell does not fulfil the cell
selection criteria,
UE initiates the measurements of all neighbour cells and if it
does not find any new suitable cell for a predefined period, UE is considered
to be "out of service area".
UE initiates cell selection procedures for the selected PLMN after
this period.
In Cell_PCH/URA_PCH this period is 12 seconds while in Cell_FACH
this period is 4 seconds.
In Service Area detection:
When a suitable cell is found (cell <re>selection criteria
is met ) , the UE considers it as having detected "in service area".
Cell Update with cause “Re-entering service Area” might be sent to
the same cell (on which UE was previously camped on) or to the different cell.
So "In Service Area" detection does not always mean that UE has detected the same cell.
So "In Service Area" detection does not always mean that UE has detected the same cell.
Coming back to the possible scenarios when UE can send Cell
update with cause “Re-entering service Area”
Scenario 1 :
UE enters into Cell_FACH/Cell_PCH/URA_PCH state ----> starts timer T305------>T 305 expires and UE detects “Out Of Service Area”-----> starts timer T307------now if before T307 expiry UE detects "In Service area" -------> UE sends cell update with cause "re entering service Area"
Scenario 1 :
UE enters into Cell_FACH/Cell_PCH/URA_PCH state ----> starts timer T305------>T 305 expires and UE detects “Out Of Service Area”-----> starts timer T307------now if before T307 expiry UE detects "In Service area" -------> UE sends cell update with cause "re entering service Area"
Scenario
2 :
UE in CELL_FACH state and detects out of service Area --->starts timer T317----->before expiry of T 317 UE detects IN Service Area --->UE sends cell update with cause "re entering service Area"
UE in CELL_FACH state and detects out of service Area --->starts timer T317----->before expiry of T 317 UE detects IN Service Area --->UE sends cell update with cause "re entering service Area"
Scenario 3 :
UE in Cell_PCH/URA_PCH state and detects “Out Of Service Area”--->starts timer T316----->Before expiry of T316 OR if T 316 expires and UE detects "In Service Area" ----->UE sends cell update with cause "re entering service Area"
Scenario 4:
UE in Cell_PCH/URA_PCH state and detects “Out Of Service Area”--->start timer T316----->T 316 expires and UE still “Out Of Service Area”----->UE starts T317 and move to CELL_FACH state -----before expiry of T 317 UE detects IN Service Area --->UE sends cell update with cause "reentering service Area"
UE in Cell_PCH/URA_PCH state and detects “Out Of Service Area”--->starts timer T316----->Before expiry of T316 OR if T 316 expires and UE detects "In Service Area" ----->UE sends cell update with cause "re entering service Area"
Scenario 4:
UE in Cell_PCH/URA_PCH state and detects “Out Of Service Area”--->start timer T316----->T 316 expires and UE still “Out Of Service Area”----->UE starts T317 and move to CELL_FACH state -----before expiry of T 317 UE detects IN Service Area --->UE sends cell update with cause "reentering service Area"
I hope this blog is helpful, I would definitely like to
include any suggestion or correction in case needed.
Ref specs : 25.133, 25.331, 25.304
Hi Rohit, good post.
ReplyDeleteWhile UE detects "Out of service Area" does it stop L2? What happens to data? If we consider cell FACH state, will data continue to be transmitted without peer being possibly able to receive anything.
If (that part of T305 when UE went OUT of service +part of T307 before detecting IN service) > (Maximum time of retransmission of PS PDU + whatever retransmission time for RESET PDU), then can it cause RLC-ErrorIndication? Might be L2 has indicated L3 of this before T307 expired and UE detects “IN service area” after this.
How will cell update with reentering with service area be handled?
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Hi Rohit, thank you for the useful post.
ReplyDeleteHowever I have additional question. What if timers T316 and T317 are not set within SIB. Does it mean they are set to infinity?According to 3GPP TS 25.331, 8.5.5.4 T317 shall never expire, i.e. all its values shall be assumed to be "infinity". T316 is not mentioned. What is the default value for T316 then? Infinity as well? How does it affect scenarious 3 and 4?
Regarding scenario 3, there is no cell update procedure with cause "re-entering service Area"
According to 25.331 8.5.5.2.1 Actions following Re-entry into "in service area" in URA_PCH or CELL_PCH state
If the UE re-enters "in service area" before T316 expiry the UE shall perform the following
1)stop T316;
2)if T307 is active, stop T307
3)perform processes described in subclause 7.2.2.
7.2.2.1 URA_PCH or CELL_PCH state:
if the UE is "in service area":
1)maintain up-to-date system information
2)perform cell reselection process
3)perform a periodic search for higher priority PLMN
Thank you in advanced.
Petr