RRC Setup Report

Presentation TimetablePresentation Timetable

Pilot

Methodology – wk5

Net. IssuesServices

Counters Alarms Drive DataNeighbours – wk5

Performance – wk5

RRC Access – wk7

SHO – wk6

ISHO –wk6

RRC Stp – wk7 Access – wk4

Common Alarms – wk4

Common Failures – wk4

Analysis – wk3

CM Service – wk2

Test No’s – wk3

Script bugs –wk3

CC Disc – wk1

Power Spikes – wk1

HC Failures – EOC

6650 ISHO – wk3

RRC Active – wk7

Inter RNC HO – wk2

TCH/BCH – wk2

Table of ContentsTable of Contents

Introduction

Detailed Descriptions Handover Control

Admission Control

Setup Fail BTS

Setup Fail Trans

Setup Fail Frozen BTS/Internal RNC

Conclusion

IntroductionIntroduction

RRC Setup PhaseRRC Setup Phase

The RRC Setup phase starts when the UE sends the “RRC CONNECTION REQUEST” message and it is completed when the RNC, after reserving all the necessary resources for the RRC Connection (RNC, BTS, Radio and Transmission), replies with DL “RRC CONNECTION SETUP” message.

During this phase, there are several processes happening between the Node B and the RNC. If any of these processes fail, the Ue will either try again or will fail. In either case the relevant counter will be triggered, giving the Network Analyst clues as to why calls are failing on a particular cell.

A schematic of the Setup phase can be seen on the RHS.

RRC: RRC Connection Request (RACH)

UE RNCNode B

NBAP: Radio Link Setup Request

NBAP: Radio Link Setup Response

ALCAP: Establish Request

ALCAP: Establish Confirm

FP: Downlink Sync

FP: Uplink Sync

NBAP: Synchronisation Indication

RRC: RRC Connection Setup (FACH)

L1 Synchronisation

RRC: RRC Connection Setup Complete (DCH)

RRC: RRC Connection Request (RACH)

UE RNCNode B

NBAP: Radio Link Setup Request

NBAP: Radio Link Setup Response

ALCAP: Establish Request

ALCAP: Establish Confirm

FP: Downlink Sync

FP: Uplink Sync

NBAP: Synchronisation Indication

RRC: RRC Connection Setup (FACH)

L1 Synchronisation

RRC: RRC Connection Setup Complete (DCH)

RRC Setup FailuresRRC Setup Failures

RRC Setup Failure due to Handover Control (HC)

RRC Setup Failure due to Admission Control (AC)

RRC Setup Failure due to BTS

RRC Setup Failure due to Transmission

RRC Setup Failure due to BTS frozen

RRC Setup Failure due to RNC internal reason

Detailed DescriptionsDetailed Descriptions

Handover ControlHandover Control

Handover Control is the algorithm that is used to allow a Ue connect to two or more cells simultaneously

Handover Control appears to start in the HA3 Program Block during RRC

Setup Procedure

RRC Setup Fail due to HCRRC Setup Fail due to HC

HC Setup Procedure in RNCHC Setup Procedure in RNC

HC Setup appears to reside in the Interface Control and Signalling Unit (ICSU)

ICSU board resides in RNC

ICSU board has redundancy where the redundancy unit is only employed if the active unit fails.

Checked RNC alarms and there were older alarms on ICSU boards

Admission ControlAdmission Control

Admission Control (AC) is the algorithm that decides whether a request to establish a RAB is admitted or not. It is also involved during the handover procedure.

RRC Setup Failure due to ACRRC Setup Failure due to AC

RRC Setup Failure for AC: AC can block the call at RRC level if there is too much noise on a cell. (i.e. Target + Offset is exceeded)

AC also blocks calls if UL or DL is blocking. Check Cell Resource counters (Prx/Ptx total)

In general it’s possible to check the AC blocking in RRC setup phase by checking the Traffic counters(M1002):

- DCH_REQ_LINK_REJ_UL_SRNC

- DCH_REQ_LINK_REJ_DL_SRNC Main failure reasons for AC

- RRC_conn_req_nack (Cell overload on UL or DL)

- No response from BRM - timer expired (indicates a cell availability problem)

RRC Setup failure AC for cell availabilityRRC Setup failure AC for cell availability

A high number of RRC failures for Admission Control does not necessary mean a high traffic or an interference problem but it might reflect a cell availability problem.

Typical detection through counters and KPIs: High RRC Setup Failure AC Rate : There may also be a huge number of RRC attempts

(see example). This indicates that the problem is continuous. Traffic counters may or may not reflect the full extent of the problem: in some cases the

traffic counters are not incremented because the request is not even received by the Admission Control.(e.g. MHA Faulty)

Cell Availability << 100% (see example). Alarm may or may not be present

Solution: Cell Availability needs to be monitored and corrected as soon as possible after it

occurs Cell Reset might help.

Cell Availability ExampleCell Availability Example

Site GLN0395 shows high number of RRC Setup Failures for Admission Control on March 15.

From Service Level counters huge number of RRC attemptsRRC repetitions.

From Cell Resource counters:

Average PrxTotal shows nearly no traffic nor interference

Cell Availability problem from 17:30

Alarm in the cell

RRC Setup Failure BTSRRC Setup Failure BTS

RRC Setup Failure for BTS: main failure reasons can be:

- RL_setup_failure (not enough resources) might be a WSP problem

- RL_setup_failure (hardware overload): might be some internal BTS issue

These fails can be seen in the counters :- SETUP_FAIL_RL_NOT_ENOUGH_RES

- SETUP_FAIL_FIRST_RL_HW_RES

BTS Resource Issue may occur here

RRC Setup Failure BTSRRC Setup Failure BTS

RRC Setup Failure TransRRC Setup Failure Trans

RRC Setup Failure due to Transmission: main failure reasons is:

- serv_req_nack_from_nrm meaning some problem in setting up AAL2 transmission resources in Iub

NRM (Network Resource Model) is in-charge of Iub capacity

After capacity check, AAL2 signalling starts

Counter RRC_CONN_STP_FAIL_TRANS is incremented if: Not enough Iub capacity

AAL2 setup fails

RRC Setup Failures TransRRC Setup Failures Trans It appears that the problem may be in this part of the setup protocol

Reason for fail is usually: Not enough Iub capacity

AAL2 setup fails

1 ©NOKIA

AAL2 user plane setup over Iub (MOC)

RNCBTS (WAM)

UE AAL2 SIG CAC CAC AAL2SIG ATM RMRRC: connection request

resource reservation

AAL2 connection establishementERQ (SUGR)

resource reservation

ECF

AAL2 conn. estab. confirmation

RRC:connection setup

RRC

Transport res. reserv. req.

Possible NACK

Possible NACK

AAL2 Setup over IubAAL2 Setup over Iub

Iub resource reservation is performed in order such that all CAC reservations for RNC internal and external (AAL2 path) resources are done before signaling.

If the CAC actions are successfully completed, the signaling is started and the RNC asks the BTS to arrange its resources accordingly.

If the CAC actions fail, the signaling phase is not started at all and the signaling counters are not aware of the connections rejected due to RNC AAL2 layer CAC.

Others RRC Setup FailuresOthers RRC Setup Failures

RRC Setup Failure Frozen BTS: the AC blocks RL establishment to allow the setup of an emergency call(in case of pre-emption, when BTS is frozen no new RRC connection are allowed)

RRC Setup Failure for RNC internal reason: all other failure reasons (e.g. parameter mismatch, timer expiry)

RRC Setup Fail RNCRRC Setup Fail RNC

•Problem should affect the entire RNC (ALL THE CELLS)•Reasons are due to RNC Software/Hardware

RRC Setup Fails on Croydon RNCRRC Setup Fails on Croydon RNC

AC, BTS and TRANS are the most common fails on Croydon RNC

Numbers of fails can be huge but this does not indicate that the issue is necessarily RNC wide. It is probably 1 site with a huge number of fails, possibly due to repeated registration attempts.

The other fails should be at or near 0.

ConclusionConclusion

Setup Fails generally occur due to hardware issues in a low traffic situation. They are the first counters triggered by any attempt to access the network

The counters give us clues where to look

Some Setup Fails will occur repeatedly in response to a single issue causing the counter appear worse than the problem really is. Conversely, in some cases, the counters may not trigger when a fault is occurring but this can be seen by a drop in traffic.