umts irat handover analsysis

58
8/18/2019 UMTS IRAT Handover Analsysis http://slidepdf.com/reader/full/umts-irat-handover-analsysis 1/58  Application Paper: iRAT Handover  Analysis 9-Dec-13, Page 1 Introduction NSA Software allows you to monitor all incoming and outgoing legs of:  3G -> 2G and 2G -> 3G iRAT handovers for CS services  3G -> 2.5G iRAT cell changes for PS services (combined RAU / LAU)  all 2.5G -> 3G iRAT cell changes for PS services (combined RAU / LAU) except for Gs interfaces NSA Software Version 4.00 does not support yet:  Network Assisted Cell Change (NACC) for PS services  PS cell changes in A/Gb mode for PS services  Combined RAU/LAU via Gs  About This Document This Application Paper covers the following topics:  An introduction about intersystem handovers and cell changes  iRAT Handover Analysis  Troubleshooting iRAT HO Using Dashboard KPIs  How to find an iRAT handover and cell change in the NSA Call Table

Upload: shahzad-waseem

Post on 07-Jul-2018

305 views

Category:

Documents


1 download

TRANSCRIPT

Page 1: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 1/58

 Application Paper: iRAT Handover Analysis

9-Dec-13, Page 1

Introduction

NSA Software allows you to monitor all incoming and outgoing legs of:

•  3G -> 2G and 2G -> 3G iRAT handovers for CS services

•  3G -> 2.5G iRAT cell changes for PS services (combined RAU / LAU)

•  all 2.5G -> 3G iRAT cell changes for PS services (combined RAU /

LAU) except for Gs interfaces

NSA Software Version 4.00 does not support yet:

•  Network Assisted Cell Change (NACC) for PS services

•  PS cell changes in A/Gb mode for PS services

•  Combined RAU/LAU via Gs

 About Th is Documen t

This Application Paper covers the following topics:

•  An introduction about intersystem handovers and cell changes

•  iRAT Handover Analysis

•  Troubleshooting iRAT HO Using Dashboard KPIs

•  How to find an iRAT handover and cell change in the NSA Call Table

Page 2: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 2/58

 Application Paper: iRAT Handover Analysis

9-Dec-13, Page 2

Evoluti on o f Intersy stem Cell Changes since 3GPP R6

In current network configurations there is no real handover of PS services from

GERAN to UTRAN or vice versa. The main reason is that there is not

dedicated traffic channel for PS data in the GSM network. PS data is sent on

 Abis and Um interface in unidirectional temporary block f lows whenever the

packet control unit in close cooperation with the base station controller (BSC)

detects that some radio resources are not occupied by CS services and hence,

are available to transport PS data. This is the big difference between 2.5G

GPRS and UMTS: if in UMTS the RNC assigns radio resources for a 64 kbps

PS radio bearer this 64 kbps bandwidth on the RLC transport channel layer is

guaranteed to be available for the particular UE as long as the spreading factor

of the channelization code is not changed. In 2.5 G GPRS there is no reserved

bandwidth for a single PS connection. If e.g. all time slots in a cell are used for

speech connections the user of a PS service needs to wait until one of the time

slots in the GSM cell is released. And if there are several PS service users

they all need to share this single time slot to transmit their data. A flow control

mechanism established in PCU and SGSN side prevents overflow of buffers in

BSSGP entities that may otherwise occur as a result of short PS transmission

resources.

The situation is changing due to the GERAN evolution. Starting with Rel. 6, the

2G SGSN is equipped with a packet flow management. A packet flow context

will be associated with the PDP context. This packet flow context can be seen

as the GERAN equivalent of the RAB. The main purpose of this enhancement

is to minimize the cell change delay of GPRS cell changes including changes

from 3G to 2/2.5G and vice versa. The duration of a 3G-2/2.5G inter-RAT cell

change from Rel. 99 UTRAN to GERAN often takes up to 10 seconds. During

this time, no IP payload is transmitted. If a Gs interface is available between

SGSN and VLR the combined location area update/routing area update

procedure, that is mandatory for each inter-RAT cell change, can be

accelerated. The delay can be reduced to approximately four seconds. A

further minimization of the cell change delay (< 3 seconds) is possible using

the network assisted cell change procedure introduced in Rel. 5. In this

scenario, the UE is able to acquire 2/2.5G system information while still

Page 3: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 3/58

iRAT Handover Analysis

9-Dec-13, Page 3

connected to the 3G system. Finally, using the Rel. 6 PS cell change

procedures it is expected to reduce the interruption of data transfer to less than

0.5 seconds, which can be called a seamless cell change.

The following examples will explain some standard procedures of inter-RAT

cell changes from UMTS to GSM and vice versa.

3G-2G Inter-RAT Handov er for CS Servi ces

  Step 1: This procedure is triggered by an RRC Measurement Report

coming from the UE. An event from event-ID group e3 (intersystem

measurement) will be sent.

  Step 2: The RANAP Relocation Required message is sent from the

SRNC to its 3G MSC. It includes information about source and target of

the cell change.

  Step 3: From the target information in the RANAP Relocation Required

message, the 3G MSC detects that the new desired cell is a GSM cell.

Hence, it is necessary to send a BSSMAP Handover Request message

to the target BSC. This BSSMAP Handover Request is part of a MAP

Prepare Handover operation.

  Step 4: The BSSMAP Handover Request message is forwarded

transparently by the 2G MSC to the target BSC.

  Step 5: The target BSC allocates radio resources, especially a time slot

for the connection, in the target cell. The signaling of this procedure

can be monitored on the Abis interface.

Figure 1. Overview of an inter-3G-2G cell change

  Step 6: After successful resource allocation via Abis, the BSC

constructs a DTAP Handover Command that is sent to the UE via the E

Page 4: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 4/58

iRAT Handover Analysis

9-Dec-13, Page 4

interface and later UTRAN. The 2G MSC receives a BSSMAP

Handover Request Acknowledge message that contains the DTAP

Handover Command.

  Step 7: The MAP Prepare Handover Acknowledge message is used to

transfer the DTAP Handover Command via the E interface.

  Step 8: The 3G MSC orders a relocation of SRNC and forwards the

DTAP Handover Command via IuCS.

  Step 9: The RRC entity of the SRNC sends an intersystem Handover

Command including the DTAP Handover Command to the UE.

  Step 10: Based on the information found in the DTAP Handover

Command the UE enters the GSM cell.

Page 5: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 5/58

iRAT Handover Analysis

9-Dec-13, Page 5

3G-2.5G Inter-RAT Cell Change fo r PS Servic es

If a UE has an active PS RAB in UMTS, its mobility is controlled by the SRNC.

The compressed mode measurement will be activated if the radio conditions in

the currently used frequency are not sufficient anymore to guarantee the

required QoS. If the UE is able to find a GSM cell that offers a sufficient radio

quality, it will send an appropriate RRC Measurement Report message to the

SRNC as shown in the figure below. This measurement report contains as a

rule the event ID 3A: "The estimated quality of the currently used UTRAN

frequency is below a certain threshold and the estimated quality of the other

system is above a certain threshold." The identifier of the measured GSM cell

is its Base Station Identity Code (BSIC). The same BSIC is found together withan indicator of the used GSM frequency band and the ARFCN, which indicates

the frequency used to transmit the GSM cells broadcast channel in the RRC

Cell Change Order from UTRAN Command message sent by the SRNC. This

message orders the UE to leave the UMTS part of the network and register in

the 2.5G areas. Since there is no dedicated traffic channel for PS data in the

GERAN it is not possible to execute a relocation procedure as in case of CS

cell changes.

System information broadcasted in the target GSM cell tells the UE if a

combined routing area update/location area update is possible or not. In other

words: if there is a Gs interface between SGSN and VLR available or not. In

the example call flow in the figure below, the Gs interface exists. But the

current NSA software version does not support intersystem cell changes on Gs

interfaces yet.

Page 6: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 6/58

iRAT Handover Analysis

9-Dec-13, Page 6

 According to this information, the UE sends a Routing Area Update Request

message that contains the update type "combined RAU/LAU procedure"

together with old RAI and old LAI as well as currently used user temporary

identities TMSI and P-TMSI. Based on the update type, the SGSN is able to

handle the query of the VLR. For this reason, it sends BSSAP+ Location

Update Request message to the VLR and receives BSSAP+ Location Update

 Accept including the new LAI and a new TMSI. These two parameters are

stored in SGSN until the routing area update procedure is completed.

Figure 2. Message flow of 3G-2.5G PS Cell Change

Page 7: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 7/58

iRAT Handover Analysis

9-Dec-13, Page 7

In the next step, the 2.5G SGSN sends a GTP SGSN Context Request to the

3G SGSN that previously served this UE.

The 3G SGSN then sends RANAP SRNS Context Request to the SRNC and

SRNC answers with RANAP SRNS Context Response (not shown in the

figure). In case the 2G and 3G SGSN are just software entities running on the

same physical network node the GTP and RANAP procedure will be handled

by primitives inside the network node and no GTP/RANAP messages will be

seen on Gn/Gp and IuPS interfaces.

Using the GTP SGSN Context Response message the 3G SGSN transmits all

parameters, that important for the particular PDP context of the UE, to the

2.5G SGSN. Now, the PDP context can be continued in the new PS core

network node. The reception of the context parameters is acknowledged.

When the 3G SGSN received the acknowledgement, it can send a RANAP

Forward SRNS Data Command to the former SRNC. Then, the downlink IP

data, that was stored in the RNC RLC buffer related to the connection, can

optionally be forwarded to the 2.5G SGSN to prevent data loss, excessive TCP

retransmissions, and long cell change delay.

When the first packet of IP data is ready to be sent by 2.5G SGSN, the Routing

 Area Update Accept message is sent including new temporary identities for the

PS and CS domain services and new routing area information (RAI) as well as

new location area information (LAI) to be stored in the UE's USIM. After this

step, the PS cell change that must be actually rather seen as a registration

procedure including optional data forwarding, is completed.

Page 8: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 8/58

iRAT Handover Analysis

9-Dec-13, Page 8

2/2.5G - 3G Inter-RAT Cell Reselection

The cell change of an UE, that has an active PDP context in 2/2.5G and is

going to move to 3G, is even simpler as shown in the following figure. Based

on the received measurement results, the BSC will decide that a cell

reselection to 3G is advisable because of the better radio quality. On the Abis

interface (not shown in the figure), a cell reselection order command is sent to

the UE and a BSSGP Radio Status message will indicate the event on behalf

of the appropriate cause value "cell reselection ordered".

The UE performs a cell reselection procedure as specified in 3GPP 25.304.

Then, it requests to establish an RRC connection. The establishment cause in

the RRC Connection Request message is "inter-RAT cell reselection". This

RRC connection is used to register to the 3G core network domains. UE and

SGSN perform the combined Routing Area Update procedure for this purpose

if the Gs interface is available. Otherwise, separate Location Area Updates and

Routing Area Updates will be performed.

Figure 3. Message flow of 2.5G-3G Inter-RAT Cell Reselection for PS Service

Typically, there is no IP data to be transmitted and the RRC connection used

for registration is released. When later the user wants to exchange the IP

payload, a new RRC connection is requested (this time e.g. "originating

background call"). In this new radio connection, you can find a Service

Request message that contains the information that on a defined NSAPI a PDP

context is already active. To service this existing PDP context, an RAB and the

appropriate radio bearer are established. Now, the IP payload can be sent/

received by the network and UE using the UMTS radio interface.

Page 9: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 9/58

iRAT Handover Analysis

9-Dec-13, Page 9

2G-3G CS inter-RAT Handover o n Iub and Iu i nterf ace

This section allows a closer look at messages and parameters used on the Iub

and IuCS interface during the CS inter-RAT handover procedure from GSM to

UTRAN. Although the procedure in general was well specified, GSM to UTRAN

CS cell changes have not been monitored for a quite long time after launch of

UMTS networks. The reason was, that the handover command message sent

via the GSM radio interface (Um) must fit into a single LAPDm frame, because

neither LAPDm transport protocol nor the GSM radio resource management

layer (as a rule represented by a proprietary radio signalling link protocol) offer

a segmentation/reassembly function. As a result, the 260 bit information field

of a LAPDm frame is the maximum length of a handover message plus thenecessary radio resource header in GSM. An RRC Handover to UTRAN

Command message, that includes all necessary settings for radio bearers,

transport channel mapping, and physical channel information, would not fit into

a single LAPDm frame. For this reason, 3GPP developed so-called default

configurations. These default configurations are stored in the UE as well as in

RNC software. They allow transmitting minimized handover messages and

parameter lists. A complete overview of possible default configuration settings

can be found in 3GPP 25.331 (RRC).

The call flow of a CS inter-RAT Handover on Iub and IuCS starts with a

RANAP Relocation Request message sent from 3G MSC to the RNC.

Figure 4. Call flow of a 2G-3G CS inter-RAT cell change

Following the ASN.1 terminology, the message is encoded as Initiating

Message of Relocation Resource Allocation procedure. This name is not

Page 10: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 10/58

iRAT Handover Analysis

9-Dec-13, Page 10

shown in the figure. The message often contains the IMSI, but this is not a

mandatory parameter.

However, the RANAP Common ID message is missed in the cell change

procedure, so to transmit the true subscriber identity to the future SRNC using

the Relocation Request message seems to be an option to close this gap. The

relocation cause indicates that this is a time-critical relocation. The relocation

type is "UE involved", which means that cell change and relocation

(assignment of a new SRNC) are done in one step. The next information

element is the target cell ID. This is the target cell ID as used in NBAP

combined with the RNC-ID found e.g. in RRC messages. Then, a RAB Setup

List follows. This RAB Setup List is similar to the one used during the RAB

 Assignment procedure to establish a RAB initially. All RABs established and

released due to relocation/cell change procedures must be taken into account

to compute the correct number of simultaneous active connections in one

UTRAN that it is a prerequisite to compute meaningful call drop rates. In

addition to the RAB-ID, the Relocation Request message contains necessary

QoS RAB parameters, as well as the RAB sub-flow parameters that determine

the QoS of single radio bearers serving the same RAB. In the RAB sub-flow

parameters of this speech RAB, you can already find the transport block sizes

for transport channels used to carry AMR A, B and C bits across the Iub and

Uu.

Part of the Relocation Request message is the source-RNC-to-target-RNC-

transparent-container that is filled with UE radio access capability information

for GSM and portion of cell change information. The cell change information is

divided into two major parts: UMTS security information including ciphering

start values and UE radio access capabilities for UMTS. Receiving this

information allows the RNC to ensure a seamless encryption of control plane

and user plane information during and after the cell change. In addition, the

RNC knows what the UE supports regarding measurement procedures,transport channels (i.e. HS-DSCH, E-DCH), modulation schemes (QPSK,

16QAM), and so on.

The reception of the RANAP Relocation Request message by the RNC triggers

a NBAP Radio Link Setup procedure on the Iub interface. Here we find for the

first time the UL scrambling code with the reduced scrambling code number.

The uplink channelization code length that is identical with the UL spreading

factor is 64 – as expected for a AMR 12.2 kbps call. DCH 31 shall carry the

signaling radio bearers (RRC information) while DCHs 8, 9 and 10 will

transport RLC blocks with AMR A, B and C bits. The radio link (RL-ID=0) is set

up in cell with NBAP c-ID=9685 as already signaled in the RANAP Relocation

Page 11: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 11/58

iRAT Handover Analysis

9-Dec-13, Page 11

Request message.

The NBAP Radio Link Setup Response indicates that the radio link setup was

successful. Now the cell's receiver is waiting to receive the UE's uplink radio

signal using the specified UL scrambling code.

 After successful radio l ink setup on Iub, the RNC sends the RANAP Relocation

Request Acknowledge message that also confirms successful establishment of

RAB with RAB-ID=1 and contains the RRC Handover to UTRAN Command

message embedded in a container. Here, the u-RNTI, that contains the

temporary s-RNTI-2, is found as well as the default configuration ID=3, that

refers to predefined settings of a 12.2 speech RAB combined with a 3.4 kbps

signaling RABs. Uplink scrambling code and uplink spreading factor are the

same as seen before in NBAP. The primary scrambling code of the UTRAN

cell is 139. And on the downlink a spreading factor 128 is used. The number of

the DL spreading code is 5. In addition to the code information the frequency

information of the cell is signaled using uplink and downlink UMTS Absolute

Radio Frequency Channel Number (uARFCN). This is a mandatory parameter

in case that there is more than one UMTS frequency used in the UTRAN,

because primary scrambling codes (only 512 of them are available) might be

the same event, if cells using different frequency directly overlap. Having

uARNFC, PSC and UL scrambling code and downlink channelization code the

UE know all necessary physical parameters to f ind the ready-to-use radio link

in the target cell.

The RRC Handover to UTRAN Command message is sent via Um (GSM radio)

interface to the mobile that now performs the cell change itself.

When the UL radio signal of the UE is finally received by the cell, the Node B

sends NBAP Radio Link Restore Indication for the specified radio link set as

shown in the figure below. The reception of this NBAP message by the RNC

triggers the transmission of the RANAP Relocation Detect message on the

IuCS interface.

Once the UE is able to use to preconfigured radio bearers of the default

configuration, it sends an RRC Handover to UTRAN Complete message to its

new SRNC. This message contains the new start value for ciphering and

integrity protection. After reception of Handover to UTRAN Complete message,

the cell change itself is successfully finished. This fact is signaled to the MSC

using the RANAP Relocation Complete message. The default configuration

parameters as well as temporary s-RNTI-2 and short UL scrambling code are

in use until now. These parameters shall be used only temporarily and need to

be replaced.

Page 12: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 12/58

iRAT Handover Analysis

9-Dec-13, Page 12

The first step into this direction is an RRC UTRAN Mobility Information

message sent by the SRNC. It contains a new u-RNTI including a full length s-

RNTI.

In addition, this message also contains a long list of all possible connection

timer and constants. Many of them are used to guard acknowledged RRC

procedures. When a UE connection is set up in the UTRAN, initially this kind of

information is read before connection setup on the broadcast channel, but

during time-critical cell change procedures there is no time to read the BCH.

For this reason the parameters need to be explicitly signaled to the UE.

Figure 5. Call flow of a 2G-3G CS inter-RAT cell change

The RRC UTRAN Mobility Information procedure is an acknowledged RRC

procedure confirmed by the UE. When the RRC UTRAN Mobility Information

Confirm message is monitored, the new settings have been activated, but still

some temporary parameters are in use. For this reason, a new NBAP

Synchronized Radio Link Reconfiguration Preparation procedure is executed

that contains new physical channel parameters that correspond to the

parameters transmitted in the following RRC Radio Bearer Reconfiguration

message form SRNC to UE.

Page 13: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 13/58

Page 14: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 14/58

 Application Paper: iRAT Handover Analysis

9-Dec-13, Page 14

iRAT HO Analysis

In the iRAT 3G-2G Handover Matrix  different panes/chapters are used to

display the data. Main chapters are:

1. iRAT 3G-2G Handover Matrix

2. Cell Radio Parameters (3G cells only)

3. Mobility Indicators (for 2G cell only)

 After a carefully case study of 3G-3G relocations scenarios it was decided that

a matrix view of 3G-3G relocations (inter-RNC hard handover) does not

provide meaningful results, because

  these procedures are rather rare in today’s network,

  in Relocation Resource Allocation the source cell of the HO cannot be

identified

  in Relocation Preparation for 3G-3G HO the target cell ID cannot be

identified (only LAC is signaling in Target ID)

  in case of failures in these procedure the existing dashboard KPIs for

HO Analysis and RANAP Abnormal Iu-Release Ratio provide sufficient

failure indicators and allow full troubleshooting workflow including drill-

down to call details.

Since best correlation results are expected with cell information provided by

operators the import of a combined 2G/3G cell info file is required as

prerequisite for the matrix algorithms.

Page 15: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 15/58

 Application Paper: iRAT Handover Analysis

9-Dec-13, Page 15

iRAT 3G-2G Handover Matrix

The iRAT HO 3G-2G Handover Matrix  consists of several sections that are

described below:

Sourc e Cell 3G

Figure 7. Source Cell 3G

This is the unambiguous identity of the source cell where the UE receives theHO Command.

In the RANAP Init.Mgs. for Relocation Preparation  the SAC of the source cell

is found together with the global cell ID (GCI) of the target ID. In the matrix

these IDs are displayed as source cell = “RNC-ID+LAC+SAC” (where RNC-ID

is derived from topology or cell info file).

Figure 8. Message Example 1. Source and target ID

Cell name, uARFCN and Primary Scrambling Code  (Primary SC) are also

derived from cell info file.

Page 16: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 16/58

iRAT Handover Analysis

9-Dec-13, Page 16

Target Cell 2G–2G Cell Identity and Radio Quality before 3G-2G

Handover

Figure 9. Target Cell 2G

In this section the cell global identity is shown as derived from Init.Mgs.

Relocation Preparation = “LAC+CI”.

The BCCH ARFCN as well as BCC and NCC belong to a target TRX of a 2G

cell and are found in RRC Measurement Control and DMTAP Handover

Command (HCOM). (Refer to the figure below.)

Figure 10. Message Example 2: RRC Measurement Control for iRAT measurements

Page 17: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 17/58

iRAT Handover Analysis

9-Dec-13, Page 17

The general relation between TRX (identified by BCCH ARFNC, BCC and

NCC) and CGI (cell) is shown in the figure below.

Figure 11. General Relation between TRX and CGI

The interRATCellID from RRC Measurement Control  is often used to indicate

the verified BSIC (BCC+NCC) in RRC Measurement Report. (See Figure

12.Message example 3.)

>>>Band indicator MP Enumerated (DCS1800 band used,PCS 1900 bandused)

Indicates how tointerpret theBCCH ARFCN

>>>BCCH ARFCN MP Integer (0..1023) [45]

BSC

TRX 0

TRX 1

TRX 2

TRX 3

TRX 255

Cell 1

Cell 2

Cell n

BTS 1

BTS m

Page 18: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 18/58

iRAT Handover Analysis

9-Dec-13, Page 18

+- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +| I D Name | Comment or Val ue |+- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +| 4999 2: 53: 38 PM, 991, 846 6- RRC- I UB RLC/ MAC AM DATA DCH RRC_DCCH_UL measur ement Repor t| TS 25. 331 DCCH- UL - V5. 9. 0 ( RRC_DCCH_UL) measurement Repor t ( = measurement Repor t ) |

| uL- DCCH- Message || 1 i nt egri t yCheckI nf o || 1. 1 messageAut hent i cat i onCode | ' 1f9185f 3' H || 1. 2 r r c- MessageSequenceNumber | 1 || 2 message || 2. 1 measurement Repor t || 2. 1. 1 measurement I dent i t y | 11 || 2. 1. 2 measuredResul t s || 2. 1. 2. 1 i nt er RATMeasur edResul t sLi st || 2. 1. 2. 1. 1 i nt erRATMeasur edResul t s || 2. 1. 2. 1. 1. 1 gsm || 2. 1. 2. 1. 1. 1. 1 gSM- Measur edResul t s || 2. 1. 2. 1. 1. 1. 1. 1 GSMCarr i erRSSI | 74 dBm + SCALE t o 73 dBm + SCALE || 2. 1. 2. 1. 1. 1. 1. 2 bsi cRepor t ed || 2. 1. 2. 1. 1. 1. 1. 2. 1 veri f i edBSI C | 0 || 2. 1. 2. 1. 1. 1. 2 gSM- Measur edResul t s || 2. 1. 2. 1. 1. 1. 2. 1 GSMCarr i erRSSI | 85 dBm + SCALE t o 84 dBm + SCALE || 2. 1. 2. 1. 1. 1. 2. 2 bsi cRepor t ed || 2. 1. 2. 1. 1. 1. 2. 2. 1 veri f i edBSI C | 2 || 2. 1. 2. 1. 1. 1. 3 gSM- Measur edResul t s || 2. 1. 2. 1. 1. 1. 3. 1 GSMCarr i erRSSI | 86 dBm + SCALE t o 85 dBm + SCALE || 2. 1. 2. 1. 1. 1. 3. 2 bsi cRepor t ed || 2. 1. 2. 1. 1. 1. 3. 2. 1 veri f i edBSI C | 3 || 2. 1. 2. 1. 1. 1. 4 gSM- Measur edResul t s || 2. 1. 2. 1. 1. 1. 4. 1 GSMCarr i erRSSI | 94 dBm + SCALE t o 93 dBm + SCALE || 2. 1. 2. 1. 1. 1. 4. 2 bsi cRepor t ed || 2. 1. 2. 1. 1. 1. 4. 2. 1 nonVeri f i edBSI C | 51 || 2. 1. 2. 1. 1. 1. 5 gSM- Measur edResul t s || 2. 1. 2. 1. 1. 1. 5. 1 GSMCarr i erRSSI | 95 dBm + SCALE t o 94 dBm + SCALE || 2. 1. 2. 1. 1. 1. 5. 2 bsi cRepor t ed || 2. 1. 2. 1. 1. 1. 5. 2. 1 nonVeri f i edBSI C | 65 || 2. 1. 3 event Resul t s || 2. 1. 3. 1 i nt er RATEvent Resul t s |

| 2. 1. 3. 1. 1 event I D | e3a || 2. 1. 3. 1. 2 cel l ToReport Li st || 2. 1. 3. 1. 2. 1 cel l ToReport || 2. 1. 3. 1. 2. 1. 1 bsi cReport ed || 2. 1. 3. 1. 2. 1. 1. 1 veri f i edBSI C | 0 || 2. 1. 3. 1. 2. 2 cel l ToReport || 2. 1. 3. 1. 2. 2. 1 bsi cReport ed || 2. 1. 3. 1. 2. 2. 1. 1 veri f i edBSI C | 2 || 2. 1. 3. 1. 2. 3 cel l ToReport || 2. 1. 3. 1. 2. 3. 1 bsi cReport ed || 2. 1. 3. 1. 2. 3. 1. 1 veri f i edBSI C | 3 |

Figure 12. Message Example 3: RRC Measurement Report with iRAT measurement results

Page 19: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 19/58

iRAT Handover Analysis

9-Dec-13, Page 19

+- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +| I D Name | Comment or Val ue |+- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +| 5097 2: 53: 39 PM, 856, 612 7- RRC- I UB RLC/ MAC AM DATA DCH RRC_DCCH_DL

handover Fr omUTRANCommand- GSM || TS 25. 331 DCCH- DL - V5. 9. 0 ( RRC_DCCH_DL) handoverFr omUTRANCommand- GSM ( =handover Fr omUTRANCommand- GSM) || TS 44. 018 Radi o Resource V5. 15. 0 ( RR- DMTAP) HCOM ( = Handover command) || Handover command || Prot ocol Di scr i mi nat or | r adi o r essour ce management messages || Sub- protocol di scr i mi nat or | Ski p I ndi cat or || Message Type | 43 || Cel l Descri pt i on || BCC | 5 || NCC | 4 || BCCH ARFCN ( hi gh) | 0 || BCCH ARFCN ( l ow) | 75 || BCCH ARFCN | 75 || Channel Descr i pt i on 2 |

Figure 13. Message Example 4: RRC Handover from UTRAN Command/DMTAP Handover Command

There is a fix mapping between BCCH ARFCN and the used GSM frequency

band:

For the sequence of messages shown in Message Examples 1 to 4 the matrix

sections looks as below:

2G cell identiy and radio quality before 3G-2G handover

2G

LAC+CI

BCCH

ARFCN/BCC/NCC GSM Frequency Band

avg. GSM RSSI @ iRAT HO

[dBm]

308-371 75/5/4 GSM 900 74

GSM Frequ. Band ARFCN Comment

GSM 400 Dynamic Not used

GSM 400 Dynamic Not used

GSM 400 259–293 Tansania only

GSM 400 306 – 340 Tansania only

GSM 700 Dynamic Not used

GSM 700 438–511 Not used

Dynamic Not used

GSM 850 128–251 America

GSM 900 1–124 Worldwide

GSM 900 0, 1–124, 975–1023 Europe

GSM 900 0, 1–124, 955–1023 Asia, Europe

GSM 900 Dynamic Not used

GSM 1800 512–885 Worldwide

GSM 1900 512–810 America

Page 20: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 20/58

iRAT Handover Analysis

9-Dec-13, Page 20

3G-2G CS Handover/Relocatio n Preparation Failu re Analys is

To request assignment of radio resources in the 2G RAN the RNC starts the

Relocation Preparation procedure.

Figure 14 shows a message flow example where first Relocation Preparation 

fails due to “semantic error” and second Relocation Preparation  sent to a

different 2G cell is successful.

Figure 14. Relocation Preparation Failure and Successful Relocation in same call

Reloc. Prep. Attempts.  This is the number of RANAP Initiating Message for

procedure code “relocation preparation”

Distinct calls with Relocation Preparation Failure: This is the number of global

call IDs in which one or more Relocation Preparation Failures have been

found. Since the Relocation Preparation procedure in case of failure is

repeated multiple times in the same call a single problem of an individual

subscriber can lead to high failure ratios of the procedure KPI while subscriber

impact is low. This column helps to validate the subscriber impact.

Top Cause Reloc. Prep. Failure: These are the cause values seen most often

in Relocation Preparation Failure message (RANAP Unsuccessful Outcome for

procedure code “relocation preparation”.

For the sample call in Figure 14 the matrix delivers the following results:

Figure 15. 3G-2G CS Handover/Relocation Preparation Failure Analysis  

Page 21: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 21/58

iRAT Handover Analysis

9-Dec-13, Page 21

NOTE: For calls that have Iu (RANAP) signaling only and miss Iub the ARFCN

cannot be detected in case of Relocation Preparation Failure. Hence, the ARFCN

values are displayed as “unknown” in such cases.

Page 22: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 22/58

iRAT Handover Analysis

9-Dec-13, Page 22

3G-2G CS Handover/Relocation Execution Failure Analysis

Figure 16 shows a sample call with multiple HO execution attempts that drops

at the end because of the handover failures.

Figure 16. Repeated Relocation Execution Failure in a voice call

The matrix view displays the following columns as follows:

Figure 17. 3G-2G CS Handover/Relocation Execution Failure Analysis

3G-2G HO Command. This is the number of DMTAP HCOM (embedded in

RANAP Successful Outcome “relocation preparation”.

Distinct Calls with HO Command.  This is the number of global call IDs that

have min. 1 DMTAP HCOM

Top Cause HO Exec. Failure.  This is the cause value seen most often in

RANAP Initiating Message Relocation Cancel. 

Number HO Failure reported by Handset. This is the number RRC Handover  

from UTRAN Failure messages (counted on the target cell of relocation

preparation procedure).

Page 23: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 23/58

iRAT Handover Analysis

9-Dec-13, Page 23

Top Cause HO Failure reported by UE.  This is the cause value seen most

often in RRC Handover from UTRAN Failure messages

Number Call Drop after failed HO.  This is the number of RANAP Iu-

ReleaseRequest messages with “BAD” cause. This is a call drop that happens

as result of failed relocation preparation or HO execution.

Top Cause for Call Drop after failed HO – the top RANAP cause value for

drops after failed relocation preparation or HO execution.

Page 24: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 24/58

iRAT Handover Analysis

9-Dec-13, Page 24

3G-2G PS Cell Change Order (CCO) Failure

Figure 18 shows a full call flow procedure of a PS cell change order to 2G.

Note that there is no relocation preparation and only identity of the target cell is

BCCH ARFCN and BSIC. As the details in the sequence of messages (figure

19 to 21) shows there are a couple of options that have been taken into

account:

1. The RRC Meas. Reports is sent periodically instead of event-triggered. If

event-triggered the proper HO trigger is event 3A.

2. The BSIC value display in RRC Meas. Report is not always the BSIC

itself. Depending on RNC NEM the BSIC value in the RRC Meas. Report

is one of the following:

a. The BSIC in combined (NCC+BCC) hex format

b. The value of the BCCH ARFCN of the cell (as seen in Figure 20)

c. The value of the inter-RATCellID seen in RRC Meas. Control before

Figure 18. CCO full call flow procedure

Figure 19. RRC Meas. Control for iRAT

Page 25: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 25/58

iRAT Handover Analysis

9-Dec-13, Page 25

Figure 20. RRC Measurement Report (periodic)

Figure 21. RRC Cell Change Order from UTRAN Command

The contents of the columns in the 3G-2G PS CCO section of the matrix is

defined as follows:

PS 3G-2G CCO Att.  This is the number of RRC Cell Change Order from

UTRAN messages.

Top Cause CCO Failure reported by UE. This is the cause value seen most

often in RRC Cell Change Order Failure.

The matrix columns representation is as shown in the figure below.

Page 26: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 26/58

iRAT Handover Analysis

9-Dec-13, Page 26

Figure 22. RRC Cell Change Order from UTRAN Command

Page 27: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 27/58

 Application Paper: iRAT Handover Analysis

9-Dec-13, Page 27

Radio Measurements (Al l reports)

In the pane Radio Measurements, in the iRAT 3G-2G Handover Matrix, the

histograms for Intra-frequency Ec/N0, Intra-frequency RSCP, GSM RSSI  and

Propagation Delay  can be viewed, for the source cell during duration of the

iRAT matrix snapshot.

The histograms show the percentage (%) value for each histogram bar and

standard deviation value.

Figure 23. Radio Measurements Histograms Representation

Page 28: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 28/58

 Application Paper: iRAT Handover Analysis

9-Dec-13, Page 28

Traff ic Load

The Traffic Load  pane, in the iRAT 3G-2G Handover Matrix, displays the

following KPIs (for the 3G source cell only).

Figure 24. Traffic Load View

iRAT CRS Mobility Ratio %. This formula works the following way: each inter-

RAT cell reselection of an UE changing from 2G to 3G leads to a combined

Location/Routing Area Update (Loc. Update cause = “normal LUP”). The

number of iRAT cell reselections is the number of successfully established

RRC connections for establishment cause “inter-RAT-cell-reselection”. The

result of the formula shows how many normal Location Updates caused by

mobility of UEs are due to iRAT cell change 2G-3G. A value > 50% indicates

that there are more UEs toggling between 2G and 3G than UEs moving from

one 3G LAC to another.

NOTE: During system test phase it was observed that in short sessions defective

handsets can leverage the equation result to values higher than 100%. If such

values are seen this is a clear indication to make a more detailed investigation of

the Location Update performance in the source cell, e.g. by filtering on all calls

with RRC Establishment Cause = "inter-RAT cell reselection" of this cell and make

column statistics on subscriber IDs.

Outgoing iRAT PDP Cntx. Mobility Ratio %. This KPI shows how many active

PDP Contexts from the source cell are leaving to 2G.

Incoming iRAT PDP Cntx. Mobility Ratio %. This KPI shows how many active

PDP Context in the cell are coming from 2G.

CS outgoing iRAT Mobility Ratio %. This KPI shows how many CS RABs that

Page 29: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 29/58

iRAT Handover Analysis

9-Dec-13, Page 29

have been successfully established in 3G are leaving to 2G while active.

This KPI shows how many CS RABs that have been successfully established

in 3G are leaving to 2G while active.

Page 30: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 30/58

iRAT Handover Analysis

9-Dec-13, Page 30

In addition to these KPIs, there are also the CS and PS Traffic Load KPIs  for

the 3G source cell as CS Traffic Load (Erlang) per hour =

PS Traffic Load (Erlang) per hour =

PS vs. CS iRAT Handover Ratio %. This KPI shows the percentage of PS iRAT

Cell Change compared to all iRAT handover (CS + PS). It allows determining if

more packet calls or more voice calls are handed over to 2G.

Page 31: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 31/58

iRAT Handover Analysis

9-Dec-13, Page 31

Sort and Fi l ter Funct ions

The following filters are available within the iRAT HO Matrix which allow to sort

and filter the top 10 (source) cells with:

  Highest iRAT CRS Mobility Ratio

  Highest Outgoing iRAT PDP Cntx. Mobility Ratio

  Highest Incoming iRAT PDP Cntx. Mobility Ratio

  Highest CS outgoing iRAT Mobility Ratio

  Highest CS Traffic Load

  Highest PS Traffic Load

  Highest PS vs. CS iRAT Handover Ratio

  Lowest PS vs. CS iRAT Handover Ratio

Page 32: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 32/58

 Application Paper: iRAT Handover Analysis

9-Dec-13, Page 32

Troubleshoo ting iRAT HO Usin g Dashboard K PIs

The TrendNavigate reports allow to analyze iRAT HO Analysis from the

several report KPIs. The following sections show these possibilities.

  Problem discovery

 As it is already known, the TrendNavigate tool provides more than one start ingpoints for discovering the network issues. One such report is the Top Cells 

report.

In our case the Top Cells report can be generated based on several KPIs, e.g.

3G-2G CS Handover Failure Ratio.

The method of getting this report is simple. From the Reports menu, click Top

Cells. In the displayed screen, in the right pane, select the report criteria and

the KPI 3G-2G CS Handover Failure Ratio from the KPI list and click Apply .

The following figure shows a view of the Top Cells Report.

Figure 25. Top Cells Report View

This report contains all the cells with 3G-2G Handover Failure, and gives

details on the number of Attempts recorded and for each attempt the sub-

Page 33: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 33/58

iRAT Handover Analysis

9-Dec-13, Page 33

cause of failure, e.g. Failure in the Radio Interface Procedure, Radio

Connection with UE lost, or Timer Expiry.

In this report we can see the SAC 22231 is the top runner of the list. So, this

deserves some deep dive analysis. The findings from this SAC shall be equally

applicable to other SACs in the network.

   Al ternate Entry Poin t

 An equally good and valuable way of discovering iRAT issues is the KPI Cause

CPT Report. This report can be opened from the toolbar menu KPI Cause. In

the displayed report window select the criteria from the right navigation panel.

In the Additional right navigation panel, select the 3G-2G CS Handover Failure

Ratio in the KPIs list.

Figure 26. Select KPI "3G-2G CS Handover Failure Ratio"

When selecting a KPI, TrendNavigate fills in the Cause and sub-causes

automatically. Next, click on the Trend button at the bottom of this panel. This

will sort the report in the descending order for all sub-causes.

Page 34: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 34/58

iRAT Handover Analysis

9-Dec-13, Page 34

The final report looks like in the figure below:

Figure 27. KPI Cause Report for "3G-2G CS Handover Failure Ratio"

In this graphic we can see the causes and sub-causes for our selected KPI.

The main cause is Relocation Cancel and the sub-causes are Failure in the

Radio Interface Procedure and Radio Connection with UE lost, along with one

case where the sub-cause is mentioned as Cause not distributed.

  Deep Dive Analysi s

Having seen the Iu Report for 3G-2G CS Handover Failure analysis, further

investigations into the messaging of call flow are possible, to find out the root

cause. The root cause can either be in the hardware (RNC, UE) or in the

planning of the network, (iRAT neighbor definitions, missing or one-way

neighbors, or poor GSM signal of the 2G neighbor).

 All of this requires a detailed analysis in the messaging to compare the values

reported in the Measurement reports for various 2G Cells.

For this purpose, the first need is to look at all those calls that contributed to

the KPI 3G-2G CS Handover Failure Ratio in the UMTs network.

TrendNavigate offers this possibility to getting the call table containing only our

intended calls.

Page 35: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 35/58

iRAT Handover Analysis

9-Dec-13, Page 35

Right-click on the bar graph under the Causes for KPI 3G-2G CS Handover

Failure Ratio. This gives you the option to open the call Table. Refer to the

figure below.

Figure 28. Right-Click on the bar graph gives the option to Open Call Table

Click the Open Call Table option to open the NSA Call Analysis application. It

gets started with the auto-filtered call table for the desired KPI.

It is worth mentioning here that if you opened the call table from the top left

graph in the KPI Cause report, the call table will contain calls for all sub-

causes. You can also open the call table for one individual sub-cause by right-

clicking on the graph for the particular sub-cause. Refer to figure 27.

The call table has the flexibility to show RANAP Release Causes. In this case

the Call Table looks like in figure below:

Figure 29. NSA-CA Call Table showing calls after filtering

Each row in the call table represents one Global Call and is capable of being

drilled down to the single message level.

Page 36: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 36/58

iRAT Handover Analysis

9-Dec-13, Page 36

  Root Causes

Delving into the Messaging Sequences revealed some interesting facts:

a. UE informs the network that 3G coverage is fading out; see figure 30

below for details.

Figure 30. UE Reporting weak 3G Coverage.

b. There are 2G neighbors in the vicinity; they are the right candidates for

iRAT Handover. The RNC gives the Relocation Preparation Command,

a signal to go ahead and perform the Handover to 2G system. This is

shown in the figure 31 below.

Figure 31. RNC gives the "Relocation Preparation" Command

Page 37: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 37/58

iRAT Handover Analysis

9-Dec-13, Page 37

But the measurement report also reveals that the strength of GSM RSSI is also

not so good: (See Figure 32)

Figure 32. UE reports weak GSM Signal for 2G neighbors

c. There is the Handover Command from UTRAN-GSM also seen on the

Iub interface, followed by Handover from UTRAN Failure, with the

cause Physical Channel Failure, as seen in the figure 33 below:

Figure 33. Handover from UTRAN Failure, cause "Physical Channel Failure"

Page 38: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 38/58

iRAT Handover Analysis

9-Dec-13, Page 38

d. As a result, the RNC gives the command Relocation Cancel with the

cause code Failure in the Radio Interface Procedure as shown in the

figure 34 below:

Figure 34. RNC sends "Relocation Cancel" Command, cause "Failure in the RadioInterface Procedure"

This process repeats several times itself (Figure 35 below) and finally the call

is released with the cause Radio Connection with UE Lost (see Figure 37)

Figure 35. UE tries several times during the call to perform iRAT HO to 2G, but failseach time.

Page 39: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 39/58

iRAT Handover Analysis

9-Dec-13, Page 39

Just before call drop, the UE reported all its 2G neighbors, none of which

seems to be strong enough to sustain the call. (See Figure 36)

Figure 36. All of the reported GSM neighbors are too weak to perform the HO.

Figure 37. Finally, RNC send the Iu-Release command, cause "Radio Connection withUE lost"

Page 40: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 40/58

iRAT Handover Analysis

9-Dec-13, Page 40

NSA Call Analysis application also shows the measured quantities in the

Measurement reports. The following figure shows the Radio environment

before the call drop with the help of NSA CA.

Figure 38. RSCP, EcNo and TrCh BER before the call dropped

Figure 38 shows clearly that the RSCP of the serving cell is deteriorating,

EcNo is below -20 dB and the TrCH BER is ~ 25%. At such Radio

environment, it is impossible for the communication to continue.

Following figure (39) shows the RSCP value only just before the call drop:

Figure 39. RSCP Value of the serving cell before the call dropped.

Obviously, with RSCP at <-114 dBm, the call cannot continue.

Page 41: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 41/58

iRAT Handover Analysis

9-Dec-13, Page 41

This analysis has been performed over several calls and was found that:

•  Some of the defined 2G neighbors are not strong enough.

•  Some of the 2G neighbors are reported having good GSM RSSI, but

still the iRAT HO fails.

The excel sheet attached below displays this analysis.

 As one can see all these details of this table that shows results of (manual)

single call analysis will be presented now in the iRAT 3G-2G Handover Matrix

in an aggregated way.

Figure 40. iRAT HO Analysis

 A snapshot of the excel sheet for iRAT HO Failure Analysis shows the GSM

RSSI is generally very low in the network, although there are some instances

of good GSM RSSI also reported in iRAT HO Failures.

Page 42: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 42/58

iRAT Handover Analysis

9-Dec-13, Page 42

   A General v iew of th e GSM RSSI in th e network

In this case, it is obvious that the 2G coverage is not so good, but to give it a

quantitative aspect, pull out some reports on RNC level and then on some

selected cell levels to see how the GSM RSSI looks like.

Figure 41 reveals that the general RMS value of the GSM RSSI is rather on the

lower side. Majority of the GSM carriers appears to be below -90 dBm, with

some of them being as low as -110 dBm. This trend indicates that the GSM

values reported by UEs are not favorable in most of the cases to support the

iRAT HO to 2G carriers.

One recommendation is to add more 2G sites to the network so as to bring the

average around -85 dBm. That will enhance significantly the chances of iRAT

HO to 2G being successful.

Figure 41. GSM RSSI for the RNC05

GSM RSSI value histogram for cell-id 61722 is shown below in the figure 19

below.

Figures 42 and figure 43 show the Histogram for GSM RSSI for two selected

cells.

Figure 42. GSM RSSI for c-id 61722

Page 43: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 43/58

iRAT Handover Analysis

9-Dec-13, Page 43

GSM RSSI for the top runner cell-id 22231 is shown in figure 20 below:

Figure 43. GSM RSSI for c-id 22231

From the sample, cells selected for GSM RSSI histogram, it is shown that the

majority of the measurements report GSM values below -90 dBm. This coupled

with the average GSM RSSI for the whole RNC (Figure 41) is sufficient to

believe that the network lacks the absolute required level of GSM coverage

and the call will keep dropping due to unsuccessful iRAT HO to 2G, unless

some measures are taken to boost the average value of 2G signal in the

network.

  PS Data Calls

There are no differences between the situations of the PS and CS calls.

It should be mentioned that it has been observed similar pattern of drop calls

while an iRAT HO was declared mandatory by the radio conditions. So, the

Cell Change Order command will be find on the Iub instead of Handover from

UTRAN command for the PS calls. The rest of the procedure and cause values

remain unchanged.

Page 44: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 44/58

 Application Paper: iRAT Handover Analysis

9-Dec-13, Page 44

RAN Optimization wit h i RAT 3G-2G Handover

  Troub leshoot and Optimi ze inter-RAT 3G-2G Handov er

Problem Statement: Failed inter-RAT Handovers prevents the subscriber from

being transferred to an environment with better radio conditions. Thus, the userplane QoS/QoE for subscriber remains poor and the risk of a call drops in the

3G radio environment rises the longer the handover is not successfully

executed.

How to proceed in TrendNavigate:

a) Open inter-RAT Handover Matrix

b) Filter on “non blanks” in column 3G-2G HO Execution Failure Ratio

Figure 44. iRAT Hanover Matrix: Set Filter for 3G-2G HO Execution Failure Ratio

c) Scroll to the left side to see names of source cell/target cell and radio

conditions in target cell. Based on this information the RAN engineers

or consultants can prepare a list of executable actions. 

Page 45: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 45/58

iRAT Handover Analysis

9-Dec-13, Page 45

  Troubleshoot 3G-2G Relocation Preparation Issues

Problem Statement:  Relocation Preparation  is a RANAP procedure that

triggers the request to allocate radio resources. As results the target systems

(e.g. BSC in GERAN) sends a DTAP Handover Command message back to

the UTRAN. If Relocation Preparation  fails the HO Command message is not

provided by BSC and hence, the handover cannot be executed and the

subscriber must continue to stay under bad radio conditions with impact on

QoS/QoE and risk of drop due to radio.

How to proceed in TrendNavigate:

a) Open inter-RAT Handover Matrix.

b) Filter on “non blanks” in column Reloc. Prep. Failure Ratio. In the

example the failure ratios really low after a successful

troubleshooting/optimization campaing in this field.

Figure 45. Set Filter for Reloc. Prep. Failure Ratio

c) Scroll to left side to see names of source/target cell and radio

conditions in target at HO decision

Figure 46. Source/ Target Cell and Radio Conditions View at HO decision

d) Further analysis must be done in the 2G RAN or core network

transport. These are the typical problem areas of Relocation

Preparation Failures.

Page 46: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 46/58

iRAT Handover Analysis

9-Dec-13, Page 46

  Wrong 2G Target Neighbor

Problem Statement:  It is aim of the handover to bring the connection of the

subscriber to an area with better radio conditions to improve QoS/QoE and

reduce the risk of drops. When a target cell does not offer good radio

conditions and the handover is executed anyway, the improvement will not

happened and – more dramatically – the risk of a drop during or after the

handover is even higher than staying in the suboptimal 3G RAN.

How to proceed in TrendNavigate:

a) Open inter-RAT handover matrix

b) Filter on target cells that have low GSM RSSI values, check impact on

HO Execution Failures. Typically the UEs react with a “physical

channel failure” when radio conditions in HO Target are insufficient or

the targeted radio signal is not found.

Figure 47. Execution Failure Analysis

Figure 48. Execution Failure Ratio

c) The typical error in such situation is that the best 2G neighbor is

missed in the inter-RAT neighbor list of the 3G source cell in the RNC.So, it is highly recommended to check this list and compare with up-to-

date 2G radio network planning data.

d) If the neighbor lists are okay then coverage issues of the 2G cell can

be assumed that are often revealed from the GSM RSSI Measurement

results reported on 3G – refer to the figure below. Here an insufficient

2G coverage of the target cell leads to “physical channel failure” during

HO/Cell Change Order   while the HO trigger conditions of the source

cell can be easily identified as 3G coverage issues based on

comparison of RSCP and Ec/N0 histograms.

Page 47: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 47/58

iRAT Handover Analysis

9-Dec-13, Page 47

Figure 49. iRAT HO Matrix. Radio Measurements (All Reports)

  Reduce Inter-RAT and Intra-RAT Location Toggli ng

Problem Statements:  During Location Update procedure, the mobile does not

listen to received Paging, so we should count the number of Paging sent to a

mobile when it is doing the LUREQ procedure (Paging received between

rrcConnectionRequest (registration) and LUACC). This sometimes is called

“collision”. Especially it is often seen that one IMSI makes 60 to more than 100

LUREQ/hour in one cell, because it is toggling in IDLE mode (not signaling

sent) between 2G and 3G LAC or between two different 3G LACs. This kind of

LAC toggling creates a lot of unwanted signaling load on RNCs.

Optimization Target:  Reduce Signaling Load  in RNC by eliminating ping-pong

Location Updates – improve customer QoE in parallel, because ping-pong LUP

leads to high number of paging failures in turn

How to proceed in TrendNavigate:

a) When looking into iRAT Matrix you see iRAT CRS Mobility Ratio, which

is number of successful RRC Conn. Setups with est. cause “inter-RAT

Cell Reselection” vs. number of Loc. Upd. “normal”.

 As we know, each inter-RAT Cell Reselection  triggers a combined

LUP/RAUP “normal” and subscribers can only come from 2G to 3G cell

or form different 3GLAU to our monitored 3G LAU (RNC). Looking at

example below it is seen 3971 LUP “normal” and 75.8% of the are due

Page 48: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 48/58

iRAT Handover Analysis

9-Dec-13, Page 48

to 2G-3G LUP, which means in turn that 100%-75.8%=24.2% are due

to 3G-3G LUP.

There also can be observed that 89.5% of the 476 PDP Contexts that

are active in this cell during the observation period come from 2G

(following iRAT CRS) and you see further that form the 197 PDP

Contexts initially established on 3G in this cell 37% are sent to 2G

using RRC Cell Change Order from UTRAN (which is forced iRAT CRS

to 2G).

45.9% of the CS RABs (voice) that are established in this cell are

handed over to 2G. Conclusion: in this cell there is a lot of 3G-2G-3G

Ping-Pong.

Figure 50. Source Cell Measurements Values

b) Now there is a possibility to verify this thesis in the following way using

the call table:

a. Filter on the first UTRAN cell = cell-ID of “Lafitte*” and RRC

Establishment Cause = “inter-RAT Cell Reselection” (3G-2G Ping-

Pong) or “registration” (3G-3G Ping-Pong)

b. Run column statistics on IMSI to see if same subscribers pop up all

the time, filter on single IMSIs for more details.

Sample: it can be seen same subscriber in same cell registering again and

again…

Page 49: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 49/58

iRAT Handover Analysis

9-Dec-13, Page 49

Figure 51. Subscriber Multiple Registration

If you need stats how often IMSI register in same cell you can run XML KPI. If

you further want to know if they have old LAC = new LAC you can also build

XML counter for this.

Figure 52. Find New/Old LAC

Page 50: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 50/58

iRAT Handover Analysis

9-Dec-13, Page 50

c) Drill-down to call details or rf5. In case of our example you see that old

LAC is default value 65534, means: old LAC was deleted on SIM card.

This happens, e.g. in case of “IMSI unknown in VLR/HLR” during

Identity Check Procedure LUP/Attach/RAUP or CMSREJect withsimilar NAS cause.

The other possible root cause is that the handset “loses” the stored

LAC due to internal error and thus, needs to register again.

Note: During system test phase using short sessions iRAT CRS Mobility Ratio

values > 100% have been observed. This was caused by a single subscriber trying to

register in PS domain ping-pong wise 3G-2G-3G etc. and always rejected by the

network. This is exceptional behavior rarely seen, but a real issue for the network in

term of rising signaling load. So values >100% shall not be seen a bug in

TrendNavigate formula, but rather they highlight exceptional problem areas.

Figure 53. Call Table View

Page 51: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 51/58

 Application Paper: iRAT Handover Analysis

9-Dec-13, Page 51

How to Find iRATs in the NSA Call Table

Circuit switched intersystem cell changes calls can be viewed in the Call Table.

When you open the Call Table, the column Handover Procedure  displays the

history of the cell change for a call as displayed in the figure below.

Figure 54. Handover Procedure shown in the Call Table

Thus any call that contains entries in the Handover Procedure column contains

intersystem cell changes.

Page 52: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 52/58

iRAT Handover Analysis

9-Dec-13, Page 52

For a detailed analysis of such calls, select the individual row and double-click

for the drill down chart. A sample screen capture is depicted below.

Figure 55. Drill-down chart

For packet switched calls the handover history is not populated. Hence, calls

with intersystem cell changes can be detected via the SGSN, BSC and RNC

columns in the Call Table. A call that has had an intersystem cell change

includes the BSC and RNC columns populated accordingly.

Page 53: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 53/58

iRAT Handover Analysis

9-Dec-13, Page 53

 An example of a PS call with an intersystem cell change is depicted in the

figure below.

Figure 56. PS Call with intersystem cell change

 Also in this view, a call that has had an intersystem cell change shows entries

in the BSC and RNC columns. 

Page 54: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 54/58

Page 55: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 55/58

iRAT Handover Analysis

9-Dec-13, Page 55

iRAT Inter radio access technology

KPIKey Performance Indicator

LACLocation Area Connection

LAI Location Area Interface

LAPD Link Access Protocol in the D channel

LAU Location Area Update

MSC Mobile Switching Centre

NAS Non Access Stratum

NBAP Node-B Application Part

NEM Network Equipment Manufacturer

NSA Network Service Analyzer

NSAP Network Service Access Point

NSAPI Network Service Access Point Identifier

PCU Packet Control Unit

PDP Packet Data Protocol (e.g. PPP, IP, X.25)

PLMN Public Land Mobile Network

PS Packet Switched

PSC Primary Scrambling Code

P-TMSI Packet TMSI

QoS Quality of Service

QPSK Quadrature Phase Shift Keying

RAB Radio Access Bearer

RAI Routing Area Interface

RAN Radio Access Network

RANAP Radio Access Network Application Part

RAU Routing Area Update

RB Radio Bearer

RL Radio link

RLC Radio Link Control; Release Complete Message

Page 56: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 56/58

iRAT Handover Analysis

9-Dec-13, Page 56

RNC Radio Network Controller

RNTI Radio Network Temporary Identity

RRC Radio Resource Control

RSSIReceived Signal Strength Indicator

SACService Area Code

SCScrambling Code

SGSN Serving GPRS Support Node

SRNC Serving Radio Network Controller

TMSI Temporary Mobile Subscriber Identity

TRX (Base Station) Transceiver

u-ARFCN UMTS Radio Frequency Channel Number

UE User Equipment

UL Up link

Um GSM Air Interface

UP User Plane

UTRAN UMTS Terrestrial Radio Access Network

VLR Visitor Location Register

Page 57: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 57/58

 Application Paper: iRAT Handover Analysis

9-Dec-13, Page 57

Technical Support

For support contact your regional Technical Assistance Center:

 EMEA

phone +373 22 879020

fax +373 22 879001

e-mail [email protected]

 Americas

phone +1 469 330 4580

e-mail [email protected] 

 Asia Pacific

phone +86 21 389 60420

e-mail [email protected]

Page 58: UMTS IRAT Handover Analsysis

8/18/2019 UMTS IRAT Handover Analsysis

http://slidepdf.com/reader/full/umts-irat-handover-analsysis 58/58

iRAT Handover Analysis

Copyright © Tektronix Communications. All rights reserved. Licensed software

products are owned by Tektronix Communications or its subsidiaries or

suppliers, and are protected by national copyright laws and international treaty

provisions.

Tektronix Communications products are covered by U.S. and foreign patents,

issued and pending. Information in this publication supersedes that in all

previously published material. Specifications and price change privileges

reserved.

TEKTRONIX, Tektronix Communications and TEK are registered trademarks of

Tektronix, Inc.

Contacting Tektronix Communications

Tektronix Communications

3033 W President George Bush Highway

Plano, TX 75075

Phone: + 1 469 330 4000 

Fax: + 1 469 330 4001

For product information, sales, service, and technical support:

+373 22 879 020

This document supports software version NSA 4.00 and above.

Revised: April 2012

Worldwide, visit http://www.tektronixcommunications.com to find contacts in

your area.