g-tch congestion rate optimization delivery guide 20070115-a-1.1.doc

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GSM TCH Congestion Rate Optimization Delivery Guide For internal use

Reference code Product name GSM RNP

Target readers Product version V1.0

Edited by Document version

GSM TCH Congestion Rate Optimization

Delivery Guide

(For internal use only)

Prepared by Wang Weiji, Yang Haiquan Date: 2006-12-20

Reviewed by Network Planning GSM TechnicalSupport Team

Date: 2006-12-29

Reviewed by Date:

Approved by Date:

Huawei Technologies Co., Ltd.

All rights reserved

2013-10-28 Huawei Technologies Proprietary Page 1 of 21


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Revision Record

Date Revised version Description Author

2006-12-20 V1.0 Completed the initial draft Wang Weiji, Yang

Haiquan

2007-01-15 V1.1 Modified the level of indexes of

Table 1 and Table 2

Wang Weiji



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Contents

1 Overview..................................................................................................................................... 5

2 Definition of TCH Congestion Rate ......................................................................................... 5

3 Application Strategy ................................................................................................................. 6

4 Optimization Strategy of TCH Congestion Rate ..................................................................... 8

5 Appendix: TCH Channel Rate Signaling Process ................................................................19



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GSM TCH Congestion Rate OptimizationDelivery Guide

Keywords: GSM, TCH congestion rate, application strategy, and optimization strategy

Abstract: This document defines the current key TCH congestion rates, and provides the

references for KPI index clarifications and index commitment related to the

project in China. In addition, this guide provides the optimization methods

briefly for the TCH congestion caused by various reasons.

Reference document

SN Document name Author Publishing house Date

1 GSM Radio Network

Planning and Optimization

Huawei People's Posts and

Telecommunications

Publishing House

June 2004

2 GSM Principles and

Network Optimization

Han Binjie Mechanical Industry

Publishing House

August 2001

3 GSM Wireless Network

Optimization Procedures

Si Fazhong 2001-03-09

5 Traffic Statistic Analysis

Guide

Bai Xiaobin 2002-04-20

6 M900/M1800 Base

Station Controller Traffic

Statistic Manual (v6.06)

Huawei

7 GSM Wireless Network

KPI Baseline (v1.38)

Huawei



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1 Overview

The TCH congestion rate is an important index for measuring the network operation

quality in the GSM network optimization. It indicates the access performance of a

mobile network. The TCH congestion rate influences on the network indexes such as

wireless put-through rate, handover success rate, and worst cell rate.

This document defines the various TCH congestion rates and introduces the

optimization strategy. Competitors and customers have different understandings on

the KPI indexes; therefore, TCH congestion rate varies with competitors and

customers.

Abstract: This document defines the current key TCH congestion rates, and provides

the references for KPI index clarifications and index commitment related to the project

in China. In addition, this guide provides the optimization methods briefly for the TCH

congestion caused by various reasons, and is helpful for the on-site handling.

The description of this document is based on GSM BSC V300R002C13.

2 Definition of TCH Congestion Rate

At present, the definitions of the TCH congestion rate are as follows:

1) TCH congestion rate (TCH overflow) (%) = [2124 Attempted TCH seizures

meeting TCH overflow/2089 Attempted TCH seizures (all)]*100%

This index indicates the percentage of the TCH allocations in the attempted TCH

seizures when there is no idle TCH for allocation in the application.

2) TCH congestion rate (including handover)(%) = [2126 TCH seizure failures for

call + 2127 TCH seizure failures for very early assignment + 2131 TCH seizure

failures for intraBSC incoming cell handover(no radio resource) +2132 TCH

seizure failures for interBSC incoming cell handover(no radio resource)]/[2090

Attempted TCH seizures for call + 2091 Attempted TCH seizures for very early

assignment + 2096 Attempted TCH seizures for intraBSC incoming cell

handover + 2097 Attempted TCH seizures for interBSC incoming cell handover].

3) TCH congestion rate (excluding handover) (%) = [2126 TCH seizure failures for

call + 2127 TCH seizure failures for very early assignment]/[2090 Attempted

TCH seizures for call + 2091 Attempted TCH seizures for very early assignment]

This index indicates the percentage of TCH seizure failures for call excluding the



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handover procedure in attempted TCH seizures for call (excluding handover).

4) Time congestion rate (%) = [2169 TCHF busy time (second) 2283 Cell

unavailable duration]/3600. This index indicates the time ratio of TCHF busy time

in the statistic duration.

5) Times congestion rate (including handover) = [[2120 TCH seizure failures for

call(no radio resource) + 2131 TCH seizure failures for intraBSC incoming cell

handover(no radio resource) + 2132 TCH seizure failures for interBSC incoming

cell handover(no radio resource) +2161 TCH seizure failures for intracell

handover (no radio resource)] /[2090 Attempted TCH seizures for call + 2091

Attempted TCH seizures for very early assignment + 2096 Attempted TCH

seizures for intraBSC incoming cell handover + 2097 Attempted TCH seizures

for interBSC incoming cell handover + 2101 Attempted TCH seizures duringintracell handover]]

This index indicates the voice channel attempted call congestion rate (including

handover).

6) Times congestion rate (excluding handover) = 2120 TCH seizure failures for call

(no radio resource)/2090 Attempted TCH seizures for call

This index indicates the voice channel attempted call congestion rate (excluding

handover).

3 Application Strategy

3.1 Distinguishing according to Index Statistic Object

It can be divided into BSC level, Cluster level, and cell level, that is, the TCH

congestion rate of the whole BSC, the TCH congestion rate of the Cluster consisting

of a series of cells, and the TCH congestion rate of each cell. In the acceptance of

TCH congestion rate index, the object requirements of the index statistic vary with

operators. In general, it is BSC level or Cluster level. The selection of BSC level or

Cluster level is subject to the specific networks and operators practical requirements.

3.2 Distinguishing according to Congestion Rate Definition

Signaling processes related to TCH congestion rate definitions are different. The

calculation value is also different. Numerator statistic reasons are different also, for

example, Attempted TCH seizures meeting TCH overflow, and seizure failure.

Attempted TCH seizures meeting TCH overflow: It is the channel request failure

caused by the real channel allocation failure. TCH seizure failure: It is the assignment



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failure caused by various causes after the assignment commands are issued.

The processes and causes of numerator TCH seizure failures are listed in the table

below:

Definition of

congestion rate

Related process Index meaning

TCH congestion rate

(TCH overflow) (%)

Call, early assignment, intra-BSC

incoming inter cell handover, inter-

BSC incoming cell handover

Indicate the idle degree of

the channel resources

TCH congestion rate

(including handover)

Call, early assignment, intra-BSC

incoming inter cell handover, inter-

BSC incoming cell handover

In the numerator, some

items indicate the idle

degree of the TCH channel

resource, and some items

indicate the TCH seizure

failure.

TCH congestion rate

(excluding handover)

Call, early assignment Indicate the TCH seizure

failure rate

Times congestion

rate (excluding

handover)

Call Indicate the TCH seizure

failure rate owing to

unavailable channel

resources. It indicates the

idle degree of channel

resources.

Times congestion rate

(including handover)

Call, intra-BSC incoming inter-cell

handover, inter-BSC incoming inter-

cell handover, and intra-BSC intra-

cell handover

Indicate the TCH seizure

failure rate owing to

unavailable channel

resources. It indicates the

idle degree of channel

resources.

In the definition of the acceptance index, TCH congestion rate (TCH overflow) (%)

and Times congestion rate (%) are used most frequently. The numerator of the time

congestion rate indicates the seizure failures owing to the unavailable TCH channel

resources. The index calculation results are better than TCH congestion rate (TCH

overflow) (%). It is the formula selected with preference in the relocation project.

Other competitors use the times congestion rate frequently in the acceptance.

The process in the times congestion rate (including handover) differs from that in the



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times congestion rate (excluding handover). The times congestion rate (excluding

handover) includes the call process only.

3.3 Time Congestion Rate

The meaning of statistic point TCHF busy time (second) of the competitor Ericsson

differs from that of Huawei.

Huawei TCHF busy time (second) indicates the no TCH allocation caused by

unavailability of TCH channel resources or BTS unavailability of a cell in the statistic

period.

Ericsson TCHF busy time (second) does not include the time of TCH channel

unavailability caused by the BTS unavailability, and only include the time of theunavailable TCH channel resources.

Therefore, Huawei TCH time congestion rate is the difference of two parts, which

subtracts the time of the unavailable TCH channel (Cell unavailable duration

(second)). Ericsson time congestion rate includes TCHF busy time (second) only.

3.4 TCH Times Congestion Rate (%)

In the numerator, TCH seizure failure in each process is caused by the unavailable

resources.

4 Optimization Strategy of TCH Congestion Rate

4.1 Optimization Analysis

The TCH congestion includes the congestion owing to TCH seizure failure caused by

Attempted TCH seizures meeting TCH overflow and other causes. Figure 1 shows the

analysis process.



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Figure 1 Analysis process of high TCH congestion rate

Check whether the TCH congestion occurs in all cells or certain cell of the BSC. If it

occurs in all cells, perform the troubleshooting from the aspects such as capacity,

transmission, and hardware. If it occurs in a certain cell, troubleshoot the specific cell.

For the TCH congestion occurred in a specific cell, locate the causes from the

aspects such as traffic load, equipment fault or transmission problem, interference,

coverage, and data configuration.



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The traffic statistic items needed for locating the TCH congestion rate problem are

listed as follows:

Table 1 TCCH congestion rate traffic statistic

Cause

classification

BSC-level traffic statistic Cell-level traffic statistic Carrier-level

traffic statistic

Attempted

TCH seizures

meeting TCH

overflow

BSC measurement

function -> Access

measurement function ->

Percentage of TRX in

good condition (%)

TCH traffic volume

(excluding very early

assignment) (ERL) ->

TCH congestion rate

(TCH overflow) (%)

Unsuccessful

assignments (no radio

resource available)

Attempted TCH seizures

(all)

TCH measurement function ->

Attempted TCH seizures meeting TCH overflow

Percentage of TRX in Good Condition (%)

TCH usability (%)

TCH traffic volume (excluding very early

assignment) (ERL)

Attempted TCH seizures (all)

None

Seizure failure BSC measurement

function -> Access


Attempted TCH seizures

(all)

TCH congestion rate

(including handover)(%)

TCH Seizure failures (all)

Attempted TCH seizures (all)

N/A.

Very early

assignment


TCH seizure failures for very

early assignment

Attempted TCH seizures for

very early assignment



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Cause

classification

BSC-level traffic statistic Cell-level traffic statistic Carrier-level

traffic statistic

TCH congestion rate

(excluding handover)(%)

Unsuccessful

assignments (radio

interface failure,

reversion to previous

channel)

Unsuccessful

assignments (equipment

failure)

Unsuccessful

assignments (requested

terrestrial resource

unavailable)

Unsuccessful

assignments (terrestrial

circuit already allocated)

Unsuccessful channel

activation duringassignment (NACK)

Channel activation

timeout during

assignment (TIMEOUT)

Assignment TCH measurement function ->

TCH Queue failures for queue

full

TCH queue failures for timeout

Directed retry after queue

failure

TCH seizure failures

(equipment failure)

TCH seizure failures (requested

terrestrial resource unavailable)

TCH seizure failures (terrestrial

circuit already allocated)

TCH seizure failures (invalid

message contents)

Incoming

BSC

handover or

intra-BSC

handover


TCH Queue failures for queue

full

TCH queue failures for timeout

Attempted TCH seizures during

intraBSC incoming cell

handover

Attempted TCH seizures for

interBSC incoming cell

handover



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Table 2 TCH congestion rate cause

Causeclassification

BSC level Cell level Carrier level

Traffic load BSC measurement

function -> Access


TCH congestion rate

(including handover)(%)

TCH congestion rate

(excluding handover)(%)

TCH congestion rate (TCHoverflow) (%)

Percentage of TRX in

good condition (%)

TCH traffic volume


assignment) (ERL) ->

Unsuccessful assignments

(no radio resource

available)

Percentage of TRX in good

condition (%)

TCH usability

TCH traffic volume


assignment) (ERL)

Full rate traffic volume

(excluding very earlyassignment) (ERL)

Half rate traffic volume


assignment) (ERL)

1800/1900 cell TCH traffic

volume (excluding very early

assignment) (ERL)

Radio

environment

and

interference

BSC measurement

function -> Access

measurement function

->Unsuccessful

assignments (radio

interface failure, reversion

to previous channel)

TCH call drop rate (%)

Cell measurement function ->

Inter-cell handover


Attempted handovers for

uplink quality


downlink quality


uplink strength


downlink strength

Cell frequency scan

Channel allocation

measurement function -> TCH

allocation measurement function

Average num. of idle TCHs in

Interf. band 1


Interf. band 2


Interf. band 3


Interf. band 4


Interf. band 5

Receive quality measurement

function

Receive level measurement

function



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Cause

classification


Hardware fault

and

transmission

problem

BSC measurement

function -> Access



(equipment failure)


(requested terrestrial

resource unavailable)


(terrestrial circuit already

allocated)

Unsuccessful channel

activation during

assignment (NACK)

Channel activation timeout

during assignment

(TIMEOUT)

Cell measurement function ->

TCH measurement function

Cell measurement function 2-

> TCHF/H measurement

function (2)

TCH seizure failures

(requested terrestrial

resource unavailable)

TCHH channel activationfailures during very early

assignment (NACK)

TCHH channel activation

failures during assignment

(TIMEOUT)

TCHF channel activation


(NACK)

TCHF channel activation


(TIMEOUT)



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Cause

classification


Coverage Cell measurement function ->

Inter-cell handover

measurement function->


uplink quality


downlink quality


uplink strength


downlink strength


timing advance


better cell

TCH measurement function

->

TCH lost radio connections

(connection failure)TCH lost radio connections

(error indication)

Power control measurement

function ->

Up-Down Link Balance

Measurement Function

Call Allocation by TA

Measurement Function

4.2 Congestion Caused by Traffic Load

If the network capacity is deficient or the radio capacity in each position of thenetwork differs from the practical traffic allocation, the TCH congestion occurs in the

cell where there are a lot of subscribers. Through the traffic statistic cell TCH

measurement task, check whether the TCH congestion rate is caused by the TCH

overflow. If the congestion rate is caused really by the too large traffic volume,

forecast its real traffic volume, and check whether the traffic can be shared by other

cells. If it exceeds the optimization adjustment capabilities, the operator should be

suggested to expand the capacity. The traffic balance measures may not conform to

the minimum radio path loss principle. The methods include the adjustment of

coverage range, adjustment of access threshold, CRO, handover threshold, and



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enabling the load handover.

4.3 Congestion Caused by Hardware Problem

The TCH channel seizure failure may be caused by hardware (carrier) or

transmission problem. As a result, the congestion may occur. The specific analysis is

as follows:

1) Check alarm information such as transmission alarm, board communication

alarm, CDU Standing Wave Ratio alarm, and clock alarm to make sure whether

the equipment is faulty.

2) Query the indexes such as Percentage of TRX in good condition (%), TCH

usability, and [TCH channel activation (NACK)]/[TCH channel activation

(TIMEOUT)] of the target cell to check whether the equipment is faulty.

3) Query the indexes such as TCH seizure failures due to A-interface failures and

TCH seizure failures due to terrestrial link failures to analyze whether the

terrestrial link is broken.

4) If the uplink channel is damaged or performance decreases, the MS resides in

the cell and cannot be accessed. As a result, the congestion occurs owing to a

lot of seizure failures. Through Incoming Inter Cell Handover Measurement

Function, you can find that handovers towards this cell fail. Through Receiving

Quality Measurement or Receiving Level Measurement Function, query each

TRX status of the cell. Query whether the uplink/downlink measurement report ofthe same TRX is abnormal to determine which carrier is associated with.

The carrier is faulty but without the fault alarm on the alarm console. This problem

can be solved by using the signaling analyzer to trace the messages of the Abis in the

cell with high TCH congestion rate. Through the analysis of the signaling, locate the

fault at the carrier.

After the fault is located at the carrier, the fault can be cleared through replacing the

carrier or temporarily disabling the carrier.

4.4 Congestion Caused by CoverageThe coverage problems such as wrong installation of BTS antenna feeder and

incorrect configuration may result in the TCH seizure failure.

1) TCH seizure failure caused by inappropriate installation of antenna feeder

The cell antenna is reversely connection, that is, the cell transmit antenna and receive

antenna are wrongly connected. As a result, the signals of the uplink and downlink

channel are severely unbalanced.

Solution: Analyze the uplink/downlink signal level and quality of the target cell by

using the DT or CQT test or using the signaling analyzer. Focus on the unbalance



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symptom.

2) TCH seizure failure caused by antenna feeder fault

The standing wave ratio of the antenna feeder may increase owing to damage,

water, and loose of the connector. The practical transmit power and receive sensitivity

decrease. As a result, the TCH channel seizure fails and the TCH congestion occurs.

This kind of problem can be located through carefully checking the antenna feeders

such as tower amplifier, power amplifier, combiner, and feeder, and check whether the

standing wave ratio alarm exists. For the cell with problem, perform the DT and CQT

test. The simple method is: Test the BTS practical transmit signals by using the test

MS at the foot of the BTS. If the signal strength is -30 to -50dBm, it is normal.

Otherwise, it is problematic.

3) TCH seizure failure caused by CDU/SCU configuration

For the BTS configuration reason, the difference of the combiner loss between the

channels passed by the BCCH and the channels passed by non-BCCH is large;

therefore, the transmit power of the channel where the non-BCCH is located is

smaller than that of the channel where the BCCH is located. If a MS originates a call

(especially, it is far away from the BTS), when the system assigns the MS with the

TCH channel of the TRX where the non-BCCH is located, the TCH channel seizure

fails owing to the low transmit power.

Solutions:

Configure the TRX of the BCCH on the channel with high combiner loss.

Optimize the configuration, to avoid the big difference of the combiner loss

between the channel where the BCCH passes and the channel where the non-

BCCH passes.

4) Congestion caused by too large coverage

Query the average level of the power control, the average level in case of the call

drop, and TA to analyze the TA value and the relation with the receive level. Judge the

coverage range by using the driving test. Query the TCH usability of the neighbor cell

to check whether the fault of the neighbor cell results in the congestion. Query the

uplink and downlink balance performance to check whether the downlink value is

greater than the uplink value, as a result, the MS cannot normally occupy the TCH.

4.5 Congestion Caused by Interference

The TCH congestion rate may be related to the interference. The call drops of the cell

occur frequently, and the SDCCH congestion rate is high. In the random access

measurement function, the RACH congestion occurs, and the immediate assignment

success rate decreases.



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1. Inter-network interference

The transmit power of illegal frequency from other networks results in theinterferences on the network. When the system assigns the TCH channel for a call,

the channel is affected by the interference. As a result, the assignment fails, and the

TCH congestion occurs.

Query Average num. of idle TCHs in Interf. band 4 and 5 to check whether it is the

uplink interference. It is used as a reference only.

To clear the interference, use the spectrum analyzer and high gain directional

antenna to search the position of the interference source, and then shut off the

interference source.

2. Intra-network interference

If the frequency plan is inappropriate, the same-/adjacent-frequency carrier-to-

Interference ratio cannot be met in some positions in the network. The

same-/adjacent-frequency interference occurs. Meanwhile, the TCCH congestion

occurs owing to the TCH seizure failure.

The uplink and downlink of the cell frequency in the network appears in pairs. The

interference band of the traffic statistic can be used as the fundament. Query the

interference band in the traffic statistic, and locate the cells with high interference

band 4 and interference band 5 values. Then, check the network frequency plan.

Locate the cell with problem, and adjust the frequency plan to solve the interference.

For the intra-network interference, the driving test should be performed in a lot of

places, to find the downlink interference and cross-cell coverage interference

according to the practical quality allocation, so as to adjust the antenna or frequency

plan specifically.

For specific troubleshooting, see G- Interference Problem Handling Guide-20050311-

A-1.0

1) Hardware fault interference: Features of the interference caused by hardware

and equipment problems: Strong interference signals and lasting for a long time.

For the interference caused by the adjustment, it may be related to the traffic

volume obviously. When the traffic volume is small, you can send the idle

BURST for verification.

2) Intra-network interference: It is the intra-frequency interference or inter-frequency

interference caused by inappropriate frequency planning, frequent frequency

multiplexing, and cross-cell coverage. In general, the intra-network interference

increases with the increase of the traffic volume.

3) For specific troubleshooting, see G- Interference Problem Handling Guide-

20050311-A-1.0



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4.6 Congestion Caused by Frequent Handover

The frequent handovers may result in the TCH channel congestion. Query thehandover times and successful call seizures, and check whether the ratio is

appropriate. Check whether the ratio of the handover-in and handover-out, and check

whether the cell congestion is caused by the inappropriate handovers.

4.7 Congestion Caused by Data Configuration

When the cell is activated with the half rate and overlaid/underlaid functions, the

congestion may be caused by the inappropriate setting of parameters. When the

GPRS service is activated, the TCH congestion occurs owing to the inappropriate

allocation of speech channel and traffic channel. In addition, the TCH channel

congestion may be caused by the wrong setting of circuit pool number.



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5 Appendix: TCH Channel Rate Signaling Process

5.1 Immediate Assignment and Assignment Channel SeizureProcess

Channel Activation Acknowledge

MS BTS BSC

Channel Request

Channel Activation

Immediate assignment Command

SABM

Channel Requiredt

UA EST IND

MSC

CR CMP Lay3 information

CCCM Service Accept

Setp

Call Proceeding

Assinnment Request

Channel Activation


SABM

UAEST IND

Assignment Complete


MS BTS BSC

Channel Request

Channel Activation

Immediate assignment Command

SABM

Channel Requiredt

UA EST IND

MSC

CR CMP Lay3 information

CCCM Service Accept

Setp

Call Proceeding

Assinnment Request

Channel Activation


SABM

UAEST IND

Assignment Complete



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5.2 BSC Intra-handover Channel Seizure Process

MS BTS2 BSC BTS1 MSC

Measurement Report from MS(1)

MS

Channel ACT(2)

Channel ACT ACK(3)

Handover CMD(4)

Handover Access(5)

Handover Detect(6)

PHY INFO(7)

UA(10)

SABM(8)

EST IND(9)

Handover Complete(11) Handover Performed(12)

MS BTS2 BSC BTS1 MSC

Measurement Report from MS(1)

MS

Channel ACT(2)

Channel ACT ACK(3)

Handover CMD(4)

Handover Access(5)

Handover Detect(6)

PHY INFO(7)

UA(10)

SABM(8)

EST IND(9)

Handover Complete(11) Handover Performed(12)



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5.3 Incoming BSC Handover Channel Seizure Process

MS Other BTS Other BSC HUWEI BSC

Measurement Report

from MS(1)

HUAWEI BTS

Channel ACT ACK(5)

Handover RQD(2)Handover REQ(3)

MSC

Channel ACT(4)

Handover REQ

ACK (6)Handover CMD(7)

HANDOVER COMMAND(8)Handover Access(9)

Handover Detect(10)Handover Detect(11)

PHY INFO(12)

PHY INFO(13)

SABM(14)Establish IND(15)

UA(16)

HANDOVER CMP(17)Handover CMP(18)Clear CMD(19)

MS

Clear CMP(20)

MS Other BTS Other BSC HUWEI BSC

Measurement Report

from MS(1)

HUAWEI BTS

Channel ACT ACK(5)

Handover RQD(2)Handover REQ(3)

MSC

Channel ACT(4)

Handover REQ

ACK (6)Handover CMD(7)

HANDOVER COMMAND(8)Handover Access(9)

Handover Detect(10)Handover Detect(11)

PHY INFO(12)

PHY INFO(13)

SABM(14)Establish IND(15)

UA(16)

HANDOVER CMP(17)Handover CMP(18)Clear CMD(19)

MS

Clear CMP(20)


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