GPRS KPIs based on network performance

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TTG Uluslararsı LTD / Registered Office: as above / Registered Number in Turkey 461414-408996

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White Paper

Performance Management and Optimization Method for

Mature GPRS Networks

Prepared by

Mehmet BEYAZ

TTG International, L.T.D.

www.ttgint.com

3/10/2015

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Contents

GPRS PERFORMANCE MANAGEMENT .................................................................................................... 5

GPRS KPIs ................................................................................................................................................ 6

GPRS BSS KPIs.......................................................................................................................................... 8

BSS Accessibility .................................................................................................................................. 8

BSS Retainability ................................................................................................................................. 9

BSS Integrity ...................................................................................................................................... 10

GPRS CN KPIs ......................................................................................................................................... 11

CN Accessibility ..................................................................................................................................... 11

CN Quality and Mobility .................................................................................................................... 12

GPRS NETWORK OPTIMIZATION ........................................................................................................... 13

GPRS RAN/BSS ....................................................................................................................................... 13

CONCLUSIONS ....................................................................................................................................... 14

ABBREVIATIONS, ACRONYMS, AND TERMS .......................................................................................... 16

REFERENCES .......................................................................................................................................... 19

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Abstract

This paper discusses performance management and optimization method for mature GPRS networks. A

selection of GPRS KPIs based on network performance counters, measurement systems, as well as possible

threshold values and their analysis and interpretation.

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INTRODUCTION

Today, many wireless operators are focused on providing mobile internet access. A large

variety of technologies has contributed to the operators’ vision of true mobility and seamless

roaming. The currently prevailing technology providing true mobile Internet access on a

European scale is general packet radio service (GPRS).

The popularity of Short Messaging (SMS), MMS and the growing demand for mobile data

services prompted the implementation of a Packet Switched (PS) overlay on Circuit Switch

(CS) network, opening the public Land Mobile Network (PLMN) operators to the data

networking market and internet-based services. However, voice services stil remains the core

business for many operators. This trend and some other factors have bringup the factor of

growing requirement to optimize GPRS.

This paper covers the performance management and optimization of GPRS networks. A

performance management overview is presented, including proposed Key Performance

Indicators (KPIs) for the RAN as well as the Core Network (CN).

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GPRS PERFORMANCE MANAGEMENT

Performance management involves collecting and analyzing performance data from different

resources and constitute set of KPIs. Network operators typically define a set of KPIs and

stratified target thresholds in line with planning, marketing and businesspriorities.

Performance management teams which are responsible, for example, for certain districts of

major cities, towns of a certain size, or large rural areas, as well as routing areas or IP

backbones periodically monitor various KPIs to isolate areas not achieving target

performance. Optimization method/s are then kick off to ensure that performance targets are

met. KPIs are usullay forumed by using performance counters provided by network elements.

Statistics based on network element counters can be verified and supplemented by additional

measurements obtained by active testing with test Mobile Station (MS), protocol analyzers at

different interfaces in the network. Such additional performance measurement systems can

emphasize aspects of GPRS performance not evident from network counters alone.

The figure 1 shows the GPRS network architecture as well possible measurement points for

network counters and protocol analyzers. Standard network surveillance or alarm monitoring,

as well as long-term trending and analysis of KPIs, can assist in identifying problems and

helping to achive desired performance.

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Figure 1. GPRS Network and interfaces

GPRS KPIs

Selecting meaningful KPIs is a pioneer to continuous performance monitoring. GPRS KPIs

should cover the Base Station System (BSS) and the CN to account for end-to-end

performance, i.e., from the MS to the Gi interface on the gateway GPRS support node

(GGSN). Typically, KPIs for the BSS and the RAN reflect accessibility, retainability, and

integrity. CN KPIs focus on Routing Area Update (RAU) behavior, congestion metrics on

several interfaces, and system accessibility. Table 1 provides an overview of GPRS KPIs.

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Network Element Attribute KPI

BSC/PCU Accessibility PRACH

BSC/PCU Accessibility PAGCH (immediate assignment

rejection rate)

BSC/PCU Accessibility Congestion time

BSC/PCU Accessibility PDCH availability

BSC/PCU Accessibility DL/UL data throuhput

BSC/PCU Accessibility PDTCH Allocation Failure Rate

BSC/PCU Accessibility TBF Set-up Failure rate

BSC/PCU Quality TBFs per PDTCH

BSC/PCU Integrity RLC BLER

BSC/PCU Quality RLC Re-transmission

BSC/PCU Quality CELL Re-selection Time

BSC/PCU/SGSN Accessibility GBL data throuhput

BSC/PCU Retainability % of TBF Preemtion

SGSN Accessibility GPRS Attached Success Rate

SGSN Accessibility GPRS Attached Time

SGSN/GGSN Quality GPRS RAU Success Rate

SGSN/GGSN Quality GPRS RAU Success Time

SGSN Accessibility PDP Context Activation on Success

Rate

SGSN Accessibility PDP Context Activation Time

SGSN/GGSN Quality Congestion on Gi, Gb and Gn

End-to-End Quality Delay

End-to-End Quality Application Throughput

Table 1. KPIs and Thresholds for GPRS System Performance

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GPRS BSS KPIs

BSS Accessibility

The RAN has an important role in overall system performance, since it typically represents

the bottleneck in terms of resources and available physical transmission rates on the air

interface as well as phishical links and PDCH. Accessibility from a RAN/BSS perspective is

mainly a function of radio resources, signaling capacity, links (Gb) and BSS parameter

configuration. Packet channel requests on the Packet Random Access Channel (PRACH),

which establish the first contact of an MS with the network. Unsuccessful access requests due

to congestions can indicate poor radio conditions.

The total number of PRACH requests may include instances where the MS sends multiple

consecutive access bursts. Immediate assignment the BSS’ response to the packet channel

request from the MS is indication for identifying congestion; the immediate assignment

rejection rate is based on the number of immediate assignment rejections over the total

number of PRACH requests. Such rejections are due mainly to the lack of Packet Data

Channel (PDCH) resources in the cell. Although extreme delays on the Um interface or lack

of resources in the Base Station Controller (BSC), Packet Control Unit (PCU), or other nodes

may cause rejections.

Cell congestion levels can be calculated as no PDCH available for a certaion time period

within the GPRS busy hour, which may be expressed as a percentage of 60 minutes. GPRS

congestion levels should be think of Traffic Channel (TCH) demand point of view for CS. On

the other hand, CS busy hour traffic can affect PS available resources for connections. This is

clearly depends on the BSS configuration in terms of preemption and the allocation of fixed

PDCHs.

The number of packet access requests and the transmitted data volume during CS busy hour

can be used to determine whether the CS resource requirements are pushing the PS traffic

into congestion or whether the GPRS traffic demand is high. On the other hand, it should be

noted that GPRS busy hour are not necessarily matching, but are offset to those of the CS. It

may, therefore, be useful to consider PS traffic expressed as bytes at the busy hour statistics

when evaluating GPRS congestion. The PDCH allocation failure rate sometimes referred as

GPRS blocking and the Temporary Block Flow (TBF) setup failure rate are additional KPIs

that can indicate congestion. A certain amount of GPRS blocking may be acceptable

considering that CS orianted cell dimensioning strategies may allow for approximately %2

blocking rate or quality of service (QoS).

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BSS Retainability

RAN CS congestion, high volumes of data traffic, or a combination of both can give rise to a

series of other symptoms that may affect not only the system accessibility but also the

retainability and quality for ongoing TBFs connections. TBFs are subscriber-specific,

bothway, logical connections associated with a varying number of PDCHs, established to

facilitate data transfer between the MS and the BSC/PCU. In other words, the data service

may be accessed successfully, but the connection quality can range from unacceptably bad to

very good. The PCU reserves the requested number of PDCHs for new connections during

the packet allocation procedure. PDCHs available in the Packet Switch Domain (PSD) at any

given moment are used for the assignment, although more than one TBF may be assigned to a

PDCH. The GPRS multiplexing capability can support multiple TBFs per PDCH and allocate

new TBFs on already used PDCHs.

Note that: The number of PDCHs requested by an MS is determined mainly by its multislot

class. On-demand PDCHs make temporary use of idle CS TCHs, which are swithed back to

the CSD when they have not been used for a defined period of time.

New GPRS users can be blocked if the PSD cannot be expanded to accommodate new TBFs

when the TBF-per-PDCH reached the limit, where resulting in PDCH allocation failures

(packet access reject) and TBF set-up failures.

Note that: The PDCH allocation failures may occur after successful immediate assignment.

These failure rates can be biased by TBF upgrade failures. System vendors may provide a

feature that allows the TBF to request additional PDCHs if it has been set up initially with

fewer PDCHs than the multislot class of the MS allows.

While TBF multiplexing and the allocation of fewer resources than the MS can support work

together to provide flexible system accessibility, both mechanisms can have an undesirable

effect on throughput performance and Bandwidth Delay Product (BDP) per user. In addition,

the absolute or average number of allocated PDCHs per TBF in a given cell provides another

standpoint on GPRS QoS. Multiplexing is point out when this KPI increases beyond 1.

Because the Logical Link Control (LLC) throughput performance per TBF is affected by the

multislot class of an MS, the distribution of different multislot MSs in the network should be

identified to set reachable throughput targets.

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BSS Integrity

The changeable radio propagation and RAN interference conditions are the main factors

affecting the BSS. The Carrier-to-Interference (C/I) ratio has a critical impact on the

maximum physical transmission rate of a PDCH and TCH. The GPRS coding schemes,

whose ranges of optimum performance are related to the C/I ratio, help to

lower the impact of radio interference on the radio channel’s BLock Error Eate (BLER).

Remaining uncorrectable bits and corrupted radio blocks set off the Radio

Link Control (RLC) Automatic Repeat Request (ARQ) mechanism if the TBF is operated in

RLC-acknowledged mode. The probability of RLC retransmissions hence reduces the

physical transmission rate of the channel, adds additional delays to the air interface, and

affects the end-to-end RoundTrip Time (RTT) of higher layer protocols. The Bit Error Rate

(BER) of a radio channel (also categorized into receive/transmit quality (Rx/TxQual) levels

ranging from 0 to 7 and the percentage of RLC retransmissions should be tightly monitored.

Desirable RLC retransmission rates are generally below %1. Moreover, TCP window sizes,

TCP retransmission time-outs, typical IP packet size distributions, and the application

protocol affect the effective throughput on the application layer.

BSS Quality and Mobility Cell reselection, as well as possible Location and Routing Area

Updates (LAUs and RAUs), has a weakening effect on throughput performance. During cell

reselection, the MS releases all channels to read the system information of the destiontion

cell. The radio outage time begins with the channel release and ends with the completion of

the cell reselection process and the establishment of downlink and uplink TBFs on PDCHs of

the destination cell. Cell reselection may also require LAUs and RAUs if the destination cell

is part of another location and routing area. This introduces extra signaling in the network as

and increases the time until new TBFs are established on the destination cell.

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GPRS CN KPIs

In the GPRS optimization CN GPRS plays a fundamental role in assessing overall system

performance. Along with the key elements of CN performance assessments are success rates

for GPRS are;

1. Attached

2. Packet Data Protocol (PDP) context activation

3. RAU

4. Paging

CN Accessibility

The GPRS attach request message is the first contact of the MS with the serving GPRS

Support Node (SGSN) then a TBF has been established successfully. It contains the

Temporary Mobile Subscriber Identity (TMSI), Mobile Network Code (MNC), and Mobile

Country Code (MCC), as well as Location and Routing Area Identity. Following identity

checks of the International Mobile Equipment Identity (IMEI) or the International Mobile

Subscriber Identity (IMSI), as well as authentication and ciphering request messages, may be

issued by the SGSN. In order to retrieve the IMSI if no entry is found for it, the SGSN uses

the old location area information to

identify the old SGSN where this terminal was last served. The SGSN may require Home

Location Register (HLR) signaling over the Gr interface to identify profiles for unknown

IMSIs. Unsuccessful authentication and ciphering may also contribute to the attach rejects.

SGSN counter statistics for the Gb interface can be used to establish the probability of attach

failures and the distribution of failure reasons. Some of the common failure reasons are:

1. Attach requests from users who are not subscribed to the GPRS network

2. International roamers from networks without roaming agreements

3. Software and hardware levels on specific MSs

4. Attach time-outs (excessive delays, failed responses from the MS)

5. Network failures.

Gb interface congestion and dimensioning should be taken in accout when assessing the

GPRS attach success rate. The Gb interface interconnecting the BSS with the SGSN is a

Frame Relay (FR) connection over an E1/T1 link. Ofcours this is depending on the CN

design strategy, this FR connection may be transported over

intermediate networks, such as Asynchronous Transfer Mode (ATM) backbones supporting

FRF5 (FR over ATM). FR congestion control and discard mechanisms, can be monitored to

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identify congestion levels on the FR data Link Connection Identifier (DLCI) carrying the

BSS GPRS protocol (BSSGP) protocol Data Units (PDUs). Measuring the PDP context

activation Success Rate (SR) is also important.

GGSN provides the allocated IP address to the SGSN via the create PDP context response,

which, in turn, sends the activate PDP context accept message to the MS. Hance, a successful

PDP context activation establishes IP connectivity between the MS and the GGSN, which

then forwards or routes the traffic to external networks. Common reasons of PDP context

activation failure include network link failures, incorrect DNS records, and inappropriate

configuration of MSs. Counters on the SGSN and GGSN provide the basis for establishing

the PDP context activation failure rates and the distribution of the causes. The IP backbone

can be based on different network architectures, QoS-aware, high-capacity backbones. When

a PDP context is in active state, GPRS tunneling protocol (GTP) encapsulation is used to

create Virtual Circuit (VC) connections between GPRS support nodes (GSNs) supporting the

data flow across the backbone. The GTP VC connection is released as soon as the context

becomes inactive.

CN Quality and Mobility

The RAU Success Rate (RAUSR) is an another important key metric for GPRS system

performance. All idea about this KPI contributes to the understanding of end user throughput

performance. A GPRS-attached MS can issue periodic, intra-SGSN and inter-SGSN RAUs,

depending on its situation. Periodic RAUs are used to ensure that the MS is stil reachable and

are so treated as a intra-SGSN updates. The non-periodic update requests can be triggered

when the MS transcends a routing area

boundary during cell reselection. The intra-SGSN RAU procedure is used when the new

routing area of the target cell is administered by the same SGSN. RAU rejects can reasons of

authentication and ciphering failures, RAU time-outs due to excessive delays, lower layer

link failures, and/or protocol failures. Performance metrics can be used for GPRS attach,

context activation, authentication time, DNS response time, TCP connection establishment

time, and additional application-specific indicators.

Throughput RTT DELAY RLC Re-

transmission

TBF Multiplexing TBF / PDTCH PS Immediate

Assignment

Reject

PDTCH

Allocation

Failures

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B

S

S

Mobility Mobility

CS Congestion PS Congestion

Poor Coverage Poor Coverage Poor Coverage PCU Capacity PCU Capacity

N-Cell Definitions N-Cell

Definitions

N-Cell Definitions CS Congestion CS Congestion CS Congestion CS Congestion

Interface Interface Interface PS Congestion PS Congestion PS Congestion PS Congestion

Fault Fault Fault Fault Fault Fault Fault

C

N

Transmission Transmission GSN Configuration GSN Configuration GSN

Configuration

GSN

Configuration

IP Backbone

Gb Congestion Gb Congestion Gb Congestion Gb Congestion Gb Congestion Gb Congestion RADIUS

Gn Congestion Gn Congestion Gn Congestion Gn Congestion Gn Congestion Gn Congestion

Packet Loss Packet Loss Gr Congestion Gr Congestion Packet Loss Packet Loss

Packet Loss Packet Loss MS

DHCP DHCP

RADIUS RADIUS

DNS DNS

PDP Context

Activation Time

PDP Context

Activation Success

RAU Time RAU Success Authentication

Figure 3. KPI Effect Matrix

GPRS NETWORK OPTIMIZATION

GPRS network optimization plans can vary from operator to operator and may be tied to

specific circumstances, although certain practices and approaches have a more universal

nature. On the one hand, optimization should be understood as an ongoing activity in the

perspective of network rollout and expansion, and one that is concerned mainly with an

optimal integration of new base stations, BSCs, and CN elements. The RAN, however, more

than any other part of the GPRS system, changes constantly as new cells are introduced and

existing ones are reconfigured, relocated, or modified. Optimization staffs are also support

network operations on an ongoing basis. On the other hand, optimization activities may be

triggered when certain areas of the network fail to meet performance thresholds.

GPRS RAN/BSS

The RAN has a leading role in GPRS optimization. Coverage planning, frequency planning,

and neighbor cell definitions and configuration have a clear impact on radio conditions and

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interference levels, where they should be carefully controlled. Such as frequency retunes and

neighbor cell corrections often improve high percentages of intra-cell and vital inter-cell

handovers or TCH drops and handovers due to downlink quality. Sufficient channel

dimensioning strategies of Base Transceiver Stations (BTSs) for CS and PS services should

make sure minimum congestion levels about %2 or less. BSS features and parameter

optimization also further improve system performance. An important feature that can be

offered by PS broadcasts, is network assisted cell change, which allows a considerably

reduced radio outage time during cell reselection process. When in packet transfer mode, the

MS sends packet cell change notifications to the BSS before entering the cell reselection

procedure. The network’s response contains minimum required system information about the

destination cell, which allows the MS to initiate TBF connections in the destination cell

before cell reselection is actually completed.

Depending on GPRS priority and preemption parameters, on-demand PDCHs can be treated

as available or blocked for CS connections, regardless of a PDCH’s current usage state.

Generally, priority and preemption algorithms are affected by using dynamic allocation or

adapting half-rate channels for CS connections, implementation vary from vendor to vendor.

Operators typically allow the preemption of on-demand PDCHs to allocate new CS

connections when needed.This methodology, conflict with a performance guarantee for

GPRS users in congested situations. Where TCH requirements are high and PDTCH

preemption causes GPRS blocking (packet immediate assignment rejects) to be %2 or higher

during the CS busy hour, a strategy for allocating FPDCHs can improve accessibility

performance. FPDCHs may not be preempted, so the defined number of time slots is

guaranteed for GPRS users. Additionally, if GPRS blocking is % 2 or higher during the

GPRS busy hour, the allocation of FPDCHs can be considered. The FPDCHs can also

increase throughput performance during blocked and congested situations. Ensuring an

acceptable level of GPRS performance without significantly affecting the CS service

becomes part of the operator’s FPDCH allocation methodology.

CONCLUSIONS GPRS system performance depends on performance management method, which identify

performance shortcomings, and on network optimization, which is conducted continuously

and also when triggered by problems. GPRS performance optimization activities should focus

on;

C/I

BLER

Radio capacity

PCU

GSN dimensioning

RADIUS

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DNS

DHCP

Mobility

Usage of BSS

Usage of CN features

Configuration

All these contribute to the end-to-end performance experienced by the user. Active

measurement systems can be used to accurately capture application throughput and RTTs.

Network operators should ensure that GSM and GPRS networks are balanced in terms of

capacity performance management and optimization, acknowledging the increasing

importance of GPRS in the context of supplementing IP services over UMTS.

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ABBREVIATIONS, ACRONYMS, AND TERMS

3G: Third generation

APN: Access point name

ARQ : Automatic repeat request

ATM : Asynchronous transfer mode

BCCH: Broadcast control channel

BDP : Bandwidth delay product

BECN : Backward explicit congestion notification

BER : Bit error rate

BLER : Block error rate

BSC : Base station controller

BSS : Base station system

BSSGP: BSS GPRS protocol

BTS : Base transceiver station

CCCH : Common control channel

C/I : Carrier-to-interference (ratio)

CN : Core network

CPU : Central processing unit

CS : Circuit switch(ed)

CSD : Circuit switch domain

DE : Discard eligible

DHCP : Dynamic host configuration protocol

DLCI: Data link connection identifier

DNS: Domain name service

FECN: Forward explicit congestion notification

FPDCH : Fixed packet data channel

FR : Frame relay

FRoATM : FR over ATM

FTP :file transfer protocol

GGSN: Gateway GPRS support node

GPRS :General packet radio service

GSM : Global system for mobile communication

GSN : GPRS support node

GTP : GPRS tunneling protocol

HLR : Home location register

IMEI : International mobile equipment identity

IMSI : International mobile subscriber identity

IP : Internet Protocol

IPoA : IP over ATM

ISP : Internet service provider

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KPI : Key performance indicator

LAN : Local area network

LAU : Location area update

LLC : Logical link control

MCC : Mobile country code

MNC : Mobile network code

MS: Mobile station

MSC: Mobile switching center

multi-RAT: Multi-radio access technology

N-cell : Neighbor cell

NMS : Network management system

NTE : Network termination equipment

PAGCH: Packet access grant channel

PBCCH : Packet broadcast control channel

PCU : Packet control unit

PDA : Personal digital assistant

PDCH : Packet data channel

PDP : Packet data protocol

PDU : Protocol data unit

PLMN : Public land mobile network

PRACH: Packet random access channel

PS : Packet switch(ed)

PSD : Packet switch domain

QoS : Quality of service

RADIUS : Remote authentication dial-in user service

RAN : Radio access network

RAND : Random number

RAU : Routing area update

RLC : Radio link control

RTT : Roundtrip time

Rx/TxQual : Receive/transmit quality

RxQual : Receive quality

SGSN : Serving GPRS support node

SI : System information

SIM : Subscriber identity module

SNMP : Simple network management protocol

SQI : Speech quality index

SRES : Signed response

TBF : Temporary block flow

TCH : Traffic channel

TCP : Transmission control protocol

TMSI : Temporary mobile subscriber identity

UMTS : Universal mobile telecommunications system

UTRAN : UMTS terrestrial radio access network

VC : Virtual circuit

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VLR : Visitor location register

WAN : Wide area network

WiMAX: Worldwide interoperability for microwave access

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REFERENCES

1. 3GPP, TS 44.060 V6.10.0 (2004–11), Technical Specification 3rd Generation Partnership Project.

2. Technical Specification Group GSM/EDGE Radio Access Network; General Packet Radio Service (GPRS); Mobile Station (MS) – Base Station System (BSS) Interface; Radio Link Control/Medium Access Control (RLC/MAC) Protocol.

3. 3GPP, TS 03.64 V8.11.0 (2003–04), Technical Specification Group GSM/EDGE Radio Access Network; General Packet Radio

Service (GPRS); Overall Description of the GPRS Radio Interface.

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5. Control (RLC/MAC) Protocol .

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December 2002, pp. 226–281.

7. S. Hoff, M. Meyer, and A. Schieder, “A Performance Evaluation of Internet Access via the General Packet Radio Service of GSM,” Vehicular Technology Conference (VTC) ’98, May 1998, Ottawa, ON, Canada.

8. D.E. Comer, “Internetworking with TCP/IP – Principles, Protocols, and Architectures,” 4th Edition, Prentice Hall, 2000.

9. D. Michel and N. Ramasarma, “GPRS Measurement Methodologies and Performance Characterization for the Railway

Environment,” Wireless Communications and Networking Conference (WCNC) 2005, March 2005, New Orleans, LA.

10. G. Heine, “GSM Networks: Protocols, Terminology, and Implementation,” Artech House Publishers, 1999.

11. 3GPP, TS 04.64 V8.7.0 (2001–12), Technical Specification Group Core Network; Digital Cellular Telecommunications System (Phase 2+); General Packet Radio Service (GPRS); Mobile Station – Serving GPRS Support Node

12. (MS-SGSN) Logical Link Control (LLC) Layer Specification (Release 1999).

13. 3GPP, TS 48.016 V6.1.0 (2004–11), Technical Specification Group GSM EDGE Radio Access Network; General Packet Radio Service (GPRS); Base Station System (BSS) – Serving GPRS Support Node (SGSN) Interface; Network Service (Release 6).

14. 3GPP, TS 07.60 V7.2.0 (2001–03), Technical Specification Group Core Network; General Packet Radio Service (GPRS); Mobile

Station (MS) Supporting GPRS (Release 1998).

15. IIR conference on “Developing An Interference Matrix to Improve GSM/GPRS Network Planning and Management” by Mehmet BEYAZ London December 2001.