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GPRS KPI MEASUREMENT TECHNIQUES FOR THE RAILWAY

ENVIRONMENT LESSONS LEARNED

Dirk Michel1

and Vaidyanathan Ramasarma2

1 Wireless Network Engineering, Nortel Networks, Maidenhead Office Park, Westacott Way, Maidenhead,

Berkshire, UK SL6 3QH Email: [email protected] Network Planning and Technology, Bechtel Telecommunications, 5275 Westview Dr., Frederick, MD,

21703, USA Email: [email protected]

Abstract - The railway environment presents a unique challenge for

cellular radio coverage. European cellular operators, among others,have realized the potential of the mobile user on railways and have

constantly sought to improve the performance of cellular networks

to accommodate current and next generation wireless services.

Improving cellular coverage and services on railways is becoming

increasingly important, considering the serious efforts of various

European nations to boost public rail usage. However, performancelimitations that certain environments such as railways might place

on current technologies such as 2.5G general packet radio service

(GPRS) have yet to be accurately verified. This paper highlightsoperational challenges and introduces strategies for measuring and

verifying GPRS key performance indicators (KPIs) on railways,

describes manual and automated field-testing concepts, and

presents selected KPI results from a trial campaign conducted inthe UK.

I. INTRODUCTION

Key performance indicators (KPIs) are becoming

increasingly important in the context of network rollouts, aswell as within optimization cycles of mature networks. KPIs

are typically used to quantify network performanceparameters and can be defined at three levels: individual cells,

cell clusters, and the entire network. Defining KPI targets

and methodologies for verifying them is critical for a cellular

operator, specifically for target coverage areas in difficult

terrain, including railway environments, where the train and

hence the mobile stations (MSs) may travel at very high

speeds. This paper presents a methodology for measuring

and verifying general packet radio service (GPRS) KPIs for

railways, with emphasis on network delay and GPRSthroughput performance. Other KPIs such as GPRS attach,

routing area update, PDP context activation, cell reselection

and packet loss are outside the scope of this paper. GPRS

throughput characteristics and specific KPI verification

issues in the railway environment are examined and possible

solutions are provided. The first section provides a brief

overview of the GPRS network architecture. Then, possiblesolutions for verifying GPRS KPIs for railway coverage are

analyzed and evaluated. Finally, sample GPRS KPI

performance metrics and the impact of mobility on

throughput are presented. The measurements presented in

this paper are part of a trial measurement campaign

conducted in the UK.

II. GPRS NETWORKARCHITECTURE

The GPRS network architecture is an extension to the

global system for mobile communication (GSM) network,

which has been introduced to enable packet switched data

services via the public land mobile network (PLMN).

Considering the overall GPRS architecture, two main

building blocks are defined: The core network and the base

station subsystem (BSS). Both blocks are important to assess

the performance of end-to-end packet data services. Figure 1shows a high-level overview of the logical GPRS networkarchitecture. The BSS governs the GSM enhanced data rates

for global evolution (EDGE) radio access network (GERAN)

and provides the wireless interface to the MS. GPRS uses the

physical air interface defined for GSM, but GPRS specific

frame formats and logical channels have been standardized.

Several channel coding schemes (CS1 to CS4) incorporating

different levels of error detection and error correction

schemes are supported, which help mitigate the undesirable

effects of a fluctuating radio environment. Commercial

GPRS networks commonly support CS1 and CS2. CS3 and

CS4 including link adaptation however are supported byseveral equipment vendors, but involve hardware upgrades of

the network and the MS. Reliable packet delivery over the airinterface is ensured by a variety of transmission-plane

protocols, including multiple access control (MAC), radio

link control (RLC), logical link control (LLC), and sub-

network dependant convergence protocol (SNDCP).

Fig. 1. Logical GPRS architecture.

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MAC enables multiple users to share the same physical

resources, RLC provides a reliable link over the physical

radio frequency (RF) channel with a selective automated

repeat request (ARQ), LLC provides reliable data transfer for

user data between the MS and the serving GPRS support

node (SGSN) along with the base station subsystem GPRS

protocol (BSSGP), which provides the necessary quality of

service (QoS) and routing information for the data transfer,and SNDCP is a convergence protocol mapping OSI Layer 3

protocols into LLC frames [1,2,3,5]. Additionally, the PLMN

operator may choose to use acknowledged or un-

acknowledged packet transfer for the RLC and LLC.

GPRS mobility management is similar to GSM. Individual

or several cells can be grouped in routing areas and every

routing area is served by one SGSN. In accordance with the

mobility management state model, the MS may be in idle,

ready, and standby state [5]. A MS can move to ready state

by attaching to the GPRS network and by activating a packet

data protocol (PDP) context, which activates GPRS userprofiles, including IP addresses and QoS parameters. Upon

request, temporary radio resources may be assigned to theMS. A temporary block flow (TBF) may then be established

to facilitate the transfer of LLC frames on one or several

PDCHs. However, the physical resources and the TBF are

only assigned for the duration of packet transfer (capacity

on demand). In the case were the packet transfer is

temporarily suspended, e.g. the user reads an Internet web

site, the allocated radio resources are released after a certain

period of time, while the PDP context remains established.

PDCHs and TBF can be re-assigned when new LLC become

available for transmission over the radio link [1]. The core

network (CN) of the packet switched domain is essentially a

cluster of routers and databases interconnected by different

interfaces and protocols. Typically, the network

interconnecting the CN devices is referred to as the packetdata network (PDN). The Gb interface connects the BSS and

the SGSN, allowing the exchange of user data and signaling

messages. The SGSN is the service access point of the GPRS

network for all MS and relays IP traffic originating from

the MS to the gateway GPRS support node (GGSN) and vice

versa. The GGSN provides interworking with external IP

networks, such as the Internet. Existing databases andinfrastructure of the GSM core network are however still

required, including home location register (HLR), visitor

location register (VLR), and the short message service center

(SMSC). For additional detail on GPRS core networks and

BSS, see [2, 4].

III. GPRS KPI TRIAL METHODOLOGY FORRAILWAY

ENVIRONMENTS

Verifying GPRS performance on railways can be

particularly challenging. Usually, KPI verificationmethodologies fall into two categories: (1) field trials and (2)

analysis of network counters. Field trials then distinguish

between automatic and manual testing methodologies as

shown in Fig. 2.

Automatic Testing

Operators can use automatic testing in urban areas by

installing test equipment in vehicles of the public transport

system, taxis etc. Automatic testing on railways however

should use the rail infrastructure to assess the performancewithin the train carriages to account for the carriage specific

penetration loss. The approximate penetration loss for current

train carriages in operation can vary between 5 dB and 25 dB,

including approximately 8dB standard deviation. In some

cases, increased losses may be introduced due to the use of

metalized carriage windows. Typically, the PLMN operator

may choose to employ a custom- built or a modified standard

passenger carriage for automatic testing on the railways,

which can be used to house the measurement equipment. The

carriage may then be attached to a train used for track

geometry measurements, usually employed by train/rail trackoperators, or trains specifically customized for radio

measurement trials.The location of the test equipment within the carriage,

especially the aerials, should be carefully chosen in order to

attain worst-case measurements. Typically, this requires the

aerials to be installed towards the aisle side of the carriage at

an approximate height of 1.5m. As continuous positional

data is generally required during post processing, a GPS

receiver should be used that provides a minimal satellite

acquisition time and continuous update. Long tunnels and

roofed railway stations can interrupt the GPS signal; hence a

minimal satellite acquisition time is highly desirable. A GPS

with dead reckoning system (DRS) capability, including

gyroscope and odometer can help to mitigate the issue, as the

sensors allow the calculation of the trains position even

when GPS signals are blocked. For carriage mounts, the GPSaerial is typically installed on the roof of the carriage,

enabling short satellite acquisition times. Such an equipment

setup within a carriage is expected to inherently produce

repeatable and consistent measurement results, as the

measurement equipment is installed in permanent positions.

Figure 3 illustrates an automatic test setup.

Field TrialsNetwork

Counters

Manual

Testing

Automatic

Testing

GPRS KPI Verification on Railways

GPRS KPI Results

Data Post Processing

- PDCH allocation(% Success)

- Volume/Cell (MB)

- PRACH requests(% Success)

- TBF assignments

- Session applicationthroughput

- Session RLC

throughput- LLC throughput- RTT delay

Fig. 2. Trial methodology.

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Fig. 3. Automatic test setup.

Manual TestingThe manual testing method however requires measures to

ensure repeatable and consistent results. Firstly, suitable test

equipment should be selected to support real-time

measurements of the GSM/GPRS air interface. Secondly, it

is considered beneficial to use test equipment from one

vendor only, which helps to maintain consistent

measurement results and post processing activities. Finally,

all equipment should be calibrated and tested before its

released to field personnel. Pre-configured equipment and a

fixed location of the test MS within the carriage will ensure a

stable measurement environment in most cases. Using a

desktop holder can help to maintain the MS in the sameupright location during the KPI measurement campaign.

Locating the MS at the aisle side of the table (approximately

1.5m height) allows for near-accurate simulation of an actualtraveling MS user. It also helps factoring effects of worst-

case scenarios of radio propagation into the measurements.

Positional data can be obtained by using a mobile GPS

receiver with minimal satellite acquisition time. The GPS

aerial can be attached to the carriage window by using

suction plugs. A possible equipment setup including

computer terminal, test MS and GPS receiver is shown in

Fig. 4.

Both methods have their own specific advantages and

disadvantages, which are listed in Table I. It should be notedthat the presented methods for field trials generally allow

introducing adjustments to the KPI results. Post processing

of the measurement data can be used e.g. to exclude anddiscard measurements from certain areas of rail routes, which

do not form part of the targeted coverage area. Such

exclusion areas can be tunnels of a certain length or cuttings

of a certain depth, depending on the required scope. The

impact and the possible bias of excluded areas on the KPI

results can thus be removed. Using network counters to

assess the GPRS performance of railway coverage is

Carriage Window

Desktop Holder

USB Hub

GPS Receiver

Test MobileStation

Laptop Computer

Carriage

GPS Aerial

Data Cables

Manual download ofmeasurement

results

Aisle area of thepassanger carriage

Fig. 4. Manual equipment setup.

generally very limited, but can provide a rough indication of

the user experience of train passengers.The difficulty of this approach is that the cells, even if

constructed for specific rail coverage employing high gaindirectional antennas with horizontal beam-widths of less than

15, may pick up other users outside of the train. Generally,

network performance counters provide cell or routing area

based counters, which are not user or session specific. GPRS

throughput on the application layer is typically not supported

either, as the GPRS network does not discriminate between

different applications, although such information is e.g.

provided within the destination port field of the transmission

control protocol (TCP) header. However, network statistics

such as PDCH allocation failures, PDCH drops, successful

TBF assignments, congestion time, as well as successful

hand-outs and hand-ins during cell reselection can beuseful in identifying low performing cells or cluster of cells.

The analysis of such network performance statistics canallow addressing corrupted hardware, congestion, RF

planning weaknesses and software configuration issues

before the actual KPI measurements take place. Table I lists

advantages and disadvantages for manual versus automatic

testing.

TABLE I

MANUAL VERSUS AUTOMATIC TESTING: ADVANTAGES AND DISADVANTAGES

Advantages Disadvantages

Automatic

Testing

-Allows a GPS system

with DRS-Intrinsically consistent-Continuousmeasurement of data

-Easier repeatability

-Cost intensive

-Travel schedule constraints-Equipment setup only incustomized coaches-Loss of Measurement Data

-Continuous performance reporting

Manual

Testing

-Cost effective

-Ad-hoc collection ofmeasurement results

-Possible inconsistency

-Possible loss of GPS fix-Incomplete measurement log files

-Possible difficulty in repeatability

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IV. SAMPLE GPRS DATA KPI RESULTS

There are two basic end-to-end parameters that helpestimating the service performance of a GPRS network: data

throughput and roundtrip time (RTT). The results discussedin this section are based on a trial area (~100 km) along

important rail routes in the UK. Network-wide results,

although important are beyond the scope of this paper. GPRSthroughput can be determined on various protocol layers,including RLC, LLC, TCP or the application layer, whereasnetwork latency is typically measured with pings, whichare Internet control management protocol (ICMP) echo

requests implemented on raw IP. Measuring the applicationthroughput however best resembles the user experience, as

the actual performance figures of the end-to-end connectionis captured. Naturally, application throughput values will beless than those measured on lower protocol layers, due toencapsulation overheads and signaling traffic [4]. There is atradeoff between mobility (fast-moving MSs) and linkperformance [5]. Phase errors (due to rapid variation in phase

of the received signal and Doppler effects) force the receiver

to incorrectly detect some bits independent of the receivedsignal-to-noise-ratio (SNR). To assess the GPRS throughputon the railway, a 100kB file can be continuously downloadedfrom a file transport protocol (FTP) server that is directlyconnected to the GPRS core network via a virtual privatenetwork (VPN). Using a small file size can be useful to

determine network accessibility, especially within a trainmoving at high speeds (e.g. 150 km/hr). In case the standard

software of the test MS does not support FTP, alternativesoftware can be used to enable repeated downloads and logfile generation. Figure 5 shows the test network used duringthe trials. Assuming that the railway coverage is provided by base stations configured as standard quasi-omni cells, itcan be estimated that a MS would spend approximately 40-

60 seconds within a given cell. Quasi-omni cells aretypically equipped with RF signal splitters and bi-directional

narrow- beam antennas pointing up and down the targetcoverage area.

Fig. 5. Test network for FTP.

Fig. 6. GPRS latency characteristics.

Such cell configurations can be beneficial when providingcoverage for linear transport networks, such as motorwaysand railways.

The time period the MS resides within a cell however

would naturally decrease with sectorized cells, as the coveredarea per cell is decreased. Typically, number of possible

GPRS cell reselections on a given rail route impacts thethroughput performance and increases the overall networklatency. Latency is a critical parameter to characterize theperformance of real-time and interactive applications such asstreaming video and network gaming. In addition to thenetwork quality requirements of different applications, the

RTT has a significant impact on the performance of higherlayer protocols above the subnet connection boundary [5].High latency and packet loss can severely degrade the

achievable application throughput. GPRS network latency istypically in the area of 700 ms to 1000 ms with a standarddeviation of approximately 150 ms [4].

Figure 6 shows the typical characteristics of RTT in acommercial GPRS network measured over successive

sessions. Traveling MSs that transcend cell boundariesexperience higher delays including higher variability ofdelays, caused by packet transfer idle mode times duringcell reselection procedures [2]. The physical radio resourcesincluding TBF are released by the old cell and re-assignedby the new reselected cell. GPRS throughput performanceis impacted in a similar fashion. Packet transfer can be

interrupted by approximately 0.25s-4s depending on howquickly the MS acquires the full set of system information of

the new cell. Also, the probability of packet loss may beincreased due to LLC frame re-routing between thereselecting cells. The protocol stack at the MS shows thatflow control and congestion control mechanisms areimplemented within RLC and TCP [5]. Packetretransmission combined with high network latency may

severely degrade throughput performance.

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Fig. 7. GPRS throughput characteristics.

Figure 7 provides session application throughput results

obtained from 950 individual 100kB file downloads on a rail

route with traveling speeds of up to 200 km/hr. The testmobile station used supports 4 PDCH channels in the

downlink and 1 PDCH channel in the uplink. It is apparent

that the mean application throughput is approximately 25

Kbps, as opposed to the theoretical maximum of nearly 40

Kbps. Various theories can explain the reason for such

performance, most notably, the criticality of an induced-

delay in the core-network, details of which will be presented

in an alternate publication. From Fig. 7, the red-curve

represents the running sum average (RSA) calculated over

the measured throughput values. Despite the high traveling

speeds of the train the RSA stabilizes approximately after

200 sessions. Figure 8 shows the error analysis for the RSA.

The difference of two consecutive RSA values has beenderived and plotted in order to identify the point at which the

measurements stabilize within a range of e.g. 0.1 Kbps. This

would be achieved after approximately 200 sessions. The

error analysis can be computed on an ongoing basis during a

measurement campaign, which can help to understand and

estimate the variability of the session application throughput.

Additionally, the error analysis can help determining the

point beyond which further measurements have negligible

impact on the mean session application throughput of a given

coverage area.

V. MOBILITY EFFECT AND TARGET LEVELS FOR THROUGHPUT

Considering the level of mobility is imperative in the contextof defining achievable targets for mean session application

throughput, as the direct comparison of throughput levels

derived from stationary and fast moving MS may lead to

unrealistic performance expectations. This emphasizes the

difficulty of employing network wide KPIs for GPRS

throughput (Tkpi). Such network wide KPIs however may still

be used if the performance impacting elements of a given

special coverage area can be quantified reliably.

Fig. 8. Throughput error analysis.

The number of available downlink PDCHs, interference (C/I),

multiplexing and the mobility effect are quantifiable

parameters [5,6]. Assuming that the railway environment andthe overall network exhibit comparable levels of mean

PDCH availability, average interference levels and

multiplexing, then the discriminating factor would be the

mobility effect. Isolating this effect and making the

appropriate adjustments to the throughput measurementswould make the adjusted values comparable to stationary

measurements. (1) provides a possibility of isolating the

mobility effect and other performance impacting conditions,

where M represents the mean level of PDCH multiplexing,

TS is the mean number of available downlink time slots, S is

the file size, ttotal denotes the total file download time, tci and

tri are packet outage time during cell reselection and routing

area / location area update respectively and n represents the

number of cell reselections.

n

0i

iitotal

Filekpi

trtct

S

4

TSMT (1)

It should be noted that packet idle time is equivalent to radio

outage, i.e. the time period in which the MS has no TBF

assigned. The effective packet outage time on the application

layer however is greater than the radio outage, as it begins

with the last TCP packet sent by the old cell and ends withthe first TCP packet sent by the new cell. The effective

packet outage time has been measured and analyzed with e.g.

windump [7] and ethereal [8] respectively.

It may be expected that the post-processed performance

(applying (1)) on the train would approach or be very close

to the performance level of stationary measurements.

Following figure presents a comparison of raw

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measurements, their post processed values and stationary

measurements of the mean session application throughput.

Fig. 9. Throughput Characteristics

Figure 9 shows that the average value of the raw

measurements on the train are 7.7 kbps lower compared tothe stationary results. It can also be seen that the average post

processed results are 3.2 kbps lower that the stationary

measurements. This outcome suggests that (1) accounts for

approximately 60% of the mobility effect. Further analysis of

the log files has shown that the average TCH availability and

the average level of multiplexing are homogeneous acrossthe test area. This however did not apply to the average level

of C/I, as regional differences have been identified. The

remaining difference between the post processed results on

the train and the stationary measurements may therefore be

attributable to the varying interference levels within the test

area. C/I levels are generally highly dependent on frequency

planning and traffic density and are likely to change on a

regular basis, especially in a mature GSM/GPRS networks,considering the ongoing introduction of new sites or re-

location or decommissioning of existing sites.

Defining railway specific KPIs that account for packetoutage time due to cell reselections and location / routing

area updates can be a possibility to determine achievable KPI

levels, although inaccuracies may be encountered due to thevariation of interference levels.

VI. CONCLUSIONS

GPRS performance verification trials on railways can be

performed using automatic and manual testing methods.

Automatic testing involves a customized railcar and isgenerally expensive. Manual testing is comparatively cost

effective but requires a field team to generate the

performance measurements and a measurement methodology

that produces consistent and repeatable results. GPRS

throughput performance is affected by cell reselection and

routing area update procedures, especially for fast-moving

MSs. Increasing mobility levels, which lead to a higher

number of cell reselections and routing area updates, increase

the amount of packet outage time. It is therefore to be

expected that GPRS throughput performance decreases with

increased mobility. Removing the mobility effect from trial

measurements on the railways improves their comparabilityto stationary measurements that are typically used for

establishing KPI levels. The linear nature of coverage along

railway tracks and the fact that trains travel through the samespace inherently provide, in time, quickly stabilizing GPRS

measurements.

REFERENCES

[1] 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; Stage 2, (Release 1999)[2] 3GPP, 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 (Release 1999)[3] 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 (MS-SGSN)

Logical Link Control (LLC) layer specification (Release 1999)[4] R. Chakravorty and I. Pratt, Performance issues with GPRS,

Journal of Communications and Networks (JCN), Vol. 4, No. 2,

December 2002, p.266-281.

[5] Timo Halonen, Javier Romero and Juan Melero, GSM, GPRS

and EDGE Performance Evolution towards 3G/UMTS,

Wiley and Sons, 2nd

ed., 2003.[6] M. Meyer, TCP performance over GPRS, Proc. 1999 IEEE

Wireless Communications and Networking Conference(WCNC), Sept, 21-24, New Orleans, LA, pp. 1248-1252.

[7] Windump, http://winpcap.polito.it[8] Ethereal, www.ethereal.com

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