Реферат: Evaluating the GPRS Radio Interface for Different Quality of Service Profiles

Evaluating the GPRS Radio Interfacefor Different Quality of Service Profiles

Abstract. This paper presents a discrete-eventsimulator for the General Packet Radio Service (GPRS) on the IP level. GPRS isa standard on packet data in GSM systems that will become commerciallyavailable by the end of this year. The simulator focuses on the communicationover the radio interface, because it is one of the central aspects of GPRS. Westudy the correlation of GSM andGPRS users by astatic and dynamic channel allocation scheme. In contrast to previous work, ourapproach represents the mobility of users through arrival rates of new GSM andGPRS users as well as handover rates of GSM and GPRS users from neighboringcells. Furthermore, we consider users with different QoSprofiles modeled by a weighted fair queueing scheme.The  simulator considers a cell clustercomprising seven hexagonal cells. We provide curves for average carried trafficand packet loss probabilities for differentchannelallocation schemes and packet priorities as well as curves for averagethroughput per GPRS user. A detailed comparison between static and dynamicchannel allocation schemes is provided.


The General Packet Radio Service(GPRS) is a standard from the European Telecommunications StandardsInstitute (ETSI) on packet data in GSM systems [6], [14]. By adding GPRSfunctionality to the existing GSM network, operators can givetheirsubscribers resource-efficient wireless access to external Internetprotocol-bases networks, such as the Internet and corporate intranets. Thebasic idea of GPRS is to provide a packet-switched bearer service in a GSMnetwork. As impressively demonstrated by the Internet, packet-switched networksmake more efficient use of the resources for burstydata applications and provide more flexibility in general. In previous work,several analytical models have been developed to study data services in a GSMnetwork. Ajmone Marsan etal. studied multimedia services in a GSM network by providing more than one channelfor data services [1]. Boucherie and Litjens developed an analytical model based on Markov chainanalysis to study the performance of GPRS under a given GSM call characteristic[4]. For analytical tractability, they assumed exponentially distributedarrival times for packets and exponential packet transfer times, respectively.On the other hand, discrete-event simulation based studies of GPRS wereconducted. Meyer et al. focused on the performance of TCP over GPRS underseveral carrier to interference conditions and coding schemes of data [10].Furthermore, they provided a detailed implementation of the GPRS protocol stack[11]. Malomsoky et al. developed a simulation basedGPRS  network dimensioning tool [9]. Stuckmann et al. studied the correlation of GSM and GPRSusers with the simulator GPRSim [13]. This paperdescribes a discrete-event simulator for GPRS on the IP level. The simulator isdeveloped using the simulation package CSIM [12] and considers a cellcluster comprising of seven hexagonal cells. Thepresented performance studies were conducted for the innermost cell of theseven cell cluster. The simulator focuses on the communication over the radiointerface, because this is one of the central aspects of GPRS. In fact, the airinterface mainly determines the performance of GPRS. We studied the correlationof GSM and GPRS users by a static and dynamic channel allocation scheme. Afirst approach of modeling dynamic channel allocation was introduced by Bianchiet al. and is known as Dynamic Channel Stealing (DCS) [3].

The basic DCS concept is totemporarily assign the traffic channels dedicated to  circuit-switched connections but unusedbecause statistical traffic fluctuations. This can be done at no expense interms of radio resource, and with no impact on circuitswitchedservices performance if the channel allocation to packet-switched services is

permitted only for idle trafficchannels, and the stolen channels are immediately released when requested bythe circuit-switched service. In contrast to the models developed in [4], [9],[10], and [11], our approach additionally represents the mobility of usersthrough arrival rates of new GSM and GPRS users as well as handover rates ofGSM and GPRS users from neighboring cells. Furthermore, we consider users withdifferent QoS profiles modeled by a weighted fair queueing scheme according to [5]. The remainder of thepaper is organized as follows. Section 2 describes the basic GPRS networkarchitecture, the radio interface, and different QoSprofiles, which will be considered in the simulator. In Section 3 we describethe software architecture of the GPRS simulator, details about the mobility ofGSM and GPRS users, the way we modeled quality of service profiles, and theworkload model we used. Results of the simulation studies are presented inSection 4. We provide curves for average carried traffic and packet lossprobabilities for different channel allocation schemes and packet priorities aswell as curves for average throughput per GPRS user.

3 The SimulationModel

We consider a cluster comprising ofsever hexadiagonal cells in an integrated GSM/GPRSnetwork, serving circuit-switched voice and packet-switched data calls. Theperformance studies presented in Section 4 were conducted for the innermost cellof the seven cell cluster. We assume that GSM and GPRS calls arrive in eachcell according to two mutually independent Poisson processes, with arrivalrates ëGSM and ëGPRS, respectively. GSM calls are handled circuit-switched, so that onephysical channel is exclusively dedicated to the corresponding mobile station.After the arrival of a GPRS call, a GPRS session begins. During thistime a GPRS user allocates no physical channel exclusively. Instead the radiointerface is scheduled among different GPRS users by the Base StationController (BSC). Every GPRS user receives packets according to a specifiedworkload model. The amount of time that a mobile station with an ongoing callremains within the area covered by the same BSC is called dwell time. Ifthe call is still active after the dwell time, a handover toward an adjacentcell takes place. The call duration is defined as the amount of timethat the call will be active, assuming it completes without being forced toterminate due to handover

failure. We assume the dwell time tobe an exponentially distributed random variable with mean 1/µh,GSMfor GSM calls and 1/µh,GPRSfor GPRS calls. The call durationsare

also exponentially distributed withmean values 1/µGSMand 1/µGPRSfor GSM and

GPRS calls, respectively. To exactlymodel the user behavior in the seven cell cluster, we have to consider thehandover flow of GSM and GPRS users from adjacent cells. At the boundary cellsof the seven cell cluster, the intensity of the incoming handover flow cannotbe

specified in advance. This is due tothe handover rate out of a cell depends on the

number of active customers withinthe cell. On the other hand, the handover rate into

the cell depends on the number ofcustomers in the neighboring cells. Thus, the

iterative procedure introduced in[2] is used to balance the incoming and outgoing

handover rates, assuming that theincoming handover rate ëh GSM

in i ,

 ( ) ( ) −1 computed at step i-1.

Since in the end-to-end path, thewireless link is typically the bottleneck, and given

the anticipated traffic asymmetry,the simulator focuses on resource contention in the

downlink (i.e., the path BSC →BTS→MS) of the radio interface. Because of theanticipated traffic asymmetry the amount of uplink traffic, e.g. induced by

acknowledgments, is assumed to benegligible. In the study we focus on the radio

interface. The functionality of theGPRS core network is not included. The arrival

stream of packets is modeled at theIP layer. Let N be the number of physical channels available in the cell. Weevaluate the performance of two types of radio resource sharing schemes, whichspecify how the cell capacity is shared by GSM and GPRS users:

•the static scheme;that is the cell capacity of N physical channels is split into

NGPRS channels reserved for GPRSdata transfer and NGSM = N — NGPRS channels

reserved for GSM circuit-switchedconnections.

•the dynamic scheme;that is the N physical channels are shared by GSM and

GPRS services, with priority for GSMcalls; given n voice calls, the remaining

N-n channels are fairly shared byall packets in transfer.

In both schemes, the PDCHs are fairly shared by all packets in transfer up to a

maximum of 8 PDCHsper IP packet («multislot mode») and a maximum of 8 packets

per PDCH [6].

The software architecture of thesimulator follows the network architecture of the

GPRS Network [14]. To accuratelymodel the communication over the radio

interface, we include thefunctionality of a BSC and a BTS. IP packets that arrive at

the BSC are logically organized intwo distinct queues. The transfer queue can hold

up to Q n = ⋅8packets that are served according to a processor sharing service

discipline, with n the number ofphysical channels that are potentially available for

data transfer, i.e. n = NGPRS underthe static scheme and n = N under the dynamic

scheme. The processor sharingservice discipline fairly shares the available channel

capacity over the packets in thetransfer queue. An arriving IP packet that cannot enter

the transfer queue immediately isheld in a first-come first-served (in case of one

priority) access queue that canstore up to K packets. The access queue models the

BSC buffer in the GPRS network. Upontermination of a packet transfer, the IP

packet at the head of the accessqueue is polled into the transfer queue, where it

immediately shares in the assignmentof available PDCHs. For this study, we fix the

modulation and coding scheme to CS-2[14]. It allows a data transfer rate of 13,4

kbit/sec on one PDCH. Figure 1 depictsthe software architecture of the simulator.

Figure 1.Software Architecture of GSM/GPRS Simulator

To model the different quality ofservice profiles GPRS provides, the simulator

implemented a Weighted Fair Queueing (WFQ) strategy. The WFQ scheduling

algorithm can easily be adopted toprovide multiple data service classes by assigning

each traffic source a weightdetermined by its class. The weight controls the amount

of traffic a source may deliverrelative to other active sources during some period of

time. From the schedulingalgorithm's point of view, a source is considered to be

active if it has data queued at theBSC. For an active packet transfer with weight wi

the portion of the bandwidth Âi(t) allocated at time t to thistransfer should be

 ( ) ( ) = ⋅∑

where the sum over all active packettransfers at time t. The overall bandwidth at time

t is denoted by B(t) which isindependent of t in the static channel allocation scheme.

The workload model used in the GPRSsimulator is a Markov-modulated Poisson

Process (MMPP) [7]. It is used to generate the IPtraffic for each individual user in

the system. The MMPP has beenextensively used for modeling arrival processes,

because it qualitatively models thetime-varying arrival rate and captures some of the

important correlations between the interarrival times. It is shown to be an accurate

model for Internet traffic whichusually shows self-similarity among different time

scales. For our purpose the MMPP isparameterized by the two-state continuous-time

Markov chain with infinitesimalgenerator matrix Q and ratematrix Ë:

The two states represent bursty mode and non-bursty mode of the arrival process.

The process resides in bursty mode for a mean time of 1/áand in non-bursty modefor

a mean time of 1/ârespectively. Such anMMPP is characterized by the average

arrival rate of packets, ëavgand the degree of burstiness, B. The former is given by:

1 2

The degree of burstinessis computed by the ratio between the burstyarrival rate and

the average arrival rate, i.e., B = ë1/ëavg.

4Simulation Results

Table 1 summarizes the parametersettings underlying the performance experiments.

We group the parameters into threeclasses: network model, mobility model, and

traffic model. The overall number ofphysical channels in a cell is set to N = 20

among which at least one channel isreserved for GPRS. The overall number of GPRS

users that can be managed by a cellis set to M = 20. As base value, we assume that

5% of the arriving calls correspondto GPRS users and the remaining 95% are GSM

calls. GSM call duration is set to120 seconds and call dwell time to 60 seconds, so

that users make 1-2 handovers onaverage. For GPRS sessions the average session

duration is set to 5 minutes and thedwell time is 120 seconds. Thus, we assume

longer “online times” and slowermovement of GPRS users than for GSM users. The

average arrival rate of data is setto 6 Kbit/sec per GPRS user corresponding to 0.73

IP packets per second of size 1 Kbyte.


Figure 2 presents curves for carrieddata traffic and packet loss probabilities due to

buffer overflow in the BSC for thestatic channel allocation scheme and one packet

priority. For GPRS 1, 2, and 4 PDCHs are reserved, respectively. The remaining

channels can be used by GSM calls.With 4 PDCHs the system overloads at an arrival

rate of 0.8 GSM/GPRS users persecond. This corresponds to an average of 12 GPRS

users in the cell (see Figure 7). InFigure 3 we present corresponding curves for the

dynamic channel allocation scheme.For GPRS 1, 2, and 4 PDCHs are reserved,

respectively but more PDCHs can be reserved «on demand». That meansthat

additional PDCHscan be reserved if they are not used for GSM voice service. From

Figure 3 we observe that for lowtraffic in the considered cell GPRS makes

effectively use of the on demand PDCHs. For example if 1 PDCH is reserved GPRS

utilizes up to 2 PDCHsat an arrival rate of 0.4 GSM/GPRS users per second. But

with increasing load the overallperformance of GPRS decreases because of

concurrency among GPRS users, andmore important, priority of GSM users over the

radio interface. In comparison withthe static channel allocation scheme we conclude

that the combination of reserved PDCHs and on demand PDCH leads to a better

utilization of the scarce radiofrequencies. The only advantage of the static channel

allocation scheme is that it can berealized more easily.

Figure 4 presents a comparison ofoverall channel utilization and average

throughput per GPRS user for thestatic and dynamic channel allocation scheme. For

the static scheme we reserved 2 and4 PDCHs respectively and for the dynamic

scheme only 1 PDCH. We observe ahigher overall utilization of physical channels by

the dynamic scheme. Comparing thedynamic with the static scheme for 2 PDCHs we

detect a slightly higher throughputfor low traffic load for dynamic channel allocation.

This results from the high radiochannel capacity available to GPRS users in this case.

They can utilize up to 8 PDCHs for their transfer (in contrast to 2 PDCHs in the static

scheme). When load increases, GSMcalls allocate most of the physical channels.

Thus, throughput for GPRS usersdecreases very fast. In the static scheme (4 PDCHs)

the decrease in throughput is not sofast, because GSM calls do not effect the PDCHs.

In an additional experiment, westudy the performance loss in the GSM voice

service due to the introduction ofGPRS. Figure 5 plots the carried voice traffic and

voice blocking probability fordifferent numbers of reserved PDCHs. The results are

valid for both channel allocationschemes because of the priority of GSM voice

service over GPRS. The presentedcurves indicate that the decrease in channel

capacity and, thus, the increase ofthe blocking probability of the GSM voice service

is negligible compared to thebenefit of reserving additional PDCHs for GPRS users.

Figure 6 shows carried data trafficand packet loss probabilities for the dynamic

channel allocation scheme anddifferent packet priorities. For GPRS1 PDCH is

reserved. Weights for packets withpriority 1 (high), 2 (medium), and 3 (low) and

percentages of GPRS users utilizingthese priorities are given in Table 1. We observe

that for low traffic in theconsidered cell most channels are covered by packets of low

priority. This is due to the highportion of low priority packets (60%) among all

packets sharing the radio interface.With increasing load medium priority packets and

at last high priority packetssuppress packets of lower priority and therefore the

utilization of PDCHsfor low and medium priority packets decreases. For a call arrival

rate of up to 2 calls per second theloss probability of high priority packets is still less

than 10-5 and therefore the correspondingcurve is omitted in Figure 6.

Figure 7 presents curves for averagenumber of GPRS users in the cell and

blocking probabilities of GPRSsession requests due to reaching the limit of M active

GPRS sessions. We observe that for2% GPRS users the maximum number of 20

active GPRS sessions is not reached.Therefore, the blocking probability remains very

low. For 10% GPRS users andincreasing call arrival rate, the average number of

sessions approaches its maximum.Thus, some GPRS users will be rejected. It is

important to note that the curves ofFigure 7 can be utilized for determining the

average number of GPRS users in thecell for a given call arrival rate. In fact, together

with the curves of Figure 2 and 3,we can provide estimates for the maximum number

of GPRS users that can be managed bythe cell without degradation of quality of

service. For example, for 5% GPRSusers and 1 PDCHs reserved, in the static

allocation scheme a packet lossprobability of 10-3 can be guarantied until the call

arrival rate exceeds 0.4 calls persecond, i.e., until there are on the average 6 active

GPRS users in the cell. For thedynamic allocation scheme a packet loss probability of

10-3 can be guarantied until thecall arrival rate exceeds 0.6 calls per second

corresponding to 9 active GPRS usersin the cell on average. Figure 8 investigates the impact of the maximum numberof GPRS user per cell to the performance of GPRS for the dynamic channelallocation scheme with 1 PDCH reserved. Of course, the expected number of GPRSusers should be less than the maximum number in order to avoid the rejection ofnew GPRS sessions. On the other hand, the maximum number of active GPRSsessions must be limited for guaranteeing quality of service for every activeGPRS session even under high traffic. The tradeoff between increasingperformance for allowing more active GPRS sessions and the

increasing blocking probability forGPRS users is illustrated by the curves of Figure 8.


This paper presented adiscrete-event simulator on the IP level for the General Packet Radio Service(GPRS). With the simulator, we provided a comprehensive performance study ofthe radio resource sharing by circuit switched GSM connections and packetswitched GPRS sessions under a static and a dynamic channel allocation

scheme. In the dynamic scheme weassumed a reserved number of physical channels permanently allocated to GPRSand the remaining channels to be on-demand channels that can be used by GSMvoice service and GPRS packets. In the static scheme no ondemandchannels exist. We investigated the impact of the number of packet data

channels reserved for GPRS users onthe performance of the cellular network. Furthermore, three different QoS profiles modeled by a weighted fair queueingscheme were considered. Comparing both channel allocation schemes, we concludedthat the dynamic scheme is preferable at all. The only advantage of the staticscheme lies in its easy implementation. Next, we studied the impact ofintroducing GPRS on GSM voice service and observed that the decrease in channelcapacity for GSM is negligible compared to the benefit of reserving additionalpacket data channels for GPRS. With the curves presented we provide estimatesfor the maximum number of GPRS users that can be managed by the cell withoutdegradation of quality of service. Such results give valuable hints for networkdesigners on how many packet data channels should be allocated for GPRS and howmany GPRS session should be allowed for a given amount of traffic in order to guaranteeappropriate quality of service.

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