Evaluating the GPRS Radio Interface for Different Quality of Service Profiles
Abstract. This paper presents a discrete-event simulator for the General Packet Radio Service (GPRS) on the IP level. GPRS is a standard on packet data in GSM systems that will become commercially available by the end of this year. The simulator focuses on the communication over the radio interface, because it is one of the central aspects of GPRS. We study the correlation of GSM andGPRS users by a static and dynamic channel allocation scheme. In contrast to previous work, our approach represents the mobility of users through arrival rates of new GSM and GPRS users as well as handover rates of GSM and GPRS users from neighboring cells. Furthermore, we consider users with different QoS profiles modeled by a weighted fair queueing scheme. The simulator considers a cell cluster comprising seven hexagonal cells. We provide curves for average carried traffic and packet loss probabilities for differentchannel allocation schemes and packet priorities as well as curves for average throughput per GPRS user. A detailed comparison between static and dynamic channel allocation schemes is provided.
The General Packet Radio Service (GPRS) is a standard from the European Telecommunications Standards Institute (ETSI) on packet data in GSM systems , . By adding GPRS functionality to the existing GSM network, operators can givetheir subscribers resource-efficient wireless access to external Internet protocol-bases networks, such as the Internet and corporate intranets. The basic idea of GPRS is to provide a packet-switched bearer service in a GSM network. As impressively demonstrated by the Internet, packet-switched networks make more efficient use of the resources for bursty data applications and provide more flexibility in general. In previous work, several analytical models have been developed to study data services in a GSM network. Ajmone Marsan et al. studied multimedia services in a GSM network by providing more than one channel for data services . Boucherie and Litjens developed an analytical model based on Markov chain analysis to study the performance of GPRS under a given GSM call characteristic . For analytical tractability, they assumed exponentially distributed arrival times for packets and exponential packet transfer times, respectively. On the other hand, discrete-event simulation based studies of GPRS were conducted. Meyer et al. focused on the performance of TCP over GPRS under several carrier to interference conditions and coding schemes of data . Furthermore, they provided a detailed implementation of the GPRS protocol stack . Malomsoky et al. developed a simulation based GPRS network dimensioning tool . Stuckmann et al. studied the correlation of GSM and GPRS users with the simulator GPRSim . This paper describes a discrete-event simulator for GPRS on the IP level. The simulator is developed using the simulation package CSIM  and considers a cellcluster comprising of seven hexagonal cells. The presented performance studies were conducted for the innermost cell of the seven cell cluster. The simulator focuses on the communication over the radio interface, because this is one of the central aspects of GPRS. In fact, the air interface mainly determines the performance of GPRS. We studied the correlation of GSM and GPRS users by a static and dynamic channel allocation scheme. A first approach of modeling dynamic channel allocation was introduced by Bianchi et al. and is known as Dynamic Channel Stealing (DCS) .
The basic DCS concept is to temporarily assign the traffic channels dedicated to circuit-switched connections but unused because statistical traffic fluctuations. This can be done at no expense in terms of radio resource, and with no impact on circuitswitched services performance if the channel allocation to packet-switched services is
permitted only for idle traffic channels, and the stolen channels are immediately released when requested by the circuit-switched service. In contrast to the models developed in , , , and , our approach additionally represents the mobility of users through arrival rates of new GSM and GPRS users as well as handover rates of GSM and GPRS users from neighboring cells. Furthermore, we consider users with different QoS profiles modeled by a weighted fair queueing scheme according to . The remainder of the paper is organized as follows. Section 2 describes the basic GPRS network architecture, the radio interface, and different QoS profiles, which will be considered in the simulator. In Section 3 we describe the software architecture of the GPRS simulator, details about the mobility of GSM and GPRS users, the way we modeled quality of service profiles, and the workload model we used. Results of the simulation studies are presented in Section 4. We provide curves for average carried traffic and packet loss probabilities for different channel allocation schemes and packet priorities as well as curves for average throughput per GPRS user.
3 The Simulation Model
We consider a cluster comprising of sever hexadiagonal cells in an integrated GSM/GPRS network, serving circuit-switched voice and packet-switched data calls. The performance studies presented in Section 4 were conducted for the innermost cell of the seven cell cluster. We assume that GSM and GPRS calls arrive in each cell according to two mutually independent Poisson processes, with arrival rates ëGSM and ëGPRS, respectively. GSM calls are handled circuit-switched, so that one physical channel is exclusively dedicated to the corresponding mobile station. After the arrival of a GPRS call, a GPRS session begins. During this time a GPRS user allocates no physical channel exclusively. Instead the radio interface is scheduled among different GPRS users by the Base Station Controller (BSC). Every GPRS user receives packets according to a specified workload model. The amount of time that a mobile station with an ongoing call remains within the area covered by the same BSC is called dwell time. If the call is still active after the dwell time, a handover toward an adjacent cell takes place. The call duration is defined as the amount of time that the call will be active, assuming it completes without being forced to terminate due to handover
failure. We assume the dwell time to be an exponentially distributed random variable with mean 1/µh,GSM for GSM calls and 1/µh,GPRS for GPRS calls. The call durations are
also exponentially distributed with mean values 1/µGSM and 1/µGPRS for GSM and
GPRS calls, respectively. To exactly model the user behavior in the seven cell cluster, we have to consider the handover flow of GSM and GPRS users from adjacent cells. At the boundary cells of the seven cell cluster, the intensity of the incoming handover flow cannot be
specified in advance. This is due to the handover rate out of a cell depends on the
number of active customers within the cell. On the other hand, the handover rate into
the cell depends on the number of customers in the neighboring cells. Thus, the
iterative procedure introduced in  is used to balance the incoming and outgoing
handover rates, assuming that the incoming handover rate ëh GSM
in i ,
( ) ( ) −1 computed at step i-1.
Since in the end-to-end path, the wireless 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 the anticipated traffic asymmetry the amount of uplink traffic, e.g. induced by
acknowledgments, is assumed to be negligible. In the study we focus on the radio
interface. The functionality of the GPRS core network is not included. The arrival
stream of packets is modeled at the IP layer. Let N be the number of physical channels available in the cell. We evaluate the performance of two types of radio resource sharing schemes, which specify 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 GPRS data transfer and NGSM = N - NGPRS channels
reserved for GSM circuit-switched connections.
• the dynamic scheme; that is the N physical channels are shared by GSM and
GPRS services, with priority for GSM calls; given n voice calls, the remaining
N-n channels are fairly shared by all packets in transfer.
In both schemes, the PDCHs are fairly shared by all packets in transfer up to a
maximum of 8 PDCHs per IP packet ("multislot mode") and a maximum of 8 packets
per PDCH .
The software architecture of the simulator follows the network architecture of the
GPRS Network . To accurately model the communication over the radio
interface, we include the functionality of a BSC and a BTS. IP packets that arrive at
the BSC are logically organized in two distinct queues. The transfer queue can hold
up to Q n = ⋅ 8 packets that are served according to a processor sharing service
discipline, with n the number of physical channels that are potentially available for
data transfer, i.e. n = NGPRS under the static scheme and n = N under the dynamic
scheme. The processor sharing service discipline fairly shares the available channel
capacity over the packets in the transfer queue. An arriving IP packet that cannot enter
the transfer queue immediately is held in a first-come first-