Above packet level admission control and bandwidth allocation for IEEE 802.16 wireless MAN☆
Introduction
IEEE 802.16 wireless Metropolitan Area Network (MAN) is a standard-based wireless network designed to provide broadband wireless access (BWA) over a long distance. Four types of scheduling services, Unsolicited Grant Service (UGS), Real-time Polling Service (rtPS), Non-Real-Time Polling Service (nrtPS) and Best Effort (BE), representing various the multimedia applications in wireless networks, are defined in the IEEE 802.16 standard [1]. Compared to conventional packet switched network (e.g., original Internet) which was designed to provide best effort service and does not provide full reliability, IEEE 802.16 wireless MAN aims at providing better service (better than best effort) to the network traffic, which is called the Quality of Service (QoS). Guaranteeing QoS for different types of services over IEEE 802.16 wireless MAN is crucial.
IEEE 802.16 MAC, defined as connection-oriented, is designed to support different QoS for these services. The QoS of such network can be managed in three ways: admission control, packet scheduling and buffer management. Admission control is used to limit the number of connections/flows (in the following paper, connection is equivalent to flow) admitted into the network so that each individual connection/flow can get its desired QoS. It is an important premise to support QoS for packet and lower levels. Once the connection is set up, when new packets arrive at the transmitter, system need to decide which packet gets transmitted first on the output link so as to achieve the promised QoS of each flow, and this process is called packet scheduling. Compared to the admission control whose decision is made based on the long term requirements, i.e., transmission rate, packet scheduling deals with the instantaneous requirements, i.e., packet end to end delay and delay jitter. Moreover, buffer management, which controls the buffer size and decides which packet to drop, is another way to provide QoS guarantee. In short, a fair admission control above the packet level and an efficient packet scheduling as well as an optimal buffer management at the packet level are essential in providing QoS for wireless networks such as IEEE 802.16 MAN. However, neither of them is addressed at all in the IEEE 802.16 standards.
In recent years, numerous multimedia admission control schemes have been proposed and evaluated [2], [3], [4], [5], [6] for existing wireless networks, such as wireless LAN (Local Area Network), General Packet Radio Service (GPRS), and CDMA based Cellular network. None of them are suitable for IEEE 802.16 wireless MAN, since these works classify network traffic into two categories: real-time and non-real-time services, and the classification is not as specific as the scheduling services classification (UGS, rtPS, nrtPS and BE) in IEEE 802.16 wireless MAN. In 802.16 wireless MAN, each connection in the uplink direction is mapped to a scheduling service which belongs to UGS, rtPS, nrtPS and BE. ATM constant bit rate (CBR) and E1/T1 over ATM are the typical connections for UGS. The rtPS is well suited for connections that carry services such as variable bit rate video and audio. The nrtPS is suitable for Internet access with minimum guaranteed rate, such as FTP. The BE is for World Wide Web (WWW) traffic. Finding accurate traffic models for these services are important in designing the IEEE 802.16 network. In a conventional telecommunication network, connection inter-arrival time and duration for voice traffic are both modeled by the exponential distribution and has been proven to be the accurate model. For other non-voice broadband traffic, the connection duration is no longer simply exponentially distributed. More and more works in the recent years show that the wide-area network traffic is correctly modeled using statistical process called self-similar due to their significant rate variability [7]. Therefore, the M/M/m analytical model that assume Poisson process traffic model for all multimedia services, can only give us an approximate performance evaluation and is not accurate enough. In fact, the variable rate voice, video and data traffic is high speed fractal traffic which is better modeled as self-similar process with substantially larger burstiness over a wide range of time scales than the Poisson process. Also, the duration of low speed data traffic connection (belongs to BE) is better modeled by Pareto distribution than the exponential distribution. Therefore, the original M/M/m queuing model turns into M/G/m model which requires much more efforts in doing analytical formulation.
In this paper, we firstly propose a dynamic admission control scheme for IEEE 802.16 wireless MAN, which guarantees QoS for various scheduling services and also improves the bandwidth utilization of the network. Then, we use an approximate analytical model to evaluate the performance of multimedia services in our proposed scheme. We also give another suitable analytical model for determining the blocking probability, considering appropriate traffic models for fractal service (rtPS and nrtPS) and non-fractal service (UGS and BE) in large networks such as IEEE 802.16 wireless MAN. Based on these theoretical results, another admission control scheme is proposed, which minimizes the blocking probabilities of all services for a given total bandwidth in the network.
The rest of this paper is organized as follows. In Section 2, we summarize the basic IEEE 802.16 MAC mechanisms specifically related to QoS provisioning. A dynamic admission control scheme based on the characteristics of schedul ing services is proposed in Section 3, where an approximate analytical model is used to evaluate the performances. In Section 4, a mathematical model for determining upper bound for blocking probability of self-similarity wireless traffic in large network, is presented. We also introduce appropriate traffic models for each scheduling service of IEEE 802.16 wireless MAN and give another admission control scheme in this section. Theoretical and simulation results are discussed in Section 5. Section 6 concludes the paper.
Section snippets
IEEE 802.16 MAC mechanisms for QoS
Wireless MAN is being developed to replace the wire line infrastructure network with more efficient deployment and lower maintenance cost. IEEE Standard 802.16-2004 [1] defines the wireless MAN air interface specification for such a network, which is called the IEEE 802.16 wireless MAN in this paper. The IEEE 802.16 wireless MAN was initially proposed as a fixed broadband access system with substantially higher bandwidth than most cellular networks. IEEE 802.16e extends its mobility support up
Scheme description
In this section, we describe our proposed admission control and bandwidth reservation scheme at the network layer. As we stated earlier, before the packet transmissions of a connection, BS should decide whether to accept or reject the request of setting up a new connection. The decision is usually made based on the long term bandwidth requirements of the connection and the residual bandwidth of current network state. The long term bandwidth requirements here means the average transmission rate
Upper bound blocking probability considering self-similarity of wireless traffic
We have analyzed some QoS metrics using an approximate mathematical model by assuming that the connection arrival process is Poisson process and the duration of the connection follows exponential distribution. In the conventional telecommunication network, the connection inter-arrival time and duration of the connection for voice traffic is modeled as an exponential distribution and it has been observed to be a perfect model for performance evaluation by both analytical and simulation
Performance evaluation
In this section, we firstly present our analytical and simulation results of QoS metrics under dynamic admission control scheme discussed in Section 3. The system parameters used in our analytical model and simulation are shown in Table 2.
Fig. 6, Fig. 7 compare the blocking probabilities of UGS and nrtPS with different value of U. We see that the reservation method is useful for decreasing CBP of UGS. In Fig. 7, the CBP of nrtPS is increased by the reservation method. As we know, UGS
Conclusions
We have proposed a dynamic admission control scheme based on the traffic characteristics of services defined in the IEEE 802.16 wireless MAN and developed an analytical model to evaluate the system performance in proposed scheme. We have assumed that the traffic arrival process and service time for all services are exponential distributions in our analytical model, while knowing the fact that the connection holding time of certain traffic is not an exponential distribution. In the later part of
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This work has been supported by the Ohio Board of Regents Doctoral Enhancements Funds.