Efficient Call Admission Control Mechanism Computer Science Essay

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The IEEE 802.16 or WiMAX networks came into existence in the year 2001 to fill the gap between Wireless Local Area Network and the global network (internet). Admission Control and Scheduling plays an important role in providing the Quality of Service (QoS) that guarantees to heterogeneous classes of traffic with different QoS requirements. Admission control is the QoS mechanism that describes whether a new connection can be admitted by the base station.

Existing CAC mechanisms were unable to provide both bandwidth and delay guarantees for the admitted connections. The goal of this research is to propose an efficient bandwidth and delay guaranteed Call Admission Control (CAC) mechanism for QoS provisioning in IEEE 802.16 networks. A new CAC mechanism that can provide both bandwidth and delay guarantees for admitted connections, called Non-Reservation based Prioritized CAC with Degradation has been proposed for Fixed and Mobile WiMAX and the performance is evaluated by comparing it with the existing CAC mechanisms. An analytical model based on Markov Chain Model has been developed to evaluate the performance of the CAC mechanisms.

The performance of the proposed scheme is evaluated in terms of blocking probabilities of the connections and bandwidth utilization of the system. We also propose an algorithm to design the optimum QoS parameters so that the blocking probabilities of service flows are less than the given threshold. This is particularly useful for network operators when they need to fix the QoS parameters, so that they can promise both QOS and performance guarantees to their customers.

1. Statement of problem and significance

1.1 General Literature Review

Keywords: WiMAX, Quality of Service (QoS), Broadband Wireless Access Network (BWA), Scheduling, Packet scheduling, Token Bucket, Uplink, Downlink, Call Admission Control (CAC), Medium Access Control (MAC), Mobile Station (MS), Base Station (BS),Voice/Data, Multiplexing, Markov Chain model, Fixed WiMAX, Mobile WiMAX .

The IEEE 802.16 or Worldwide Interoperability for Microwave Access (WiMAX)[1] networks came into existence in the year 2001 to fill the gap between Wireless Local Area Network (WLAN) and the global network (internet)[2]. The term Quality of Service (QoS) [1] refers to the ability of service operator to support user's application requirements with respect to service categories like bandwidth, latency (delay), jitter, and traffic (packet) loss [3].

Admission Control and Scheduling plays an important role in providing the Quality of Service (QoS) guarantees to heterogeneous classes of traffic with different QoS requirements [4]. "Admission control is the QoS mechanism that describes whether a new connection can be admitted by the base station [5]". The IEEE 802.16 standard specifies five Unsolicited Grant Service (UGS), real-time Polling Service (rtPS), non real-time Polling Service (nrtPS), Best Effort Service (BE), and extended real-time Polling Service (ertPS) [6,7].There are various applications supported by the WiMAX scheduling services like Voice over Internet Protocol (VoIP), Moving Pictures Expert Group (MPEG) video, File Transfer Protocol (FTP) [6].

1.2 Needs area

Admission Control and Scheduling plays an important role in providing the QoS guarantees to heterogeneous classes of traffic with different QoS requirements [4]. Bandwidth guarantee for heterogeneous classes of traffic results in bandwidth utilization of the system and bandwidth utilization represents the level of network operator satisfaction. Delay guarantee helps in reducing the blocking probabilities. The Blocking Probabilities represents the level of user satisfaction. Lesser the Blocking Probabilities of connections, the more will be the user satisfaction [8].

The existing CAC mechanisms were unable to provide both bandwidth and delay guarantees for the admitted connections. Thus there is a definite need for a more efficient CAC mechanism that can provide both bandwidth and delay guarantees for admitted connections for heterogeneous classes of traffic in fixed and mobile WiMAX when compared to the existing CAC mechanisms.

1.3 Technology gaps

Gap 1: Understanding the MAC protocol, data transfer between the different layers to develop the QoS assurance mechanisms for the QoS in IEEE 802.16 BWA networks.

Providing Quality of Service (QoS) guarantees for heterogeneous classes of traffic with different QoS requirements in IEEE 802.16 Broadband Wireless Access (BWA) system is very important and challenging problem, since the standard has not specified any QoS module and left it for the manufacturers for their own implementation [2]. Although IEEE 802.16 standard defined QoS parameters for different types of services like UGS, rtPS, it has not provided the actual values of the QoS parameters and has left it for the network operators to decide on that. Therefore if we can understand and find out the bandwidth values that can be assigned to each service flow in order to keep the blocking probabilities below the given threshold we can develop a suitable algorithm.

Gap2: Develop a CAC mechanism for QoS provisioning in WiMAX for all classes of traffic.

The main function of CAC mechanism is to provide bandwidth and delay guarantee. CAC mechanisms proposed by Wongthavarawat and Ganz [5], Jiang and Tsai [9] provide both bandwidth and delay guarantees to the admitted connections in their CAC mechanisms. But the CAC mechanisms proposed by them were simple and they did not take the properties of various types of services into consideration. In that the CAC scheme provides both bandwidth guarantees to all service flows and delay guarantees to rtPS service flows. Wang et al. [10], Yin Ge and Geng-Sheng (G.S.) Kuo [11], proposed a CAC scheme for IEEE 802.16e Mobile WiMAX. The drawbacks of these schemes are that they do not provide any delay guarantees to admitted connections [10, 11].

This proposal addresses the Gap 2. To the best of our knowledge there is no proposed CAC mechanism that can provide both bandwidth and delay guarantees to the connections. The solution is to propose an efficient CAC mechanism for both fixed as well as Mobile WiMAX in such a way that QoS guarantees in terms of both bandwidth and delay are satisfied in the proposed CAC mechanisms.

1.4 Objectives and methods

The main objective of this research is to propose efficient CAC mechanisms for both fixed as well as Mobile WiMAX for all classes of traffic. An efficient CAC mechanism is one which decreases the Blocking Probabilities of the connections and increases the bandwidth utilization of the system. A new mechanism called Non-Reservation based Prioritized CAC with Degradation (NRP-CAC-Deg) is proposed for QoS provisioning in WiMAX. This mechanism does not reserve any bandwidth to UGS connections but provides priority to the UGS connections. Providing the priority to UGS connections benefit both users and network operators because the lesser the blocking probabilities the more satisfied are the customers and the more he uses the service. This will automatically improve the revenues of the service provider.

2. Introduction

2.1 Overview of IEEE 802.16 Networks

Wireless Communication came into existence when Guglielmo Marconi successfully transmitted a message across the Bristol Channel in 1896, using the so-called wireless telegraph [3]. The full exploitation of "wireless experience" requires a connection to the internet, which is becoming more and more vital due to the countless services upon which we all rely, such as email, instant messaging, and Voice over IP (VoIP). This lack of technology to bridge the gap between local (Wireless Local Area Network (WLAN) and Wireless Personal Area Network (WPAN)) and global (i.e., internet), wireless technologies drove the development of the first version of the IEEE 802.16 [2], which was published in 2001.

Initially the IEEE 802.16 was aimed at providing high speed internet access in the Point-to-Multipoint (PMP) manner only. The support of Quality of Service (QoS) [12] was embedded, since the first release which clearly stated the role of IEEE 802.16 as a leading technology for the support to advanced multimedia applications. During the subsequent years the standard has been

Amended and published in 2004 as IEEE 802.16d [1], so as to include support to non-LOS (Line Of Sight) deployment. The support to mobile terminals was only added in the 2005 release of the IEEE 802.16 standard, i.e. IEEE 802.16e [13].

2.2 Objective of Work

Providing Quality of Service (QoS) guarantees for heterogeneous classes of traffic with different QoS requirements in IEEE 802.16 Broadband Wireless Access (BWA) system is very important and challenging problem, since the standard has not specified any QoS module and left it for the manufacturers for their own implementation. Admission Control and Scheduling plays an important role in providing the QoS guarantees to heterogeneous classes of traffic with different QoS requirements [5]. There are three conditions for establishment of a new connection. They are namely "Presence of sufficient bandwidth to accommodate it, bandwidth and delay guarantee received, maintenance of Quality of Service of already existing ones [5]".

Present state Desired StateInefficient CAC mechanisms do not provide bandwidth and delay guarantees for heterogeneous classes of traffic.To design an efficient algorithm for CAC mechanism that provides both bandwidth and delay guarantees for all heterogeneous classes of traffic for both fixed and mobile WiMAX.Although IEEE 802.16 standard defined QoS parameters for different types of services, it has not provided the actual values to the QoS parameters and left it for the network operator to decide on that. Therefore in this thesis, an algorithm to find the optimum bandwidth values required for each type service, while maintaining the blocking probabilities of each type of service below a given threshold has been proposed. This helps the network operators to promise both the QoS guarantees and Performance guarantees to their customers.

Fig.1: Objective of work using Present State/Desired State Technique [17]

2.3 Related Work

This proposal addresses the gap 2 in which an efficient CAC mechanism is to be developed for QoS provisioning in WiMAX for all classes of traffic. There are different approaches by various authors but they have their disadvantages. These different approaches are discussed in this related work.

2.3.1 CAC for QoS Provisioning in Fixed WiMAX

1. Leong et al. [14] proposed a Call Admission Control (CAC) policy for the cellular system integrating on/off voice and best-effort data. It also develops a model to characterize the interaction of the voice and data traffic flows which leads to better resource utilization and QoS. The proposed CAC is complex as it consists of two sub-parts, one handling admissions for voice calls and another for data calls.

2. An approach to mix pricing as one of the parameters for designing an admission control policy was proposed by Hou et al. [15]. It uses dynamic network conditions to distribute incentives and thus shaping the aggregate traffic in the network. The performance of the system is analyzed in terms of congestion prevention, achievable user utility, and earned revenues.

3. Niyato and Hossian [16] discussed general models and basic admission control approaches and then extended them into 4G wireless networks. But these existing schemes are not directly applicable to IEEE 802.16 BWA networks.

4. Wang et al. [11] proposed a dynamic CAC scheme for IEEE 802.16d Fixed WiMAX. The proposed CAC scheme uses bandwidth reservation and degradation policies. Bandwidth reservation policy is used to prioritize the UGS connections. Degradation is the method of decreasing the bandwidth allocated to the admitted connections in order to accommodate more number of connections. Two QoS parameters Maximum Sustained Traffic Rate (MSTR) and Minimum Reserved Traffic Rate (MRTR) specified in the standard allows us to degrade the bandwidth. Initially when there is few numbers of connections in the network, the connections are given maximum amount of bandwidth (which is equal to the MSTR). In this paper authors shows that degradation model improves the bandwidth utilization of the system as well as decreases the blocking probabilities of the service flows than that of the constant rate CAC scheme. But this scheme does not provide any delay guarantees to the admitted connections.

5. Wongthavarawat and Ganz [5], Jiang and Tsai [9], proposed a scheduling algorithm and CAC policy for QoS in IEEE 802.16 Fixed WiMAX. In that the CAC scheme provides both bandwidth guarantees to all service flows and delay guarantees to rtPS service flows. Both the CAC schemes differ in the way they provide delay guarantees to rtPS connections. Both the CAC schemes were token bucket based CAC schemes, but the CAC schemes proposed were simple, in which the BS admits or rejects a connection request only based on the available bandwidth. The differentiation of the supported services of different priorities is considered only in the scheduling algorithm instead of in the CAC policy.

2.3.2 CAC for QoS Provisioning in Mobile WiMAX

The CAC mechanisms for Mobile WiMAX are different from that of Fixed WiMAX because in Mobile WiMAX BS receives requests from the Handover connections from the neighboring BS also. Therefore a mechanism for handling handover requests should also be present in the Mobile WiMAX BS. Also the handover connections should be given higher priority than the newly originated connections because it is more annoying to drop an ongoing connection than blocking a newly originated connection.

1. Liping Wang et al. [10] proposed a CAC scheme based on the IEEE 802.16e Mobile WiMAX standard. In that Handoff connections are given higher priority than newly originated connections, because it is more annoying to drop an ongoing connection rather than blocking a newly originated connection. The priority to Handoff connections is given by exclusively reserving a fixed amount of bandwidth to Handover connections. The authors also use degradation for increasing Bandwidth Utilization and decreasing Blocking Probabilities and Dropping Probabilities. But the main drawback of this scheme is that it does not provide any delay guarantees to the admitted connections.

2. Ge and Kuo [11] proposed a CAC scheme for IEEE 802.16e Mobile WiMAX. In that the authors gave priority to handoff connections by allowing them to use the degraded bandwidth of the admitted connections, contrary to the above scheme where bandwidth reservation is used. The drawback of this scheme is that it also does not provide any delay guarantees to admitted connections.

To the best of our knowledge there is no proposed CAC mechanism that can provide both bandwidth and delay guarantees to the connections. This proposal addresses an efficient CAC scheme for both Fixed and Mobile WiMAX which provides both bandwidth and delay guarantees to the admitted connections for all classes of traffic.

3. Research Plan

3.1 Overview of Research Plan

Planning is a key factor in any research project. Planning is done considering the objectives, processes involved, resources available like number of people involved and the equipment, time and budget. For this project the basic plan can be given by the following steps represented in a block diagram as follows.

Collecting and studying the relevant literature.

Analyzing QoS support in WiMAX networks.

.

Availability and Preliminary investigation of materials to be used.

Developing CAC mechanism for Fixed WiMAX for given QoS requirements.

Developing CAC mechanism for Mobile WiMAX for given QoS requirements.

Analyzing and testing the developed CAC mechanism for fixed WiMAX satisfying the given QoS requirements.

Analyzing and testing the results.

Analyzing and testing the developed CAC mechanism for Mobile WiMAX satisfying the given QoS requirements.

Designing of algorithm for obtaining optimum QoS parameter values.

Fig. 2: Research Plan Overview

3.2 Research Objectives, Hypothesis, Materials and Methods

3.2.1 Objectives

Medium Term Objective : To develop an efficient CAC mechanism that can provide both bandwidth and delay guarantees for admitted connections for heterogeneous classes of traffic in fixed and mobile WiMAX when compared to the existing CAC mechanisms.

Long term objective: To design 100% efficient VoIP (Voice over Internet Phone) without silence suppression [1].

WiMAX defines five scheduling services that should be supported by the BS MAC scheduler for data transport over a connection. VoIP uses the Unsolicited Grant Service (UGS) of WiMAX [5]. The existing methods developed based on the assurance mechanisms were unable to provide 100% efficiency. The measurable QoS service flow parameters for this scheduling service are Minimum Reserved Traffic Rate, Maximum Request/Transmission Policy, Maximum Sustained Traffic Rate, Maximum Latency, and Tolerated Jitter [1]. When the efficient scheduling and CAC mechanisms are developed, then this long term objective is achieved. This proposal addresses a part of the long term objective where an efficient CAC mechanism is developed.

3.2.2 Working Hypothesis

Designing Non-Reservation based Prioritized CAC with Degradation for Quality of Service (QoS) provisioning in IEEE 802.16 Network provides more efficient bandwidth and delay guarantee for all classes of traffic for bandwidth ranging from 1MHz to 20MHz (WiMAX range of frequencies) than previously reported CAC mechanisms [5, 9, 10, 11, 14, 15, 16].

Testability:

The hypothesis can be tested by evaluating the performance of the Call Admission Control (CAC) mechanism by using the Markov Chain Model. In this each BS is represented in the form of an analytical model. The obtained analytical model is simulated using the MATLAB tool. The output generated is compared with the outputs of the CAC mechanisms reported by the previous authors [5, 9, 10, 11, 14, 15, and 16]. If the results show that the proposed CAC outperforms the already reports ones, it is a good hypothesis.

Falsification of hypothesis:

a) There are five types of service classes "Unsolicited grant service (UGS), real-time polling service (rtPS), extended real-time polling service (ertPS), non-real-time polling service (nrtPS), and best effort service (BE) [7]". If we can prove that the CAC mechanisms don't provide both efficient bandwidth and delay guarantee for at least one class of traffic then the hypothesis is falsified.

b) If we can prove that there is at least one frequency between 1MHz to 20MHz [1] where the CAC mechanisms don't provide both efficient bandwidth and delay guarantee for all classes or at least one class of traffic, then the hypothesis is falsified.

c) There are two types of WiMAX, namely the Fixed WiMAX and Mobile WiMAX. The mobile WiMAX requires an additional parameter in the CAC mechanism namely the handoff parameter coming from the Handover mechanism. If we can prove that CAC mechanisms don't provide both efficient bandwidth and delay guarantee for at least one class of traffic in mobile WiMAX, then the hypothesis is falsified.

Fundamental Basis:

The above hypothesis generated depends on fundamental basis. The principles used are orthogonal frequency division multiplexing (OFDM) and orthogonal frequency division multiple access (OFDMA) [1]. "OFDM is based on the idea of dividing a given high-bit-rate data stream into several parallel lower bit-rate streams and modulating each stream on separate carriers-called subcarriers, or tones [1]". OFDMA is a principle that allows "Sub channelization in both the uplink and the downlink [1]".The sub channels can be allotted to various users. Fixed WiMAX (IEEE 802.16d) is based on OFDM and Mobile WiMAX (IEEE 802.16e) is based on OFDMA. These communication principles help in designing the CAC algorithms for Quality of Service (QoS) provisioning in IEEE 802.16 with regard to bandwidth and delay guarantee.

3.2.3 Materials and Methods to test the Hypothesis

Materials Used:

1. TreeAge Pro (TreeAge Software, Inc.) is the software used for the development and analysis of the Markov models [18].

2. MATLAB7.1 (The MathWorksTM) is the software used for the simulation of the Markov models [19, 20].

3. Hidden Markov model tool is the software that supports the simulation of the Markov model by the MATLAB. This tool can be found in the MATLAB statistics toolbox.

4. A computer with the minimum configuration required and the requirements are:

a. Operating System such as XP, Vista. b. Intel Pentium IV processor. c. 2GB free Hard disk space. d. Monitor that supports 256 Color Graphics. e. 2 GB RAM.

Method Used:

The performance evaluation of the CAC mechanism is obtained by using the Markov Chain Model [5] which is a stochastic process. A Markov Chain is used to find out the distribution for the future state. The distribution for the future state Xn+1, depends only on the present state Xn and is independent of the past states X0, X1,---,Xn. This analytical model resembles an ergodic Markov Chain [20]. We consider a single Fixed WiMAX BS in isolation. This BS will receive the bandwidth requests from the SSs within the coverage area of a BS. Three types of services UGS, rtPS, nrtPS need QoS guarantees and request for connection admission. Consider the simple CAC mechanism in which the BS will admit the connection whenever there is sufficient amount of bandwidth left at the BS, to accommodate the connection. Otherwise the BS will reject the connection.

In this scenario the BS can be uniquely represented in the form of a State S = (nu, nr, nn) based on the number of admitted connections of each type. State S = (nu, nr, nn) represents that the BS has currently admitted nu, nr, and nn number of UGS, rtPS and nrtPS connections respectively into the network. The BS will be in a particular State S = (nu, nr, nn) until a new connection of either type (UGS, rtPS, nrtPS) is admitted into the network or an ongoing connection of either type (UGS, rtPS, nrtPS) is terminated.

For calculation of steady state probabilities the following steps are followed:

1. Define the proposed CAC mechanism.

2. Based on the proposed CAC mechanism, obtain the state space S of the Markov Chain. The state space will be different for different CAC mechanisms.

3. Obtain the state balance equation for each state S in the state space S of the Markov Chain.

4. Obtain the Steady State Probabilities of the all the states in the state space S of the Markov Chain by solving the state balance equations. The steady state probability of the state S represents the probability of the BS being in that particular state s at any given instant of time.

a. Calculation of new connection blocking probability:

Using the steady state probabilities the blocking probabilities of the connections can be calculated. The new connection blocking probability is the probability of rejecting a new request for connection.

b. Calculation of Bandwidth Utilization

The bandwidth utilization is the average ratio of the used bandwidth to the total bandwidth.

The simulation of this Markov model can be done by various simulators like the ns-2, Omnet++, OPNET, MATLAB. We prefer to simulate the Markov model using the familiar MATLAB.

3.3 Analysis and Interpretation of Expected Results

Markov model developed using the TreeAge Pro (TreeAge Software, Inc.) is analyzed first. We are going to use MATLAB7.1 (The MathWorksTM) [19, 20] software in this proposal to simulate our Markov model. The frequency of operation considered is 1-20 MHz. The arrival process of new connection requests for UGS, rtPS and nrtPS is considered Poisson with arrival rates λUGS, λrtPS, λnrtPS and the arrival rates of all the connections are assumed to be same. The service time for UGS, rtPS, and nrtPS connections are exponentially distributed. Maximum Sustained Rate (in Kbps), Minimum Reserved Rate (Kbps), Bucket Size (bi bits), Delay (ms) are taken as the input QoS parameters. The MATLAB program is given the above input parameters in the numerical form and the various graphs are plotted.

We will first select the best model of the already existing CAC models and compare them with the proposed model. The graphs give the clear visualization of the results for all range of frequencies. The XY-Plot we analyze for fixes WiMAX includes Blocking Probability vs. Total Arrival Rate (1/sec), Bandwidth Utilization of system vs. Total Arrival Rate (1/sec). A new Non-Reservation based Prioritized CAC with Degradation (NRP-CAC-Deg) mechanism which provides Bandwidth Guarantees and also satisfies the delay guarantees to all connections is proposed and this is compared with the existing ones [5, 9, 10, 11, 14, 15, and 16].

The CAC mechanisms for Mobile WiMAX are different from that of Fixed WiMAX because in Mobile WiMAX Base Station receives requests from the Handover connections from the neighboring BS also. The NRP-CAC-Deg mechanism proposed for Fixed WiMAX can be extended for Mobile WiMAX by including the mechanism for handling handover connection requests also [1]. The XY-Plot we analyze for fixes WiMAX includes Blocking Probability and dropping probability vs. Total Arrival Rate (1/sec), Bandwidth Utilization of system vs. Total Arrival Rate (1/sec).

The Blocking Probabilities represents the level of user satisfaction. Lesser the Blocking Probabilities of connections, the more will be the user satisfaction. Bandwidth Utilization represents the level of network operator satisfaction. The more the Bandwidth Utilization, the more the revenues the network operator will get. If the graphs of the proposed CAC mechanism shows the best amount of decrease for Blocking Probability vs. Total Arrival Rate (1/sec) and best amount of increase for Bandwidth Utilization of system vs. Total Arrival Rate (1/sec), then the hypothesis tested is verified.

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