CHAPTER 4 CALL ADMISSION CONTROL BASED ON BANDWIDTH ALLOCATION (CACBA)

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1 92 CHAPTER 4 CALL ADMISSION CONTROL BASED ON BANDWIDTH ALLOCATION (CACBA) 4.1 INTRODUCTION In our previous work, we have presented a cross-layer based routing protocol with a power saving technique (CBRP-PS) for UWB ad hoc networks. This protocol uses MAC and physical layer (PHY) specific information and maintains a separate route per Access Category (AC) for destination in its routing tables. The route selection is performed based on available bandwidth, contention delay, transmission delay and queuing delay at every link per AC from the available routes to the destination. In addition to this, we have also proposed an efficient power saving mechanism at the MAC layer which reduces the power consumption on useless tasks, such as idle listening, collision, overhearing, and control overhead. The power saving technique uses an adaptive duty cycle mechanism based on the channel condition of a node. Even though CBRP-PS provides increased bandwidth, reduced delay and energy consumption, there is a need for resource management and admission control in the routing technique. The call admission control technique can be employed to reduce the network congestion and guarantee the quality of service (QoS). The resource management in UWB networks can be employed to solve near-sender blocking problem and improve the throughput in such a way that best effort traffic can be transferred through the

2 93 network. Thus, as an extension to CBRP-PS, we have proposed to implement a routing technique in UWB networks with admission control and resource management schemes. 4.2 NEED FOR ADMISSION CONTROL IN UWB NETWORKS In the recent past, there is a progressing trend of increased multimedia usage in mobile networks. This has created an interest to run realtime applications in UWB ad hoc networks. However, running real time applications in UWB ad hoc network is possible only if UWB ad hoc network satisfies the QoS requirements such as high delivery rate and low end to end delay. If these QoS requirements are not met by the UWB ad hoc networks, the QoS experienced by the user will be degraded and the application will become useless. When compared to wired network counterpart, the UWB network performance is worst affected due to various factors such as frequent topology changes, mobility induced communication failure, high latencies due to contention, transmission and queuing delays. The UWB ad hoc network requires new techniques to meet the QoS requirements and the constraints of ad hoc network. In the existing systems, the service differentiation techniques have been proposed in MAC layer which assigns higher priority for real time traffic when compared to the best effort traffic. In the presence of real time traffic, the best effort traffic is never allowed to be transferred through the network. Suppose, the network allows the real-time traffic without any control, the network performance will be degraded. The call admission control techniques can be employed to overcome this problem by preventing the over utilization of the wireless channel. In the presence of existing traffic flows, the admission control allows the new flow only if the admission of new flow does not affect the QoS

3 94 requirements of all the flows. Thus, the admission control algorithm does not allow a flow to be started at will. The channel gain ratio in the admission control algorithm rejects the secondary users by provoking infeasible power control or low sum data rate. Resolving infeasible power control and improving sum data rate of an UWB ad hoc network with less fairness loss are the major objective of admission control. The QoS of unlicensed secondary users need to be guaranteed by the power and admission control in the UWB ad hoc networks without interfering with licensed primary users. Admission decision needs to be performed by a node for each request in the distributed power/rate allocation scheme. On admitting the request, local measurements of the system and control message exchanges information helps in determining the transmission power and the rate. The network performance requirements can be met by admission control which eliminates the congestion. Due to unregulated access, the performance guarantees can be violated and thus the QoS aware networks require admission control. But the admission control algorithm is not required by the best effort networks. Like in cellular networks and centralized wireless networks, centralized schemes require the admission control schemes. The broadcast channel at each slot is heard initially by the sender during a call request arrival. The admission of a call can be determined by using this and the resources are allocated to according to the power/rate levels for each sender.

4 NEED FOR RESOURCE MANAGEMENT IN UWB NETWORKS The long acquisition time in UWB transmissions can be rectified by resource management scheme which solves the near sender blocking problem. Resource management is required in order to consider the degradation of the achieved throughput which is caused due to long acquisition time. The IP pure best effort traffic like , WEB browsing, file transfer and data services having specific quality-of-service (QoS) requirements, are supported by radio resource sharing principles. In network access point (AP) of wireless LAN the resource management module is designed which is based on UWB as the fundamental physical transmission technique. Here AP is responsible for medium access control (MAC) functionalities which also helps in transmission coordination and resource allocation of all devices. 4.4 PRELIMINARIES IEEE e QoS Enhancements The call admission control and Hybrid Coordination Function (HCF) are the two new functionalities introduced by IEEE e working group for QoS service level differentiation in IEEE WLAN. All the stations satisfying the e specifications are called enhanced stations or QoS stations (QSTA). IEEE e defines 8 Traffic categories (TCs) with the priority values of IEEE d standard. TCs are characterized by traffic specifications (TSPEC) similar to those introduced in for IP FlowSPecs definition and adopted in IntServ and DiffServ architectures. Four Access Categories (ACs) have been introduced in order to support the mentioned

5 96 eight TCs. The concept of TXOP (Transmission Opportunity) is introduced in order to satisfy the QoS requirements of each AC. The TXOP is defined as the time interval during which a station is allowed to send the data. The contiguous time during which TXOPs are granted to the same QSTA is called Service Period (SP). The interval between two successive SPs is called Service Interval. The proposed system makes use of the QOS specification given IEEE e The Hybrid Coordination Function (HCF) The HCF is made of a contention-based channel access, known as the Enhanced Distributed Coordination Access (EDCA), and of a HCF Controlled Channel Access (HCCA). The use of the HCF requires a centralized controller, which is called the Hybrid Coordinator (HC) and is generally located at the access point. The EDCA method operates as the basic Distributed Coordination Function (DCF) access method but using different contention parameters per access category. In this way, a service differentiation among ACs is statistically pursued. A queue is associated to each AC at any QSTA, which acts as a virtual station with its own QoS parameters. Each queue within a station contends for a TXOP and starts a back off timer after detecting that the channel is idle for an Arbitration Inter frame Space (AIFS), which is at least equal to a DIFS. For each class AC(i), a contention window CW(i) and an AIFS(i) are defined. If several back off timers reach zero within the same station at the same time slot, then the highest priority frame will be transmitted and any lower priority frame will be deferred with the retry procedure and modifying the back off timer. EDCA parameters have to be properly set to provide prioritization of ACs. Tuning them in order to meet specific QoS needs is a current research topic.

6 97 Regarding the goal of providing delay guarantees, several papers have pointed out that the EDCA behavior is very sensitive to the value of the contention parameters and that the Inter frame space based priority scheme used by the EDCA mechanism can provide only a relative differentiation among service classes, but not absolute guarantees on throughput/delay performance. EDCA can starve lower priority flows. To overcome these limitations, adaptive algorithms that dynamically tune EDCA parameters have been recently proposed, however, the effectiveness of these heuristic schemes have been proved only using simulations and no theoretical bounds on their performance in a general scenario has been derived. The HCCA method combines some of the EDCA characteristics with some of the Point coordination Function (PCF) basic features. Each super frame consists of a beacon frame and a contention free period for PCF access. The remaining part of the super frame forms the CP, during which the QSTAs contend to access the radio channel using the EDCA mechanism. After the medium remains idle for at least a PIFS interval during the CP, the HC can start a Contention Access Phase (CAP). During the CAP, only QSTAs polled and granted with a special frame, known as QoS CF-Poll frame, are allowed to transmit during their TXOPs. Thus, the HC implements a prioritized medium access control. Notice that PIFS is shorter than each AIFS. The number of CAPs and their locations in each super frame are chosen by the HC in order to satisfy QoS needs of each station. Moreover, at least one CP interval, long enough to transmit a maximum size data frame at the minimum BSS rate, must be contained in a super frame. This CP interval can be used for management tasks, such as associations of new stations, new traffic negotiations, and so on. CAP length cannot exceed the value of the

7 98 system variable dot11caplimit, which is advertised by the HC in the Beacon frame when each super frame starts. The IEEE e specifications allow QSTAs to feed back queue lengths of each AC to the HC. This information is carried on an 8 bits long subfield contained in the QoS control field of each frame header. During data transmission both in the CAPs and in the CPs, the queue lengths are reported in units of 256 octets. This information can be used to design novel HCCA-based dynamic bandwidth allocation algorithms using feedback control. In fact, the e draft does not specify how to schedule TXOPs in order to provide the required QoS; it only suggests a simple scheduler that uses static values declared in TSPECs for assigning fixed TXOPs. The proposed system described in the next section makes use of the HCF Controlled Channel Access (HCCA) technique for bandwidth allocation. 4.5 PROPOSED CALL ADMISSION CONTROL BASED ON BANDWIDTH ALLOCATION (CACBA) We have proposed to develop a call admission control in UWB ad hoc networks. The basic idea behind the call admission control is bandwidth allocation and bandwidth redistribution. The bandwidth is allocated with the help of the HCF Controlled Channel Access (HCCA). In HCCA, the hybrid coordinator (HC) assigns the TXOPs to the Access Categories (ACs) based on the time constraints specified for each AC. We will refer to an UWB mobile Ad hoc network with a set of quality of service enabled nodes (QNs). Each QN has N queues, with N = 4, one for any AC in the e proposal. The queue length is estimated based on the time interval between two successive contention access points, average depletion rate and the disturbance. Then the transmission slots are assigned

8 99 and the bandwidth is allocated accordingly. When the bandwidth allocated is more than the required bandwidth then the extra bandwidth is allocated to the other nodes. The extra bandwidth is calculated by estimating the traffic load. Thus the bandwidth redistribution helps in call admission control in the UWB ad hoc networks Queue Length Estimation Initially, the contention window (CW) of a node is used to estimate the queue length. For every T seconds, the CW ( ) of a node is calculated. The quantity in equation (4.1) (Xuemei Gao et al 2007) determines the CW by exponentially weighing moving average method to the old contention window (o) and sample contention window (s). Thus we have * (o) (1 ) * (s) (4.1) where is the contention window of the node calculated at every T seconds, (o) denotes old contention window, (s) denotes current contention window, is a constant and is set to 0.3 in our simulation which grants higher priority to current sample (s). The traffic load of this mobile node is calculated as the number of packets in the queue. More packets are passed through the interface queue when the traffic flows of the mobile node are higher. The equation (4.2) calculates the average queue length which gets updated every T seconds. Q = * Q (o) + (1- ) * Q(s) (4.2)

9 100 Where Q denotes the average queue length, Q (o) denotes the old queue length; Q(s) denotes the current queue length, is a constant and is set to 0.3 in our simulation which grants higher priority to current sample Q(s). The and are the constants which have influence on the average values and Q, the current traffic condition will impose. The and can be any number selected from the range [0, 1]. However, the smaller values of and allow the current traffic conditions to influence on the average value Bandwidth Allocation HCCA functionalities are employed to perform the bandwidth allocation in UWB ad hoc networks. Based on the time constraints specified to each Access Categories (AC), the Hybrid Coordinator (HC) assigns TXOPs to each AC Let I p be the time interval between two consecutive contention access points (CAPs). The bandwidth allocated to the node at every time interval I p should be in a level such that it will drain each queue during next CAP. So the bandwidth assigned to drain the i th queue can be represented as the average depletion rate, A i. The node is conscious of all the queue levels H i ; i=1,2,3n at the beginning of each CAP where N is the total number of traffic queues in the network Transmission Opportunity (TXOP) Assignment In order to meet QoS constraints, HC allocates TXOPs to mobile nodes for every time interval I p. Here the bandwidth A i is transformed into a

10 101 TXOP i assignment. The following relation (4.3) holds if the i th queue is drained at data rate Ri (Boggia et al 2007). TXOP (S) i A (S).I i R i p (4.3) where Ai(s) is the bandwidth assigned during s th service interval, TXOP i (s) is the TXOP assigned to the i th queue during the s th service interval and is the time overhead due to ACK packets, SIFS and PIFS time intervals. i p The number of MSDUs corresponding to the amount of data A (s).i to be transmitted determines the extra quota of TXOP which is due to the overhead. On assuming all MSDUs with same nominal size which is specified in the TSPEC, it is possible to estimate the value. The queuing delay needs to be guaranteed if A i(s).i p does not correspond to a multiple of MSDUs. Here the TXOP assignment is rounded in excess which equals or lessens the target delay T i Call Admission Control In this section, Call Admission Control scheme is proposed to guarantee QoS. After the TXOP is allocated to the active traffic streams in each CAP, the equation (4.4) should be satisfied for admitting a new flow request (Boggia et al 2007). TXOP I p s 1 S i 1 TXOPi I p t (4.4)

11 102 where indicates the super frame duration t is the time used for HCCA traffic during the super frame. Here the standard CAC test is modified by constant TXOPs by using a simple scheduler along with the bandwidth allocation algorithm. In addition to the sum of average source rates in TSPECs, the bandwidth is also taken into account for the CAC test Bandwidth Redistribution After allocating the bandwidth, the remaining bandwidth in a station are redistributed to the other stations whenever the resources are not required locally for some time. The distribution of the free resources is calculated based upon the traffic load estimation. In bandwidth redistribution, each node which has reserved resources, determines its communication load at discrete time intervals. If sufficiently large amount of resources are observed to be temporarily free, the node will hand over some of these resources to its neighbors, by broadcasting an Extra Bandwidth (EB) message. Each node will continue to observe the arrival rate of its transmission queue. If its local load is increasing again, the node broadcasts a Bandwidth Recollection (RB) message to inform its neighbors that they are no longer allowed to make use of the extra bandwidth allocated. For each stream, HC reassigns its resources on receiving the Eb and Rb messages. Here, free bandwidth is added to the Contention Period, so it is fairly distributed since all back off entities can compete for access according to their priorities. Selfish nodes are the nodes that do not forward others packets. These nodes use the network and its services but they do not cooperate with other nodes. Such selfish nodes do not consume any energy such as CPU power, battery and also bandwidth for retransmitting the data of other nodes and they

12 103 reserve them only for themselves. In the bandwidth redistribution scenario, selfish nodes may misuse the strategy to gain more resources. Detecting and dealing with selfish nodes is a separate research area. In our research work, we have not proposed any technique to deal with selfish nodes. It has been assumed that the network is fully secured and there is no selfish node in the network under consideration Traffic Load Estimation The traffic load in UWB ad hoc networks are calculated based upon the contention window of a node. The busy schedule of the medium is determined by BW and the contention and traffic situation of the node in ad hoc networks can be considered by CW. The channel cannot be accessed when the node has idle neighbors. The traffic load around the node can be determined by estimating the average contention of the channel around a mobile node. Based on the CW and queue length estimated in section 4.5.1, the local load of node i can be calculated as follows, W i = e* / max + (1- e) * Q/Q max (4.5) where W i is the local load of node i, and Q denotes the current value of contention window and queue length, max and Q max denotes the maximum value of contention window and queue length. The selection of constant e is to balance the effects of the two factors and Q. In our simulation we set e to 0.5, which grants the same priority to the two factors. There are two thresholds W min and W max which indicate the minimum and maximum threshold values for W i, respectively.

13 104 Let W(t i ) denote the local load W at time interval t i. If W(t i ) < Wmin, then the neighbors. EB message can be broadcasted to allocate the free bandwidth to Else if W(t i ) > Wmax, then RB message can be broadcasted to withdraw the allocated free bandwidth from the neighbors. The time interval t i can be incremented as t i = t i + dt (4.6) where dt represents the small increment for the time interval. Selfish nodes are the nodes that do not forward others packets. These nodes use the network and its services but they do not cooperate with other nodes. Such selfish nodes do not consume any energy such as CPU power, battery and also bandwidth for retransmitting the data of other nodes and they reserve them only for themselves. Detecting and dealing with selfish nodes is a separate research area. In our research work, we have not proposed any technique to deal with selfish nodes. It has been assumed that the network is fully secured and there is no selfish node in the network under consideration. 4.6 SUMMARY In this chapter, we have proposed to develop a call admission control in UWB ad hoc networks. The basic idea behind the call admission control is bandwidth allocation and bandwidth redistribution. The bandwidth

14 105 is allocated with the help of the HCF Controlled Channel Access (HCCA). In HCCA, the hybrid coordinator (HC) assigns the TXOPs to the Access Categories (ACs) based on the time constraints specified for each AC. The queue length is estimated based on the time interval between two successive contention access points, average depletion rate and the disturbance. Z-transformation is used to determine steady state queuing delay. Then the transmission slots are assigned and the bandwidth is allocated accordingly. When the bandwidth allocated is more than the required bandwidth then the extra bandwidth is allocated to the other stations. The excess bandwidth availability is computed with the help of current the traffic load.

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