2340 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 7, NO. 6, JUNE 2008 A Renewal Theory Based Analytical Model for the Contention Access Period of IEEE 802.15.4 MAC Xinhua Ling, Yu Cheng, Member, IEEE, Jon W. Mark, Life Fellow, IEEE, and Xuemin (Sherman) Shen, Senior Member, IEEE
Abstract—In this paper, we propose a simple yet accurate reaching zero. Moreover, when the BC reaches zero, a node analytical model for the slotted non-persistent carrier sense adopting DCF transmits immediately, while a node with CAP- multiple access protocol with binary exponential backoff, as MAC does so only after sensing two consecutive idle slots. specified in the medium access control (MAC) protocol of the IEEE 802.15.4 standard for the contention access period. The Intuitively, the different protocol behavior of 802.15.4 will model is based on a three-level renewal process, which leads to a result in performance different from the two well-known MAC general analytical framework applicable to the protocol variants protocols. of either single or double sensing, in a saturated or unsaturated Several simulation studies have been conducted [4]–[7] case, under a general traffic arrival distribution and with various to understand the performance of the 802.15.4 protocol. In backoff policies. The analytical model can be used to obtain some important performance metrics, such as MAC throughput and addition, efforts have also been made in analytically model- average frame service time. The accuracy of the analytical model ing the protocol, especially the slotted non-persistent CSMA is demonstrated by extensive simulation results. The applicability with binary-exponential-backoff (BEB) MAC protocol for the of this model to the performance analysis of other slotted MAC contention access period defined in the standard [8]–[11]. protocols is also briefly discussed. While simulation studies, usually time consuming, may only Index Terms—Analytical model, CSMA/CA, IEEE 802.15.4, address particular scenarios under specific conditions, analyt- MAC, renewal theory, wireless networks. ical modeling enables one to gain a clearer insight into the characteristics of the protocol. In this paper, a simple and yet accurate analytical model I. INTRODUCTION for the IEEE 802.15.4 MAC protocol is proposed. Instead of HE FAST growth of public interest in wireless sensor modeling the channel, we model the behavior of an individual T networks and wireless personal area networks (WPAN) node based on a novel concept of three-level renewal process, in recent years has led to the standardization of the IEEE which can be solved by the fixed-point technique [12]. The 802.15.4 [1], which contains a new protocol stack targeting new modeling approach significantly simplifies the mathemati- at low-power low-rate wireless networks. The standard has cal analysis, where the important performance metrics of MAC been quickly accepted by industry, and many products have throughput and average frame service time (also referred to appeared in the market since its ratification. as access delay in [13]) can be directly obtained. We also The IEEE 802.15.4 standard, especially its medium access show that the proposed model is in fact a general analytical control (MAC) protocol for the contention access period framework which enables us to analyze different protocol (CAP), also draws great interest from the academia. A salient variants of either single or double sensing, in a saturated or difference between the CAP MAC specified in this standard unsaturated case, under a general traffic arrival distribution, and the classical slotted non-persistent carrier sense multiple and with various backoff policies. Extensive computer simu- access (CSMA) [2] is that a node can transmit only after lation results are presented to demonstrate the accuracy of the two consecutive sensing of an idle channel in the former, proposed analytical model. while just one channel sensing is required in the latter. In The remainder of the paper is organized as follows. The addition, it differs from the well-known IEEE 802.11 [3] DCF 802.15.4 MAC protocol is briefly reviewed in Sec. II. The MAC protocol for wireless local area networks (WLANs) analytical model for saturated nodes is presented in Sec. III. in that the backoff counter (BC) of a node does not freeze In Sec. IV, we derive the normalized MAC throughput and when the channel is busy; instead, it keeps decreasing until the average frame service time. In Sec. V, we extend the analytical model to the case where nodes are unsaturated. Manuscript received January 12, 2007; revised June 7, 2007; accepted July 28, 2007. The associate editor coordinating the review of this paper and Analytical results are compared with simulation results in approving it for publication was Y. Fang. This work has been supported jointly Sec. VI to assess analytical accuracy; the impact of the by the Natural Sciences and Engineering Council (NSERC) of Canada under key MAC parameters on the protocol performance is also Strategic Grant # STPGP 257682 and Research In Motion (RIM). X. Ling, J. W. Mark, and X. S. Shen are with the Department of Electrical discussed. Related work is given in Sec. VII, followed by and Computer Engineering, University of Waterloo, Waterloo, ON, N2L 3G1, concluding remarks in Sec. VIII. Canada (e-mail: {x2ling, jwmark, xshen}@bbcr.uwaterloo.ca). Y. Cheng is with the Department of Electrical and Computer Engineering, II. THE IEEE 802.15.4 MAC PROTOCOL Illinois Institute of Technology, Chicago, IL 60616-3793, USA (e-mail: [email protected]). In this section, we briefly review the MAC protocol spec- Digital Object Identifier 10.1109/TWC.2008.070048. ified in the IEEE 802.15.4 standard. More details of the 1536-1276/08$25.00 c 2008 IEEE LING et al.: A RENEWAL THEORY BASED ANALYTICAL MODEL FOR THE CONTENTION ACCESS PERIOD OF IEEE 802.15.4 MAC 2341
Beacon Beacon the node will do the second CCA in the next slot. Only when Contention Access Period Contention Free Period both CCAs indicate an idle channel (thus CW reaches zero), will the node start the transmission in the next slot; otherwise, ... GTS... GTS inactive portion it will enter the next backoff stage and reset CW to two. NB,0 ≤ NB ≤ Superframe Durataion The number of backoff slots in stage NB [0, 2BENB − Beacon Interval max, is drawn from a uniform distribution over 1],whereBE0 = macMinBE is the initial and minimum BE = BE +1 Fig. 1. IEEE 802.15.4 superframe structure in the beacon-enabled mode backoff exponent for each frame. NB+1 NB is upper-bounded by aMaxBE which is the default maximum value of backoff exponent, and NBmax is the maximum protocol, such as specific parameter settings or physical layer number of backoff stages allowed for a frame. If all the NB related information, can be found in [1], [4]. max backoff stages end up with a busy channel indicated The MAC layer in the IEEE 802.15.4 standard specifies by the associated CCAs, a Channel Access Failure event two operating modes: an ad hoc non-beacon-enabled mode will be reported to the upper layer; the node may then and a beacon-enabled mode. In the ad hoc mode, nodes in start the above procedure again for the next frame. The the network use a non-slotted CSMA with collision avoidance standard specifies the following default parameter values: macMinBE =3,aMaxBE =5 NB =5 (CSMA/CA) mechanism to contend for channel access. If the and max .Their channel is assessed to be idle, the transmission of a frame impact on the protocol performance will be discussed in will begin immediately; otherwise the node will backoff and Sec. VI. During the backoff procedure, if the node succeeds try to access the channel in a future slot. This mechanism has in accessing the channel, it will reset the three parameters NB,BE CW been extensively studied in the literature and its performance and to the default values for initial transmission is well understood [2], [14]. In the beacon-enabled mode, a of the next frame. personal area network (PAN) coordinator transmits a beacon periodically to form the so-called “superframe” time structure, III. THE ANALYTICAL MODEL as shown in Fig. 1. A superframe consists of a beacon that In this section, we first describe the network considered, enables the beacon-enabled mode, contention access period then present the 3-level renewal process, which is the foun- (CAP), contention free period (CFP), and an optional inactive dation of the proposed analytical model for the CAP-MAC. portion in which all the nodes may enter a sleep mode to We then develop the model by first considering the single reduce power consumption. The CAP and CFP together form channel sensing (SS) case, to illustrate the essence of our the active portion of the superframe, during which all com- model. We further extend the model to the double channel munication among the nodes should take place. In the CFP, sensing (DS) case specified in the standard. Notice that the the network coordinator alone controls entirely the contention- only difference between the SS and the DS is that the former free channel access by assigning guaranteed time slots (GTS) just requires one successful CCA before the node can start to to those nodes with their GTS requests granted. According transmit2. In the analysis, it is assumed that for each node, the to the default values specified in the standard [1], the active probability τ to start channel sensing in a randomly chosen slot portion of each superframe contains 48 backoff slots, 15 of is constant, regardless of the number of retransmission trials which are occupied by the CFP. The assignment of the GTS to it has experienced. This assumption is widely adopted for the those nodes is determined by the scheduling scheme adopted study of IEEE 802.15.4 [9]–[11], which is the counterpart of by the network coordinator, which is open in the standard. the assumption regarding the channel access probability taken Therefore, depending on the specific scheduling scheme used, in the IEEE 802.11 MAC protocol modeling [15], [16]. Two the performance analysis of CFP is actually the same as that important first-order MAC performance metrics, normalized of the well-studied centralized scheduling schemes in cellular throughput and average frame service time, are the primary systems. targeting results of the proposed model. Hence, we have used In the CAP, a non-persistent slotted CSMA/CA with binary average values wherever possible, as in [15], [17]. exponential backoff multiple access protocol, termed CAP- MAC in the sequel, is defined in the standard. Three variables A. System Model need to be maintained for each frame before it is successfully transmitted. They are respectively the number of random back- A single-hop wireless network consisting of N functionally off stages experienced (NB), the current backoff exponent identical nodes is considered. Specifically, all the nodes are (BE), and the contention window (CW)1. According to this within the same transmission range of one another so there protocol, a node with a frame waiting for transmission at the are no hidden terminals in the network. In addition, all MAC buffer is required to backoff a random number of slots the nodes are synchronized and they can correctly sense first, with CW set to two. At the end of this backoff stage, the channel status during the CCA slots. An ideal wireless the node will do the first channel clear assessment (CCA). If channel without transmission error is assumed so that all the channel is sensed idle, CW is decremented by one and transmitted frames may be lost only due to collisions caused by simultaneous transmissions from multiple nodes. However, 1The term contention window is the number of slots that the channel has non-ideal channels that may cause unsuccessful reception of to be sensed idle by a node before its transmission of a frame, which is completely different from the contention window defined in IEEE 802.11. 2Some issues of SS will be addressed in Sec. VI-B2
2342 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 7, NO. 6, JUNE 2008
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