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Designing a Channel Access Mechanism for Wireless Sensor Network

, Abdellatif I. Moustafa, , Hussein A. Konber

Abstract: Although there are various Medium Access Control (MAC) protocols proposed for Wireless Sensor Network (WSN), there is no protocol accepted as a standard specific to it. This paper deals with completing the design of our previously proposed MAC for WSN by proposing a channel access mechanism (CAM). The CAM is based on developing a backoff mechanism which mainly differentiates nodes’ backoffs depending on their different identification numbers, and it employs a performance tuning parameter for reaching a required performance objective. The probability distribution of the backoff period is constructed and Markov chain modeling is used to analyze and evaluate the CAM against the IEEE802.15.4 slotted CSMA/CA based on single- and multihop communication with respect to the reliability, the average delay, the power consumption, and the throughput. The analysis reveals that the required performance of CAM against the IEEE slotted CSMA/CA can be obtained by choosing the maximum backoff stages number and the tuning parameter value and that CAM performs better than the IEEE with larger nodes number. The multihop scenario results in a good end-to-end performance of CAM with respect to the reliability and delay becomes better with lengthier paths at the expense of increasing the energy consumption.1. IntroductionMAC [1–3] is the rudiment for any wireless communication system to function properly. It coordinates access to and transmission over the medium common to several nodes and puts rules to minimize interference and packet collisions among them under imposed constraints and desired performance goals.It is not highly true to say that the collision cause is the concurrent transmissions, because concurrent transmissions may not cause collision even if the transmitters reside in the same radio range. It is better not to point to the sender in clarifying the cause of the collision, but referring it to the receiver where the collision occurs at a receiver due to its reception to more than one signal at the same time because of its residence in the common transmission area of more than one transmitter whether it is the intended receiver of one or more of them.The MAC protocols can be divided into two main approaches, contention-based [4, 5] (random assignment protocols) and contention-free [6, 7] (schedule-based) of which indoors may be classified into fixed-assignment protocols and demand-assignment protocols. Far from the bad channel utilization of the fixed-assignment and its other cons and far from the additional overhead of the demand-assignment through polling and reservation, the contention-based MAC protocol is more logical for accessing the channel; however, it is more prone to fail in successful medium allocation and collision prevention. This depends on the characteristics of the contention-based MAC protocol itself and another high importance factor which is the logical topology that determines the number of talkers, who can talk to who, when and where they can talk, at what range, and so forth.Based on that, it is preferred to use a contention-based MAC with a good performance works on a logical topology paved for it especially with respect to predictability and number of contending nodes, where the condition under which these protocols may fail in preventing collisions is the sources’ number increase or the sources’ transmission rate increase.The MAC layer design intended by the work proposed in this paper is based on the physical layer of the IEEE802.15.4 standard [8–10] and composed of two techniques, a timing structure mechanism (TSM), proposed by our previous work [11], including the setup of the logical topology by dividing the network into subnetworks (sub-NWs) using multichannels and identifying the time structure of the sub-NW members’ work and the contention-based CAM proposed in this paper. The main TSM idea was to construct a receive schedule which makes, at a time, only one node from a group of nodes (sub-NW) listen to the channel, and each node takes its turn successively to listen for a small period. At any time a node wants to transmit, it can turn its radio to the transmit state and transmit directly in its maximum range or in a range suitable to the currently listening node using the CAM. The backoff periods are aligned with a reference time common to the nodes.The CAM is designed to be suitable to the proposed TSM and benefits from it, and it is based on developing a backoff mechanism, resorting to the common manner of increasing the backoff stages (i.e., repeating the trials of accessing the channel if it is found busy rather than announcing channel access failure and discarding the packet), and using a number of transmission trials to cope with the transmission failure rather than discarding the packet.The rest of this paper is organized as follows. Section 2 includes a brief literature review for wireless MAC protocols. Section 3 begins with giving an overview of the beacon-enabled IEEE802.15.4 slotted CSMA/CA; then it illustrates the proposed CAM idea and its modeling. The performance assessment of CAM is depicted in Section 4, where the CAM performance is evaluated against the slotted CSMA/CA in terms of single- and multihop communication; also the effect of different parameters on CAM performance and its tuning is considered. Finally, Section 5 concludes the paper and suggests open issues for future work.2. Literature ReviewThe wireless medium access schemes used in different types of wireless networks are based on carrier sensing, backoff algorithms and mechanisms for avoiding hidden and exposed terminal problems. The Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) with its two versions, nonpersistent and -persistent, represents the basic form of channel access control. In nonpersistent CSMA, if the device senses the channel busy, it backs off before trying to transmit again. When the channel is idle, the device transmits immediately. In -persistent CSMA, the device continues sensing the busy channel until it becomes idle, and in case of idle channel, it transmits or defers transmission according to a probability . Keeping devices in the receive state when not transmitting consumes a large amount of energy. Multiple Access with Collision Avoidance (MACA) [12] uses two additional packets, Request-to-Send (RTS) and Clear-to-Send (CTS) before the transmission to reduce the occurrence of the hidden and exposed terminal problems. The RTS is sent by the sender, and the receiver willing to accept data responds with CTS; the other devices hear the RTS or the CTS and avoid interfering the involved devices until end of transmission. The RTS/CTS represents overload on the network and causes additional delay.Modifications to these schemes were then proposed, such as using acknowledgment, using Request-for-Request-to-Send packet by a busy RTS receiver after finishing its transaction, employing waiting intervals other than the backoff time providing priority levels for wireless channel access as used in the IEEE802.11 [13] distributed coordination function, and using variations in backoff time computation method such as binary exponential backoff, multiplicative increase and linear decrease, balanced backoff algorithm, and waiting time based backoff. Wireless networks do not only use contention-based schemes but also use contention-free access such as the point coordination function defined in IEEE802.11 in which a coordinator device polls other devices for data.Due to the energy constraint in WSN, the design of WSN MAC considers other mechanisms, in addition to that used in coordinating the shared medium allocation and controls nodes’ activation to allow them to sleep saving their energy wasted in idle listening and overhearing. The used medium allocation scheme itself should be energy-efficient; for example, it does not employ large overhead. The MAC protocols proposed in literature for WSN can be broadly classified according to the scheme depicted in Figure 1.Figure 1: Different approaches for WSN MAC protocols.The contention-based synchronous sleep-scheduling [14] can be through having each node following a periodic active/sleep cycle; the nodes that are close to one another synchronize their active cycles together, and if the next hop of a transmission overhears it, it remains awake until receiving the forwarded data rather than sleeping and delaying data forwarding up to its next active cycle. But this is not always the case, the next-hop node may be out of the hearing range of both the sender and the receiver making data forwarding interruption problem unavoidable; the staggered wake-up scheduling [15, 16] is used to address this problem which creates a pipeline for data propagation based on the depth-level of nodes in a data-gathering tree, where the active period of one level partially overlaps with that of the lower level.In the asynchronous sender-initiated MAC [17, 18], the sender transmits a preamble to indicate a pending transmission. The receiver wakes up occasionally to listen to such a preamble for appropriately responding. In receiver-initiated schemes [19], instead of long preambles, the sender listens to the channel waiting for the receiver small beacons, transmitted in duty cycle fashion, to synchronize with the receiver. The asynchronous schemes are simpler to implement than the synchronous but it may result in very long delay. WSN MAC can be contention-free using Time Division Multiple Access (TDMA) or Frequency Division Multiple Access (FDMA) or hybrid. In multichannel MAC [20, 21], some issues are raised such as limited number of available channels, channel selection and assignment policy, and recursive channel switching overhead. Radio-triggered MAC [22, 23] and cross-layer MAC [24, 25] designs are other approaches proposed for WSN which can be employed with different types of channel access mechanisms. If radio-triggered ID [26, 27] is used, an additional wa
Keywords: probability distribution / Backoff / energy consumption / WSN / Wireless Sensor Network / transaction / TDMA / collision avoidance / MAC protocols

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