QoS-aware MAC protocols for wireless sensor networks: A survey <s> Differentiation methods <s> We present an innovative MAC protocol (Q-MAC) that minimizes the energy consumption in multi-hop wireless sensor networks (WSNs) and provides quality of service (QoS) by differentiating network services based on priority levels. The priority levels reflect application priority and the state of system resources, namely residual energy and queue occupancies. The Q-MAC utilizes both intra-node and inter-node arbitration. The intra-node packet scheduling is a multiple queuing architecture with packet classification and weighted arbitration. We also introduce the power conservation MACAW (PC-MACAW) - a power-aware scheduling mechanism that, together with the loosely prioritized random access (LPRA) algorithm, govern the inter-node scheduling. Performance evaluation are conducted between Q-MAC and S-MAC with respect to two performance metrics: energy consumption and average latency. Simulation results indicate that the performance of the Q-MAC is comparable to that of the S-MAC in non-prioritized traffic scenarios; when packets with different priorities are present, Q-MAC supiors in average latency differentiation between the classes of service, while maintaining the same energy level as that of S-MAC <s> BIB001 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> Differentiation methods <s> This paper presents a new prioritized medium access control protocol (PSIFT) that improves the performance of event-driven wireless sensor networks by suppressing the redundant reports of sensor nodes in proximity. This suppression of similar reports results in low collision probability, shorter delay and accurate detection of events by the sink. PSIFT is a CSMA-based MAC protocol that provides traffic differentiation by varying the inter frame space and contention window size for each traffic class. We also introduce the new report suppression mechanism based on the broadcast nature of wireless transmission in PSIFT. This mechanism works very efficiently in large-scale, event-driven wireless sensor networks. Simulation results show that the new PSIFT MAC protocol can bring significant improvement in delay performance of the networks. When the sensing range grows up to transmission range, PSIFT can reduce the latency as much as three times in delivering reports as compared to the existing IEEE802.11 MAC protocol. <s> BIB002 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> Differentiation methods <s> Wireless sensor networks are very application specific and have to be developed according to the requirements of a particular problem. They are usually deployed in harsh and mobile environments, with tasks ranging from just collecting environment readings to highly crucial tasks like tracking an enemy vehicle. This means the sensor networks can monitor or sense a very wide range of parameters and tasks. However, all these measurements and events that are read by the sensors are never of equal importance. So we propose PR-MAC, which provides a scheme in which we prioritize the events that are being recognized. Our aim is to design a MAC layer protocol which would reduce the delay of transmitting high priority tasks or measurements. <s> BIB003 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> Differentiation methods <s> Abstract In sensor networks, data packets transmitted by different devices in home networking and industrial applications maintain different levels of importance. In this paper, we propose two mechanisms for IEEE 802.15.4 sensor networks to provide multi-level differentiated services which are required by each and every device. Mathematical model based on the discrete-time Markov chain is presented and is analyzed to measure the performance of the proposed mechanisms. Numerical results show the effects of the variation of contention window size and backoff exponent for service differentiation on 802.15.4 sensor networks. From the results, we derive that contention window size is more affective than backoff exponent on the saturation throughput while backoff exponent is more affective than contention window size on the average delay of every device. Simulation results are given to verify the accuracy of the numerical model. <s> BIB004 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> Differentiation methods <s> Energy is a major concern in ad hoc networks as it dictates the lifetime of a terminal and consequently the whole network. In an ad hoc network, not all terminals are equally solicited : depending on their position, some serve as relay nodes more than others. In order to maximize the lifetime of the whole network, we propose in this work to give priority in accessing the medium to the ones with larger number of neighbors, as done in Reference [1] but for routing only. In doing so, not only do we save energy for hard working nodes, as energy is mostly used by the MAC access, but we also take advantage at most out of these central nodes before they move to other, less dense locations. As far as the MAC access is concerned, our results show up to 82% increase in the lifetime of the whole network. <s> BIB005 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> Differentiation methods <s> The paper proposes a MAC layer protocol called QoMOR (QoS-aware MAC protocol using optimal retransmission) that is designed to provide QoS in a decentralized network with multiple sensor (source) nodes that do not have the capability to receive acknowledgements from the sink node. A minimum data delivery probability between the sensor and sink nodes is achieved by allowing each source node to transmit each new packet an optimal number of times in every interval of time. The paper first discusses the single QoS class case and derives the maximum achievable frame delivery probability and then extends the concepts to the multiple QoS class case. It also addresses a few design optimization criteria. Simulation and numerical analysis results show that QoMOR can effectively provide QoS guarantees under distributed control without using conventional ARQ-based schemes. <s> BIB006 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> Differentiation methods <s> Rapid penetration of small, customized wireless devices and enormous growth of wireless communication technologies has already set the stage for large-scale deployment of wireless sensor networks. While the need to minimize the energy consumption has driven significant researches in wireless sensor networks, offering some precise quality of service (QoS) for multimedia transmission over sensor networks has not received significant attention. However, the emerging new applications like video surveillance, telemedicine and traffic monitoring needs transmission of wireless multimedia over sensor networks. Naturally, offering some better QoS for wireless multimedia over sensor networks raises significant challenges. The network needs to cope up with battery-constraints, while providing improved QoS (end-to-end delay and bandwidth requirement). This calls for a suitable sensory MAC protocol capable of achieving application-specific QoS. In this paper, we have proposed a new QoS-based sensory MAC protocol, which not only adapts to application-oriented QoS, but also attempts to conserve energy without violating QoS-constraints. Performance modeling, analysis and simulation results demonstrate that the proposed protocol is capable of providing lower delay and better throughput, at the cost of reasonable energy consumption, in comparison to other existing sensory MAC protocols. <s> BIB007 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> Differentiation methods <s> Ultra-wideband (UWB) is a physical (PHY) layer technology that promises high transmission rates, as well as high resistance to noise and multipath effects. However, the impulse-based nature of UWB, coupled with its low transmission power, makes it difficult to enable efficient detection of the signals. Consequently, conventional carrier-sensing based MAC protocols cannot be used with a UWB PHY. In this paper, we propose SASW-CR - a slotted Aloha MAC protocol for UWB networks with sliding contention window and cooperative retransmissions, which provides QoS support without the use of carrier sensing. SASW-CR utilizes the slotted-Aloha technique to avoid carrier sensing and reduce packet collisions. In addition, it makes use of differentiated contention windows to provide varying classes of QoS for different traffic classes. A cooperative retransmission technique is also introduced to improve the overall traffic throughput and reduce end-to-end delay. The efficacy of our protocol is demonstrated through simulations. <s> BIB008 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> Differentiation methods <s> This paper introduces I-MAC, a new medium access control protocol for wireless sensor networks. I-MAC targets at improving both channel utilisation and energy efficiency while taking into account traffic load for each sensor node according to its role in the network. I-MAC reaches its objectives through prioritized and adaptive access to the channel. I-MAC performances obtained through simulations for different network topologies, scenarios and traffic loads show significant improvements in energy efficiency, channel utilization, loss ratio and delay compared to existing protocols. <s> BIB009 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> Differentiation methods <s> The media access control (MAC) protocol in wireless sensor networks provides a periodic listen/sleep state for protection from overhearing and idle listening. However, many scenarios and applications exist in which sensor nodes must send data quickly to destination nodes. This paper proposes the priority-based quality-of-service MAC (PQMAC) protocol for wireless sensor networks. We use data priority levels to differentiate among data transmissions, and propose a MAC protocol based on these levels. This protocol manages scheduling by adaptively controlling network traffic and the priority level. We focused on reducing the latency of the message transmission from the source to the destination. Simulation results showed that PQMAC reduces latency problems in wireless sensor networks while maintaining energy efficiency. <s> BIB010 </s> QoS-aware MAC protocols for wireless sensor networks: A survey <s> Differentiation methods <s> Data aggregation has been one of important key techniques to increase energy efficiency and bandwidth utilization in wireless sensor networks. There were many research works in this area, but most of them have a disadvantage that cannot optimally meet practical requirements, such as the heterogeneity of application data and latency requirements. In this paper, we propose a novel and simple data aggregation protocol, referred to as Lump, which enables to support QoS requirements of applications. For this purpose, it prioritizes packets for differentiated services and facilitates aggregation decision. Its architecture has a cross-layered design that mitigates overheads of in-network processing, and it is completely an independent module residing on between data-link layer and network layer so that it can be applicable to a variety of applications. Our experiments show that, when an intermediate node has four neighbors, Lump can reduce radio traffic up to 30% while satisfying QoS requirements. <s> BIB011
After priority assignment, the second and crucial phase of the service differentiation is resource sharing according to the importance of the carried data. There are some techniques at the MAC layer to provide different quality of services to different traffic classes and can be listed as follows: Changing Contention Window (CW) size: Contention based medium access schemes necessitate a contention period between the sensor nodes that attempt to send data concurrently, in order not to interfere with each other's transmission. Following the contention period, one of these sensor nodes wins the contention and qualifies to reserve the communication channel and sends its data. Since contention period determines the sensor node which will be served next, it has a direct effect on the medium sharing among all sensor nodes. We can extend this medium sharing also among all traffic classes carried in the network if sensor nodes are assessed according to their data waiting to be transmitted. Hence, the desired service quality can be provided to specific traffic classes by favoring the sensor nodes which have data belonging to that particular traffic class during contention period as in BIB007 BIB008 BIB003 BIB002 BIB010 BIB009 BIB004 . Traditionally, each contender node sets a timer or selects a contention slot. The first sensor node whose timer expires or whose slot time arrives reserves the medium and starts sending its data. By setting relatively shorter CW sizes for sensor nodes with higher priority traffic, it can be assured that the timer or slot of that sensor node will expire before others. Similarly, setting longer CW sizes for sensor nodes with lower priority traffic decreases their medium reservation chance. This method also has an indirect contribution to more qualified service provisioning by reducing the probability of collision since contentions mostly occur within the reduced set of nodes belonging to the same priority group BIB005 . Changing contention slot selection probability: In random access MAC schemes, contender nodes normally select a contention slot in a random fashion. However, employing non-uniform probability distributions for contention slot selection makes significant difference BIB001 . For instance, using a decreasing geometric distribution can increase the chance of medium reservation for a node since smaller contention slots are most likely to be selected. Changing inter-frame space (IFS) duration: In contention based medium access schemes, IFS is defined as the amount of time that sensor nodes stay quiet just before the contention or backoff period. Employing different IFS values for sensor nodes having different kinds of traffic classes provides service differentiation among them and gives precedence to the ones using shorter IFS BIB009 BIB002 BIB007 BIB003 . Changing backoff exponent: Although IFS and contention periods are utilized to overcome collisions in contention based medium access schemes, it is impossible to totally eliminate collisions because more than one sensor nodes may set their timers to the same time or select the same contention slot. Therefore, backoff mechanism is used to alleviate the congestion and reduce the probability of collision by increasing the contention duration. This increase is controlled by an exponent and takes the number of consecutive collisions into account. Hence, using different backoff exponents for different traffic classes can also be considered as a technique for service differentiation as in BIB004 . Transmission slot scheduling: Reservation-based medium access schemes divide the time into small portions called slot. Although there are plenty of slot assignment techniques in the literature, specific slot assignment methods can be derived according to the requirements of the application. For example, reserving consecutive slots for a video sensor node which transmits delay sensitive real-time video frames can increase the service quality considerably. Changing active time: MAC protocols employing sleeplisten schedule for energy saving can set the active time of the sensor nodes according to their priority level BIB007  . For example; sensor nodes processing besteffort data may work with 1% duty cycle while nodes processing real-time data are working with 50%. Eventually, lower latency and packet dropping ratio and higher throughput can be achieved for higher priority traffic. Changing adaptation speeds: Some protocols dynamically adapt themselves to the current network conditions by changing some parameters like CW size or backoff exponent during operation of the sensor node. Using different coefficients for the adaptation of parameters can control the speed of convergence to local optimums, hence can provide service differentiation BIB007 . Setting smaller coefficients for low priority traffic and bigger coefficients for high priority traffic in case of down-scale adaptation of CW size might be a good example. Changing error correction strength: MAC protocols utilizing error control mechanisms to provide QoS support can accommodate service differentiation by changing either persistency of retransmissions BIB006 or strength of the error control codes as mentioned in Section 5.2. Error resiliency of the traffic belonging to different priority classes can be controlled easily and hence, desired level of reliability can be assured for each traffic class. Changing DoA: As mentioned in Section 5.3, higher DoA needs accumulation of packets at the buffer of the router node which causes longer delays. On the other hand, lower DoA decreases the quality of redundancy elimination and increases the energy consumption. Therefore, employing variable DoA for each traffic class can be a technique for service differentiation in terms of delivery latency BIB011 .