A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> QoS Provisioning Mechanisms <s> Wireless body area networks (WBANs) form a new and interesting area in the world of remote health monitoring. An important concern in such networks is the communication between the sensors. This communication needs to be energy efficient and highly reliable while keeping delays low. Mobility also has to be supported as the nodes are positioned on different parts of the body that move with regard to each other. In this paper, we present a new cross-layer communication protocol for WBANs: CICADA or Cascading Information retrieval by Controlling Access with Distributed slot Assignment. The protocol sets up a network tree in a distributed manner. This tree structure is subsequently used to guarantee collision free access to the medium and to route data towards the sink. The paper analyzes CICADA and shows simulation results. The protocol offers low delay and good resilience to mobility. The energy usage is low as the nodes can sleep in slots where they are not transmitting or receiving. <s> BIB001 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> QoS Provisioning Mechanisms <s> This paper presents a novel energy-efficient MAC Protocol designed specifically for wireless body area sensor networks (WBASN) focused towards pervasive healthcare applications. Wireless body area networks consist of wireless sensor nodes attached to the human body to monitor vital signs such as body temperature, activity or heart-rate. The network adopts a master-slave architecture, where the body-worn slave node periodically sends sensor readings to a central master node. Unlike traditional peer-to-peer wireless sensor networks, the nodes in this biomedical WBASN are not deployed in an ad hoc fashion. Joining a network is centrally managed and all communications are single-hop. To reduce energy consumption, all the sensor nodes are in standby or sleep mode until the centrally assigned time slot. Once a node has joined a network, there is no possibility of collision within a cluster as all communication is initiated by the central node and is addressed uniquely to a slave node. To avoid collisions with nearby transmitters, a clear channel assessment algorithm based on standard listen-before-transmit (LBT) is used. To handle time slot overlaps, the novel concept of a wakeup fallback time is introduced. Using single-hop communication and centrally controlled sleep/wakeup times leads to significant energy reductions for this application compared to more ldquoflexiblerdquo network MAC protocols such as 802.11 or Zigbee. As duty cycle is reduced, the overall power consumption approaches the standby power. The protocol is implemented in hardware as part of the Sensiumtrade system-on-chip WBASN ASIC, in a 0.13- mum CMOS process. <s> BIB002 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> QoS Provisioning Mechanisms <s> Wireless body sensor networks (BSNs) in healthcare systems operate under conflicting requirements. These are the maintenance of the desired reliability and message latency of data transmissions, while simultaneously maximizing battery lifetime of individual body sensors. In doing so, the characteristics of the entire system, including physical, medium access control (MAC), and application layers have to be considered. The aim of this paper is to develop a new MAC model for BSNs to fulfill all these specific rigorous requirements under realistic medical settings. For that purpose, a novel cross-layer fuzzy-rule scheduling algorithm and energy-aware radio activation policies are introduced. The main idea is to integrate a fuzzy-logic system in each body sensor to deal with multiple cross-layer input variables of diverse nature in an independent manner. By being autonomously aware of their current condition, body sensors are able to demand a "collision-free" time slot, whenever they consider it strictly required (e.g. high system packet delay or low body sensor residual battery lifetime). Similarly, they may refuse to transmit, if there is a bad channel link, thus permitting another body sensor to do so. This results in improving the system overall performance. The proposed MAC model is evaluated by computer simulations in terms of quality of service and energy consumption under specific healthcare scenarios. <s> BIB003 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> QoS Provisioning Mechanisms <s> We propose the cross-layer based battery-aware time division multiple access (TDMA) medium access control (MAC) protocols for wireless body-area monitoring networks in wireless healthcare applications. By taking into account the joint effect of electrochemical properties of the battery, time-varying wireless fading channels, and packet queuing characteristics, our proposed schemes are designed to prolong the battery lifespan of the wireless sensor nodes while guaranteeing the reliable and timely message delivery, which is critically important for the patient monitoring networks. In addition, we develop a Markov chain model to analyze the performance of our proposed schemes. Both the obtained analytical and simulation results show that our proposed schemes can significantly increase the battery lifespan of sensor nodes while satisfying the reliability and delay-bound quality of service (QoS) requirements for wireless body-area monitoring networks. Furthermore, the case study of the electrocardiogram (ECG) monitoring application shows that besides meeting the delay requirements, our proposed schemes outperform the IEEE 802.15.4 and Bluetooth protocols in terms of battery lifespan. <s> BIB004 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> QoS Provisioning Mechanisms <s> In this paper, an Urgency-based MAC (U-MAC) protocol, in which sensor nodes reporting urgent health information are given higher priority by cutting-off the number of packet retransmission of sensor nodes with non urgent health information, is proposed. The main consideration of this work is providing Quality of Service (QoS) support in medical wireless sensor networks through differentiating nodal access to the medium. The proposed MAC protocol is mathematically analyzed considering a beacon-enabled star network configuration of the IEEE 802.15.4a standard at 2.4 GHz. The used wireless body area network (WBAN) consists of N sensor nodes controlled by a single network coordinator. The obtained performance results show the capability of the proposed UMAC protocol in providing service differentiation in medical WBAN. Also, the results show that the number of critical nodes that can be supported by WBAN and their packet arrival rates decrease as the number of packet retransmission of such nodes is increased. <s> BIB005 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> QoS Provisioning Mechanisms <s> Wireless Body Area Network (WBAN) consists of low-power, miniaturized, and autonomous wireless sensor nodes that enable physicians to remotely monitor vital signs of patients and provide real-time feedback with medical diagnosis and consultations. It is the most reliable and cheaper way to take care of patients suffering from chronic diseases such as asthma, diabetes and cardiovascular diseases. Some of the most important attributes of WBAN is low-power consumption and delay. This can be achieved by introducing flexible duty cycling techniques on the energy constraint sensor nodes. Stated otherwise, low duty cycle nodes should not receive frequent synchronization and control packets if they have no data to send/receive. In this paper, we introduce a Traffic-adaptive MAC protocol (TaMAC) by taking into account the traffic information of the sensor nodes. The protocol dynamically adjusts the duty cycle of the sensor nodes according to their traffic-patterns, thus solving the idle listening and overhearing problems. The traffic-patterns of all sensor nodes are organized and maintained by the coordinator. The TaMAC protocol is supported by a wakeup radio that is used to accommodate emergency and on-demand events in a reliable manner. The wakeup radio uses a separate control channel along with the data channel and therefore it has considerably low power consumption requirements. Analytical expressions are derived to analyze and compare the performance of the TaMAC protocol with the well-known beacon-enabled IEEE 802.15.4 MAC, WiseMAC, and SMAC protocols. The analytical derivations are further validated by simulation results. It is shown that the TaMAC protocol outperforms all other protocols in terms of power consumption and delay. <s> BIB006 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> QoS Provisioning Mechanisms <s> As wireless devices and sensors are increasingly deployed on people, researchers have begun to focus on wireless body-area networks. Applications of wireless body sensor networks include healthcare, entertainment, and personal assistance, in which sensors collect physiological and activity data from people and their environments. In these body sensor networks, quality of service is needed to provide reliable data communication over prioritized data streams. This article proposes BodyQoS, the first running QoS system demonstrated on an emulated body sensor network. BodyQoS adopts an asymmetric architecture, in which most processing is done on a resource-rich aggregator, minimizing the load on resource-limited sensor nodes. A virtual MAC is developed in BodyQoS to make it radio-agnostic, allowing a BodyQoS to schedule wireless resources without knowing the implementation details of the underlying MAC protocols. Another unique property of BodyQoS is its ability to provide adaptive resource scheduling. When the effective bandwidth of the channel degrades due to RF interference or body fading effect, BodyQoS adaptively schedules remaining bandwidth to meet QoS requirements. We have implemented BodyQoS in NesC on top of TinyOS, and evaluated its performance on MicaZ devices. Our system performance study shows that BodyQoS delivers significantly improved performance over conventional solutions in combating channel impairment. <s> BIB007 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> QoS Provisioning Mechanisms <s> This work outlines challenges and requirements of designing a Medium Access Control (MAC) protocol for Body Area Networks, where sensor nodes send their measurement data to a central gateway device (also known as sink or hub) regularly on TDMA basis. While a number of protocols has already been proposed specifically for BAN's, they fail to account for unique characteristics of wireless channel around the human body, namely the deep and long fades that hinder communication. As such, they can not provide reliability and latency guarantees that are essential for many BAN applications. This work proposes to employ variable TDMA scheduling that allows the order of transmissions within each TDMA round to be decided dynamically, rather than be fixed in advance. Accordingly, the hub is able to account for variations of the wireless channel when allocating TDMA slots, thus increasing the probability of successful transmissions. Initial evaluations based on extensive experimental measurements have shown that, under strict latency constraints, variable scheduling approach allows to reduce transmission losses by up to 15% without accounting for retransmissions, and achieve over 99.5% reliability when retransmissions are enabled. <s> BIB008 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> QoS Provisioning Mechanisms <s> We consider reliable communications in Body Area Networks (BAN), where a set of nodes placed on human body are connected using wireless links. In order to keep the Specific Absorption Rate (SAR) as low as possible for health safety reasons, these networks operate in low transmit power regime, which however, is known to be error prone. It has been observed that the fluctuations of the Received Signal Strength (RSS) at the nodes of a BAN on a moving person show certain regularities and that the magnitude of these fluctuations are significant (5 -- 20 dB). In this paper, we present BANMAC, a MAC protocol that monitors and predicts the channel fluctuations and schedules transmissions opportunistically when the RSS is likely to be higher. The MAC protocol is capable of providing differentiated service and resolves co-channel interference in the event of multiple co-located BANs in a vicinity. We report the design and implementation details of BANMAC integrated with the IEEE 802.15.4 protocol stack. We present experimental data which show that the packet loss rate (PLR) of BANMAC is significantly lower as compared to that of the IEEE~802.15.4 MAC. For comparable PLR, the power consumption of BANMAC is also significantly lower than that of the IEEE~802.15.4. For co-located networks, the convergence time to find a conflict-free channel allocation was approximately 1 s for the centralized coordination mechanism and was approximately 4 s for the distributed coordination mechanism. <s> BIB009 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> QoS Provisioning Mechanisms <s> In this paper, we aim at the optimal use of the scarce energy collected by a node powered by human energy harvesting in order to improve the provided Quality of Service (QoS). To achieve this goal, we have developed a Power-QoS control scheme, called PEH-QoS. PEH-QoS is composed of three sub-modules that interact with each other in order to make optimal use of energy and get the best possible QoS. The scheme intends to ensure that a node can both capture/detect the medical events and transmit the respective data packets efficiently. One of the main features of our mechanism is that only useful data sequences are transmitted, discarding data packets that have lost their clinical validity (i.e., out of date). Extensive simulations have been conducted in order to evaluate the behavior of PEH-QoS in a typical medical node under energy harvesting conditions. <s> BIB010 </s> A Survey on M2M Systems for mHealth: A Wireless Communications Perspective <s> QoS Provisioning Mechanisms <s> In this paper, BSN-MAC, a medium access control (MAC) protocol designed for Body Sensor Networks (BSNs) is proposed. Due to the traffic coupling and sensor diversity characteristics of BSNs, common MAC protocols can not satisfy the unique requirements of the biomedical sensors in BSNs. BSN-MAC exploits the feedback information from the deployed sensors to form a closed-loop control of the MAC parameters. A control algorithm is proposed to enable the BSN coordinator to adjust parameters of the IEEE 802.15.4 superframe to achieve both energy efficiency and low latency on energy critical nodes. We evaluate the performance of BSN-MAC by comparing it with the IEEE 802.15.4 MAC protocol using energy efficiency as the primary metric. <s> BIB011
As mentioned in Section 3, healthcare applications have very heterogeneous traffic characteristics. Usually, most applications produce low-rate and periodic traffic, thus encouraging reservation-based scheduling solutions for uplink communication at the MAC layer. However, high traffic applications, such as video monitoring or aggregated data of given sensors (e.g., ECG, EMG, etc.) must also be supported. Hence, MAC protocols should define different levels of priority and support advanced scheduling policies, often based on the cross-layer design paradigm. Service differentiation and access priorities: Priority-based scheduling based on service differentiation is one of the most widely employed mechanisms for QoS. The main idea lies in the assignment of different access priorities to traffic flows in order to achieve specific performance goals. Different criteria can be employed for the priority assignment, such as QoS requirements (e.g., latency constraints) or data criticality (e.g., emergency alarms versus normal data traffic). In the context of WBANs, a common approach is to distinguish between normal and emergency traffic and assign a higher level of access priority to the latter. This concept is adopted in the Urgency-based MAC (U-MAC) BIB005 , a protocol based on the slotted Aloha mechanism employed in the IEEE 802.15.4. U-MAC classifies all traffic flows as either critical or non-critical. Critical data flows are allowed a higher number of transmission attempts, thus increasing their channel access probability. A different way to handle emergency data has been proposed in BIB002 . The idea is that whenever the master is not busy communicating with a slave that is awake, it sequentially sends polls to all associated slaves. If a slave node wants to communicate an alarm, it stays awake until a poll is received (otherwise, under normal traffic, the slave would wake up according to its sleep pattern). In BIB006 , alarms are handled with the help of a secondary wakeup radio: the node informs the coordinator on the presence of emergency data by sending a control packet in the wakeup radio, and then the coordinator immediately polls the node in the main channel to retrieve the alarm message. More advanced service differentiation schemes define more priority levels. For example, in BodyQoS BIB007 , three classes of descending priority are defined: reserved downlink data from the coordinator to the nodes, reserved uplink data from the nodes to the coordinator and best-effort data. Reserved downlink data are scheduled first in each superframe, followed by the reserved uplink data, if there is still available bandwidth. Any remaining bandwidth is devoted to best-effort traffic. Adaptive resource scheduling that takes into consideration the channel quality is also provided. Two levels of scheduling are defined in  . At the first level, traffic flows are classified according to their criticality level (i.e., high, medium or low urgency). At the second level, each priority class is allocated a dynamically adjusted non-zero portion of the bandwidth according to the Generalized Processor Sharing principle, whereas Earliest Deadline First scheduling is adopted for traffic flows of the same class. Cross-layer scheduling: In addition to service differentiation, advanced scheduling algorithms can be designed by taking into account cross-layer information acquired from the interaction of the MAC with other protocol layers. Cross-layer parameters often include channel state information, provided by the PHY layer, or routing information from the network layer, among others. A simple channel-aware method, called Flipping algorithm, that can improve the performance of TDMA-based schemes, has been proposed in BIB008 . The solution applies to channels that can be modeled as a two-state Markov chain: good channel condition ensures successful data transmissions, whereas packet losses occur when the link quality is bad. The Flipping algorithm schedules the transmissions within a TDMA superframe based on the outcome of the previous superframe. All good links of the previous frame are scheduled first, but with reversed order, in order to exploit the most recently observed good quality links. On the other hand, all bad links are scheduled last in the new superframe, thus increasing the probability of having their channel quality changed to the best. BANMAC BIB009 is a more advanced cross-layer scheme that exploits the fact that, even though the channel quality of WBANs experiences many fluctuations (e.g., due to the movement of body parts), some regular patterns can still be observed. BANMAC is able to predict the channel fluctuations and schedule the transmissions of the nodes opportunistically when the channel conditions are more favorable. Another cross-layer approach is adopted in CICADA BIB001 , where a joint MAC scheduling and routing scheme is employed to ensure ensure low-latency and network flexibility to topology changes. Although QoS is the main objective of the scheduling mechanisms presented in this section, sometimes scheduling may also result to a reduction of energy consumption. In continuation, we present some schemes that include energy-related parameters such as power constraints (e.g., power limitations of implantable devices) or battery lifetime levels in the scheduling decisions. In BSN-MAC BIB011 , for example, four priority classes are defined to match four energy levels: (i) unconstrained, for the coordinator node that is assumed to have no energy constraints; (ii) constrained, for wearable sensor devices; (iii) highly constrained, for ingestible sensor devices; and (iv) extremely constrained, for implantable devices. The protocol also defines three levels of data criticality (low, normal and high). These parameters, along with the remaining battery lifetime level and the buffer occupancy, are combined to produce a cross-layer metric used to adjust the length of the IEEE 802.15.4 superframe with the objective of reducing latency while enhancing energy efficiency. DQBAN takes into account the quality of the wireless link, the residual battery lifetime and the waiting time of the nodes in the transmission queue to implement a cross-layer, fuzzy-logic scheduler BIB003 . The main idea is that each node can affect the scheduling order by sending a specific request in the scheduling part of the frame. For example, a node may request to be given priority if its energy level becomes critical or to delay its transmission if bad channel quality is detected. A cross-layer TDMA scheme that aims to increase the battery lifespan of the nodes without compromising reliability and QoS requirements, has been proposed in BIB004 . The main driver of the proposed scheme is based on battery discharge dynamics which dictate that battery lifetime can be increased if the battery remains idle for a certain amount of time. In order to maximize the idle time between transmissions, nodes may choose to accumulate several packets in their buffers. A decision on whether to transmit in a given time slot is made depending on cross-layer information regarding the battery recovery level, the buffer occupancy and the wireless channel state. Finally, PEH-QoS BIB010 promotes energy-aware management and QoS provisioning in a WBAN with energy harvesting capabilities by proposing a three-module architecture that comprises: (i) the Power-EH Aware Management (PHAM), which ensures the optimal use of the harvested energy, by preventing transmission attempts by nodes with insufficient energy levels and encouraging the detection of all events by the sensor nodes (to ensure the detection of critical medical events); (ii) the Data Queue Aware Control (DQAC), which prevents the saturation of the data packet queue and maintains the clinical validity of the data by discarding outdated packets; (iii) the Packet Aggregator/Scheduling System (PASS) that maximizes throughput and energy efficiency by supporting the transmission of aggregated data packets.