On the Implant Communication and MAC Protocols for a WBAN

On the Implant Communication and MAC Protocols for a WBAN Abstract Recent advances in micro-electro-mechanical systems (MEMS), wireless communication, low-power intelligent sensors, and semiconductor technologies have allowed the realization of a wireless body area network (WBAN). A WBAN provides unobtrusive health monitoring for a long period of time with real-time updates to the physician. It is widely used for ubiquitous healthcare, entertainment, and military applications. The implantable and wearable medical devices have several critical requirements such as power consumption, data rate, size, and low-power medium access control (MAC) protocols. This article consists of two parts: body implant communication, which is concerned with the communication to and from a human body using RF technology, and WBAN MAC protocols, which presents several low-power MAC protocols for a WBAN with useful guidelines. In body implant communication, the in-body radio frequency (RF) performance is affected considerably by the implants depth inside the human body as well as by the muscle and fat. We observe best performance at a depth of 3cm and not close to the human skin. Furthermore, the study of low-power MAC protocols highlights the most important aspects of developing a single, a low-power, and a reliable MAC protocol for a WBAN. Keywords: In-body, on-body, RF communication, Implant, WBAN 1. Introduction Cardiovascular diseases are the foremost cause of deaths in the United States and Europe since 1900. More than ten million people are affected in Europe, one million in the US, and twenty two million people in the world [1]. The number is projected to be triple by 2020, resulting in an expenditure of around 20% of the gross domestic product (GDP). The ratio is 17% in South Korea and 39% in the UK [2]. The healthcare expenditure in the US is expected to be increased from $2.9 trillion in 2009 to $4 trillion US dollars in 2015 [3]. The impending health crisis attracts researchers, industrialists, and economists towards optimal and quick health solutions. The non-intrusive and ambulatory health monitoring of patients vital signs with real time updates of medical records via internet provide economical solutions to the health care systems. A wireless body area network (WBAN) is becoming increasingly important for healthcare systems, sporting activities, and members of emergency as well as military services. WBAN is an integration of in-body (implants) and on-body (wearable) sensors that allow inexpensive, unobtrusive, and long-term health monitoring of a patient during normal daily activities for prolonged periods of time. In-body radio frequency (RF) communications have the potential to dramatically change the future of healthcare. For example, they allow an implanted pacemaker to regularly transmit performance data and the patients health status to the physician. However, the human body poses many wireless transmission challenges. This is partially conductive and consists of materials having different dielectric constants and characteristics impedance. The interface of muscles and fats may reflect the RF wave rather than transmitting it. The key elements of an RF-linked implant are the in-body antenna and the communi cation link performance. Also, in the case of many implants and wearable sensors, a low-power MAC protocol is required to accommodate the heterogeneous traffic in a power-efficient manner. This article is divided into two parts: body implant communication and WBAN MAC protocols. In the body implant communication part, we look at the RF communication link performance at various depths inside a human (artificial) body. In the MAC part, we review the existing low-power MAC protocols and discuss their pros and cons in the context of a WBAN. We further provide alternative MAC solutions for in-body and on-body communication systems. The rest of the article is divided into three sections. In section 2, we present a discussion on body implant communication including in-body electromagnetic induction, RF communication, antenna design, and the communication link performance. Section 3 discusses several low-power MAC protocols and realizes a need for a new, a low-power, and a reliable MAC protocol for a WBAN. The final section concludes our work. 2. Body Implant Communication There are several ways to communicate with an implant that includes the use of electromagnetic induction and RF technology. Both are wireless and their use depends on the application requirements. Further, the key elements of an RF-linked implant are the in-body antenna and the communication link performance. The following part discusses in-body electromagnetic induction, RF communication, antenna design, and the communication link performance. 2.1. In-body Electromagnetic Induction Several applications still use electromagnetic coupling to provide a communication link to an implant device. In this scheme, an external coil is held very close to the body that couples to a coil implanted just below the skin surface. The implant is powered by the coupled magnetic field and requires no battery for communication. Data is transferred from the implant by altering the impedance of the implanted loop that is detected by the external coil and electronics. This type of communication is commonly used to identify animals that have been injected with an electronic tag. Electromagnetic induction is used when continuous, long-term communication is required. The base band for electromagnetic communication is typically 13.56 MHz or 28 MHz, with other frequencies also available. The choice of a particular band is subject to regulation for maximum specific absorption rate (SAR). The inductive coupling achieves best power transfer efficiency when uses large transmit and receive coil s. It, however, becomes less efficient when the space is an issue of the device is implanted deep inside the human body. Furthermore, inductive coupling technique does not support a very high data rate and cannot initiate a communication session from inside of the body. 2.2. In-body RF Communication Compared with the electromagnetic induction, RF communication dramatically increases bandwidth and supports a two-way data communication. The band designated for the in-body RF communication is medical implant communication service (MICS) band and is around 403 to 405 MHz. This band has a power limit of 25  µW in the air and is usually split into ten channels of 300 kHz bandwidth each. The human body is a medium that poses numerous wireless transmission challenges. It consists of various components that are not predictable and will change as the patient ages, gains or losses weight, or even changes posture. Values of dielectric constant (ÃŽ µr), conductivity (ÏÆ') and characteristic impedance (Zo) for some body tissue are given in table 1 [4]. This demonstrates that these two tissue types are very different. Also, the dielectric constant affects the wavelength of a signal. At 403 MHz, the wavelength in the air is 744mm, but in muscle with ÃŽ µr = 50 the wavelength reduces to 105mm, which helps in designing implanted antennas. 2.3. In-body Antenna Design A modern in-body antenna should be tuneable by using an intelligent transceiver and software routine. This enables the antenna coupling circuit to be optimised. Due to the frequency, and available volume, a non-resonant antenna is commonly used. It has a lower gain than a resonant antenna. This makes design of the antenna coupling circuit very important. Antenna options are dictated by the location of the implant. A patch antenna can be used when the implant is flat. Patch antennas are comprised of a flat insulating substrate coated on both sides with a conductor. The substrate is a body compatible material with a platinum or a platinum/iridium conductor. The upper surface is the active face and is connected to the transceiver. The connection to the transceiver needs to pass through the case where the hermetic seal is maintained, requiring a feed-through. The feed-through must have no filter capacitors present; these are common on other devices. An implanted patch antenna is electrically larger than its physical size because it is immersed in a high (ÃŽ µr) medium. It can be much larger electrically if the substrate is of higher (ÃŽ µr), such as titania or zirconia. A loop antenna can also be attached to the implant. This antenna operates mostly by the magnetic field, whereas the patch operates mostly by the electric field. The loop antenna delivers performance comparable to that of a dipole, but with a considerably smaller size. In addition, the magnetic permeability of muscle or fat is very similar to that of an air, unlike the dielectric constant that varies considerably. This property enables an antenna to be built and used with much less need for retuning. A loop antenna can be mounted on the case in a biocompatible structure. 2.4. In-body Link Performance The demonstration system consists of a base-station, an implant, antennas, and a controlling laptop. The base-station contains a printed circuit board (PCB) with a wakeup RF circuit, a Zarlink ZL70101 IC, and a micro-controller. It sends a wakeup signal on industrial, scientific, and medical (ISM) 2.4 GHz band to power up the implant to communicate. It also supports communication within the MICS band. The implant contains a Zarlink ZL70101 IC, a micro-controller, and a battery. The power limits of the wakeup signal for ISM and MICS bands transmitters are 100mW and 25  µW respectively. Experiments that measure the performance of an implant inside a living body are difficult to arrange. The alternative is to use 3D simulation software or a body phantom defined in [5]. The use of 3D simulation software is time consuming and hence practically not valuable. Therefore, measurements are generally performed using the body phantom and immersing a battery-powered implant into it [6]. Since no additional cables are attached to the test implant, the interference errors in the measurements are minimal. The body phantom is filled with a liquid that mimics the electrical properties of the human body tissues. The test environment is an anechoic chamber that includes a screened room. The interior walls of the room have sound-absorbent cones to minimize any reflections from walls or the floor that could distort the results. In real life, however, the results will be affected by the reflections from walls, desks, and other equipment and hardware. The body phantom is mounted on a woo den stand (non-conductive). The distance from the body phantom to the base-station is 3m. The MICS base-station dipole antenna is mounted on a stand. 1(a) shows the anechoic chamber with a body phantom (on the wooden stand), a log periodic test antenna (foreground), and a base-station dipole (right). The log periodic antenna is used to calculate the power radiated from the body phantom. A depth is defined as the horizontal distance between the outer skin of the phantom and the test implant. Vertical polarization of the implant is the case when the long side of the box and the patch antenna is vertical. The link performance is measured once the communication link is established. The measurements include the effective radiated power (ERP) from the implant, the received signal at the implant from the base-station, and the link quality. Measurements are made over a set distance with all the combinations of implant and test antenna polarisations, i.e., vertical-vertical (V-V), horizontal-vertical (H-V), vertical-horizontal (V-H), and horizontal-horizontal (H-H) polarisations. Typical results are shown in 1(b) where the ERP is calculated from the received signal power and the antenna characteristics. The measurement of the signal levels is done with the log periodic antenna and the spectrum analyzer. It can be seen in the that there is a significant difference in signal levels with polarisation combinations and depths. For a V-V polarisation, the ERP increases from a 1cm depth to a maximum between 2 and 7 cm, and then it decreases. The gradual increase is due to the simulated body acti ng as a parasitic antenna. The also shows how the signal level is affected by the depth with different polarisation. Such a test needs to be done with the antenna that is to be used in the final product. To measure the received signal at the implant, the Zarlink ZL70101 has an inbuilt receive signal strength indication (RSSI) function that gives a measure of the signal level detected. RSSI is a relative measurement with no calibration. The implant receives and measures a continuous wave signal transmitted by the base-station. In this case, the implant and the base-station antennas are vertically polarised. 1(c) shows an increase in the signal level at a depth between 3 and 4cm for a 15dec power. The power settings refer to the base-station and are cond to set the ERP to 25  µW. Signal levels are not valuable unless they are related to data transmission. One way to maintain the link quality is to measure the number of times the error correction is invoked during the transmission of 100 blocks of data. Two types of error correction codes, i.e., error correction code (ECC) and cyclic redundancy code (CRC) are invoked to maintain data integrity and reliability. The fewer ECC and CRC invocations result in better link quality. In 1(d), the error correction is lowest at a depth between 3 and 5 cm. A sample of ECC data collected at a 3cm implant depth is given in Table 2. The Count indicates the number of data blocks, the Time (ms) indicates the block transmission time, and the ECC indicates the number of times it is invoked. During the transmission of 100 blocks of data at a 3cm depth, the ECC is invoked 368 times, which is further equivalent to an average 3.68 times (as given in 1(d)). 2.5. Discussion The ERP, RSSI, as well as the ECC and CRC plots show that the implant demonstrates the best performance at a depth between 3 and 5 cm. The depth and position of an implant is not chosen for engineering performance but for the best clinical reasons. The implant designer must be aware of the possible losses through the human body. The attenuation and the parasitic antenna effects vary from patient to patient, with the position of the implant and with the time as the patient gains, or looses weight. Therefore, these factors need to be built into the link budget. 3. WBAN MAC Protocols Some of the common objectives in a WBAN are to achieve maximum throughput, minimum delay, and to maximize the network lifetime by controlling the main sources of energy waste, i.e., collision, idle listening, overhearing, and control packet overhead. A collision occurs when more than one packet transmits data at the same time. The collided packets have to be retransmitted, which consumes extra energy. The second source of energy waste is idle listening, meaning that a node listens to an idle channel to receive data. The third source is overhearing, i.e., to receive packets that are destined to other nodes. The last source is control packet overhead, meaning that control information area added to the payload. Minimal number of control packets should be used for data transmission. Generally MAC protocols are grouped into contention-based and schedule-based MAC protocols. In contention-based MAC protocols such as carrier sense multiple access/collision avoidance (CSMA/CA) protocols, nodes contend for the channel to transmit data. If the channel is busy, the node defers its transmission until it becomes idle. These protocols are scalable with no strict time synchronization constraint. However, they incur significant protocol overhead. In schedule-based protocols such as time division multiple access (TDMA) protocols, the channel is divided into time slots of fixed or variable duration. These slots are assigned to nodes and each node transmits during its slot period. These protocols are energy conserving protocols. Since the duty cycle of radio is reduced, there is no contention, idle listening and overhearing problems. But these protocols require frequent synchronization. Table 3 compares CSMA/CA and TDMA protocols. 3.1. WBAN MAC Requirements The most important attribute of a good MAC protocol for a WBAN is energy efficiency. In some applications, the device should support a battery life of months or years without interventions, while others may require a battery life of tens of hours due to the nature of the applications. For example, cardiac defibrillators and pacemakers should have a lifetime of more than 5 years, while swallowable camera pills have a lifetime of 12 hours. Power-efficient and flexible duty cycling techniques are required to minimize the idle listening, overhearing, packet collisions and control packet overhead. Furthermore, low duty cycle nodes should not receive frequent synchronization and control information (beacon frames) if they have no data to send or receive. The WBAN MAC should also support simultaneous operation on in-body (MICS) and on-body channels (ISM or UWB) at the same time. In other words, it should support multiple physical layer (Multi-PHYs) communication or MAC transparency. Other important factors are scalability and adaptability to changes in the network, delay, throughput, and bandwidth utilization. Changes in the network topology, the position of the human body, and the node density should be handled rapidly and successfully. The MAC protocol for a WBAN should consider the electrical properties of the human body and the diverse traffic nature of in-body and on-body nodes. For example, the data rate of in-body nodes varies, ranging from few kbps in pacemaker to several Mbps in capsular endoscope. In the following sections, we discuss proposed MAC protocols for a WBAN with useful guidelines. We also present a case study of IEEE 802.15.4, PB-TDMA, and S-MAC protocols for a WBAN using NS2 simulator. 3.2. Proposed MAC Protocols for a WBAN In this section, we study proposed MAC protocols for a WBAN followed by useful suggestions/comments. Many of the proposed MAC protocols are the extension of existing MAC protocols originally proposed for wireless sensor networks (WSNs). 3.2.1. IEEE 802.15.4 IEEE 802.15.4 has remained the main focus of many researchers during the past few years. Some of the main reasons of selecting IEEE 802.15.4 for a WBAN were low-power communication and support of low data rate WBAN applications. Nicolas et.al investigated the performance of a non-beacon IEEE 802.15.4 in [7], where low upload/download rates (mostly per hour) are considered. They concluded that the non-beacon IEEE 802.15.4 results in 10 to 15 years sensor lifetime for low data rate and asymmetric WBAN traffic. However, their work considers data transmission on the basis of periodic intervals which is not a perfect scenario in a real WBAN. Furthermore, the data rate of in-body and on-body nodes are not always low, i.e., it ranges from 10 Kbps to 10 Mbps, and hence reduces the lifetime of the sensor nodes. Li et.al studied the behavior of slotted and unslotted CSMA/CA mechanisms and concluded that the unslotted mechanism performs better than the slotted one in terms of throughput and lat ency but with high cost of power consumption [8]. Intel Corporation conducted a series of experiments to analyze the performance of IEEE 802.15.4 for a WBAN [9]. They deployed a number of Intel Mote 2 [10] nodes on chest, waist, and the right ankle. Table 4 shows the throughput at a 0dBm transmit power when a person is standing and sitting on a chair. The connection between ankle and waist cannot be established, even for a short distance of 1.5m. All other connections show favourable performance. Dave et al. studied the energy efficiency and QoS performance of IEEE 802.15.4 and IEEE 802.11e [11] MAC protocols under two generic applications: a wave-form real time stream and a real-time parameter measurement stream [12]. Table 5 shows the throughput and the Power (in mW) for both applications. The AC_BE and AC_VO represent the access categories voice and best-effort in the IEEE 802.11e. Since the IEEE 802.15.4 operates in the 2.4 GHz unlicensed band, the possibilities of interference from other devices such as IEEE 802.11 and microwave are inevitable. A series of experiments to evaluate the impact of IEEE 802.11 and microwave ovens on the IEEE 802.15.4 transmission are carried out in [13]. The authors considered XBee 802.15.4 development kit that has two XBee modules. Table 6 shows the affects of microwave oven on the XBee remote module. When the microwave oven is ON, the packet success rate and the standard deviation is degraded to 96.85% and 3.22% respectively. However, there is no loss when the XBee modules are taken 2 meters away from the microwave oven. 3.2.2. Heartbeat Driven MAC Protocol (H-MAC) A Heartbeat Driven MAC protocol (H-MAC) [14] is a TDMA-based protocol originally proposed for a star topology WBAN. The energy efficiency is improved by exploiting heartbeat rhythm information in order to synchronize the nodes. The nodes do not need to receive periodic information to perform synchronization. The heartbeat rhythm can be extracted from the sensory data and hence all the rhythms represented by peak sequences are naturally synchronized. The H-MAC protocol assigns dedicated time slots to each node to guarantee collision-free transmission. In addition, this protocol is supported by an active synchronization recovery scheme where two resynchronization schemes are implemented. Although H-MAC protocol reduces the extra energy cost required for synchronization, it does not support sporadic events. Since the TDMA slots are dedicated and not traffic adaptive, H-MAC protocol encounters low spectral/bandwidth efficiency in case of a low traffic. For example, a blood pressure node may not need a dedicated time slot while an endoscope pill may require a number of dedicated time slots when deployed in a WBAN. But the slots should be released when the endoscope pill is expelled. The heartbeat rhythm information varies depending on the patient condition. It may not reveal valid information for synchronization all the time. One of the solutions is to assign the time slots based on the nodes traffic information and to receive synchronization packets when required, i.e., when a node has data to transmit/receive. 3.2.3. Reservation-based Dynamic TDMA Protocol (DTDMA) A Reservation-based Dynamic TDMA Protocol (DTDMA) [15] is originally proposed for a normal (periodic) WBAN traffic where slots are allocated to the nodes which have buffered packets and are released to other nodes when the data transmission/reception is completed. The channel is bounded by superframe structures. Each superframe consists of a beacon used to carry control information including slot allocation information, a CFP period a configurable period used for data transmission, a CAP period a fixed period used for short command packets using slotted aloha protocol, and a configurable inactive period used to save energy. Unlike a beacon-enabled IEEE 802.15.4 superframe structure where the CAP duration is followed by CFP duration, in DTDMA protocol the CFP duration is followed by CAP duration in order to enable the nodes to send CFP traffic earlier than CAP traffic. In addition, the duration of inactive period is configurable based on the CFP slot duration. If there is no CFP t raffic, the inactive period will be increased. The DTDMA superframe structure is given in 2(a). It has been shown that for a normal (periodic) traffic, the DTDMA protocol provides more dependability in terms of low packet dropping rate and low energy consumption when compared with IEEE 802.15.4. However, it does not support emergency and on-demand traffic. Although the slot allocation based on the traffic information is a good approach, the DTDMA protocol has several limitations when considered for the MICS band. The MICS band has ten channels where each channel has 300 Kbps bandwidth. The DTDMA protocol is valid only for one channel and cannot operate on ten channels simultaneously. In addition, the DTDMA protocol does not support the channel allocation mechanism in the MICS band. This protocol can be further investigated for the MICS band by integrating the channel information in the beacon frame. The new concept may be called Frequency-based DTDMA (F-DTDMA), i.e., the coordinator first selects one of the channels in the MICS band and then divides the selected channel in TDMA superframe (s) according to the DTDMA protocol. However the FCC has imposed several restrictions on the channel selection/allocation mechanism in the MICS band, which further creates problems for the MAC designers. 3.2.4. BodyMAC Protocol A BodyMAC protocol is a TDMA-based protocol where the channel is bounded by TDMA superframe structures with downlink and uplink subframes as given in 2(b) [16]. The downlink frame is used to accommodate the on-demand traffic and the uplink frame is used to accommodate the normal traffic. There is no proper mechanism to handle the emergency traffic. The uplink frame is further divided into CAP and CFP periods. The CAP period is used to transmit small size MAC packets. The CFP period is used to transmit the normal data in a TDMA slot. The duration of the downlink and uplink superframes are defined by the coordinator. The advantage of the BodyMAC protocol is that it accommodates the on-demand traffic using the downlink subframe. However, in case of low-power implants (which should not receive beacons periodically), the coordinator has to wake up the implant first and then send synchronization packets. After synchronization, the coordinator can request/send data in the downlink subframe. The wake up procedure for low-power implants is not defined in the BodyMAC protocol. One of the solutions is to use a wakeup radio in order to wake up low-power implants before using the downlink subframe. In addition the wakeup packets can be used to carry control information such as channel (MICS band) and slot allocation information from the coordinator to the nodes. Finally, the BodyMAC protocol uses the CSMA/CA protocol in the CAP period which is not reliable for a WBAN. This should be replaced by slotted-ALOHA as done in DTDMA. Further details on low-power MAC protocols (originally proposed for WSNs) for a WBAN are given in Appendix I. 3.3. Case Study: IEEE 802.15.4, PB-TDMA, and SMAC Protocols for a WBAN In this section, we investigate the performance of a beacon-enabled IEEE 802.15.4, preamble-based TDMA [17], and SMAC protocols for an on-body communication system. Our analysis is verified by extensive simulations using NS-2. The wireless physical parameters are considered according to a low-power Nordic nRF2401 transceiver (Chipcon CC2420 radio [18] is considered in case of IEEE 802.15.4) [19]. This radio transceiver operates in the 2.4-2.5 GHz band with an optimum transmission power of -5dBm. We use the shadowing propagation model throughout the simulations. We consider a total of 7 nodes firmly placed on a human body. The nodes are connected to the coordinator in a star topology. The distribution of the nodes and the coordinator is given in 3(a). The initial nodes energy is 5 Joules. The packet size is 120 bytes. The average data transmission rate of ECG, EEG, and EMG is 10, 70, and 100 kbps. The transport agent is a user datagram protocol (UDP). Since the traffic is an uplink t raffic, the buffer size at the coordinator is considered unlimited. In a real WBAN, the buffer size should be configurable based on the application requirements. For energy calculation, we use the existing energy model defined in NS-2. The simulation area is 33 meter and each node generates constant bit rate (CBR) traffic. The CBR traffic is an ideal model for some of the medical applications, where the nodes send data based on pre-defined traffic patterns. However, most of the nodes in a WBAN have heterogeneous traffic characteristics and they generate periodic and aperiodic traffic. In this case, they will have many traffic models operating at the same time, ranging from CBR to variable bit rate (VBR). 3(b) shows the throughput of the IEEE 802.15.4, PB-TDMA, and S-MAC protocols. The performance of the IEEE 802.15.4, when cond in a beacon-enabled mode, outperforms PB-TDMA and S-MAC protocols. The efficiency of a MAC protocol depends on the traffic pattern. In this case, S-MAC protocol results poor performance because the traffic scenario that we generated is not an ideal scenario for the S-MAC. 3(c) shows the residual energy at various nodes during simulation time. When nodes finish their transmission, they go into sleep mode, as indicated by the horizontal line. The coordinator has a considerable energy loss because it always listens to the other nodes. However, the energy consumption of the coordinator is not a critical issue in a WBAN. We further analyze the residual energy at the ECG node for different transmission powers. There is a minor change in energy loss for three different transmission powers as given in 3(d). This concludes that reducing the transmission power only d oes not save energy unless supported by an efficient power management scheme. The IEEE 802.15.4 can be considered for certain on-body medical applications, but it does not achieve the level of power required for in-body nodes. It is not sufficient for high data rate medical and non-medical applications due to its limitations to 250 kbps. Furthermore, it uses slotted or unslotted CSMA/CA where the nodes are required to sense the channel before transmission. However, the channel sensing is not guaranteed in MICS band because the path loss inside the human body due to tissue heating is much higher than in free space. Bin et.al studied the clear channel assessment (CCA) range of in-body nodes which is only 0.5 meters [20]. This unreliability in CCA indicates that CSMA/CA is not an ideal technique for the in-body communication system. An alternative approach is to use a TDMA-based protocol that contains a beacon, a configurable contention access period (CCAP), and a contention free period (CFP) [21]. Unlike the IEEE 802.15.4, this protocol is required to use a slot ted-ALOHA protocol in the CCAP instead of CSMA/CA. The CCAP period should contain few slots (3 or 4) of equal duration and can be used for short data transmission and a guaranteed time slot (GTS) allocation. To enable a logical connection between the in-body and the on-body communication systems, a method called bridging function can be used as discussed in [21]. The bridging function can integrate in-body and on-body nodes into a WBAN, thus satisfying the MAC transparency requirement. Further details about bridging function are given in [22]. 3.4. Discussion Since the CSMA/CA is not suitable due to unreliable CCA and heavy collision problems, it can be seen that the most reliable power-efficient protocol is a TDMA-based protocol. Many protocols have been proposed for a WBAN and most of them are based on a TDMA-based mechanism. However, all of them have pros and cons for a real WBAN system that should operate on Multi-PHYs (MICS, ISM, and UWB) simultaneously. The MAC transparency has been a hot topic for the MAC designers since different bands have different characteristics in terms of data rate, number of channels in a particular frequency band, and data prioritization. A good MAC protocol should enable reliable operation on MICS, ISM, and UWB etc bands simultaneously. The main problems are related to MICS band due to FCC restrictions [23]. According to FCC, â€Å"Within 5 seconds prior to initiating a communications session, circuitry associated with a medical implant programmer/control transmitter must monitor the channel or channels the MICS system devices intend to occupy for a minimum of 10 milliseconds per channel.† In other words, the coordinator must perform Listen-before-talking (LBT) criteria prior to a MICS communication sessions. The implants are not allowed to On the Implant Communication and MAC Protocols for a WBAN On the Implant Communication and MAC Protocols for a WBAN Abstract Recent advances in micro-electro-mechanical systems (MEMS), wireless communication, low-power intelligent sensors, and semiconductor technologies have allowed the realization of a wireless body area network (WBAN). A WBAN provides unobtrusive health monitoring for a long period of time with real-time updates to the physician. It is widely used for ubiquitous healthcare, entertainment, and military applications. The implantable and wearable medical devices have several critical requirements such as power consumption, data rate, size, and low-power medium access control (MAC) protocols. This article consists of two parts: body implant communication, which is concerned with the communication to and from a human body using RF technology, and WBAN MAC protocols, which presents several low-power MAC protocols for a WBAN with useful guidelines. In body implant communication, the in-body radio frequency (RF) performance is affected considerably by the implants depth inside the human body as well as by the muscle and fat. We observe best performance at a depth of 3cm and not close to the human skin. Furthermore, the study of low-power MAC protocols highlights the most important aspects of developing a single, a low-power, and a reliable MAC protocol for a WBAN. Keywords: In-body, on-body, RF communication, Implant, WBAN 1. Introduction Cardiovascular diseases are the foremost cause of deaths in the United States and Europe since 1900. More than ten million people are affected in Europe, one million in the US, and twenty two million people in the world [1]. The number is projected to be triple by 2020, resulting in an expenditure of around 20% of the gross domestic product (GDP). The ratio is 17% in South Korea and 39% in the UK [2]. The healthcare expenditure in the US is expected to be increased from $2.9 trillion in 2009 to $4 trillion US dollars in 2015 [3]. The impending health crisis attracts researchers, industrialists, and economists towards optimal and quick health solutions. The non-intrusive and ambulatory health monitoring of patients vital signs with real time updates of medical records via internet provide economical solutions to the health care systems. A wireless body area network (WBAN) is becoming increasingly important for healthcare systems, sporting activities, and members of emergency as well as military services. WBAN is an integration of in-body (implants) and on-body (wearable) sensors that allow inexpensive, unobtrusive, and long-term health monitoring of a patient during normal daily activities for prolonged periods of time. In-body radio frequency (RF) communications have the potential to dramatically change the future of healthcare. For example, they allow an implanted pacemaker to regularly transmit performance data and the patients health status to the physician. However, the human body poses many wireless transmission challenges. This is partially conductive and consists of materials having different dielectric constants and characteristics impedance. The interface of muscles and fats may reflect the RF wave rather than transmitting it. The key elements of an RF-linked implant are the in-body antenna and the communi cation link performance. Also, in the case of many implants and wearable sensors, a low-power MAC protocol is required to accommodate the heterogeneous traffic in a power-efficient manner. This article is divided into two parts: body implant communication and WBAN MAC protocols. In the body implant communication part, we look at the RF communication link performance at various depths inside a human (artificial) body. In the MAC part, we review the existing low-power MAC protocols and discuss their pros and cons in the context of a WBAN. We further provide alternative MAC solutions for in-body and on-body communication systems. The rest of the article is divided into three sections. In section 2, we present a discussion on body implant communication including in-body electromagnetic induction, RF communication, antenna design, and the communication link performance. Section 3 discusses several low-power MAC protocols and realizes a need for a new, a low-power, and a reliable MAC protocol for a WBAN. The final section concludes our work. 2. Body Implant Communication There are several ways to communicate with an implant that includes the use of electromagnetic induction and RF technology. Both are wireless and their use depends on the application requirements. Further, the key elements of an RF-linked implant are the in-body antenna and the communication link performance. The following part discusses in-body electromagnetic induction, RF communication, antenna design, and the communication link performance. 2.1. In-body Electromagnetic Induction Several applications still use electromagnetic coupling to provide a communication link to an implant device. In this scheme, an external coil is held very close to the body that couples to a coil implanted just below the skin surface. The implant is powered by the coupled magnetic field and requires no battery for communication. Data is transferred from the implant by altering the impedance of the implanted loop that is detected by the external coil and electronics. This type of communication is commonly used to identify animals that have been injected with an electronic tag. Electromagnetic induction is used when continuous, long-term communication is required. The base band for electromagnetic communication is typically 13.56 MHz or 28 MHz, with other frequencies also available. The choice of a particular band is subject to regulation for maximum specific absorption rate (SAR). The inductive coupling achieves best power transfer efficiency when uses large transmit and receive coil s. It, however, becomes less efficient when the space is an issue of the device is implanted deep inside the human body. Furthermore, inductive coupling technique does not support a very high data rate and cannot initiate a communication session from inside of the body. 2.2. In-body RF Communication Compared with the electromagnetic induction, RF communication dramatically increases bandwidth and supports a two-way data communication. The band designated for the in-body RF communication is medical implant communication service (MICS) band and is around 403 to 405 MHz. This band has a power limit of 25  µW in the air and is usually split into ten channels of 300 kHz bandwidth each. The human body is a medium that poses numerous wireless transmission challenges. It consists of various components that are not predictable and will change as the patient ages, gains or losses weight, or even changes posture. Values of dielectric constant (ÃŽ µr), conductivity (ÏÆ') and characteristic impedance (Zo) for some body tissue are given in table 1 [4]. This demonstrates that these two tissue types are very different. Also, the dielectric constant affects the wavelength of a signal. At 403 MHz, the wavelength in the air is 744mm, but in muscle with ÃŽ µr = 50 the wavelength reduces to 105mm, which helps in designing implanted antennas. 2.3. In-body Antenna Design A modern in-body antenna should be tuneable by using an intelligent transceiver and software routine. This enables the antenna coupling circuit to be optimised. Due to the frequency, and available volume, a non-resonant antenna is commonly used. It has a lower gain than a resonant antenna. This makes design of the antenna coupling circuit very important. Antenna options are dictated by the location of the implant. A patch antenna can be used when the implant is flat. Patch antennas are comprised of a flat insulating substrate coated on both sides with a conductor. The substrate is a body compatible material with a platinum or a platinum/iridium conductor. The upper surface is the active face and is connected to the transceiver. The connection to the transceiver needs to pass through the case where the hermetic seal is maintained, requiring a feed-through. The feed-through must have no filter capacitors present; these are common on other devices. An implanted patch antenna is electrically larger than its physical size because it is immersed in a high (ÃŽ µr) medium. It can be much larger electrically if the substrate is of higher (ÃŽ µr), such as titania or zirconia. A loop antenna can also be attached to the implant. This antenna operates mostly by the magnetic field, whereas the patch operates mostly by the electric field. The loop antenna delivers performance comparable to that of a dipole, but with a considerably smaller size. In addition, the magnetic permeability of muscle or fat is very similar to that of an air, unlike the dielectric constant that varies considerably. This property enables an antenna to be built and used with much less need for retuning. A loop antenna can be mounted on the case in a biocompatible structure. 2.4. In-body Link Performance The demonstration system consists of a base-station, an implant, antennas, and a controlling laptop. The base-station contains a printed circuit board (PCB) with a wakeup RF circuit, a Zarlink ZL70101 IC, and a micro-controller. It sends a wakeup signal on industrial, scientific, and medical (ISM) 2.4 GHz band to power up the implant to communicate. It also supports communication within the MICS band. The implant contains a Zarlink ZL70101 IC, a micro-controller, and a battery. The power limits of the wakeup signal for ISM and MICS bands transmitters are 100mW and 25  µW respectively. Experiments that measure the performance of an implant inside a living body are difficult to arrange. The alternative is to use 3D simulation software or a body phantom defined in [5]. The use of 3D simulation software is time consuming and hence practically not valuable. Therefore, measurements are generally performed using the body phantom and immersing a battery-powered implant into it [6]. Since no additional cables are attached to the test implant, the interference errors in the measurements are minimal. The body phantom is filled with a liquid that mimics the electrical properties of the human body tissues. The test environment is an anechoic chamber that includes a screened room. The interior walls of the room have sound-absorbent cones to minimize any reflections from walls or the floor that could distort the results. In real life, however, the results will be affected by the reflections from walls, desks, and other equipment and hardware. The body phantom is mounted on a woo den stand (non-conductive). The distance from the body phantom to the base-station is 3m. The MICS base-station dipole antenna is mounted on a stand. 1(a) shows the anechoic chamber with a body phantom (on the wooden stand), a log periodic test antenna (foreground), and a base-station dipole (right). The log periodic antenna is used to calculate the power radiated from the body phantom. A depth is defined as the horizontal distance between the outer skin of the phantom and the test implant. Vertical polarization of the implant is the case when the long side of the box and the patch antenna is vertical. The link performance is measured once the communication link is established. The measurements include the effective radiated power (ERP) from the implant, the received signal at the implant from the base-station, and the link quality. Measurements are made over a set distance with all the combinations of implant and test antenna polarisations, i.e., vertical-vertical (V-V), horizontal-vertical (H-V), vertical-horizontal (V-H), and horizontal-horizontal (H-H) polarisations. Typical results are shown in 1(b) where the ERP is calculated from the received signal power and the antenna characteristics. The measurement of the signal levels is done with the log periodic antenna and the spectrum analyzer. It can be seen in the that there is a significant difference in signal levels with polarisation combinations and depths. For a V-V polarisation, the ERP increases from a 1cm depth to a maximum between 2 and 7 cm, and then it decreases. The gradual increase is due to the simulated body acti ng as a parasitic antenna. The also shows how the signal level is affected by the depth with different polarisation. Such a test needs to be done with the antenna that is to be used in the final product. To measure the received signal at the implant, the Zarlink ZL70101 has an inbuilt receive signal strength indication (RSSI) function that gives a measure of the signal level detected. RSSI is a relative measurement with no calibration. The implant receives and measures a continuous wave signal transmitted by the base-station. In this case, the implant and the base-station antennas are vertically polarised. 1(c) shows an increase in the signal level at a depth between 3 and 4cm for a 15dec power. The power settings refer to the base-station and are cond to set the ERP to 25  µW. Signal levels are not valuable unless they are related to data transmission. One way to maintain the link quality is to measure the number of times the error correction is invoked during the transmission of 100 blocks of data. Two types of error correction codes, i.e., error correction code (ECC) and cyclic redundancy code (CRC) are invoked to maintain data integrity and reliability. The fewer ECC and CRC invocations result in better link quality. In 1(d), the error correction is lowest at a depth between 3 and 5 cm. A sample of ECC data collected at a 3cm implant depth is given in Table 2. The Count indicates the number of data blocks, the Time (ms) indicates the block transmission time, and the ECC indicates the number of times it is invoked. During the transmission of 100 blocks of data at a 3cm depth, the ECC is invoked 368 times, which is further equivalent to an average 3.68 times (as given in 1(d)). 2.5. Discussion The ERP, RSSI, as well as the ECC and CRC plots show that the implant demonstrates the best performance at a depth between 3 and 5 cm. The depth and position of an implant is not chosen for engineering performance but for the best clinical reasons. The implant designer must be aware of the possible losses through the human body. The attenuation and the parasitic antenna effects vary from patient to patient, with the position of the implant and with the time as the patient gains, or looses weight. Therefore, these factors need to be built into the link budget. 3. WBAN MAC Protocols Some of the common objectives in a WBAN are to achieve maximum throughput, minimum delay, and to maximize the network lifetime by controlling the main sources of energy waste, i.e., collision, idle listening, overhearing, and control packet overhead. A collision occurs when more than one packet transmits data at the same time. The collided packets have to be retransmitted, which consumes extra energy. The second source of energy waste is idle listening, meaning that a node listens to an idle channel to receive data. The third source is overhearing, i.e., to receive packets that are destined to other nodes. The last source is control packet overhead, meaning that control information area added to the payload. Minimal number of control packets should be used for data transmission. Generally MAC protocols are grouped into contention-based and schedule-based MAC protocols. In contention-based MAC protocols such as carrier sense multiple access/collision avoidance (CSMA/CA) protocols, nodes contend for the channel to transmit data. If the channel is busy, the node defers its transmission until it becomes idle. These protocols are scalable with no strict time synchronization constraint. However, they incur significant protocol overhead. In schedule-based protocols such as time division multiple access (TDMA) protocols, the channel is divided into time slots of fixed or variable duration. These slots are assigned to nodes and each node transmits during its slot period. These protocols are energy conserving protocols. Since the duty cycle of radio is reduced, there is no contention, idle listening and overhearing problems. But these protocols require frequent synchronization. Table 3 compares CSMA/CA and TDMA protocols. 3.1. WBAN MAC Requirements The most important attribute of a good MAC protocol for a WBAN is energy efficiency. In some applications, the device should support a battery life of months or years without interventions, while others may require a battery life of tens of hours due to the nature of the applications. For example, cardiac defibrillators and pacemakers should have a lifetime of more than 5 years, while swallowable camera pills have a lifetime of 12 hours. Power-efficient and flexible duty cycling techniques are required to minimize the idle listening, overhearing, packet collisions and control packet overhead. Furthermore, low duty cycle nodes should not receive frequent synchronization and control information (beacon frames) if they have no data to send or receive. The WBAN MAC should also support simultaneous operation on in-body (MICS) and on-body channels (ISM or UWB) at the same time. In other words, it should support multiple physical layer (Multi-PHYs) communication or MAC transparency. Other important factors are scalability and adaptability to changes in the network, delay, throughput, and bandwidth utilization. Changes in the network topology, the position of the human body, and the node density should be handled rapidly and successfully. The MAC protocol for a WBAN should consider the electrical properties of the human body and the diverse traffic nature of in-body and on-body nodes. For example, the data rate of in-body nodes varies, ranging from few kbps in pacemaker to several Mbps in capsular endoscope. In the following sections, we discuss proposed MAC protocols for a WBAN with useful guidelines. We also present a case study of IEEE 802.15.4, PB-TDMA, and S-MAC protocols for a WBAN using NS2 simulator. 3.2. Proposed MAC Protocols for a WBAN In this section, we study proposed MAC protocols for a WBAN followed by useful suggestions/comments. Many of the proposed MAC protocols are the extension of existing MAC protocols originally proposed for wireless sensor networks (WSNs). 3.2.1. IEEE 802.15.4 IEEE 802.15.4 has remained the main focus of many researchers during the past few years. Some of the main reasons of selecting IEEE 802.15.4 for a WBAN were low-power communication and support of low data rate WBAN applications. Nicolas et.al investigated the performance of a non-beacon IEEE 802.15.4 in [7], where low upload/download rates (mostly per hour) are considered. They concluded that the non-beacon IEEE 802.15.4 results in 10 to 15 years sensor lifetime for low data rate and asymmetric WBAN traffic. However, their work considers data transmission on the basis of periodic intervals which is not a perfect scenario in a real WBAN. Furthermore, the data rate of in-body and on-body nodes are not always low, i.e., it ranges from 10 Kbps to 10 Mbps, and hence reduces the lifetime of the sensor nodes. Li et.al studied the behavior of slotted and unslotted CSMA/CA mechanisms and concluded that the unslotted mechanism performs better than the slotted one in terms of throughput and lat ency but with high cost of power consumption [8]. Intel Corporation conducted a series of experiments to analyze the performance of IEEE 802.15.4 for a WBAN [9]. They deployed a number of Intel Mote 2 [10] nodes on chest, waist, and the right ankle. Table 4 shows the throughput at a 0dBm transmit power when a person is standing and sitting on a chair. The connection between ankle and waist cannot be established, even for a short distance of 1.5m. All other connections show favourable performance. Dave et al. studied the energy efficiency and QoS performance of IEEE 802.15.4 and IEEE 802.11e [11] MAC protocols under two generic applications: a wave-form real time stream and a real-time parameter measurement stream [12]. Table 5 shows the throughput and the Power (in mW) for both applications. The AC_BE and AC_VO represent the access categories voice and best-effort in the IEEE 802.11e. Since the IEEE 802.15.4 operates in the 2.4 GHz unlicensed band, the possibilities of interference from other devices such as IEEE 802.11 and microwave are inevitable. A series of experiments to evaluate the impact of IEEE 802.11 and microwave ovens on the IEEE 802.15.4 transmission are carried out in [13]. The authors considered XBee 802.15.4 development kit that has two XBee modules. Table 6 shows the affects of microwave oven on the XBee remote module. When the microwave oven is ON, the packet success rate and the standard deviation is degraded to 96.85% and 3.22% respectively. However, there is no loss when the XBee modules are taken 2 meters away from the microwave oven. 3.2.2. Heartbeat Driven MAC Protocol (H-MAC) A Heartbeat Driven MAC protocol (H-MAC) [14] is a TDMA-based protocol originally proposed for a star topology WBAN. The energy efficiency is improved by exploiting heartbeat rhythm information in order to synchronize the nodes. The nodes do not need to receive periodic information to perform synchronization. The heartbeat rhythm can be extracted from the sensory data and hence all the rhythms represented by peak sequences are naturally synchronized. The H-MAC protocol assigns dedicated time slots to each node to guarantee collision-free transmission. In addition, this protocol is supported by an active synchronization recovery scheme where two resynchronization schemes are implemented. Although H-MAC protocol reduces the extra energy cost required for synchronization, it does not support sporadic events. Since the TDMA slots are dedicated and not traffic adaptive, H-MAC protocol encounters low spectral/bandwidth efficiency in case of a low traffic. For example, a blood pressure node may not need a dedicated time slot while an endoscope pill may require a number of dedicated time slots when deployed in a WBAN. But the slots should be released when the endoscope pill is expelled. The heartbeat rhythm information varies depending on the patient condition. It may not reveal valid information for synchronization all the time. One of the solutions is to assign the time slots based on the nodes traffic information and to receive synchronization packets when required, i.e., when a node has data to transmit/receive. 3.2.3. Reservation-based Dynamic TDMA Protocol (DTDMA) A Reservation-based Dynamic TDMA Protocol (DTDMA) [15] is originally proposed for a normal (periodic) WBAN traffic where slots are allocated to the nodes which have buffered packets and are released to other nodes when the data transmission/reception is completed. The channel is bounded by superframe structures. Each superframe consists of a beacon used to carry control information including slot allocation information, a CFP period a configurable period used for data transmission, a CAP period a fixed period used for short command packets using slotted aloha protocol, and a configurable inactive period used to save energy. Unlike a beacon-enabled IEEE 802.15.4 superframe structure where the CAP duration is followed by CFP duration, in DTDMA protocol the CFP duration is followed by CAP duration in order to enable the nodes to send CFP traffic earlier than CAP traffic. In addition, the duration of inactive period is configurable based on the CFP slot duration. If there is no CFP t raffic, the inactive period will be increased. The DTDMA superframe structure is given in 2(a). It has been shown that for a normal (periodic) traffic, the DTDMA protocol provides more dependability in terms of low packet dropping rate and low energy consumption when compared with IEEE 802.15.4. However, it does not support emergency and on-demand traffic. Although the slot allocation based on the traffic information is a good approach, the DTDMA protocol has several limitations when considered for the MICS band. The MICS band has ten channels where each channel has 300 Kbps bandwidth. The DTDMA protocol is valid only for one channel and cannot operate on ten channels simultaneously. In addition, the DTDMA protocol does not support the channel allocation mechanism in the MICS band. This protocol can be further investigated for the MICS band by integrating the channel information in the beacon frame. The new concept may be called Frequency-based DTDMA (F-DTDMA), i.e., the coordinator first selects one of the channels in the MICS band and then divides the selected channel in TDMA superframe (s) according to the DTDMA protocol. However the FCC has imposed several restrictions on the channel selection/allocation mechanism in the MICS band, which further creates problems for the MAC designers. 3.2.4. BodyMAC Protocol A BodyMAC protocol is a TDMA-based protocol where the channel is bounded by TDMA superframe structures with downlink and uplink subframes as given in 2(b) [16]. The downlink frame is used to accommodate the on-demand traffic and the uplink frame is used to accommodate the normal traffic. There is no proper mechanism to handle the emergency traffic. The uplink frame is further divided into CAP and CFP periods. The CAP period is used to transmit small size MAC packets. The CFP period is used to transmit the normal data in a TDMA slot. The duration of the downlink and uplink superframes are defined by the coordinator. The advantage of the BodyMAC protocol is that it accommodates the on-demand traffic using the downlink subframe. However, in case of low-power implants (which should not receive beacons periodically), the coordinator has to wake up the implant first and then send synchronization packets. After synchronization, the coordinator can request/send data in the downlink subframe. The wake up procedure for low-power implants is not defined in the BodyMAC protocol. One of the solutions is to use a wakeup radio in order to wake up low-power implants before using the downlink subframe. In addition the wakeup packets can be used to carry control information such as channel (MICS band) and slot allocation information from the coordinator to the nodes. Finally, the BodyMAC protocol uses the CSMA/CA protocol in the CAP period which is not reliable for a WBAN. This should be replaced by slotted-ALOHA as done in DTDMA. Further details on low-power MAC protocols (originally proposed for WSNs) for a WBAN are given in Appendix I. 3.3. Case Study: IEEE 802.15.4, PB-TDMA, and SMAC Protocols for a WBAN In this section, we investigate the performance of a beacon-enabled IEEE 802.15.4, preamble-based TDMA [17], and SMAC protocols for an on-body communication system. Our analysis is verified by extensive simulations using NS-2. The wireless physical parameters are considered according to a low-power Nordic nRF2401 transceiver (Chipcon CC2420 radio [18] is considered in case of IEEE 802.15.4) [19]. This radio transceiver operates in the 2.4-2.5 GHz band with an optimum transmission power of -5dBm. We use the shadowing propagation model throughout the simulations. We consider a total of 7 nodes firmly placed on a human body. The nodes are connected to the coordinator in a star topology. The distribution of the nodes and the coordinator is given in 3(a). The initial nodes energy is 5 Joules. The packet size is 120 bytes. The average data transmission rate of ECG, EEG, and EMG is 10, 70, and 100 kbps. The transport agent is a user datagram protocol (UDP). Since the traffic is an uplink t raffic, the buffer size at the coordinator is considered unlimited. In a real WBAN, the buffer size should be configurable based on the application requirements. For energy calculation, we use the existing energy model defined in NS-2. The simulation area is 33 meter and each node generates constant bit rate (CBR) traffic. The CBR traffic is an ideal model for some of the medical applications, where the nodes send data based on pre-defined traffic patterns. However, most of the nodes in a WBAN have heterogeneous traffic characteristics and they generate periodic and aperiodic traffic. In this case, they will have many traffic models operating at the same time, ranging from CBR to variable bit rate (VBR). 3(b) shows the throughput of the IEEE 802.15.4, PB-TDMA, and S-MAC protocols. The performance of the IEEE 802.15.4, when cond in a beacon-enabled mode, outperforms PB-TDMA and S-MAC protocols. The efficiency of a MAC protocol depends on the traffic pattern. In this case, S-MAC protocol results poor performance because the traffic scenario that we generated is not an ideal scenario for the S-MAC. 3(c) shows the residual energy at various nodes during simulation time. When nodes finish their transmission, they go into sleep mode, as indicated by the horizontal line. The coordinator has a considerable energy loss because it always listens to the other nodes. However, the energy consumption of the coordinator is not a critical issue in a WBAN. We further analyze the residual energy at the ECG node for different transmission powers. There is a minor change in energy loss for three different transmission powers as given in 3(d). This concludes that reducing the transmission power only d oes not save energy unless supported by an efficient power management scheme. The IEEE 802.15.4 can be considered for certain on-body medical applications, but it does not achieve the level of power required for in-body nodes. It is not sufficient for high data rate medical and non-medical applications due to its limitations to 250 kbps. Furthermore, it uses slotted or unslotted CSMA/CA where the nodes are required to sense the channel before transmission. However, the channel sensing is not guaranteed in MICS band because the path loss inside the human body due to tissue heating is much higher than in free space. Bin et.al studied the clear channel assessment (CCA) range of in-body nodes which is only 0.5 meters [20]. This unreliability in CCA indicates that CSMA/CA is not an ideal technique for the in-body communication system. An alternative approach is to use a TDMA-based protocol that contains a beacon, a configurable contention access period (CCAP), and a contention free period (CFP) [21]. Unlike the IEEE 802.15.4, this protocol is required to use a slot ted-ALOHA protocol in the CCAP instead of CSMA/CA. The CCAP period should contain few slots (3 or 4) of equal duration and can be used for short data transmission and a guaranteed time slot (GTS) allocation. To enable a logical connection between the in-body and the on-body communication systems, a method called bridging function can be used as discussed in [21]. The bridging function can integrate in-body and on-body nodes into a WBAN, thus satisfying the MAC transparency requirement. Further details about bridging function are given in [22]. 3.4. Discussion Since the CSMA/CA is not suitable due to unreliable CCA and heavy collision problems, it can be seen that the most reliable power-efficient protocol is a TDMA-based protocol. Many protocols have been proposed for a WBAN and most of them are based on a TDMA-based mechanism. However, all of them have pros and cons for a real WBAN system that should operate on Multi-PHYs (MICS, ISM, and UWB) simultaneously. The MAC transparency has been a hot topic for the MAC designers since different bands have different characteristics in terms of data rate, number of channels in a particular frequency band, and data prioritization. A good MAC protocol should enable reliable operation on MICS, ISM, and UWB etc bands simultaneously. The main problems are related to MICS band due to FCC restrictions [23]. According to FCC, â€Å"Within 5 seconds prior to initiating a communications session, circuitry associated with a medical implant programmer/control transmitter must monitor the channel or channels the MICS system devices intend to occupy for a minimum of 10 milliseconds per channel.† In other words, the coordinator must perform Listen-before-talking (LBT) criteria prior to a MICS communication sessions. The implants are not allowed to

Moma Visit – Compare and Contrast Paintings

MoMA visit – Visual analysis Art is an effective way to express beauty. Artists have different ways to express their ideas and communicate nonverbally through their work. In the Early Modern Art period, artists were free to create their pieces in diverse ways that never had been done before. For this essay, I chose the works of two artists of this period to compare and contrast so that the diversity of Modern art can be demonstrated.Even though Vincent Van Gogh and Rene Magritte was born forty five years apart from each other and had different styles, I am interested in analyzing â€Å"The Starry Night† (Vincent van Gogh) and â€Å"The Empire of Light, ii† (Rene Magritte) after my trip to the Museum of Modern Art. At the first analysis, I would like to discuss the famous painting â€Å"The Starry Night† (1889) by Van Gogh. This painting is in oil on canvas and has the dimensions of 73. 7  cm ? 92. 1  cm. Van Gogh depicts the town in Rhone under gasligh t and reflected in the blue river.As it is displayed at MoMA, â€Å"The Starry Night† attracts a great number of visitors to its mysterious blue spirals. The focus of the painting is quite high as the first point drawn to the viewer is at the sky. The spirals meet with each other and the yellow stars are bright throughout the dark blue sky. Van Gogh uses thick brush strokes to draw the mysterious spirals, which occupy most of the center of the painting. The enormous spirals are reproduced images of moving waves in the ocean, but also induce a whirling movement. This technique plays a significant part to create the chaotically moving effect for the picture.Most people who see this picture for the first time feel that the whole scenery somehow shifts, even though it is a still life painting. Moving my view point from the mysterious spirals, I can see the big, extremely dark green cypress tree located in the right part of the picture. Cypress appears in Van gogh’s picture often such as in the â€Å"Cypresses† and â€Å"Wheatfield with cypresses. † Unlike these two pictures above, he uses similar sinuous curves tortured from full cycles one after another in this one to reproduce the brush stroke he uses in the spirals.In the bottom one third of the painting, Rhone town at midnight is peaceful in blue. The clearest part we can see at this town is the church and its bell tower. Although the church only consists of simple sketches, it seems to be the most complicated fragment in the painting. The bell tower points up to the sky and draws attention of the viewers to the spirals once again. Secondly, the next painting I would like to mention is â€Å"The Empire of Light ii† (1950) of Rene Magritte. Its dimensions are of 78. 8 x 99. 1 cm. Magritte wants to demonstrate a nocturnal scene under a daytime sky.When I first saw the painting, the initial thought that came through my mind was to admire the peaceful scenery of an ordinary small town. Then I noticed the paradox in the painting. The upper half of the painting is the light blue sky of a peaceful day with floating white clouds. However, the bottom half is a small town at midnight. Houses and trees are all covered with dark hue as if it is late at night. The town looks as if it has already gone to sleep. The thin and precise brush strokes that Magritte adroitly use make the picture look as real as a photo. The closer and longer I look at the painting, the more confused I become.The serenity of the deep-sleeping town, which is dimly lit by the antique street light, along with the floating clouds, makes me feel so calm and peaceful that I am persuaded into its reality. However, I am still able to notice the contrast between day and night in two halves of the painting. This confusion is the result of the eyes tricked to believe in what we see. The placement of day and night remarkably reminds us about our conscious mind that we use in everyday life, and about our lit unconscious mind that waits for us to use it to brighten our senses.This painting conceals Magritte’s psyche that â€Å"The mind loves the unknown. It loves images whose meaning is unknown, since the meaning of the mind itself is unknown. † On the one hand, there are similarities between two paintings that I notice. Firstly, both painters use oil on canvas for the paintings. They also have the approximate dimensions of about 70cmx95cm. Secondly, mystery is an important fact that makes the two paintings worth seeing. People cannot resist taking a look at the spirals and following the moving effect of the whole picture when seeing â€Å"The Starry Night. The spirals themselves are mysterious since they are echoed the movement of ocean waves. Similarly, ‘The Empire of Light, ii† makes spectators wonder the reality of a nocturnal scene of a small town under a sunny day. Their minds are tricked to believe in the true-to-life scene as well as to question th e paradox. In addition, luminosity is created successfully in two paintings. In â€Å"The Starry Night†, the moon and the stars illuminate the sky as stars and moon are in concentric circles and contrary to the dark blue sky surrounding.The moon is placed at the top right corner of the painting, where the sun usually appears, and produces light together with the stars. Correspondingly, in Magritte’s, light comes from the title of the painting itself. The lamppost, as well as the windows’ dim light, creates the luminosity standing alone in the dark night. Finally, even though both paintings have no sign of human and animals, motion still appears clearly. The whole â€Å"Starry Night† is a chaos, moving and moving freely, while the floating clouds in the sunny sky in the upper half of the painting prevents â€Å"The Empire of Light, ii† from lacking in movement.On the other hand, each picture has its own quality. The first difference between the two paintings is the brush stroke. Vincent van Gogh uses the thick brush stroke in â€Å"The Starry Night,† especially when painting the spirals and the cypress, while Magritte’s extremely thin brush stroke appears everywhere in the painting. As a result, the surfaces of two paintings are different; rough in â€Å"The Starry Night† and smooth in â€Å"The Empire of light, ii. † Also, while â€Å"The Starry Night† has an open shape, â€Å"The Empire of Light, ii† is closed-shaped. In â€Å"Starry Night,† there are no edges coming between the stars and the sky.They just merge with one another and create a beautiful â€Å"Starry Night. † However, the thin brush strokes in â€Å"The Empire of Light† identify all objects distinctively. Edges are clear between houses; clouds and sky are clearly recognized. Thirdly, although both painters use blue, the hue, value and intensity are different. Blue is the spiritual color in â€Å"Th e starry night,† and Van Gogh uses blue that varies from the darkest in the town to the lightest in the inside of the spirals. Blue is also diverse in the sky, together with yellow to make a sky lively.Magritte, however, uses just one value of light blue to paint his sky in a sunny day and the sky looks serene. Last but not least, even though both paintings depict the same subject of a small town at midnight, each painting represents a different style. While â€Å"the starry night† of Van Gogh follows post-impressionism – a style that combines the elements of Impressionism with those of non-Europe to create their own way – â€Å"The Empire of Light, ii† of Magritte is surrealist, whose purposes are to describe the irrational, to examine perception and to create the word and image relationship.In conclusion, â€Å"The starry night† of Vincent van Gogh and â€Å"The Empire of Light, ii† are expressions of aesthetic values. The paintings, either post-impressionist or surrealist, represent the painters’ own way of self-expression and their own way to communicate with the spectators nonverbally. Reference 1. Van Gogh’s letters http://vangoghletters. org/vg/letters/let693/letter. html http://vangoghletters. org/vg/letters/let782/letter. html 2. Magritte’s quotes http://www. rene-magritte. org/rene-magritte-quotes. jsp 3. Other http://www. moma. org/collection/object. php? object_id=78456 http://www. all-art. rg/art_20th_century/magritte1. html The starry night – Vincent van Gogh The Empire of Light, ii – Rene Magritte Claude Monet – Contextual factors influenced his style Claude Monet (1840-1926) was a French artist, who was considered a leader of the Impressionism movement as this style was named after his painting â€Å"Impression, Sunrise† (1872). He is one of the typical artists to mention when we discuss the Early Modern art era. I would like to analyze the contextu al factors that affect his style of painting. First of all, the political circumstances in France from 1870s had effects on Monet’s style.During the Franco-Prussian War (1871-1872), Monet came to England with Pissarro for shelter. He met the dealer Durand Ruel, who then became a great advocator of the Impressionists. Also in England, Monet had chance to study the works of John Constable and Joseph Mallord William Turner, which inspired his innovations in study of colors. After spending a summer (1971) in Amsterdam, Holland, he returned to French, which was damaged by the civil war. He stayed in Argenteuil, which was an industrially growing town near the Seine river until 1878.From 1872-1878, Argenteuil was a center for Impressionist painters to gather after Monet published his painting â€Å"Impression, Sunrise† depicted the Seine river. In addition, innovations in painting manufacture were introduced in public in 19th century. In 1841, John Rahn invented the collapsib le tin paint tube, which revolutionized the color palette and technique of  plein-air oil painting  by offering a range of pre-mixed colors in a convenient medium. This is a major factor to emerge Impressionism. Monet used this plein-air painting as a elementary principle of his art in 1860s,.He sought to combine his Realist investigation with the possibilities of creating a new style of painting. In the paintings produced at La Grenouillere at 1869, Monet found a way establish a challenging tension between appearances and painterly invention that proved sufficient basis for the developed Impressionist manner of the 1870s and after. In 1870s, Monet’s brush stroke became smaller and more diverse. He also tried to conduct experiments to control value contrasts. These experiments with color were played off against and within a traditional framework, essentially perspectives in nature.Finally, 19th century marked the emergence of photography, which had a major effect on Moder n Art. In the first half of the century, cameras were introduced continuously and rapidly improved. For example, in 1814, Joseph Niepce used his  camera obscura  to take picture, but the image required eight hours of light exposure and later faded. In 1851, Frederick Scott Archer invented the  Collodion process, in which images required only two or three seconds of light exposure. Thus, painters were in need of seeking new techniques to innovate their painting. They had to prove that paintings might not be replaced by photos.Thus, Monet sought a way to break the old rules of traditional painting. His brush strokes became thinner; the closed shape in traditional painting became more open as the boundaries between objects were more obscure, as we can see in his â€Å"Impression, Sunrise. † According to the contemporary critics, the painting was rather a sketch than a complete work of art. However, this painting was the first impressionist to be presented. In conclusion, t he political circumstances and developing technique are two crucial factors that affect the style of Monet Claude.After his first painting in 1972, Impressionism became his style for his later life. Work cited MoMA – Monet Claud’s biography http://www. moma. org/collection/artist. php? artist_id=4058 http://www. claude-monet. com/index. jsp House, John, et al. :  Monet in the 20th century, page 2, Yale University Press, 1998 http://www. visual-arts-cork. com/artist-paints/colour-palette-nineteenth-century. htm http://library. brown. edu/cds/paris/finearts. html#impressionism http://inventors. about. com/od/pstartinventions/a/Photography. htm

Music Appreciation Essay

1. What are figured bass and basso continuo? How are they related? Figured bass is a musical notation using numbers to indicate chords, intervals, and other aspects in relation to the bass note of the music and basso continuo is the harmony of the music itself. (this has nothing to do with the question; when I took AP music theory, figured bass was my favorite thing to do!) 2. What is ornamentation? Ornamentation is the use of non-necessary musical flourishes, such as trills and grace notes, to the basic melody or harmony. 3. What is an oratorio? How does it differ from an opera? Oratorios are basically just concerts, whereas operas are more theatrical. Oratorios have a different focus and don’t use scenery like operas. 4. What is an orchestra? How did the development of orchestras influence Baroque music? Orchestras are larger instrumental ensembles or groups that contain brass, string, percussion, and woodwind instruments. The development of orchestras brought along the development of new instruments, like the violin and cello & it also brought along the composition of orchestral music specifically. 5. What is an instrumental suite? A composition that consists of a succession of short pieces, forming into a large composition. Critical thinking questions: 1. What are the characteristics of Baroque music? How would you describe Baroque music? Baroque music is defined as â€Å"that in which the harmony is confused, charged with modulations and dissonances, the melody is harsh and little natural, the intonation difficult, and the movement constrained† (Jean-Jacques Rousseau). To me, baroque music is highly eccentric and decorative; extremely difficult and concise, yet beautiful all at the same time. 2. Choose one of the composers discussed in the unit and listen to several of the composer’s works. Which works did you listen to? How would you describe this composer’s music? Why do you think this composer was an influential figure in Baroque music? I chose to listen to one of my favorite composers of all time, Antonio Vivaldi. I listened to The Four Seasons (every season & every movement). I find his compositions astounding, able to tell stories without the use of words. Small things such as the incorporation of animals and sounds from nature truly paint a picture in the mind. In addition to this, the sudden change from a soothing violin soloist playing to a group of string instruments playing at once completely dramatizes the piece, making it more meaningful. Overall, I believe Vivaldi is an amazing composer, who was highly influential in the Baroque period due to his use of mostly stringed instruments, and he was highly innovative. His music was highly decorated and creative, and in most cases, very melodic and smooth. Vivaldi continues to be an influential composer to this day. 3. How did composers and musicians think about themselves during the Baroque period? How did this influence the music that they created? 4. What advantages and disadvantages did Baroque composers have in the patronage system? What did they gain from this practice? What limitations did it place on them? They were able to showcase their musical genius, yet they were limited to what they were able to play because of what the patron wanted to hear. They did gain a lot of fame and publicity through the patronage system despite the limitations that were placed on them. 5. Baroque music often tried to capture and reflect a particular emotion or feeling. Choose one of the musical works in the unit. Identify the work that you chose. What feeling or emotion is the composer trying to capture or reflect in the work? What aspects of the music lead you to this emotion or feeling?

Video Games Effects On Children - 906 Words

Video games have become an important part of children lives these days. As parents we need to help our children spend a reasonable amount of time on gaming. Allowing too much time on video games can hinder a child’s developmental process. Obsessive amounts of video game playing can cause; attention span problems, inhibit social interaction, aggression, poor social skills, and games are not intellectually stimulating. Parents need to take control and limit the time their children spend on video games each day. Paying Attention School children already have enough on their plates while in school and need to pay as much attention as they can to their teachers. Sixty-seven percent of elementary children are more liable to have attention span problems from playing video games for more than two hours (Klein, 2010). Some parents seem to believe that video games will not harm their attention like TV does. Researchers say that children who watch TV or play games for long periods struggle to pay attention to their teachers while at school (Klein, 2010). Parents will even say that video games do not affect their children’s attention because they can sit for hours playing with no problem. Dr. Ron Steingard, says â€Å"Continuous activity doesn’t mean sustained attention†. Dr. Steingard, also states that â€Å"The task is changing so rapidly, shorts burst of attention are all that’s involved† (Miller, n.d., para. 6). Yalda Uhls, a senior researcher with the UCLA’s Children’s Digital Media Center,Show MoreRelatedThe Effects Of Video Games On Children Essay1279 Words   |  6 PagesIntroduction Video games have always been a controversial type of entertainment, that may come from how relatively new video games are compared to other mediums of entertainment[1]. 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