IEEE WBAN Beaconing for Wireless USB Protocol Adaptation

Transcription

1 IEEE WBA Beaconing for Wireless USB Protocol Adaptation Kyeong Hur 1, Won-Sung Sohn 1 *, Jae-Kyung Kim 1 and YangSun Lee 2 1* Dept. of Computer Education, Gyeongin ational University of Education, Gyesan- Dong San 59-12, 45 Gyodae-Gil, Gyeyang-Gu, Incheon, , Korea Telephone: , Fax: Dept. of Computer Engineering, Seokyeong University Seoul, Korea sohnws@ginue.ac.kr Abstract In this paper, an IEEE wireless body area networks (WBA) medium control protocol is developed to support a wireless USB (WUSB) application as a protocol adaptation layer (PAL). Even though we can avoid colliding packets using CSMA/CA, hazard of beacon collisions still remains in the WUSB over WBA networks. Further, in order to solve beacon conflict problem, we introduce a multi-channel beaconing (MCB) for avoidance of beacon collision. The proposed MCB can minimize the possibility of beacon collision by efficiently managing the multiple available channels in a hybrid manner combining proactive and reactive method. Keywords: Hierarchical MAC, Wireless USB, Wireless Body Area etworks (WBA) 1. Introduction There is a need for a standard optimized for ultra-low power devices and operation on, in or around the human body to serve a variety of applications including medical and personal entertainment. Examples of the applications served by the proposed standard are: Electroencephalogram (EEG), Electrocardiogram (ECG), Electromyography (EMG), vital signals monitoring (temperature (wearable thermometer), respiratory, wearable heart rate monitor, wearable pulse oximeter, wearable blood pressure monitor, oxygen, ph value, wearable glucose sensor, implanted glucose sensor, cardiac arrhythmia), wireless capsule endoscope (gastrointestinal), wireless capsule for drug delivery, deep brain stimulator, cortical stimulator (visual neuro-stimulator, audio neuro stimulator, Parkinson s disease, etc.), remote control of medical devices such as pacemaker, actuators, insulin pump, hearing aid (wearable and implanted), retina implants, disability assistance, such as muscle tension sensing and stimulation, wearable weighing scale, fall detection, aiding sport training. This will include body-centric solutions for future wearable computers [1]. In a similar vein, the same technology can provide effective solutions for personal entertainment as well. The existence of a body area network standard will provide opportunities to expand these product features, better healthcare and wellbeing for the users. It will therefore result in economic opportunity for technology component suppliers and equipment manufacturers. WUSB (Wireless USB) is designed to provide the same function as wired USB and supports high bandwidth (480Mb/s). High bandwidth property makes WUSB an essential technology for indoor high-speed multimedia applications such as Internet 1

2 Protocol Television (IPTV) and Personal Video Recorder (PVR) services. Also, WUSB system gets a lot of attention in home multimedia network since its low transmission power doesn t cause interference with other devices and its wide bandwidth isn t affected by interference from adjacent other devices. Recently, many researches were carried out for high-quality video transmission over UWB link. Applications like HDTV (High Definition Television) and PVR (Personal Video Recorder), generate high data rates and can easily saturate the bandwidth even at the rates provided by UWB transmissions. However, these researches can t add new service traffic in wireless channel when there are no available resources [2-6]. Figure 1 shows the situation that three adjacent WUSB clusters shares one WiMedia superframe. As shown in Figure 1, the situation that multiple WUSB clusters share one superframe can occur in several places such as office and school that can be used various PCs and office equipments. Also, multiple adjacent WUSB clusters cause the interference among WUSB clusters by device s mobility or changes in the wireless environment [4]. Figure 1. WiMedia MAC Superframe Architecture in which Multiple WUSB Clusters Operate Simultaneously The IEEE Wireless Body Area etwork (WBA) is a standard for short range, wireless communication in the vicinity of, or inside, a human body (but not limited to humans) [7]. The IEEE Wireless Body Area etwork (WBA) standard is used in or around a body. It is designed to serve advanced medical and entertainment options enabled by this standard. It will allow medical equipment manufacturers and consumer electronics manufacturers to have small, power-efficient, inexpensive solutions to be implemented for a wide range of devices. It uses existing ISM bands as well as frequency bands approved by national medical and/or regulatory authorities. Support for Quality of Service (QoS), extremely low power, and data rates up to 10 Mbps is required while simultaneously complying with strict non-interference guidelines where needed. This standard considers effects on portable antennas due to the presence of a person (varying with male, female, skinny, heavy, etc.), radiation pattern shaping to minimize Specific Absorption Rate (SAR) into the body, and changes in characteristics as a result of the user motions. In our study, we integrate the wireless body area networks (WBA) with the wireless USB (WUSB) system to develop wireless communication technologies and localizationbased input functions for wireless wearable computer systems. Even though we can avoid colliding packets using CSMA/CA, hazard of beacon collisions still remains in the WUSB over WBA networks. In spite of its importance, so far, there have been no fundamental solutions for beacon collision problem in the WUSB over WBA networks. In order to solve beacon conflict problem, we introduce a multi-channel beaconing (MCB) for avoidance of beacon collision. The proposed MCB can minimize the possibility of beacon collision by efficiently managing the multiple available channels in a hybrid manner combining proactive and reactive method. 2

3 2. Characteristic of WUSB Protocol WUSB protocol works on WiMedia MAC superframe, and WUSB Channel consists of a set of Private DRP reservations within WiMedia MAC superframe. Since Private DRP reservation blocks can only be used by member devices related to specific application, devices using other application can t obtain the information of the corresponding private DRP reservation blocks [4]. Beacon Period(BP) PCA Private DRP DRP DEV0 DEV1 DEV2 BPST BPST WCTA 1 WCTA n MS- CTA1 WCTA m WUSB Channel WiMedia MAC superframe WCTA 1 WCTA o ext ext ext Transaction Group 1 Transaction Group 2 Transaction Group n Figure 2. The Architecture of WUSB Channel As shown in Figure 2, WUSB defines a WUSB Channel which is encapsulated within a set of WiMedia MAC superframes via a set of Private DRPs. The WUSB Channel is a continuous sequence of control packets, called s (Micro-scheduled Management Commands), which are broadcast by the host within Private DRP reservations in WiMedia MAC superframe. s contain host identifying information, I/O control structures and a time reference to the next in the sequence. includes the information of scheduling for all I and OUT transaction in WUSB Channel and it is used primarily to dynamically schedule channel time for data communications between WUSB host and WUSB devices. An specifies the sequence of micro-scheduled channel time s (MS-CTAs) up to the next within a private DRP reservation block or to the end of a Private DRP reservation block [4]. For transactions between wired USB and WUSB, the wired USB protocol completes an entire I or OUT transaction (Token, Data and Handshake phases) before continuing to the next bus transaction for the next scheduled function endpoint. The Wireless USB protocol broadcasts USB Token (equivalents) in the and utilizes TDMA time slots for the Data and Handshake phases as appropriate for the transfer type and direction of data communication. Utilizing this method, a host can start a group of transactions at the same time because the may contain Tokens for more than one Wireless USB transaction [4]. Within the context of the Wireless USB application, the Micro-scheduled sequence (e.g., plus associated time slots) is called a Transaction Group. A WUSB Host determines how individual transactions are scheduled into individual transaction groups in order to satisfy the needs (and priorities) of the applications controlling the devices in the WUSB Cluster. The wired USB protocol allows a maximum of one data packet per USB transaction. Due to the significant packet delimiter overheads for wireless (long packet preambles, MIFS, SIFS, etc.), WUSB includes the capability to send multiple data packets during a transaction s data phase. This feature allows for potentially better efficiency because packet delimiters and inter-packet gaps can be reduced. The general term for this capability is a Burst Mode Data Phase. Wired USB protocol needs n transactions so that USB host transmits n data frames to WUSB device. However, WUSB completes the same data transmission in smaller transactions using Data Burst Mode. This Data Burst Mode can improve the resource utilization in 3

4 WUSB protocol using the limited resource since it can reduce overheads such as long packet preambles, MIFS, and SIFS [4]. 3. WUSB over WBA Protocol The purpose of IEEE standard is to provide an international standard for a short range (i.e., about human body range), low power and highly reliable wireless communication for use in close proximity to, or inside, a human body. Data rates, typically up to 10Mbps, can be offered to satisfy an evolutionary set of entertainment and healthcare services. Current Personal Area etworks (PA)s do not meet the medical (proximity to human tissue) and relevant communication regulations for some application environments. They also do not support the combination of reliability, Quality of Service (QoS), low power, data rate and noninterference required to broadly address the breadth of body area network applications [7]. To provide or support time referenced s in its wireless body area network (WBA), a hub shall establish a time base as specified in [7] which divides the time axis into beacon periods (superframes) regardless of whether it is to transmit beacons. In such cases, the hub shall transmit a beacon in each beacon period, except in inactive superframes, unless prohibited by regulations such as imposed in MICS band. The hub may shift (rotate) its beacon transmission time from one offset from the start of current beacon period to another offset from the start of next beacon period, thereby shifting the time reference for all scheduled s, to prevent large-scale repeated transmission collisions between its WBA and neighbor WBAs [7]. In cases where a hub is not to provide or support time referenced s in its WBA, it may operate without establishing a time base nor superframe boundaries and hence without transmitting beacons at all. Equivalently, a hub shall operate in beacon mode transmitting a beacon in every beacon period other than in inactive superframes to enable time referenced s, unless regulations as applicable in the MICS band disallow beacon transmission. In the latter case, a hub shall operate in non-beacon mode transmitting no beacons, with superframe and slot boundaries established if to the medium in its WBA involves time referencing, or without superframe or slot boundaries if to the medium in its WBA involves no time referencing. In the beacon mode, a hub shall divide each active beacon period into applicable phases as illustrated in Figure 3. The hub may announce some superframes (beacon periods) as inactive superframes where it transmits no beacons and provides no phases, if there are no s scheduled in those superframes [7]. B B2 EAP1 RAP1 Type-I/II phase EAP2 RAP2 Type-I/II phase CAP Beacon period (superframe) n Figure 3. Layout of Access Phases in a Beacon Period (Superframe) for Beacon Mode The hub shall place the phases exclusive phase 1 (EAP1), random phase 1 (RAP1), type-i/ii phase, exclusive phase 2 (EAP2), random phase 2 (RAP2), type-i/ii phase, and contention phase in the order stated and shown above. The hub may set to zero the length of any of these phases, but shall not have RAP1 shorter than the guaranteed minimum length 4

5 communicated in Connection Assignment frames sent to nodes that are still connected with it. To provide a non-zero length CAP, the hub shall transmit a preceding B2 frame [7]. A type-i/ii phase is either a type-i or type-ii phase as described below but not both. The two type-i/ii phases may be both of type-i, both of type-ii, or one of type-i and the other of type-ii. If one is of type-i and the other is of type-ii, either one may appear before the other. The hub may schedule uplink s, downlink s, and bi-link s anywhere in a type-i phase. It may improvise type-i and type-ii polled s as well as posted s anywhere outside the scheduled s in the type-i phase. It may also provide type-i and type-ii polled s within scheduled bi-link s in a type-i phase. A type-i polled is conveyed in terms of its time duration (the maximum time the polled node may use for its frame transactions in the ). And a type-ii polled is conveyed in terms of a frame count (the maximum number of frames the polled node may transmit in the ). These s along with the corresponding methods by which they are obtained are illustrated in Figure 4. Scheduled Improvised Type-I/II polling Scheduled-polling Scheduled Improvised posting Improvised Type-I/II polling Scheduled uplink Type-I/II polled (uplink) Scheduled bilink Type-I/II polled (uplink) Scheduled downlink Posted (downlink) Type-I/II polled (uplink) Type-I phase Figure 4. Allocation Intervals and Access Methods Permitted in a Type-I Access Phase The hub may schedule bi-link s and delayed bi-link s anywhere in a type-ii phase, except that it shall not schedule any bilink s after delayed bi-link s in the same type-ii phase. It may improvise type-ii, but not type-i, polled s as well as posted s anywhere outside the scheduled bi-link s and delayed bi-link s in the type-ii phase. It may also provide type-ii, but not type-i, polled s within scheduled bi-link s and delayed bi-link s. These s along with the corresponding methods by which they are obtained are illustrated in Figure 5 [7]. Scheduled-polling Improvised Type-II polling Scheduled-polling Improvised posting Delayed-polling Delayed-polling Improvised Type-II polling Scheduled bilink Type-II polled (uplink) Type-II polled (uplink) Scheduled bilink Type-II polled (uplink) Posted (downlink) Delayed bilink Type-II polled (uplink) Delayed bilink Type-II polled (uplink) Type-II phase Figure 5. Allocation Intervals and Access Methods Permitted in a Type-II Access Phase In exclusive phase 1(EAP1), random phase 1 (RAP1), exclusive phase 2(EAP2), random phase 2 (RAP2), and contention phase (CAP), s may only be contended s, which are non-reoccurring time s valid per instance of. The method for obtaining the contended s 5

6 shall be CSMA/CA if prandonaccess is set to CSMA/CA, or slotted Aloha if prandonaccess is set to Slotted Aloha. A hub or nodes may obtain contended s in EAP1 and EAP1, if it needs to send data type frames of the highest user priority (i.e., containing an emergency or medical event report). The hub may obtain such a contended psifs after the start of EAP1 or EAP2 without actually performing the CSMA/CA or slotted Aloha procedure. Only nodes may obtain contended s in RAP1, RAP2, and CAP, to send management or data type frames [7]. To send data type frames of the highest user priority based on CSMA/CA, a hub or node may treat the combined EAP1 and RAP1 as a single EAP1, and the combined EAP2 and RAP2 as a single EAP2, so as to allow continual invocation of CSMA/CA and improve channel utilization. To send data type frames of the highest user priority based on slotted Aloha, a hub or node may treat RAP1 as another EAP1 but not a continuation of EAP1, and RAP2 as another EAP2 but not a continuation of EAP2, due to the time slotted attribute of slotted Aloha. Figure 6 shows the WUSB over WBA architecture. Here, the IEEE WBA superframe begins with a beacon period (BP) in which the WBA hub performing the WUSB host s role sends the beacon. This beacon mode of the WBA is operated in both non-medical and medical traffic environments. The data transmission period in each superframe is divided into the exclusive phase 1 (EAP1), random phase 1 (RAP1), Type-I/II phase, EAP2, RAP2, Type-I/II phase, and contention phase (CAP) periods. The EAP1 and EAP2 periods are assigned through contention to data traffic with higher priorities. Further, the RAP1, RAP2, and CAP periods are assigned through contention to data traffic with lower priorities. In the Type-I/II phase periods, the WBA hub reserves time slots without contention to exchange data with its input-sensor nodes. Beacon Mode with Superframe B B2 EAP1 RAP1 Type-I/II phase EAP2 RAP2 Type-I/II phase CAP MS- CTA1 MS- CTAn MS- CTA1 MS- CTAm MS- CTA1 MS- CTAo WUSB Channel Transfers WBA IEEE Private Period WUSB Host s Beacon Control Transfer Isochronous/ Bulk Data Transfer Interrupt Transfer WUSB Host s Beacon ext ext ext Transaction Group 1 Transaction Group 2 Transaction Group n Figure 6. WUSB over WBA Architecture In the WUSB over WBA Architecture, in order to set up a wireless communication link to wearable computer systems, the WUSB channel is encapsulated within a WBA superframe via Type-I/II phase periods that enables the WUSB host and the input-sensor nodes to reserve time slots without contention through scheduling. In the user scenario of a wearable computer system when using the WUSB over WBA architecture, the user carries a portable or wearable computing host device. This host device performs roles of the WUSB host and the WBA hub simultaneously. Therefore, a wearable WUSB cluster and a WBA cluster are formed. The attached input-sensor nodes perform the functions of localization-based input interfaces for wearable computer systems and healthcare monitoring. Furthermore, the attached wireless nodes comprise the peripherals of a wearable computer system, and the central WUSB host exchanges data with the outer peripherals of the WUSB slave devices [4]. 6

7 4. Multi-Channel Beaconing for WUSB over WBA Architecture 4.1. Idle Listening for Beacon Collision Detection In this Section, we introduce a novel beacon collision avoidance algorithm which utilizes multiple channels through dynamic transition. And also, WUSB over WBA MAC on the same channel can be dynamically scheduled by overhearing each other. Therefore, we call this multi-channel beaconing (MCB). A beacon issued by a coordinator, periodically or by request, plays an important role in constructing a WBA and in synchronizing all of the devices in the network. Since all of the devices synchronized by the same beacon operate with the same period, they can make the promised duty cycle. As a result, the power saving function of 15.6 devices can be used. According to the IEEE standard, a duty cycle of less than 10% can be achieved. Due to the importance of issuing the beacon frames punctually, they have the highest priority in the network. Therefore, unlike in the case of normal data or control packets, carrier sensing or back-off algorithms are not used for beacons, except for their initial transmission. If there exists a WBA, the beacon transmission will be protected by other devices in the network. However, in reality, it is possible that a number of WBAs using IEEE15.6 coexist or are overlapped in the same area. In this case, since the standard does not provide any method of mutual information exchange, collisions may occur between the beacon frames that the different WBAs transmit. Therefore, it might result in unexpected network panic. Direct beacon frame collisions occur when two or more WUSB/WBA hosts are in the transmission range of each other (direct neighbors) and send their beacon frames at approximately the same time, as shown in Figure 7(a). Assume that WUSB/WBA slave node 1 is associated with C1 WUSB/WBA host. And C2 is a WUSB/WBA host of other wearable computer systems. In this case, if C1 and C2 transmit their beacon frame at the approximately same time, node 1 may lose beacon information by the collision of the two beacons. If the superframe duration of the two Systems is the same, beacons will be continuously conflicted. Unfortunately, these two hosts cannot identify the collision. In contrast to the direct beacon collision, Indirect beacon frame collisions occur when two or more WUSB/WBA hosts cannot hear each other, but they have overlapped transmission ranges (indirect neighbors) and transmit their beacon frames at the approximately same time, as shown in Figure 7(b). Assume that node 1, which is located in the overlapped region of both transmission ranges of C1 and C2, will not be able to correctly receive their beacon frames, since the beacons will collide each other. each WUSB/WBA host cannot listen to the other s beacon information. 1 C1 C2 C1 1 C2 C1 time C1 time C2 time C2 time (a) Direct beacon collision. (b) Indirect beacon collision. Figure 7. Direct and Indirect Beacon Frame Collision Problems 7

8 Core idea in the MCB is to scan other candidate channel during idle period, and then if a device identifies the beacon collision, the WUSB/WBA host re-associates with the slave device via the candidate channel, which is already verified to be clean, while original channel are used for other devices during active period. To allocate a new clean channel for beacon collision-free, a WUSB/WBA host has to know the status of other channel. Therefore, the WUSB/WBA host checks whether other device uses the channel. We newly define a (CCS) clean channel searching scan for searching a new candidate channel. Since no mechanism to avoid beacon frame collisions is considered in the current IEEE standard, some proposals have been discussed in Task Group These approaches were proposed as pattern ideas to trigger the design of solutions to the beacon frame collision problem. Two approaches were proposed to avoid the direct beacon frame collision problem [7]. The first approach is the time division approach. Time is divided such that beacon frames and the superframe duration of a given WBA coordinator are scheduled during the inactive period of its neighbor coordinators. The idea is that each coordinator uses a starting time Beacon_Tx_Offset to transmit its beacon frames, which must be different from the starting times of its neighbor coordinators and their parents. This approach requires that a coordinator wakes up both in its active period and in its parent s active period. Observe that Beacon_Tx_Offset must be chosen adequately, not only to avoid beacon frame collisions, but also to enable efficient utilization of inactive periods, thus maximizing the number of clusters in the same network. This problem is more challenging when the superframe orders and beacon orders are different from one cluster to another. The second approach is the beacon-only period approach. In this approach, a time window, denoted as Beacon-Only Period, is reserved at the beginning of each superframe for the transmission of beacon frames in a contention-free fashion. Each coordinator chooses a sending time offset by selecting a contention-free time slot (CFTS) such that its beacon frame does not collide with beacon frames sent by its neighbors. The advantage of this approach as compared to the previous one is that the active periods of the different clusters start at the same time, thus direct communication between neighbor nodes is possible, and there is no constraint on the duty-cycle. The main complexity of this approach is the dimensioning of the duration of the beacon-only period for a given network topology. This duration depends on the number of nodes in the network, their parent-child relationship and also the scheduling mechanism used to allocate the CFTS to each coordinator. Additionally, the GTS mechanism cannot be implemented (at least in accordance to the specification), since transmission from nodes belonging to different clusters may collide. Thus, transmissions are only allowed during the CAP, which will be shared by different clusters. Importantly, oppositely to the time division approach, the beacon-only period approach implies a non-negligible change to the standard protocol. The problem of indirect beacon frame collisions is more complex than the one of direct beacon frame collisions. There is a need to not only know the neighbor coordinators, but also all other coordinators that are two hops away. Two alternatives were proposed by the Task Group 15.6 [7]. In the reactive approach, a coordinator does not carry any specific procedure to avoid indirect beacon frame collision during the association with its parent. Once a beacon frame conflict is detected by a given node, it initiates a recovery procedure to resolve the problem, which may take a long time. In the proactive approach, coordinators try to 8

9 avoid the indirect beacon frame conflict at the association phase by the collection of specific data about beacon frame transmission times of their neighbors. In this approach, each potential coordinator must have the ability to forward the beacon frame time offset of its parent to its neighbor WBA coordinators. This approach is more complex than the reactive approach, but it completely avoids beacon frame collisions during network run-time. The approaches proposed by the Task Group 15.6 show how to extend the standard to take beacon frame scheduling into account, but how to choose the time offsets of different beacons is not addressed. Moreover, these approaches only focus on how to avoid conflicts between beacons. Collisions between beacon frames and data frames may also occur, because while the time of beacon transmissions was considered, the superframe duration of other WBAs was not. In addition, if a beacon frame and a data frame collide again after adjusting the time of the beacon transmission, the WBA coordinator (PC) has to adjust the transmission time of the beacon frame again [7]. Originally, MAC sub-layer shall discard all frames received over the PHY data service excluding beacon frame. However, MCB hears all of the frames on the candidate channel during clean channel searching scan. The WUSB/WBA host can know the existence of other systems indirectly by analyzing data frame received during the scan. Figure 8 presents idle channel listening of MCB. A WUSB/WBA host has Active and Inactive periods which are used to make a duty cycle of devices. MCB performs the CCS scan repetitively during the inactive period. If a device notifies the beacon collision in the next Active period, the WUSB/WBA host commands the channel change into the candidate channel in the next Inactive period. WUSB / WBA Host WUSB / WBA Host Figure 8. Idle Channel Listening of MCB 4.2. Adaptive Channel Switching for Fast Recovery WUSB / WBA Host WUSB / WBA Host Figure 9. Channel Switching Operation of MCB 9

10 Figure 9 illustrates the operation of MCB. In Figure 9, it is assumed that the physical deployment is indirect collision environment in Figure 7(b). Let s assume that device 1 has been associated to WUSB/WBA host 1 (WH1). The beacon of WH1 is equal to the beacon of WH2. WH1 operates normally on channel X in the active period and know that it can allocate superframe on channel Y, through CCS scanning on channel Y during its inactive period. In MCB, a WUSB/WBA slave device can detect a beacon conflict and the WUSB/WBA host recovers the device fast from the collision. The device 1 (D1) in the overlapped region of the transmission ranges of WH1 and WH2 cannot receive a beacon frame. If the number of beacon frames that D1 loses consecutively is more than amaxlostbeacons (default value is 5), D1 performs an orphan scan. The WH1 that has received this orphan notification command frame sends the host realignment command frame. However, two beacons collide again because a beacon of WH1 is equal to a beacon of WH2. Therefore, D1 cannot receive the beacon frame that WH1 transmitted. If the status of D1 is orphan and the number of lost beacon frames is more than amaxlostbeacons again, D1 concludes the status of beacon conflict and sends a beacon conflict notification command frame using CSMA/CA. WH1 that has received the beacon conflict notification command frame sends D1 channel switching command frame. The channel switching command frame contains a clean channel Y and time offset equal to the difference between the original beacon transmission time and the transmission time of new beacon frame. WH1 switches its channel to the channel Y and transmits its beacon frame. After receiving a beacon frame that WH1 transmits on channel Y, D1 can communicates normally with WH1. Initial WUSB/WBA Construction Sending a WBA beacon frame Collision detection Yes o Clear channel searching scan during inactive period Recovery procedure Figure 10. Flow Chart of WUSB/WBA Host using MCB 5. Performance Evaluation We used WUSB/WBA S2 simulator developed in the environment where indirect beacon collision happens [8-13]. Table 1 shows WBA PHY/MAC simulation parameters used in this paper; the network size is 10m*10m; the maximum 30 devices are randomly deployed into this area. Each WUSB/WBA slave device transmits data randomly when receiving a beacon frame. In this simulation, the beacon order of each device and host is equal to 4 an d the superframe order (SO) is equal to 1. To evaluate the performance, we use re-associated ratio equal to number of recovered beacons over number of lost beacons. In Figure 11(a), we present the re-associated ratio versus the number of adjacent WUSB/WBA hosts. Simulation results show that the re-associated ratio is more than 84% irrespective 10

11 Average Recovery Time (ms) Reassociated ratio (%) International Journal of Software Engineering and Its Applications of the number of adjacent hosts. In Figure 11(b), we show the average recovery time after a device detects beacon collision. The average recovery time is related to the SO because WUSB/WBA host using MCB transmits new beacon frame for the device detecting beacon collision, on the new clean channel in the inactive period. Table 1. WBA PHY/MAC Parameters Parameter Value LPreamble 64 bits LPHY_Hdr 15 bits LMAC_Hdr 56 bits LFCS 16 bits pmifs 20 μs psifs 75 μs pallocationslotmin 16 μs pallocationslotresolution 16 μs pallocationslotlength 32 μs msuperframelength 256 psuperframelength msuperframelength*pallocati onslotlength mbeaconslotlength 96 μs mbpextention 24 μs mbeacon2slotlength 80 μs mb2pextention 24 μs umber of adjacent WUSB/WBA Hosts (a) Re-associated ratio SO = 1 SO = umber of adjacent WUSB/WBA Hosts (b) Average Recovery Time. Figure 11. Re-associated Ratio and Average Recovery Time Performances of MCB 11

12 6. Conclusion In this paper, an IEEE wireless body area networks (WBA) medium control protocol is developed to support a wireless USB (WUSB) application as a protocol adaptation layer (PAL). Further, we propose multi-channel beaconing (MCB) method in order to solve beacon conflict problem in the environment of WUSB over WBA. MCB reduces recovery time after a beacon collision happens through scanning an available channel during the inactive period. The simulation results show that MCB can solve indirect beacon conflict problem efficiently irrespective of the number of adjacent WUSB/WBA hosts. Acknowledgements This work was supported in part by Basic Science Research Program through the ational Research Foundation of Korea (RF) funded by the Ministry of Education, Science and Technology (MEST) ( ) and in part by Mid-career Researcher Program through RF grant funded by the MEST ( ). This paper is a revised and expanded version of a paper entitled Adaptive Fast Recovery Technique for Avoidance of Beacon Collision in WUSB over WBA Protocol for Wearable Computer Systems presented at Advanced Information Technology and Sensor Application, Daejun, Korea, , References [1] R. Rosenberg and M. Slater, The Chording Glove: A Glove-Based Text Input Device, IEEE Transaction on Systems, Man, and Cybernetics-Part C: Applications and Review, vol. 29, no. 2, (2009). [2] USB 2.0, USB-IF, [3] WiMedia Alliance, [4] Certified Wireless USB 1.1, USB-IF (2010), [5] WiMedia MAC Release Spec. 1.5, Distributed Medium Access Control (MAC) for Wireless etworks, WiMedia Alliance, (2009). [6] K.-I. Kim, Adjusting Transmission Power for Real-Time Communications in Wireless Sensor etworks, Journal of Information and Communication Convergence Engineering, vol. 10, no. 1, (2012). [7] IEEE WPA Task Group 6 Body Area etworks (BA), IEEE, (2010). [8] A. Varga, The OMeT++ Discrete Event Simulation System, Proceedings of European Simulation Multiconference (ESM 2001), (2001). [9] S.-R. Kim, D.-Y. Lee and C.-W. Lee, An Adaptive MAC Scheduling Algorithm for Guaranteed QoS in IEEE e HCCA, International Journal of Future Generation Communication and etworking, vol. 1, no. 1, (2008). [10] A. andi and S. Kundu, Energy Level Performance of Packet Delivery Schemes in Wireless Sensor etworks over Fading Channels, International Journal of Future Generation Communication and etworking, vol. 4, no. 2, (2011). [11]. Karthikeyan, V. Palanisamy and K. Duraiswamy, Performance Comparison of Broadcasting methods in Mobile Ad Hoc etwork, International Journal of Future Generation Communication and etworking, vol. 2, no. 2, (2009). [12] V. Cuong guyen, V. Thuan Pham and B.-K. Moon, A ew Energy Saving Mechanism in IEEE e/m, International Journal of Energy, Information and Communications, vol. 2, no. 4, (2011). [13] R. Palit, A. Singh and K. aik, An Architecture for Enhancing Capability and Energy Efficiency of Wireless Handheld Devices, International Journal of Energy, Information and Communications, vol. 2, no. 4, (2011). 12

13 Authors Kyeong Hur he is currently an Associate Professor in the Department of Computer Education at Gyeongin ational University of Education, Republic of Korea. He was senior researcher with Samsung Advanced Institute of Technology (SAIT), Korea from September 2004 to August He received a M.S. and Ph.D. in Department of Electronics and Computer Engineering from Korea University, Seoul, Korea, in 2000 and 2004, respectively. His research interests include; computer network designs, next generation Internet, Internet QoS, and future All-IP networks. Won-Sung Sohn he received the B.S. and M.S. degrees in Department of Computer Engineering from Dongguk University in 1998 and 2000 and the Ph.D degree in Department of Computer Science from Yonsei University in From 2004 to 2006 he was a postdoctoral associate in the Computational Design Laboratory at Carnegie Mellon University. He is currently a professor at Department of Computer Education, Gyeongin ational University of Education. His research interests include educational design research, human-computer interaction and computer education. Jae-Kyung Kim he received the B.S. degree in Statistical Computing/Chemistry from Dankook University in 2000 and the M.S, and Ph.D degrees in Computer Science from Yonsei University in 2002 and He is currently a researcher at Smart Education Research Center at Gyeongin ational University of Education. His research interests include smart education, human-computer interaction, annotation, and electronic textbooks. YangSun Lee he received his B.S., M.S. and ph.d degrees in Department of Computer Engineering from Dongguk University, He is currently a professor at Department of Computer Engineering, Seokyeong University. He is also a member of board of directors of Smart Developer Association, Korea Multimedia Society and Korea Information Processing Society. His research interests include smart system solutions, programming languages, and embedded systems. 13

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