iA-MAC: improved Adaptive Medium Access Control ... - IEEE Xplore

2014 International Symposium on Communications and Information Technologies (ISCIT)

iA-MAC: improved Adaptive Medium Access Control protocol for Wireless Body Area Networks Ashfaq Ahmad1, Nadeem Javaid1, Zahoor Ali Khan2, Muhammad Imran3 , Mohammed Alnuem3 1 COMSATS

Institute of Information Technology, Islamabad, Pakistan Program, FE, Dalhousie University, Halifax, Canada 3 College of Computer and Information Sciences, King Saud University, Ryadh, Saudi Arabia 2 Internetworking

Abstract—This paper presents improved Adaptive Medium Access Control (iA-MAC) protocol for Wireless Body Area Networks (WBANs). In addition to the adaptive guard band assignment technique, the newly proposed protocol has an improved as well as adaptive sleep/wakeup mechanism. We consider a simple temperature measuring application, where sensors (nodes) sense human body for updated information. If the current readings are within normal range, nodes stay in idle state (do not access channel). On the other hand, if the current sensed information is within high range then nodes switch to active state and access the channel because critical information needs to be transmitted as soon as possible. Besides the normal and high ranges of temperature, if the current sensed temperature lies with pre-high range then nodes access for channel if and only if the current sensed data is not duplicated version of the previously sensed data. Moreover, iA-MAC uses well defined synchronization mechanism to avoid collisions between data as well as control packets. Simulation results show that performance of the newly proposed iA-MAC protocol is better than its existing counter part protocols in terms of the selected performance metrics. Keywords Channel Access, TDMA, Wireless, Body Sensor Networks, IEEE 802.15.4

I.

I NTRODUCTION

The concept of knowing tiny details to deduce great results is not new. In every era, the technique of attaining such information is modified from previous one and today we are able to get information wirelessly and ubiquitously. The field of wireless sensor network is emerged with in last decade and huge work is done over this subject. Furthermore, WSN gives birth to several other application arenas as underwater area sensor networks, terrestrial area sensor networks and one of the most critical of all, body area sensor networks. Body area sensor networks as the name indicates is such a network in which small sensing nodes are attached or implant in a body to sense different attributes of that body. These sensed data is transmitted to a Base Station that forward it in readable form to appropriate person or server. Apparently this may seem very simple and easy task but in actual, it is not. These tiny sensor devices are constrained with many things, most prominent of them are: limited battery and low computational power. Numerous researchers have worked on energy conservation to optimize network functional time [1]. One major way to preserve energy is to limit number of transmissions over the network. Network topology has a huge 156

impact on energy conservation. In Body Area Sensor networks, the best suited is start or one hop communication topology. Avoiding multi hop topologies results in minimal transmissions and receptions leading to optimum battery usage. Besides topology, there must be such algorithms that provide efficient network resources usage. For that purpose, IEEE 802.15.4 is designed considering low power and low rate applications. This standard ultimately suits best for body area sensor networks. This standard works on the concept of superframe as in previous standards like IEEE 802.11. The super frame of this standard is divided into two parts, i.e. Active and Inactive Part. All the activities are done in active mode, while in inactive mode, sensor is programed to sleep to preserve energy. Considering Active mode, it is again classified into two portions. One is based upon contention free period (CFP) which is divided into equal time slots for each sensor. Sensor nodes of network use these time slots using TDMA. While the other portion is termed as Contention Access Period (CAP). In CAP every sensor node has to content to access channel using CSMA/CA. This CSMA/CA is slightly changed in IEEE 802.15.4 with respect to CSMA/CA used in IEEE 802.11. There is only one clear channel assessment and then data is to be transmitted in view of IEEE 802.11. Considering IEEE 802.15.4, if a node succeeds to get two consecutive clear channel assessments, only than data will be transmitted. This slight change in CSMA/CA algorithm results in efficient power consumption of node [1], [2]. II.

R ELATED W ORK

The IEEE 802.11standard and its improvements are designed for high speed Wireless Local Area Networks (WLANs) [1], [2]. The IEEE 802.11 standard also supports high data rates. Furthermore, IEEE 802.11 standard requires high energy during its operation and has poor bandwidth management which is not feasible for WBANs. For low power and energy efficient transmission, IEEE 802.15.4 standard has been designed for Wireless Personal Area Networks (WPANs). IEEE 802.15.4 standard has gained attraction due to its low duty cycle, low bandwidth, and low power consumption during data transmission. Due to low energy consumption and low duty cycling, IEEE 802.15.4 standard can be used in modern health care applications. IEEE 802.15.4 standard uses two channel access mechanisms; Beacon Enabled (BE) and Non Beacon Enabled (NBE). In BE mode, IEEE 802.15.4 MAC does not work properly due to beacon broadcast overhead.

The NBE mode of IEEE 802.15.4 standard uses a CSMA/CA mechanism for channel access which consume extra energy for Clear Channel Assessment (CCA). A duty cycling based MAC protocol is proposed in [3], with efficiently wakeup and fall-back time. During Contention Free Period (CFP), the coordinator node assigns Time Division Multiple Access (TDMA) based time slots to its child nodes. Node communicates data during its assigned time slot. During no data, child node turns into sleep mode to save energy. Furthermore, overhearing and collision are avoided due to the central control algorithm. In [4], the authors proposed a hybrid and secure MAC protocol (PMAC) for WBANs. PMAC uses two contention access periods (CAPs) for accommodating normal and lifecritical traffic and one contention-free period (CFP) for accommodating large amount of data packets. H-MAC protocol proposed in [5] works on synchronization mechanism. In this protocol, natural heart beats are used for synchronization purpose. Moreover, collisions are avoided by dedicated time slot assignment mechanism. Another TDMA based MedMAC scheme is proposed by Timmons and Scanlon in [6]. In this protocol, overlapping is avoided between two adjacent time slots by introducing the Guard band. Collisions can also be avoided by introducing TDMA based time slots. Multi-constrained QoS issues in WBANs are discussed in McMAC [7]. In this protocol, a superframe structure is proposed which depends on the traffic load of a sensor node. If the QoS is achievable, then the node transmits data during a particular period. This protocol also provides an emergency data handling mechanism. In [8], authors proposed A-MAC protocol with a centralized coordinator node and TDMA based data transmission mechanism. The coordinator node assigns fixed time slots for data transmission. To avoid collision, adaptive Guard band allocation technique is used. III.

I A-MAC

As the proposed iA-MAC protocol is based on efficient utilization of the available resources for specific application, thereby it enables the nodes to communicate with CN at reduced energy consumption cost. Our proposed protocol assumes star topology due to simplicity in implementation. In iAMAC, CN assigns Guaranteed Time Slot (GTS) to each node, where nodes scan the body of patient for fresh information and send these data to CN which is assumed to be equipped with enough energy. The CN, equipped with a single transceiver, then transmits received data to Monitoring Station (MS). The communication mode from CN to MS can be either direct or indirect through an access point. We divide the total time frame T F rame into three parts: Contention Free Period (CFP), Contention Access Period (CAP) and time T MS . CFP is used for communication with nodes, CAP to accommodate ondemand traffic and T MS to convey nodes’ information to MS. During the protocol operation, iA-MAC uses two types of packets; Data Packets and Control Packets. Data packets contain the sensed data of node(s), and Control packets are given as follows: 157

Channel Packet: This packet includes address of CN along with channel information. Data Request Packet: In order to meet on-demand traffic, CN sends this type of packet to node. Acknowledgment Packet: The purpose of this packet is to ACKnowledge data packet. Time Slot Request Packet: It embeds request information to CN about guaranteed time slots. This packet is sent by node. Time Slot Request Reply Packet: This packet holds information about CN’s reply to node as well as guaranteed time slot information. Synchronization-Acknowledgment Packet: Drift value along with ACKnowledment of last received data are both embedded in this packet. Detailed description of the proposed methodology is in the following subsections. A. Improved Adaptive Sleep and Wakeup Mechanism Nodes scan human body for updated information which vary from application to application. Application examples include: temperature, pulse rate, blood pressure, etc. This scanning process is continuous, however, the channel access mechanism depends on scanned information. We categorize the scanned information into three categories; normal, prehigh and high. Let us consider a simple temperature measuring application where nodes regularly scan for updated information. Based on the three categories of sensed information, the improved sleep and wakeup mechanism is adjusted in an adaptive manner as follows. Case I: This case assumes the sensed information to be in normal range. In response, the node(s) is(are) in idle state. Case II: In this case, the sensed information falls within prehigh range. When this condition is met for the first time, the concerned node switches to active state and tries for channel access. However, if the condition is met continuously then before switching to active state the node checks for duplicated data. If the the sensed data is within permissible duplication range then the node stays in the idle state. Incase, if converse to the previous statement is true then the node switches to active state. Case III: In case III, the sensed information falls within high range. As this information is very critical so the node(s) wakeup and switch(es) to active state. In order to transmit data (other than control packet) to CN, node must be in active state. In this way, our improved adaptive sleep and active mechanism further minimizes the number of transmissions and thus huge amount of nodes’ energy is saved. B. Channel Selection Soon after network establishment, scan process for free Radio Frequency (RF) channels is initiated by CN. If the channel is busy, then CN turns off the current RF channel and switches on one of the available RF channels. This process is halted for the time being only if discovery of free RF channel is successful. In response to positive result of free channel search, channel packet is broadcast by CN which is intended to reach nodes. The Channel packet includes very important information; CN’s address and information about the channel. Meanwhile, node(s) also scan for free RF channel(s). If the

where GF denotes guard factor. Similarly, the nth guard band; TGn , is calculated as: TGn =

GF × T S n 100

(3)

TGn is placed before N th time slot. When the of time slots assignment is completed, nodes enter the sleep state. Nodes wakeup if and only if these have data to send within their allocated time slots. This adaptive approach provides reliable and almost collision free communication while keeping in mind the reduced energy consumption cost. Fig. 2 shows guard band assignment.




 










 

 


Fig. 2.



  

Time Slots Assignment with Guard-band Time

D. Synchronization Mechanism

Fig. 1.

Channel Selection

channel is busy, the concerned node waits for duration T CP in which it listens for channel packet. If the channel packet fails to reach the receiving end, the concerned node switches off from the current RF channel and switches on to one of the available free channels. On the other hand, upon successful reception of the channel packet, the node in response sends an acknowledgment (ACK) packet to CN. Detailed flow chart is shown in Fig. 1.

Synchronization mechanism is the essence of collision free communication between sender and receiver within their assigned time slots. Within the expected time slot, CN listens for data packet. If the data packet is received with the assigned time slot, CN time stamps the current reception time and compares it with the expected value of reception time. For an acceptable delay D, the drift value DV is calculated from expected and current reception times of data packet. Depending on the magnitude of difference value, the following approaches are used: If (difference value > D): CN sends DV within SYNChronization ACKnowledgment packet for future synchronization (piggy back approach), If (difference value ≤ D): CN sends simple ACK. IV.

C. Time Slot Assignment Soon after the selection of free RF channel, node(s) send(s) Time Slot Request packet at CN’s address. This packet includes information about node’s data rate as well as information about Time Slot. In subject to efficient utilization of the available resources, CN assigns variable time slots to and variable Guard Band Time TG to nodes in proportion to their traffic, thus the approach is adaptive. Once the assignment process of variable time slots and variable guard band is completed, CN sends time slot request reply to to nodes. We insert TG between two successive time slots to avoid interference. The TG is calculated like [8] as follows:

E NERGY C ONSUMPTION A NALYSIS

Sensing and processing energy consumption is taken as constant regarding transceivers activity. Sleep and awake mechanism plays an important role to reduce the energy consumption of the sensor node. Let Etotal be the total energy consumed in one cycle, Esleep and Eactive be energy consumed in sleep and awake mechanisms, respectively. Such that, Etotal = Esleep + Eactive Energy consumption for N number of cycles is given by, ET =

TGn,n+1 =

GF 1 × (T S n + T S n+1 ) 100 2

GF × T S 1 100

N X

Etotal

(5)

k=1

(1)

where, GF denotes guard band factor which depends upon the average drift value. T S n and T S n+1 represent the nth and (n + 1)th time slots, respectively. Similarly, the first guard band time slot; TG1 , which is inserted before first time slot is calculated in terms of guard factor as follows: TG1 =

(4)

(2) 158

Energy consumption is a function of power and time, while power is a function of voltage and current. Energy consumption in sleep mode is less as compared to active mode. Esleep = Tsleep × Isleep × V

(6)

Tsleep = Tf rame − Tactive

(7)

where Tf rame is the duration of the frame and Isleep is the sleep mode current drawn from voltage source V . Let Tactive

be active time duration for nodes. Sensor node consumes switching energy Eswitch during Tactive period, transmission energy Etrans , reception energy Ereceive , and time-out energy Etime−out :

Body temperature 95 F 0

97.7 – 99.5 F

99.5 – 100.9 F

0

0

Hyperthermia

Normal

Fever

(10)

Let, Ereceive is the energy consumed at the receiver side and can be calculated as: Ereceive = l × Tbyte × Ireceive × V

Hyperthermia

Hyperpyrexia

(9)

During Tswitch time duration, node draw Iswitch current from voltage source V . Let L be length of packet, let Tbyte is the time required for single byte transmission, and let Itrans the amount of current drawn from V during transmission. Energy consumed during transmission is given by: Etrans = l × Tbyte × Itrans × V

0

0

Transceiver consumes Eswitch energy during switching process. Eswitch = Tswitch × Iswitch × V

99.5 – 100.9 F

104 – 106 F

Eactive = 2 × Eswitch + Etrans + Ereceive + Etime−out (8)

Fig. 3.

Temperature ranges

10

(11)

9.5

Etout = Ttout × Itout × V

Energy Consumption (milliJoule)

The interval which is called the time-out (Ttout ) interval after the ACK transmission and before its reception on the receiver side. Energy consumed during Ttout is termed as time-out energy (Etout ): (12)

Where, Itout is the current drawn from voltage source V during Ttout .

8.5 8 7.5 7 6.5 6

V.

R ESULTS AND D ISCUSSIONS

Fig. 4.

In this section, our proposed protocol iA-MAC is compared with A-MAC and IEEE 802.15.4. A total of 10 nodes are deployed on human body forming star topology. In this simulation scenario, we focus on the measurement of body temperature only. Fig. 3 describes a range of body temperatures. Crossbow MICAZ energy model (TF rame =1 s, V =3 volts, Iswitch =1 µ− A, Iidle =20 mA, Itrans =17.4 mA, Ireceive =19.7 mA, C=1000) is used in this simulation setup. Simulation results, shown in this section, are average results which are obtained after 5 time executing the protocol. In addition, Random Uniformed Model is used for packet drop calculation. Plot in Fig. 4 shows improved performance of iA-MAC over A-MAC and IEEE 802.15.4. Number of back-offs of CSMA/CA operation of IEEE 802.15.4 increases with an increase in average packet error rate. The consequence of which is consumption of extra energy on every additional backoff. In A-MAC, energy consumption is minimized through minimum number of channel accesses, adaptive guard band allocation and GTSs communication. Within normal range of body temperature nodes remain in sleep state and do not access channel, thus conserving energy. iA-MAC further minimizes energy consumption cost by minimizing the number of transmissions due minimum number of channel accesses. In addition to the characteristics of A-MAC, iA-MAC does not access channel incase of duplicated data while considering prehigh range of body temperature. In subject to minimum energy consumption, the performance order of the three compared protocols is: iA-MAC>A-MAC>IEEE 802.15.4. 159

IEEE 802.15.4 A−MAC iA−MAC

9

0

5

10 Packet Error Rate(%)

15

20

Energy consumption analysis for N = 1000

To analyse throughput and lifetime, 0.5 joules of energy is provided to each node. Fig. 5 depicts that our protocols lifetime is improved. IEEE 802.15.4 transmit data to CN periodically without considering the type of the data, that is whether it is important or not. In A-MAC, nodes sense data continuously however, channel accesses are performed only when a certain threshold of temperature is met. iA-MAC further improves the network lifetime by not accessing channel whenever the scanned information in pre-high range is with the duplication range. Moreover, (in iA-MAC) packet collision and overhearing is controlled by assigning guard bands according to weight of data and guaranteed time slots assignment to nodes. Therefore, and adaptive approach in A-MAC enhances its network lifetime. In this regard, performance of the three protocols is: iA-MAC>A-MAC>IEEE 802.15.4. CSMA/CA approach is used in IEEE 802.15.4 in which a packet is discarded if channel is found busy after maximum number of back-offs. Curves in Fig. 6 show that the total number of packets sent to CN by iA-MAC are more than A-MAC as well as IEEE 802.15.4. Prior to this point, we have stated throughout the paper that iA-MAC minimizes the number of channel access tries which means that the number of packets sent by using this protocol are minimum. However, Fig. 6 shows that converse of this statement is true. To answer this ambiguous case, take any specific cycle number under consideration (suppose cycle number 20). At this cycle, the

5

10

IEEE 802.15.4 A−MAC iA−MAC

9

4

7

No. of Pkts dropped

No. of dead nodes

8

6 5 4 3

Fig. 5.

3.5 3 2.5 2 1.5

2

1

1

0.5

0

0

10

20

IEEE 802.15.4 A−MAC iA−MAC

4.5

30 40 No. of cycles

50

60

0

70

Network lifetime

Fig. 7.

number of packets sent by iA-MAC are less than that of AMAC and IEEE 802.15.4, respectively. This result agrees with our statement on which we focus throughout the paper (i.i., the channel access tries are further minimized in iA-MAC). Furthermore, the accumulated packet sensing rate of iA-MAC is more than that of A-MAC as well as IEEE 802.15.4. This is obvious due to increased network lifetime; alive nodes positively contribute to packet sending rate.

0

10

20

30 40 No. of cycles

50

60

70

Packets drop rate

of the previously sensed data and vise versa. In this way, the sleep state of nodes is prolonged and energy consumption of the network in minimized. Besides the improved adaptive sleep and wakeup mechanism, adaptive guard band allocation and wisely organized synchronization mechanism minimize the over all packet drop rate of the network. Simulation results justify improved performance of our proposed iA-MAC protocol as compared the other selected protocols in terms of network lifetime, packet sending rate and packet drop rate.

350 IEEE 802.15.4 A−MAC iA−MAC

No. of Pkts sent to CN

300

VII.

This research work is supported by the Research Centre of College of Computer and Information Sciences at King Saud University through Project No. RC121244. The authors are grateful for this support.

250 200 150

R EFERENCES

100

[1]

50 0

Fig. 6.

ACKNOWLEDGEMENT

0

10

20

30 40 No. of cycles

50

60

70

Packet sending rate

[2]

To resemble the situation of packet drop in real scenario, we adopted two link states in our simulations, that is 70% of good link state probability and 30% bad. Fig. 7 shows packet drop rate with IEEE 802.15.4, A-MAC, and iA-MAC in comparison. Higher packet drop of IEEE 802.15.4 is because of CSMA/CA and fixed guard band assignment which causes high contention. In A-MAC, the packet drop rate is minimized due to less number of transmissions. iA-MAC further minimizes the packet drop rate by using an intelligent approach i.e., not accessing channel incase of duplicated data in pre-high body temperature range and adaptive guard band allocation.

[3]

[4]

[5] [6]

VI.

C ONCLUSION

In modern health-care systems, nodes are used to monitor human body. These nodes regularly scan for fresh information. Depending on the type of sensed data, our newly proposed iA-MAC protocol intelligently organizes the sleep and active states. Nodes do not access the channel (idle state) if the current sensed data is within normal range, and access channel if the currently sensed data falls within high range (active state). In between these two ranges; the pre-high range, nodes access the channel if the current sensed data is not duplicated version 160

[7]

[8]

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