PPME - New priority MAC protocol for multi event wireless sensor network

In multievent wireless sensor networks, different priority level events can appear at the same time and require different QoS (Quality of Service) provision based on various priority levels. So, in this paper, we propose a new MAC protocol using beacon and CSMA p-persistent varied by packet priority levels to reduce collisions for different priority level packets in multiple event wireless sensor networks.

PPME - NEW PRIORITY MAC PROTOCOL FOR MULTI-EVENT

WIRELESS SENSOR NETWORK

Nguyen Thi Thu Hang*, Nguyen Chien Trinh, Nguyen Tien Ban

Abstract: In multievent wireless sensor networks, different priority level events

can appear at the same time and require different QoS (Quality of Service) provision based on various priority levels So, in this paper, we propose a new MAC protocol using beacon and CSMA p-persistent varied by packet priority levels to reduce collisions for different priority level packets in multiple event wireless sensor networks Simulation results show that our proposed protocol significantly reduces packet latency for all four priority level packets and saves more energy than QAEE and MPQ MAC protocols Moreover, the protocol keeps high packet success rate compared to the other two protocols

Keywords: Priority MAC, CSMA p-persistent, Multievent, Wireless sensor network

1 INTRODUCTION

In some wireless sensor networks (WSN), different types of events can appear at the same time Higher priority levels are mostly assigned to the important or emergency events such as wildfires in forest fire alarm systems, earhquake warning or first aid for stroke patients [1-3] Lower priority levels are mainly set for normal events as regular measurement of temperature, wind power, water level or heartbeat

High priority events often require higher quality of service than normal ones such as more real-time, higher reliability and also require energy consumption efficiently, especially in WSN with the limitation of power and processing capacity [4-7]

To meet these complex requirements of QoS and energy efficiency, there are many proposed QoS solutions for wireless sensor networks using MAC technology SMAC [8] and TMAC [9] focus on energy saving in networking, RI-MAC [10] improves network performance as packet delay and transmission rate, ERI-MAC is the protocol initiated by the receiver and uses the technique of multiplexing small packets into large packets before transmission for energy saving [11], PQMAC considers packet priority in the network but requires synchronization between nodes [12], QAEE also considers packet priority and has two priority packet levels that are high and low [13], and MPQ takes into account four packet priority levels [14]

The above solutions have limitation in meeting multi-priority requirements or if they have already consider different packet priority, priority packets still have to wait for a certain period of time until the end of the disputed window to be transmitted Especially in emergency cases, there will be multiple conflicting packets which reduce the packet success rate as well as prolong the packet delay for retransmission after the collision

In order to solve the problem of priority transmission of concurrent important packet types, ensuring small delay and good packet success rate, in this paper we propose the Priority MAC protocol for Multi-Event wireless sensor network which combines Beacon and priority MAC mechanisms (PMME) The contributions of the paper are as follows:

1 Propose a combination of beacon mechanism and a novel CSMA p-persistent mechanism varied by different packet priority levels

2 Implement the proposed PMME protocol in Castalia simulation to evaluate and compare with other two MAC protocols (QAEE and MPQ) on the ability to meet the multi QoS requirements of four different packet priority levels in a multi-event wireless sensor network

The paper is organized as follows: Section 2 describes the operation mechanism and analyzes the advantages and disadvantages of QAEE and MPQ protocols that consider the priority of packets Section 3 introduces the proposed PMME MAC protocol with two enhancements The evaluation of PMME based on computer simulation is presented in Section 4 Finally the last section is the summarization and our future research work

2 RELATED WORK

Several research works have been considered to ensure QoS for multi-priority wireless sensor networks such as PQMAC [12], QAEE [13], and MPQ [14]

PQMAC (Priority-based QoS) considers four packet priority levels in the network but requires synchronization between nodes [12] and the packet delay is still higher than the requirement of realtime application

Figure 1 Description of the operation of QAEE-MAC protocol

QAEE is the protocol that allows the receiver to initiate communication [13] It considers two priority levels of packets and allows high priority packets to be transmitted faster than low priority ones In this protocol, receiver node will wake up periodically to receive packets sent from the sender After waking up, the node will listen to the environment for a guaranteed period of time T g and then sends Wakeup-Beacon to notify senders After transmitting the Wakeup-Beacon, the receiver will wait for a while to receive the all Tx-Beacons (which adds the priority field) of senders The senders will insert the packet priority bit and the Network Allocation Vector (NAV) field in the Tx-Beacon Then, the senders will wait for the Rx-Beacon with NAV field from the receiver The receiver will receive multiple Tx-Beacons with different priority levels in a period of time Tw and select one sender based on the highest packet priority It then propagates the Beacon carrying the address of the selected sender Based on receiving this

Rx-Beacon, the selected sender will be allowed to send data while other senders will not be active during this time In the aftermath of the competition, senders with untransmitted data will wake up and continue randomly competing as before Figure 1 describes QAEE communication operation in which SIFS (Short Interframe Space) is the required interval for processing a packet and switching the radio state of the sensor node

QAEE has some disadvantages First, it considers only two priority levels as high (1) and low (0) Second, the receiver node must wait until it receives the entire Tx-Beacons from the sending nodes and then sends the Rx-Beacon to all senders This means that even

if the receiver has received the highest priority Tx-Beacon then it still has to wait until

w

T runs out Therefore, the higher priority sender still has to wait and other sending nodes consume energy during idle listening time for Rx-Beacon

MPQ has improved over QAEE by considering four different priority levels (Table 1) and significantly reduces the latency for the highest priority packets by accepting first Tx- Beacon with the highest priority and then sends an Rx-Beacon acknowledgment to the selected sender without waiting until Tw runs out [14] If there is no highest priority packet arrives before Tw runs out, packets of lower priority still have to wait until

w

T expires The MPQ protocol uses CSMA p-persistent mechanism with p is inversely proportional to the number of senders ns, this mechanism spreads Tx-Beacon frames from

s

n sendersevenly to reduce collision

Table 1 Different levels of packet priority [15]

Data Category Priority

The MPQ protocol uses the general format of IEEE 802.15.4 frame for Wakeup-Beacon, Tx-Beacon, and Rx-Beacon frames with some special fields highlighted in Figure 2

SA is the source address of the packet to be sent to the destination address DA In the Wakeup-Beacon, SA is the receiver’s address, this beacon is used to broadcast to the surrounding environment, so there is no specific destination address DA In the Tx-Beacon, SA is the address of the sender has sensor data to be sent, the DA is the receiver node’s address, and the priority is the priority of the sensor data to be sent In the Rx-Beacon, SA is the address of the node that wants to receive data, the DA is the sender node’s address that has been selected by the receiver

However, the MPQ protocol still has some limitations First, only the highest priority packets will be processed earlier, while the lower priority packets still have to wait until

w

T expires to be considered for delivery Thus, the nodes do not have the highest priority packets also have to spend time waiting for the Rx-Beacon Secondly, assigning pvalues

is rigid and unrealistic which requires receivers in the network have to know exactly the number of competing senders

With the remaining shortcomings of the QAEE and MPQ protocols, further improvements to the MAC protocols should be addressed to further improve WSN performance

Figure 2 Frame format of all Beacons [14]

3 PROPOSED SOLUTION

In order to achieve the quality of service required by packets of different priority levels, our proposed MAC protocol in the paper has two variations compared to the MPQ protocols

Firstly, in order to prioritize data packets according to the priority level of the packet, the proposed MAC protocol allows the sending nodes to send a Tx-Beacon with the sending frequency proportional to the priority level of packet

Secondly, the receiver shortens the waiting time for sending Rx-Beacon When the receiver receives the first Tx-Beacon from any sending node, it sends the Rx-Beacon to the first sender node This Rx-Beacon also informs other sender nodes to sleep during the transmitting data of the selected sender

Figure 3 describes the proposed PMME operation with two enhancements: the CSMA p-persistent mechanism, which adapts to the priority level of the packet, and the earliest Tx-Beacon accepting mechanism

3.1 CSMA p-persistent mechanism with p varied by the priority level

To prioritize packets, in the PMME protocol, a new CSMA p-persistent mechanism is applied, Tx-Beacon is sent from sender with p varied by priority level (Figure 4) With this mechanism, if a sender receives its Rx-Beacon (allowing it to send data frame), first, it listens to the medium (or check the state of the medium) before deciding to send data frame or not Second, if the sender finds the medium is idle it follows these steps:

1 With probability p, the sender sends its frame The value p is varied by the priority level, the higher the priority level, the larger the p (largest p is 1, smallest

p is 0) In our proposal, there are two different types of the value p: linear and non linear

a) For linear value, sender i th will have its value p n i as

1

i

j

i p

j







Wakeup-Beacon

Tx-Beacon

FC SA DA Priority NAV FCS

Rx-Beacon FC: Frame Control FCS: Frame Check Sequence

SA: Source Address DA: Destination Address

NAV: Network Allocation Vector

where n is the number of priority levels

b) For non linear value, sender i th will have its value pi a n, as

1 ,

1 1

i i

j j

a p

a











(2)

where a is the distinguishing base, and n is the number of priority levels

Of cource, the summarization of all senders’ p i must equal 1 (means one hundred per cent) as follows:

1 1

n i i

p





1 1

n i n i

p





1

1

n i

a n i

p





 ) (3)

Figure 3 Description of PMME operation

2 With probability q 1 p, the sender waits for the beginning of the next time slot and checks the state of the medium again

a If the medium is idle, it goes to step 1

b If the medium is busy, it goes to step 2

Wakeup

Tg

TX:

Wakeup Beacon

Receiver

Packet Generation

Listening

RX:

Wakeup Beacon

Sender 1

(N1)

TX: Transmit RX: Receive

Sender 2

(N2)

RX:

Tx Beacon

Packet Generation

TX:

Rx Beacon (to N1)

ACK

TX: DATA

RX:

Rx Beacon (to N1)

RX:

Rx Beacon (to N1)

RX: ACK

TX:

Tx Beacon

RX:

Wakeup Beacon

Sleep, NAV+random time considering priority level

SIFS

SIFS

Listening

SIFS

Accept earliest Tx-Beacon, no need to wait until T w expired

Time slot changed by priority level

Tw

Figure 4 CSMA p-persistent in PMME

The Tx-Beacon of a packet with higher priority level will have more chance to appear than the Tx-Beacon of a packet of the lower priority level, so that the accepting rate of the higher priority level Tx-Beacon would be higher than the lower one with limited times of retransmission

3.2 The earliest Tx-Beacon accepting mechanism

To reduce the waiting time in the collising window after a sender sending Wakeup-Beacon, the PMME protocol uses the earliest Tx-Beacon accepting mechanism by sending Rx-Beacon right after receiving the first Tx-Beacon This Rx-Beacon also announces all other senders not sending their frames during the NAV time By doing so, our protocol will shorten the waiting time comparing to Twof QAEE and MPQ protocols, the earlier sending Rx-Beacon will also help other senders know to avoid sending their frames and do not make the collision in the following NAV time, neither waste the energy to sense the medium or send frames and make the collision become worst

4 PERFORMANCE EVALUATION

This part introduces the simulation results for evaluating, comparing multi-event wireless sensor network performance using the proposed PMME, QAEE and MPQ protocols based on Castalia 3.3 simulation [16] and OMNeT ++ 4.6 [17] using the CC2420 transceiver standard [18]

Sense the medium

Idle Busy

prand ≤ pn

Y

Send the frame

N

Wait for a

time slot t

(pn ~ priority level)

Wait for CCA check delay

4.1 Simulation Parameters

Table 2 Simulation Parameters for PMME

Number of concurrent sender nodes 1-10

w

g

Table 2 shows the main parameters in PMME simulation Sensor nodes are randomly distributed in the sensor field, at one time there would be 1 to 10 sender nodes sending data, one sink is in the center Each sender node sends data packets at a rate of 1 packet per second, with equal packets of different types

Performance parameters are evaluated in our simulation:

 Average packet delay: The packet delay is an expression of how much time it

takes for a packet of data to get from the source node to the destination node There are several contributing sources to the delay encountered in transmitting a packet For all three protocols, they are propagation delay, the time the sender node has a packet wait for the Wakeup-Beacon, time to send Tx-Beacon, time

to wait and receive the corresponding Rx-Beacon (this time can be extended by the NAV if it is not the selected node to send data packet), bandwidth delay (the time to send the packet data over a channel with limitted bandwidth) and other status transition times Average packet delay is the averate delay of all packets received by the receiver

- Average packet delay Davrformula is calculated as follows:

1

N i i avr

D D

N







(4)

whereN , Diare the total number of received packets and the delay of the

th

i received packet

 Packet success rate (PSR): It is a ratio of the total number of packets received at

the destination node NR (excluding duplicate packets) to the total packets sent from all sender nodes NS

100%

R S

N PSR

N

 Energy Efficiency: It is considered as the inversion of the average power

consumption for successfully transmitting a data bit (μj/bit) The less energy consumed, the higher the efficiency

o The formula for calculating average energy consumption is as follows:

T

E E



Where ET, NR, and DS are the total power consumption, the total number of packets received, and the packet size in bits respectedly

o Total energy consumption is calculated as:

1

m

k



where m represents the number of states, kis the radio status (four states: state of play, state of reception, state of hearing, and state of sleep) Pk is the power consumption at state k and tk is the lifetime of the state k

4.2 Result analyses

In this section, simulation results show that our PMME could adapt to the different QoS requirements of multiple event types especially with lower delay for all types of packets

4.2.1 Average packet delay

Average packet delay is shown in Figure 5 and Figure 6 with packet sending rate of 1

packet per second and maxTxRetries = 10, n 4, and a 3 It can be seen that when the number of sending node increases, packet delay will be higher because many packages are sent at the same time and cause a collision QAEE uses Twto be able to receive multiple requests at the same time and then sort the requests in order of priority level, then accepts the highest priority packets, so it will take more time to receive all the requests Since the packets compete for sending packets, there will be a collision that results in the retransmission of these packets And since no priority time is given for sender nodes to send Tx-Beacons, the probability of collision is high even at the next sending cycle With the proposed PMME solution, receiving the first request would greatly reduce the delay compared to QAEE and MPQ, and using the p-persistent collision avoidance mechanism would help to disperse the Tx-Beacon The PMME energy efficiency increases because

this protocol avoids sending many repeated Tx-Beacons before the actual data packet is transmitted

Figure 5 Comparing the average package latency for all types of packages

Figure 6 Average packet delay using the PMME protocol versus

using the QAEE and MPQ protocols

However, the influence of maxTxRetries, after trying to send Tx-Beacon to the maximum retry number, the data packet will be destroyed On the other side, the packet delay will only count for the received packets, thus reducing the degree of differentiation

of packet types when the number of sending nodes increases In addition, it can be seen

that the delays of different priority packages of QAEE and MPQ do not significantly differ from each other and from the 4 PMME priority packets when the number of nodes is small The QAEE and MPQ assume that p is the inversion of the number of sender nodes, when the number of send nodes is 1, p is always 1 and the Tx-Beacon is not delayed if it knows that the medium is idle, while the PMME does not distinguish the number of sender nodes, and there would always be delay time for Tx-Beacon to be sent even when the medium is idle In fact, it is impossible to know exactly the number of nodes that send data simultaneously at a time so the assumption of PMME is more reasonable

4.2.2 Packet delay of different priority levels

a) Linear p, n=4

b) Nonlinear p, a=3, n=4

Figure 7 Average packet delay using the PMME protocol with 4 priority levels

Packet delay according to PMME packet priority is shown in Figure 7 with maxTxRetries = 10, number of senders is from 1 to 10 In order to be able to perform a different priority mechanism, the scenario selects linear type with

1

i

j

i p

j







and none