Dynamic Channel Assignment with Reservation Mechanism ... · A Hybrid Multi-channel MAC protocol...

Ubiquitous Computing and Communication Journal

Volume 3 Number 3 Page143 www.ubicc.org

A Hybrid Multi-channel MAC protocol with Virtual Mechanism and Power Control for Wireless Sensor Networks

Yang Yuwang*1, Ju Yutao2, Jin Baoshen3,Yu Jimin1 ,Sun Yamin1,Yang Jingyu1

1Computer Department of Nanjing University of Science and Technology,Jiangsu,China 2Mechanical Electrical Engineering Department of Nanjing University of Science and

Technology,Jiangsu,China 3 R&D center, ZTE communication Company,Nanjing,Jiangsu,China *Corresponding author, E-mail address: [email protected].

**other authors: E-mail address:{yutaoju,Baoshenjin,jiminyu,yamingsun,jingyuwang}@mail.njust.edu.cn

ABSTRACT A Hybrid Multi-channel MAC protocol with Virtual Mechanism and Power Control for Wireless Sensor Networks (HM-VMPC) is designed and implemented in this paper. This kind of protocol integrates dynamic channel assignment mechanism and quasi-reservation mechanism effectively. It employs a virtual MAC frame mechanism to support larger network layer packets, and a multi-channel virtual carrier sensing mechanism to estimate idle or busy channels effectively, and has the function of intelligent power control which adjusts the transmission power levels automatically according to the distance among network nodes, therefore reduces the energy consumption and prolongs the life of the entire network. This protocol provides proper solution to the hidden and exposed terminal problem in wireless sensor network, and improves the network performance. HM-VMPC is compatible with the physical layer of IEEE 802.15.4 standards, and is able to run on SARD (Sensor Applications Reference Design) board from Freescale company. The performance of HM-VMPC is tested and compared with other classical MAC protocols. Experiments show that HM-VMPC is suitable to solve the problem of hidden and exposed terminal in multi-hop wireless networks, and can control the power more effectively to reduce the energy consumption of network nodes for prolonging the life of the entire network.

Keywords: multi-hop wireless network, MAC protocol, dynamic channel assignment, sensor network

1 INTRODUCTION

MAC layer of wireless sensor networks is

particularly important for controlling all the incoming and outgoing packet directly. MAC protocols have direct impact on the utilization of channels, QoS (Quality of Service) of the entire network and node battery life [1].The difference of MAC protocols between wireless and traditional wired network is that, besides the consideration of fair channel access and collision-free data transmission, it has to focus on saving the battery power and improving the scalability of MAC protocols. In addition, because of the restriction of sensor node capacity, the MAC protocol itself cannot be too complicated.

In order to solve the exposed and hidden terminal problem, channel access fairness, QoS guarantee, power control and other issues faced by multi-hop wireless MAC protocols, many wireless MAC protocols have been put forward which can be divided into four types. They are schedule based, competition based, collision-free, and hybrid MAC

protocols. Schedule based MAC protocols include

Self-Organizing Medium Access Control for Sensor Networks(SMACS)[2],Eavesdrop-And-Register (EAR)[2] algorithm, Distributed Energy Aware MAC Layer Protocol (DE-MAC)[3], Energy Efficient MAC protocol for Sensor Networks (EMACS)[4] protocols. To this kind of protocols, the time period staring from network nodes embedded with this protocol is decided by the scheduling algorithm, which fully adapts to topology change and maintains optimal network attributes. The disadvantage of SMACS is that the node coming from different sub-network may never have communication opportunity. The EAR algorithm maintains seamless connection among mobile nodes, but only suitable for those networks which remain overall static. The core idea of DE-MAC is to allow nodes to exchange the energy-level information, but the drawback is that even while in the time slot of its neighbor node possession, it must keep awake, too.

Competition based MAC protocols generally

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employ the broadcast channel, and CSMA ( Carrier Sense Multiple Access ) operation mode, deal with hidden and exposed terminal problem by signaling control with additional information. Such protocols include sensor MAC(S-MAC)[5], Timeout MAC(T-MAC) Protocol[6], Directional MAC (D-MAC) [7], Adaptive Rate control protocol [8], Radio protocol[9], Wise MAC[10], Aloha[11], extended IEEE 802.11 based Route Access Protocol (RAP) MAC[12] and non-persistent CSMA with preamble sample(NP-SCMA-PS)[13]. The S-MAC protocol employs IEEE802.11 standards to avoid the collision. However it does not give enough protection to nodes with less energy. T-MAC employs adaptive performance parameters based on S-MAC, but it can not overcome problems such as node early sleep, virtual cluster and multi-hop synchronization. DMAC is a MAC layer protocol of high energy efficiency and low delay based on data assemblage, but it isn't suitable for data communication among arbitrary nodes.

Collision-free MAC protocols include Spatial TDMA (Time Division Multiple Access) [13], the Implicit Prioritized Access Protocol[14] which has high throughout especially under the circumstance of high payload. The traffic-adaptive medium access protocol (TRAMA)[15] which avoids competition due to concealing interrupt, but brings long delay.

Hybrid MAC protocols include TDMA-DMA[16]

and Contention Aware Transport (CAT)[17]. TDMA-DMA employs the advantages of physical layer, but its channel utilization rate is low. The power utilization rate is high for CAT protocol, but its cost is large for implementing mobile performance..

The earlier multi-channel MAC protocols mostly adopt fixed channel assignment mode by using different band width [16,18] or spreading frequency codes [19] to divide a single channel into multiple ones. These protocols reduce collision effectively, but increase the complication, node cost and energy consumption.

The study has found that the idle listening is the major energy consumption in the wireless multi-hop network, so the protocols with any kind of channel access mechanism closes the communication modules while no node data transmission in order to reduce idle listening for idle energy consumption reduction. To prevent the sleep mechanism impacting on neighboring node connection, frame structure is used to divide into equal interval frame. Nodes in the entire network or in the same virtual cluster use the synchronized frame to obtain connection[21]. Intra-node in the slot allocation generally is fair, but it also selects the key points, and gives the key points more transmission time slots to adapt the network communication payload or balance the nodes from the view of energy consumption [20].

Some MAC protocols such as S-MAC, T-MAC,

DMAC are single channel protocols whose performance become worse when the network size increases. And the unique exposed and hidden terminal problem cannot always been resolved by single-channel MAC protocols. The traditional multi-channel MAC protocols are based on fixed channel allocation and require more complicated and expensive hardware. The application of multi-channel and dynamic channel allocation technology can completely solve the problem of hidden and exposed terminals, solve the channel distribution, access control, collision and competition issues effectively, further can significantly increase network capacity, overall performance and network life.

Wireless sensor network nodes are usually powered by batteries with limited amount of energy. Some techniques to reduce energy consumption are of interest. One way to conserve energy is to use power saving mechanisms[23, 24]. Another technique is to use power control schemes which suitably change transmitting power to reduce energy consumption [25,

26, 27,28,29,30]. Moreover, power control can potentially be used to improve spatial reuse of the wireless channel.

In this paper, we design a protocol of multi-channel MAC protocol-HM-VMPC (a Hybrid Multi-channel MAC protocol with Virtual Mechanism and Power Control) based on half-duplex transceiver.

In second section of this paper a new multi-channel MAC protocol called HM-VMPC is designed, which is compatible with the standard IEEE 802.15.4 physical layer of wireless sensor networks as well as power control capability. The overall design of HM-VMPC protocol is analyzed followed by the main mechanism which the protocol uses such as virtual carrier sensing, virtual data frame, dormant mechanism and intelligent power control strategy.

In third section the HM-VMPC protocol is implemented under the CodeWarrior IDE by SARD development board used as multi-hop wireless network nodes.

In forth section experiments are finished and compared with several classic single-channel MAC protocols, so analysis of the advantages and disadvantages of HM-VMPC is obtained. The experiment uses four SARD boards as the sensor node hardware to make up a multi-hop sensor network to confirm the HM-VMPC protocol performance, and one personal computer which have serial port as task management center to demonstrate the data received by sink nodes. This paper uses energy consumption and the packets loss rate to measure the performance of HM-VMPC protocol. 2 HM-VMPC design 2.1 Overall Design of the HM-VMPC protocol



The overall design of HM-VMPC protocol is as following：

The whole channels are divided into N sub-channels, and one of these taken as a control channel is reserved to send control orders, book data channels. The other sub-channels are taken as data channels. Dynamic channel assignment and quasi-reservation mechanism from references [31,32,33,34] similar to those from multi-channel CSMA protocols are applied to manage and maintain these data channels. The traditional virtual sensing mechanism [35] is improved, and virtual multi-channel carrier sensing scheme for this protocol is designed. This scheme takes the Multiplicative Increase Linear Decrease(MILD) as the retreat algorithm, and is supported by RTS(request-to-send)/CTS(clear-to- -send)/RES(Reservation) Mechanism.

Data channels are reserved by the RTS / CTS / RES mechanism which runs on the control channel, and are acceptable to both sides of the receiver and transmitter.

Traditional stop-wait protocol[36] is improved to transmit sequence data of virtual sub-frame produced from its original virtual data frame. After a receiver completes the reception, it reorganizes the sub-frame into a complete virtual frame. Further this virtual data frame is transmitted to the upper network layer, and then the function of MAC protocol is finished.

Moreover, in this protocol design, the thought of energy consumption reduction from S-MAC[5] is improved to reduce energy consumption of nodes by the periodic dormant mechanism. The protocol improves the time synchronization of TPSN (Timing-sync Protocol for Sensor Networks) [21] to be more concise and efficient.

Intelligent power control technology is used to save energy due to battery life limited, reduce the packet collision and improve the utilization of channels.

2.2 Main mechanisms adopted by HM-VMPC

protocol 2.2.1 Dynamic channel assignment and channel

reservation mechanisms This protocol divides the channel into N

unduplicated sub-channels, and takes a public sub-channel as the control channel, which is used to send control command, book data channel, send broadcasting and so on. Then, the remaining N-1 sub-channels are taken as data channels, and the dynamic channel assignment technique is adopted to manage these channels. HM-VMPC protocol can support 16 sub-channels correspondent to the hardware design based on IEEE802.15.4 from Freescale company. Each node maintains a list which records the current usage of sub-channels, assigns

dynamic channel for data exchange among nodes on demand in time. When the nodes need to transmit data, the protocol books a data channel by RTS/CTS/RES handshake mechanism, and releases the channel when the mission completes.

HM-VMPC protocol cites the reservation mechanism of multi-channel CSMA protocol, and this kind of multi-channel mechanism based on the reservation mechanism has better capability compared with pure stochastic election for idle channel mechanism. Even the band width of each sub-channels is very small, the advantage of adopting reservation mechanism still exists. 2.2.2 RTS-CTS-RES handshake mechanism

HM-VMPC protocol uses sequent three handshake mechanism of RTS/CTS/RES on the control channel to book a data channel accepted by both communication sides. In addition, the adjacent nodes receive CTS frame and set the promissory channel busy, use this collaboration to solve the hidden terminal problem. RES frame is designed to solve exposed terminal problem, which has the main contents of promissory channel number and data length of this communication, it receives adjacent nodes which send RES frame, gains promissory channel number and transmission data length, estimates the sustaining time of exchange data. After three-time handshake, promissory data channels are assigned to the two nodes which transmit data this time. The timing sequence can be seen in Fig.2.1. 2.2.3 Virtual data frame mechanism

After three-times handshake, transmitter and receiver will monopolize promissory data channel, begin to send data frame at promissory data channel. IEEE 802.11 Distributed Coordination Function (DCF)、S-MAC、T-MAC protocols all book to send a long message data frame after establishing the communication. HM-VMPC protocol improves this kind of classical algorithms.

In HM-VMPC, a virtual data frame mechanism is applied to divide the original virtual data frame into many sub-virtual frames, and then the improved stop- wait protocol is used to transmit sequence data of virtual sub-frames. The main improvement is in the following:

At the transmitter node: 1) The MAC layer constructs a sub-data frame

which is waiting to be sent in order, initializes the frame, and the sequence number is set to be zero,

2) Let the data frame be delivered to the transmission buffer,

3) The data frame in the buffer is transmitted, and the channel is sensing to enter wait and acknowledgment state, and the timer starts at the same time,



Transmitter A B

Control Channel

Receiver B

Control Channel

Transmitter

&Receiver

Promissory Channel

L

RTS

S

RTS

S

CTS CTS

RES

RES

DATA

ACK

Nearby node of A

Control channel

RTS RES

Nearby node of A

Promissory Channel

NAV(RES)

Nearby node of B

Control channel CTS

Nearby node of B

Promissory Channel

NAV(CTS)

Figure 2.1: time sequence of node A transmitting data to node B with HM-VMPC protocol

4) If acknowledgment frame ACK is received,

the node judges whether the total number of the data frame reaches. If it is true, it proves that the virtual data frame has been sent out completely, then this data transmission is ended and the node starts to sense the control channel again. If not, the node continues to obtain a new data frame in order. If a denial frame is received or the timing counter is overtime, then it changes to the step 3) to re-transmit data frame.

At the receiver node: 1) Parameters are initialized, 2) The node starts to wait for data frame. If it

receives a data frame, examines whether there is transmission mistake on the CRC check. If there is no mistake, it continues the next step. If there exists mistake, then it structures a denial frame to transmit and goes to step 2) again,

3) The node picks out the data frame ID, constructs and transmits the ACK frame. If the ID is not the expected one, then it transfers to step 2). If the ID is the one, then it takes data section from the received data frame into the buffer in order,

4) The node judges whether the total number of the data frame reaches. if it does, it shows that the virtual data frame has been all received completely, the node can pass the payload to the upper network layer. Otherwise, it goes to step 2) and continues to wait for receiving a new sub-data frame.

HM-VMPC employs virtual frame mechanism for performance improvement. This kind of virtual frame mechanism is compatible with long message division mechanism of S-MAC protocol, as long as sub-data-frame of virtual frame is defined as the short message length of S-MAC, and at the same time, the virtual frame length itself is defined as the long message length of S-MAC. But the design concept of frame treatment from HM-VMPC and S-MAC protocols is different, and the virtual frame

mechanism in HM-VMPC is much more suitable for network transmission with large traffic.

Virtual frame mechanism in HM-VMPC can reduce packet collision, increase network throughout, shorten the time delay and so on. 2.2.4 Multi-channel virtual carrier sensing

technique A multi-channel virtual carrier sensing

mechanism is designed to estimate idle or busy channels effectively. HM-VMPC protocol design takes CSMA/CS method as basic channel competition mechanism, but HM-VMPC protocol uses a list of Idle and Busy Channel(IBC), a list of Network Assignment Vectors (NAVs) and a timer to replace NAV of the single channel protocol, because the protocol is used for network nodes of multi-channel, it should assign a NAV for each data channel of the node to record their idle-busy state.

HM-VMPC protocol adopts an IBC list, NAVs list and a timer to replace the NAV in single-channel protocol. Because the protocol is for multi-channel mechanism, it should assign a NAV about idle and busy state for each channel of nodes. The basic principle is as following.

When a sensing cycle starts, NAVs and IBC lists are initialized. That is the physical verification of the data channel. If the channel will be idle, the corresponding bit in the list is set to 1, if the channel is busy, let the corresponding bit be 0. Each bit in the IBC list is correspondent to an element in NAVs. When the node receives the CTS and RES from adjacent nodes, it will amend the NAV correspondent to the reservation channel in NAV list, and based on time interval set by the timer, non-0 in the NAV value is constantly revised in the list of NAV. Value 0 stands for a idle channel. If the node hopes to send or receive data, the NAVs list is checked to determine whether a channel is idle.



Receiver

i-1th level

Transmitter

i-th level

δ1 T2 T3 δ2 T1 SYNC-REQ T1 SYNC-ACK T2,T3 T4

Figure 2.2: Time synchronization timing

This process is shown in Fig.2.1. B、S、L denotes retreat time, interval between short frames(SIFS), interval between long frames(LIFS) respectively.

2.2.5 Synchronization mechanism

HM-VMPC protocol takes some thoughts from TPSN to synchronize time, and makes improvement to obtain concision and efficiency. Improved TPSN saves much more network resources, and does not affect synchronal results, which is described as following.

The root node first broadcasts synchronization request frame TSR, and after the neighbor node receives the TSR of the root node, sets the its own

system resources. The time difference ∆ between transmitter and

receiver is computed in following: Let δ1 denote transmission delay of synchronized

request frame. Let δ2 denote transmission delay of synchronized response frame. T1, T2, T3, T4 are local time of request frame at the transmitter, local time of receiving request by receiver, local time of response at the receiver, local time of response receiving by the transmitter respectively.

From reference[21]: (T - T ) - (T - T ) ∆ = 2 1 4 3

level as 1, records the sequence number of the root node. They wait for a period of stochastic time separately, exchange synchronization information with the root node. After the 2nd level node senses

δ

2 = (T2 - T1 ) + (T4 - T3 )

(2.1)

exchange information of the first level node, adds 1 to its own level value in the TSR frame, records the source ID in the TSR frame as its upper node number, withdraws and waits for a random time, requests again to implement the time synchronism with the first level node. Likewise, after the i-th level node sends out the request synchronization frame, its lower level node records the ID in TSR frame, sets its own level as i+1, withdraws and waits for a random time, so it can guarantee the higher level node complete the time synchronization, then requests again to carry on the synchronization with the i-th level node. Finally all the nodes synchronize with source node. In synchronization process, if the node receives many synchronized request frames, it should choose the

Here the value of ∆ and δ=δ1+ δ2is only relevant to （T2 – T1）and（T4 – T3）, and there is no direct relationship with time of processing and response of receiver node, so it is very suitable for the wireless sensor network with unstable performance. Transmitter can compute time difference ∆ with upper level node according to T1, T2, T3, T4, and adjust the local clock and make itself with upper level node synchronous. 2.2.6 Mechanism of the power control

All the handshake control frames （RTS/CTS/RES）are transmitted with the biggest power level in HM-VMPC. After the node receives

smallest level node as its own higher level node.

RTS, according to receiving power pr and the

The packet of HM-VMPC adds level-established information based on original synchronized request frame in TPSN, so the packet itself has level information when requested synchronization is transmitted to higher layers. Two stages in TPSN are merged to one, and then this mechanism reduces protocol complexity, improves the entire network synchronization speed, saves energy and network

current total noise power pnoise of the receiver node, The sending level PLev of sending node used as current data communication is computed through the intelligence power control processing, and this information is informed to the transmitter node through CTS frame. The node sending information receives a CTS frame, analyzes a power level Plev and adjusts the transmitting power to send out a data



n

n

frame. If this time fails to send out the data frame, transmitter node will adopt higher transmitting power level to carry on again until the transmitter sends packets successfully.

Receiver node computes the transmitting power level PLev as following:

Supposed that the sensor nodes all have same transmitting power and level, maximum and

implements the clock synchronization at the dormancy time interval. If the entire network clock synchronization is not finished in a dormancy period, it may continue in the next action period. The period value of clock synchronization relies on the timer precision of node hardware.

The node joining latest will continue to maintain the active sensing condition in order to maintain

minimum transmitting power are

pt ,max and

synchronization with the network as soon as possible, Once the node finds any active node though sensing,

pt ,min

respectively. From reference[22] , pt ,max

and

it transmits the request of synchronization, completes the network time synchronism, and enters the active

pr satisfy the following relationship: dormancy period. 3 The implement of HM-VMPC protocol

λ pr = pt ,max . 4π d

gt .gr （2.2） 3.1 The implement platform of hardware The hardware of network nodes is SARD board provided by Freescale Company, which can be used

Among them , λ is the wavelength of carrier

wave, d is the distance of receiver A and transmitter B, n is a path loss coefficient, and gt, gr are antenna gain of the transmitter and receiver respectively. Under the normal condition, λ 、gt and gr are constant and n value is between 2 and 4.

to implement the design of the wireless multi-hop sensor network compatible with IEEE 802.15.4 standards. The SARD is the platform solution of Zigbee-ready from Freescale Company. Figure 3.1 shows the main components of SARD.

After receiver node A receives a RTS, according to the background total power pnoise, receiving

ON/OFF

Buttons

Programming Port

sensitive level and the limiter

Rs ,n of receiver

LED

Signal-to-Noise(SNR), this mechanism can compute

the minimum power of signal pa for correct MCU

receiving information by node A, then node B should adopt the minimum transmitting power pb which satisfies:

RS232 MC13192

Antenna

λ DC IN

pa = pb . 4π d

gt .gr （2.3） Transformer Sensor

Although, two parameters n and d are unknown, they can be regarded as constants in the very short time intervals. Further pb can be expressed as:

Figure 3.1: SARD Board Application

p ≥ p pt ,max p

p

HM-VMPC

b min max a . , t ,min , t ,max （2.4） Initialization/ Simple MAC Layer

pr Configration Phisical Layer

Then, the node A searches minimum transmitting power level PLev to satisfy formula(2.4) in the power level list, and fill the power control information field within CTS frame, further transmits the CTS to the node B.

2.2.7 Periodic dormancy

As continuous sensing is an important factor

Hardware Configration

File

Hardware Drivers

Hardware

affecting energy consumption for sensor networks. In order to not let the clock synchronization increase the extra network burden, HM-VMPC protocol

Figure 3.2: Architectures of simple MAC and HM-VMPC



SYNC start

Send TSR？

Avoid collision for a while

Set local level, record upper node

ID Y

Y

Send TSR by CSMA，set Timer N Start listening

channel

A frame detected

Need to modify ID

of upper node? Y Is TSR？ N

Is local

TSA？

Y N N Target ID is

Local ID?

Y Send

TSA

N

Get time difference,

sync with upper node

N

End？

Y

End

Figure 3.3: Flow chart of synchronization mechanism

Several nodes can constitute various types of network such as star network and mesh network through different network protocols.

3.2 The software development implementation

The HM-VMPC development platform of the software is the Development Studio of the Freescale CodeWarrior 5.7.0.

SARD itself has a simple MAC protocol stack, and the layer relationship with HM-VMPC is shown in figure 3.2 .The simple MAC layer provides some interfaces for the HM-VMPC.

Control flow is implemented like the time synchronization mechanism in HM-VMPC taken as an example represented by fig.3.3. 4 Experimental design and result analysis

Fig 4.1 stands for the experiment environment. In fig.4.1, the sensor network node is

encapsulated in order to be deployed. The battery is outside of the box for replacing easily.



Pack

ets l

oss

rate

(/10

00)

Single to Multiple Channel

60 Competition

40

20 Single Channel CSMA

HM-VMPC Protocol

0 20 40 60 80 100 120

Frame Length(Bytes)

Figure 4.1: experiment environment of multi-hop sensor networks

Figure 4.2: Single to multiple channel competition

4.1 Experiment of channel competition

The collision of HM-VMPC data packet is analyzed through packet loss rate comparison of HM-VMPC protocol with CSMA protocol under one to multi-node communication.

Let three nodes send data frame to one master node at the same time. Every node uses the same biggest transmitting power of 3.6dBm, and the distance between nodes and the master node is 5 meters. The time-gap of inter-frame is a random number which is not more than 2000 millisecond.

The frame size is taken as 20B, 40B, 60B, 80B, 100B, 120B respectively for 6 group tests, and each group test lasts for 1000 seconds, and the packet loss rate is recorded.

In figure 4.2, when the MAC frame length is short, the packet loss rate of the non-handshake single channel CSMA protocol is lower than that of HM-VMPC protocol. This is because when the frame size is short, the network traffic is small, and the probability that the data frame collision is small. Although HM-VMPC protocol is multi-channel, but three-times handshake control packet transmitting on the control channel exists more collision.

But along with MAC frame size increases, the increasing trend of the packet loss rate of the simple CSMA network is faster than that of HM-VMPC protocol. This is because along with the increase of MAC frame, the entire network traffic increases, and

the probability of collision of frames in simple CSMA transmission channel is also increasing, which causes the packet loss rate rising. However, HM-VMPC protocol is multi-channel, and there is no collision on data channels, only the frame collision on the control channel causes packet loss. Therefore, the trend of HM-VMPC packet loss rate changing with frame size is not obvious. Its packet loss rate increases mainly due to the slow response speed of master node caused by the network traffic increase. 4.2 Exposed terminal experiment

The exposed terminal problem solved by HM-VMPC protocol is analyzed through loss rate comparison test of HM-VMPC with single channel IEEE 802.11 under exposed terminal environment.

Let the master node as the exposed transmitting terminal and the exposed receiving terminal respectively. Sending nodes in the network use the same transmitting power of 3.6dbm, and the time-gap of inter-frame is a random number which is not more than 1500 millisecond. Figure 4.3 and the Figure 4.4 stand for the network topology of the exposed terminal experiment. Here “0” stands for the master node, “pc” stands for a personal computer.

The frame size is taken as 20B, 40B, 60B, 80B, 100B, 120B respectively for 6 group tests, and each group test counts 1000 data packets, and the test results are in figure 4.5 and figure 4.6.

pc pc

1 0 2 3 1 0 2 3

Figure 4.3: Topology of receiving exposed problem Figure 4.4: Topology of sending exposed problem



Pack

ets

loss

ra

te(/1

000)

P

acke

ts l

oss

rate

(/10

00)

Pack

et lo

ss ra

te(/

1000

)

Packets loss rate when Receiver exposed pc

50

802.11 40 30 HM-VMPC 20 10

0 20 40 60 80 100 120

Data length(Bytes)

1 0 2 3

Figure 4.5: Packets loss rate when receiver exposed Figure 4.7: Topology of receiver hidden

Packets Loss Rate of Transmitter exposed

40 802.11

packet loss rate of hidden receiving terminal

35

30

25

30 HM-VMPC

20

10

0 20 40 60 80 100 120

20

15

10 802.11

5 HM-VMPC

0 20 40 60 80 100 120

MAC frame length(Bytes)

MAC frame length(Bytes)

Figure 4.6: Packets loss rate when transmitter exposed

Multi-channel MAC protocol-HM-VMPC is much better than the single channel protocol-IEEE 802.11 in the solution for exposed receiving terminal problem. As the network payload increases, the packet loss rate of single channel protocol increases quickly, while that of HM-VMPC increases slowly. This is because when the network payload increases, the data amount transmitted on single channel grows sharply, so the channel becomes quite jam. Therefore, probability of data frame collision increases, and this leads to the loss rate increasing obviously. HM-VMPC employs multi-channel scheme, the data and control information is transmitted separately, and more than one data channel may transmit data at the same time, therefore it is not very obvious to MAC payload affecting multi-channel HM-VMPC protocol.

From Fig. 4.6, when the node is under the exposed terminal condition, the packet loss rate of single channel IEEE 802.11 MAC protocol with the handshake mechanism is still high, and changes obviously with the network payload. But the packet loss rate of multi-channel HM-VMPC is lower than that of IEEE 802.11, and has no obvious change with the network payload. This result shows the advantage of the multi-channel HM-VMPC protocol in solving

Figure 4.8: Packet loss rate of hidden receiving Terminal exposed terminal problem. 4.3 Experiment of hidden receiving terminal

Let the master node as the hidden receiving terminal, Sending nodes in the network use the same transmitting power of 3.6dbm, and the time-gap of inter-frame is a random number which is not more than 1500 millisecond. Figure 4.7 stands for the network topology of the hidden terminal experiment.

The frame size is taken as 20B, 40B, 60B, 80B, 100B, 120B respectively for 6 group tests, and each group test counts 1000 data packets, and the test results are in Figure 4.8.

From the figure 4.8, multi-channel HM-VMPC has some advantage to solve the hidden terminal problem. Under the condition of multi-channel, the hidden receiver terminal can shake hands when the surrounding nodes transmit the data. Compared with other protocols, this operation mode can avoid the collision between data frames and control frames when handshake mechanism works. Thus the packet loss rate is reduced effectively. But, it is possible that the collision on the control channel between the frames of the HM-VMPC can happen, so there is still some packet loss.



Tran

smiss

ion

pow

er

leve

l

Pack

et lo

ss r

ate(

/100

0)

T ransmission power level wit h int elligent power cont rol

Packet loss rate with power control on MAC

16

14

12

10

8 With

power 6

control 4 Without

power 2

control 0

0 5 10 15 20 25 30

With 50 power 45 control 40

Without 35

power 30

control 25

20

15

10

5

0

layer

Transmission range(/m) 0 5 10 15 20 25 30

Transmission range(/m)

Figure 4.9: Effectiveness of intelligent power control

Figure 4.10: Rule of packet loss rate change

4.4 Power control experiment

By the measurement of the transmitting power levels of nodes embedded HM-VMPC protocol with power control mechanism under different communication distance, and the comparison of packet loss rate under the same distance with and without power control mechanism, the effectiveness of the power control in HM-VMPC is verified.

Nodes transmit data frames to the master node in the different distance, the length of MAC frame is 125B, the interval time to send frame is 500 milliseconds (to ensure same number packets in the same time), the transmission distance has 7 groups which are 0,5,10,15,20,25 and 30 meters respectively, each group test lasts 600 seconds. The average transmitting power levels of each experiment are computed, and the packet loss rate in HM-VMPC with and without power control is compared. Nodes without power control transmit data frames with maximum power level.

Figure 4.9 shows the average transmitting power level of intelligent power control nodes in different communication distance. When the distance increases, the transmitting power increases, but the relationship of transmitting power level and communication distance is not linear. In half position of the maximum distance (about 15 meters, the normal communication distance of the SARD nodes is about 30 meters) does not change significantly. Clearly, intelligent power control mechanism can choose the right transmitting power level based on the distance.

From Figure 4.10, in various communication distances, HM-VMPC has almost the same packet loss after having intelligent power control. This shows that saving energy with power control does not bring the decline of network performance.

5 Conclusion

In this paper, HM-VMPC protocol employs

many effective mechanisms to improve the

performance of the MAC protocol. It avoids collision by the improvement method from CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance), and the assistance with three handshake mechanism of RTS/CTS/RES, and also modifies periodic dormant monitoring mechanism of S-MAC protocol. By improving the time synchronization mechanism from TPSN, nodes achieve absolute time synchronization. Besides, it also introduces reservation, dynamic channel allocation as well as an intelligent power control technique according to the distance between the nodes is to adjust transmission power dynamically.

The collision of HM-VMPC is tested through packet loss rate comparison of HM-VMPC protocol with CSMA protocol under one to multi-node communication.

The exposed and hidden terminal problem solved by HM-VMPC protocol is analyzed through loss rate comparison test of HM-VMPC with single channel IEEE 802.11.

By the measurement of the transmitting power levels of nodes under different communication distance, and the comparison of packet loss rate with and without power control mechanism, the effectiveness of the power control in HM-VMPC is tested.

A large number of experiment show that HM-VMPC is suitable to solve exposed and hidden terminal problem, and effectively reduce the energy consumption through the power control. Acknowledgment

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