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A Multi-Channel MAC Protocol for Wireless Sensor Networks

Chen xun1,2, Han peng1,He qiu-sheng1, Tu shi-liang1, Chen zhang-long1

1.Department of computer science and engineering Fudan University Shanghai, 200433, China

2.School of electronic and information Jiangsu University of Sci&Tech Zhenjiang ,Jiangsu ,212003,China

(xunchen ,041021070,heqiusheng)@fudan.edu.cn

Abstract In this paper, we propose a novel multi-channel

medium access control protocol for wireless sensor networks. Our protocol can dynamically assign multiple channels to nodes, thereby significantly increasing network energy efficiency, network lifetime and data throughput as well. The protocol requires only one transceiver per node, but solves the multi-channel hidden terminal problem through distributed coordinator node. Finally we evaluate the performance of this protocol through simulations. The performance results show that protocol significantly increase network energy efficiency, network lifetime and data throughput and exhibits prominent ability to utilize multi-channel transceiver among neighboring nodes.

Keywords: multi-channel, media access control, wireless sensor networks, channel assignment

1. Introduction

Wireless sensor networks may consist of many different types of sensors such as seismic, low sampling rate magnetic, thermal, visual, infrared, acoustic and radar, which are able to monitor a wide variety of ambient conditions [1]. Thus, wireless sensor network has the potential for many applications: for military purpose, it can be used for monitoring, tracking and surveillance of borders; in industry for factory instrumentation; in environment to monitor forest, ocean, etc.

In wireless sensor networks (WSNs), energy saving is becoming more and more important, due to nodes’ limited energy resource. Some solutions for saving energy at MAC layer for WSNs are put forward. As we known, on one hand, the radio bandwidth in WSNs is quite limited. On the Other hand, many new radio transceivers already provide multiple channels function, such as CC2420[14] or MC13192[17]. However most of existing MAC

protocols focus on the single channel solution in wireless sensor networks and don’t take into account the multi-channel transceivers. For example, IEEE 802.15.4 standard allows for use of multiple channels available at the physical layer, but its MAC protocol is designed only for single channel. A single-channel MAC protocol doesn’t work well in the multi-channel environment, so it is imperative to design multi-channel MAC protocols in wireless sensor networks taking full advantage of multi-channel parallel transmission mechanisms.

In this paper, we propose a novel multi-channel protocol, which has the potential to dynamically assign channel to sensor nodes so that multiple communication links can be used for transmission simultaneously in the same region. Simulation results show that protocol significantly increase network energy efficiency, network lifetime and data throughput

The main contribution of this paper can be summarized as follows:

propose a novel coordinator-based multi-channel MAC protocol for WSNs.

provide efficient broadcast support. The rest of this paper is organized as follows:

Section 2 reviews the related work. Section 3 provides some background information. Section 4 presents our multi-channel MAC protocol in detail. Section 5 describes the simulation model and discusses the results of our simulations. Section 6 concludes the whole paper.

2. Related Work

Recently a number of MAC protocols have been proposed for Wireless Sensor Networks. Existing work in this area can be classified into the three categories: contention-based multiple access, Time-Division Multiple Access (TDMA) and multi-channel MAC protocols.

S-MAC[2], one of the famous energy efficient protocols in sensor network, is a contention-based

Proceedings of The Sixth IEEE International Conference on Computer and Information Technology (CIT'06)0-7695-2687-X/06 $20.00 © 2006

random access protocol with a fixed listen/sleep cycle. It uses a coordinated sleeping mechanism, similar to the power saving mechanism of IEEE 802.11. A time frame in S-MAC is divided into two parts: one for a listening session and the other for a sleeping session. Only for a listen period, sensor nodes are able to communicate with other nodes. The fixed listen/sleep cycle avoids any wasteful idle listening

TRAMA[3] is a TDMA-based algorithm, intended to increase the utilization of classical TDMA in an energy efficient manner. It is similar to Node Activation Multiple Access (NAMA) [4], in which for each time slot a distributed election algorithm is used to select one transmitter within two-hop neighborhood. This kind of election eliminates the hidden terminal problem and hence, that all nodes in the one-hop neighborhood of the transmitter will receive data without any collision.

There are a few multi-channel MAC protocol in wireless sensor network, Matthew J. Miller proposed a multi-channel MAC protocol[5]. The protocol assumes that each sensor node is equipped with two transceivers The primary transceiver is used for sending data and control packets, while the second transceiver is capable of transmitting a busy tone, which is named wakeup radio and just used to wakeup neighboring nodes and named wakeup radio. To some extent, these protocols are not unalloyed multi-channel MAC protocol, because the wakeup radio can’t be used to convey actual data packets. MMSN[6] protocol is the first multi-channels MAC protocol in wireless sensor node. However in this protocol the channels assignment is fixed. These reduce the channel utilization.

Most of present MAC protocols in wireless sensor network are designed on the basis of the single-channel transceivers and can’t take advantage of multi-channel transceiver. We need to consider how to utilize the multi-channel function of modern transceiver to improve communication performance and decrease the energy consumption.

3. PRELIMINARIES

Energy waste in MAC layerIn MAC layer, we can classify energy waste into

four types. Collision : When a receiver node receives more

than one packet at the same time, these packets are called “collided packets”. Packets that are damaged in collision have to be discarded and be re--transmitted. Therefore the networks would

spend more energy on the packet re--transmitted. Overhearing,: A node receives packets that are

destined to other nodes, which is named overhearing

Control packet overhead: The MAC protocols need that the nodes exchange some control packet with each other in MAC layer. We must do our best to reduce the control packet exchange

Idle listening, the nodes have to listening to an idle channel to receive possible traffic. When designing the MAC protocol for WSNs, we must reduce energy waste and increase energy efficiency.

4. Multi-Channel MAC Protocol

4.1 Assumption

In this section ,we present our multi-channel MAC MCMAC protocol. We first depict our assumptions before describing the protocol in detail.

N channels are available for use and all channels have the same bandwidth. None of channels overlap, so the packets transmitted on different channels do not interfere with each other. Sensor nodes have prior knowledge about how many channels are available.

the number of the data channel is N-1 and that of the control channel is one. The control channel can be used to exchange the control packets and data packets. But the data channels can only deliver the data packets.

Each sensor node is equipped with a half-duplex transceiver, so a node can either transmit or receive, but can not do both simultaneously. Also the node can transmit or receive on only one channel at the same time. So when receiving on one channel, the node can’t carrier sense on other channel.

The sensor node can switch the transceiver to different channel dynamically, the switching time for channel is less than 200us[14].

The sensor network is divided into cluster in order to save energy. The nodes within the same cluster are synchronized. A cluster comprises 64 sensor nodes at most. Each cluster head act as media access coordinator.

The cluster head can communicate with each other at stronger power.

Synchronous error among the nodes is less than 100us.[15]

To eliminate the bottleneck problem of single sink node we assume that there are many sink node in the networks.


4.2 Protocol Framework

C lu s te r H e a d

S e n s o r N o d e

A

B

C

Figure 1. Overlapped cluster

We now depict the proposed multi-channel protocol framework. The sensor network is divided into many overlapped clusters as shown in Figure 1. The cluster establishment scheme has been proposed in many papers. Our paper does not focus on this problem and adopt the elementary LEACH[16] scheme.

Synchronous Beadcon Trasnsmission Rquest Windows

Channel Schedule Windows Data Convey Windows Sleep Windows

Frame Structure

Active Period Sleep Period

1 2 3 4

Figure 2. Communication frame structure

Since the energy saving is a critical issue, we establish the low-duty-cycle communication operation within the cluster in order to reduce energy waste.

As shown In Figure 2 , every communication cycle consists of an active period and a sleep period, then the active period is divided into four stages. These four stages are synchronous beacon, transmission request, channel schedule and data convey respectively. The nodes are sleep at most time. Under the light-load condition, the sleep time of the nodes is beyond 98% of the total time.

The neighboring cluster heads negotiate about sleep time with each other in order to avoid inter-cluster interference. If the neighboring clusters interlaced enter the active period, the neighboring clusters do not interfere with each other.

At same time, the cluster heads need to negotiate to reserve a Contact-Time for inter-cluster-head communication. This Contact-Time is in the sleep period of the intra-cluster communication, as shown in figure 3. The inter-cluster-head communication is on the control channel.

Inter-Cluster - head Contact Time

Intra-Cluster Communication Sleep Period

Cluster Head A

Cluster Head B

Cluster Head C

Figure 3. Inter-cluster negotiationCluster establishment

We assume that the cluster heads and the member node can get some cluster information, such as the number of the member nodes, member node ID, the neighboring node lists and the cluster ID that the neighboring node is attached to. The node can be attached to different cluster in the same time, which can act as the gate-node to implement inter-cluster communication. The cluster member node is in the radio range of the cluster head. The source node can select the request transmission time according to which cluster the destination node belongs to.

Once established the clusters, the cluster head broadcast the synchronous information within the cluster. In this duration the sensor node keep in listening mode to attain synchronous information. After the sensor nodes get the synchronous information, they setup their wake up clock and get into sleep.

normal communication process The nodes wake up by the wakeup clock and

keep listening on the control channel. As shown in Figure 4, we describe the multi-channel MAC protocol in detail.

CH1

CH2

CH2

CH1

8Node ID1

Node ID2

Node ID7

Node ID8

7

Cluster Head

Beacon Receive

Broadcast

Data Convey in Channel1

Sleep Data Convey in Channel1

Figure 4. Protocol demonstrate

1. Synchronous beacon stage The cluster heads transmit the synchronous

beacon signal on the control channel at the head of the active period in order that the sensor nodes can adjust their wakeup clocks. 2.Transmission request stage

In this stage, the cluster head switches to the listening mode on the control channel. The Transmission requested stage is divided into many time slots. The number of slot is equal to the number of the cluster member nodes. The cluster head distributes a fix time slot to every node. The slots are assigned according to the sensor node ID. The first slot belongs to the node whose ID is smallest and the last slot belongs to the node which ID is largest.

The member node and cluster head keep a table as shown in Table 1. The cluster head can deduce the request node ID according to the time slot position number. Then the node which requests communication leaves out the source ID field in


the request packet and use the time slot position number instead of the destination node ID, thereby, reduces communication control overhead.

If one sensor node wants to send data packet, it send the require control packet to the cluster head on the control channel in its appointed time slot. The cluster head deduce the source ID and destination ID according to the time slot position number. If the node has no message to send, it can switch off the radio in the whole transmission request stage in order to save energy.

Table 1.Node ID to slot number Node ID Assigned Slot ID

1010 1 1C0A 2 201C 3

……. ……. The request packet is very simple, as shown in

Figure5, which includes some flag and time slot position number of destination node. The B flag denotes that the source ID requests a broadcast or unicast. The P flag denotes the request priority. In this way the protocol can provide simply priority support in MAC layer.

Type Dest Node(Slot Number)B P Check

Sum

B flag: 1 broadcast 0. unicastP flag: 1. high prior 0. low prior

Figure 5. Request packet

3.Schedule stage During the transmission request stage, the

cluster head collects all the request information from the member node. In this stage all member nodes switch to the listening mode on the control channel. The cluster heads distribute channels for the source node and destination node. The cluster heads, then, broadcast the channel assignment information packets on the control channel. After member nodes receive the channel assignment information packets, they switch their transceiver to the appointed channel according to the content of the channel assignment information packets. In the remained time of the schedule stage, all nodes turn off the transceivers and turn to sleep for saving energy.

When the number of transmission request is greater than the channel number, the cluster would store superfluous request in a queue buffer according to the priority of request or other factor. Once idle channel resource exists, the cluster head takes out these request from the queue buffer and assign channel resource for the source node and destination node according to these request. This method reduces the request retransmission.

The cluster head informs the source nodes whether it receives the request through the

request-bit-map field of schedule packet. This field has 64 bits and one bit denote a source node. For example, when the cluster head receives a request packet from node A, the cluster head will set “1” in the bit which denoted node A. The source node can check corresponding bit to find request state and decide to whether retransmission the request or wait for the cluster head schedule in the next communication cycle.

This field may be leaved out in order to reducing the schedule packet size, when channel resource contents the request from source nodes. We try our best to reduce the control overhead and increase the energy efficiency. As shown in Figure6, the schedule packet comprises packet type, request-bit-map (option), schedule block length and schedule block. The schedule block includes the source node ID, destination node ID and the appointed channel information.

Type Schdule Block Length

Src ID(Slot

Number)

Dest ID(Slot

number)

ScheduledChannel ChecksumRequest

Bit-Map

Figure 6. Channel schedule packet

4. Data delivery stage In the data delivery stage, if a node is assigned

the channel resource they wakeup to exchange the message on the appointed channel, and then turn off the transceiver and return to sleep for energy saving.

If a node is not assigned channel resource, the node has to keep sleep state throughout the whole data delivery stage.

Out protocol provides the support of message broadcast. If one node wants to broadcast a message, it can set the flag “B” in the request packet, the cluster head will assign an appropriative channel for source node. The source node can transmits the broadcast message to the cluster head on this channel, and then the cluster head send this broadcast message to other cluster heads in the contact time. In the next active period, the cluster head should piggyback the broadcast packet within the channel assignment information packets and broadcast them in the schedule stage. Our protocol requires only cluster head to relay the broadcast the message. Compared with other protocols, our protocol provides the efficient support of broadcast with low energy.

5. Analysis of Protocol

5.1 Simulation Parameters and Performance Metric.

We simulate our protocol in the OMNET++ environment using similar configurations. Table 2


shows the default value of each parameter.

Table 2. Simulation parameterArea 200m X 200m SquareNode Number 400 Node Placement Uniform Communication rate 250 Kbps Tx\Rx\Sleep Mode 17.4mA\19.7mA\20uA Radio Range 40m Load 2 packet/node/second Packet length 100byte

5.2 Energy Efficiency vs Channel Variety

Figure 7. Energy efficiency vs channel variety

We take energy consumption per byte as an metric to measure our protocol performance. The energy consumption ration is calculated as the total energy consumption over the total effectual data successfully delivered by MAC layer. Figure 7 shows that our protocol becomes more energy efficient when we increase the number of channel. We see that the energy consumption per byte of successful efficient data decrease from 35uJ/byte to 5uJ/byte when the number of channels increases from 1 to 20.

According to the transceiver component characteristic, the transceiver consumption power is 60mW in transmission or receiver mode. Thus without any addition control communication overhead, the transceiver consumes 1.8uJ when it transmits or receives one byte data and the total energy consumption is 3.6uJ/byte both the source node and destination node.

At the same time, we can find the speed of energy consumption decrease slows down when channel increases more. We think that the proportion of energy consumption spent on the control overhead becomes little with increasing channel number so that the energy consumption on one efficient byte data decreases slowly.

5.3 Energy Efficiency vs Packet Length

We compare the energy efficiency of our

protocol with MMSN. The number of channel is 8 in both protocols. The data packet length changes from 32 bytes to 256 bytes .The result is shown in Figure 8.

Figure 8. Energy efficiency vs packet length

We discover that the energy efficiency performance of our protocol is better than that of MMSN. We select the cluster head to coordinate the intra-communication, which reduce the network-wide idle listening and increase the member node sleep time. The nodes act as cluster head in turn to avoid the cluster head node deplete earlier. Figure 8 shows the MMSN spend more energy than our protocol, when the packet length increases, and therefore the energy efficiency of both protocol is improved. We think that network spend more time on the deliver the data packet and reduce the idle listening time, so energy efficiency increase.

5.4 Throughput vs Source Node

Figure 9. Throughput vs source node

We compare the performance protocol when the source node number and packet size variety. There are 8 communication channels in this simulation. Figure 9 shows that the data throughput increases with an increase in number of source nodes, because more nodes get involved in the communication and more parallel data transmission occur. When the source node increases beyond certain value transmission, the congestion phenomena appear and the increase of


throughput slows down. If the source node increases sequentially, the data throughput would reach the saturated state. From figure 10 we can also find that the data throughput can increase with the packet size adding. Which can be explained from the increase in utilization per active period. We can increase the data throughput by the bigger packet size under the better channel condition. But the bigger size packet has lower probability of successful transmission. We must choose a rational packet size for a trade-off between throughput and energy efficiency.

6 Conclusion and Future Work

In this paper, we propose a novel multi-channel MAC protocol for wireless sensor network application, which is a coordinator-based multi-channel MAC protocol. Energy Efficiency is the primary goal in the protocol design. This protocol reduces the network-wide idle and increases the sleep time of node, and hence, it obtains significant energy saving and prolong the network lifetime. In addition, it improves multi-channel utilization, decreases the packet collision and increases the data throughput through the dynamical channel assignment. Finally we evaluate the performance of our protocol through simulation and the result show that our protocol achieves energy efficiency and better throughput under the multi-channel condition.

In future, we will study QoS support in Multi-channel MAC layer and evaluate the impact of higher layer on the proposed protocol in this paper.

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