Synchronous receiver initiated MAC protocol for long-lived sensor networks

Abstract

In this paper, we discuss the design of a new Medium Access Control (MAC) protocol, SRI-MAC (Synchronous Receiver Initiated MAC), for wireless sensor networks whose goal is to extend the lifetime of the network by avoiding major energy waste causes, such as collisions, overhearing and idle listening.

SRI-MAC is designed on the basis of synchronized duty cycled MAC protocols. The nodes in SRI-MAC synchronize each other to avoid data packet collisions. SRI-MAC uses receiver-initiated data transmission in order to efficiently and effectively operate over a wide range of traffic loads. It employs an adaptive beacon and a series of RTS/CTS (Request To Send/Clear To Send) packets to reduce duty cycle and minimize idle listening. In SRI-MAC, the senders remain active and wait silently until the receiver explicitly signifies when to start data transmission by sending CTS packet. If the node is not the intended sender, it returns to sleep immediately and continues its sleep as if the medium had been idle. Analytical and Simulation results show that SRI-MAC reduces the energy consumption and achieves an obvious improvement on the network lifetime.

Introduction

With the recent advances in MicroElectro-Mechanical Systems (MEMS) technology, wireless communications, and digital electronics, the design and development of low-cost, low-power, multifunctional sensor nodes that are small in size and communicate untethered in short distances have become feasible. The ever-increasing capabilities of these tiny sensor nodes, which include sensing, data processing, and coble the realization of wireless sensor networks (WSNs) based on the collaborative effort of a large number of sensor nodes [1]. WSNs have great potential for many applications in scenarios such as military target tracking and surveillance [2], [3], natural disaster relief [4], biomedical health monitoring [5], [6], and hazardous environment exploration and seismic sensing [7].

Due to low power support for sensor nodes, energy efficiency becomes one of the core problems. From analysis on the sensor nodes, the communication module is the part consuming most energy, which is the main optimization goal. The Medium Access Control (MAC) protocol directly controls the communication module, so it has important effect on the nodes’ energy consumption. Many reasons related to MAC paradigms lead to energy waste and WSN life reduction, such as [8], [9], [10], [11], [12]:

• Collision: The first one is the collision. When a transmitted packet is corrupted due to interference, it has to be discarded and the follow on retransmissions increase energy consumption. Collision increases latency also.

• Overhearing: The second is overhearing, meaning that a node picks up packets that are destined to other nodes.

• Packet overhead: The third source is control packet overhead. Sending and receiving control packets consumes energy too and less useful data packets can be transmitted.

• Overmitting: The four source is overmitting, it occurs when a sensor node sends data to a recipient who is not ready to receive them. Indeed, the sent messages are considered useless and consume an additional energy.

• Packets size: The five source is size of the messages. The size of the messages has an effect on the energy consumption of the emitting and receiving nodes. Thus, the size of the packets must not be too elevated nor too weak. Indeed, if it is small, the number of control packets increases the overhead. In the other case, a high transmission power is necessary for large size packets.

• Idle listening: The last major source of inefficiency is idle listening i.e., listening to receive possible traffic that is not sent. This is especially true in many sensor network applications. If nothing is sensed, the sensor node will be in idle state for most of the time. Many solutions to the problem of idle listening have been proposed utilizing the technique of duty cycling [13]. In this technique, each sensor node turns its radio on only periodically, alternating between active and sleeping states.

The main goal of any MAC protocol for sensor network is to minimize the several major sources of energy waste including collision, control packet overhead, overhearing, overmitting, and idle listening. Collision and packet overhead are common issues and should be addressed by every MAC protocol. In this paper, we will put our focus on idle listening, overmitting and overhearing, which are issues really important in a wireless sensor network.

The medium access control protocols for the wireless sensor networks can be classified broadly into two categories: Contention based and Schedule based. The schedule based protocol can avoid collisions, overhearing and idle listening by scheduling transmit and listen periods but have strict time synchronization requirements. The contention based protocols on the other hand relax time synchronization requirements and can easily adjust to the topology changes as some new nodes may join and others may die few years after deployment.

Contention-based duty cycle MAC protocols in the literature can be roughly categorized into two categories: synchronized and unsynchronized approaches.

Synchronized protocols, such as S-MAC (Sensor-MAC) [8], T-MAC (Time-MAC) [9], and U-MAC [14], negotiate a schedule that specifies when nodes are awake and asleep within a frame.

Unsynchronized protocols, such as B-MAC (Berkeley-MAC) [13], X-MAC [15] and RI-MAC (Receiver Initiated-MAC) [16], use the preamble sampling techniques that periodically wake up for a very short duration and sample the medium for activities by the long preamble. Because of high overhead introduced by synchronized wake/sleep schedules, unsynchronized protocols are used widely. While unsynchronized protocols are simple and energy-efficient, the long preamble exhibits several disadvantages. First, the receiver typically has to wait the full period until the preamble is finished before the data exchange can begin. This wastes energy at both the receiver and sender. Second, the unsynchronized protocol suffers from the overhearing problem, where non-targeted receivers also wake up during the long preamble and have to stay awake until the end of the preamble to find out if the packet is destined for them. This wastes energy and leads to the latency problem [15].

Our protocol SRI-MAC (Synchronous Receiver Initiated-MAC) is designed on the basis of synchronized duty cycled MAC protocols. SRI-MAC uses receiver-initiated data transmission in order to efficiently and effectively operate over a wide range of traffic loads. SRI-MAC employs an adaptive beacon and a series of CTS/RTS packets to reduce duty cycle and minimize idle listening. In SRI-MAC, the senders remain active and wait silently until the receiver explicitly signifies when to start data transmission by sending a CTS packet. If the node is not the intended sender, the node returns to sleep immediately and continues its sleep as if the medium had been idle.

The remainder of this paper is organized as follows. Section 2 reviews the related works. Section 3 exhibits the details of SRI-MAC with an illustrative example SRI-MAC behavior. Section 4 provides a Markov chain based model for estimating the power consumption of SRI-MAC. Section 5 presents the results of a number of simulation experiments carried out to validate our analytical results, as well as to provide additional insight on the performance of SRI-MAC. Section 6 concludes the paper.