Transmission Control Protocols for Wireless Sensor Networks

Introduction

The transmission control protocol plays an important role in achieving transport reliability and guaranteeing information fidelity. Given that an interesting event has indeed been detected, the relevant information has to be reported reliably from the event spot to the aggregating sinks, which are often located several hops away. Meanwhile, users may often want to disseminate task settings or query commands through the sink node, which also requires reliable data delivery from sinks to individual or groups of sensor nodes. Failure to deliver important data in the network can hurt the applications’ objective, and even defeat the very reason why the sensor network was deployed.

In the Internet, both the network layer and the transport layer play an important role in achieving data transport reliability. The network layer offers a best-effort service to provide available routing paths among network nodes, while the transport layer is responsible for achieving reliable delivery, congestion alleviation and fairness guarantee. Unlike in the Internet, transmission reliability in the sensor network can’t be provided predominantly by transport layer mechanisms of end systems, such as TCP and UDP. Rather, the inherent tradeoffs between transmission reliability, QoS provision and resource constraints require the adoption of cross-layer solutions.

To address the transmission control issues in wireless sensor networks, several protocols have been proposed in the literature. According to the specific functionality, they can be categorized into the following three types:

• (1)

  Congestion Control: Congestion occurs when offered traffic load exceeds available network capacity. It can not only degrade information fidelity but also waste node energy. Therefore, congestion must be effectively and efficiently controlled, either to avoid it or alleviate it. Generally, there are three mechanisms that deal with this problem: congestion detection, congestion notification and congestion control.

• (2)

  Reliability Guarantee: Reliability guarantee requires the ability to detect and repair packet loss in the network. The protocols for dealing with this issue in WSNs can be sorted into two main categories: retransmission-based and redundancy-based.

• (3)

  Fairness Guarantee: The constraint resources in WSNs are shared with all sensor nodes. The transmission control protocols need to provide fair resource allocation among all sensor nodes according to the specific fairness policy, so that the sinks can obtain a fair amount of observation data from the whole network. In general, the fairness can be implemented as an integrated part of the congestion control protocol or as an independent control protocol.

Currently, there is no single solution covering all the above design points. However, relevant solutions have been developed for a single point or small point sets. A detailed taxonomy of the transmission control paradigms in WSNs is shown in Figure 1.

Figure 1.

Transmission Control Protocols: A taxonomy

Background

Transmission control protocols in WSNs are confronting some unique environments and problems. There are some factors that negatively impact the data transmission in WSNs:

• (1)

  The wireless link is an open and lossy media, in which the transmission error rate is relatively higher than that in the wired network, due to path loss via distance attenuation and shadowing by obstacles;

• (2)

  The packet reception at the receiving node can be interfered by the other nodes that transmit simultaneously in the same area;

• (3)

  The routing paths may become unavailable due to death, movement or energy exhaustion of nodes;

• (4)

  The node buffer for the packets to be transmitted will overflow when the traffic load exceeds the channel capacity;

• (5)

  The transmission control has to comply with the stringent resource constraints in energy, memory and communication of sensor nodes.