Wireless Sensor Network

Focus on:

• Ubiquitous Computing.

• Ubiquitous Network Society.

• (often) Human-centric.

Ubiquitous.

• Anytime.

• Anyone.

• Any Device.

• Affordable.

• All Security.

• Any Information/Service.

MAC

Challenges for wireless networking

MAC is a critical layer for networking.

Traditional problems.

• Fairness.

• Latency.

• Throughput.

For Sensor Networks, added.

• Power efficiency.

• Scalability.

MAC challenges for WSN

Sensor networks are deployed in an ad hoc fashion, with individual nodes remaining largely inactive for long periods of time, but then becoming suddenly active when something is detected.

These characteristics of sensor networks and applications motivate a MAC that is different from traditional wireless MACs:

• Energy conservation and self-configuration are primary goals.

• Per-node fairness and latency are less important.

Challenges in WSN’s

• Energy and Power Consumption.

• Self-organization.

• Communication Heterogeneity.

• Adaptability.

• Security.

• Scalability.

Design Challenges

Typically, severely energy constrained.

• Limited energy sources (e.g., batteries).

• Trade-off between performance and lifetime.

Self-organizing and self-healing.

• Remote deployments.

Scalable.

• Arbitrarily large number of nodes.

Heterogeneity.

• Devices with varied capabilities.

• Different sensors.

• Hierarchical deployments.

Adaptability.

• Adjust to operating conditions and changes in application requirements.

Security and privacy.

• Potentially sensitive information.

• Hostile environments.

Sensor Network MAC Protocols

The major sources of energy wastage are:

• Collisions – interfering packets.

• Overhearing – hearing more than required from a packet.

• Control packet overhead – control versus data.

• Idle listening – hearing for nothing.

Typical solutions in wireless MACs.

• Carrier Sensing.

  • Only during low traffic load.

• Contention

  • RTS-CTS only during high traffic load.

• Backoff

  • Backoff in application layer is desired other than in MAC layer.

Achieving good scalability and collision avoidance capability is necessary.

Challenges

1. Energy Efficiency:

   1. Sensor nodes are not connected to any energy source.

   2. Energy efficiency is a dominant consideration no matter what the problem is.

   3. Many solutions, both hardware and software related, have been proposed to optimize energy usage.

2. Ad hoc deployment (adaptability):

   1. Most sensor nodes are deployed in regions which have no infrastructure.

   2. We must cope with the changes of connectivity and distribution.

3. Unattended operation:

   1. Generally, once sensors are deployed, there is no human intervention for a long time.

   2. Sensor network must reconfigure by itself when certain errors occur.

4. Dynamic changes (self-healing and scalability).

   1. As changes of connectivity due to addition of more nodes or failure of nodes, Sensor network must be able to adapt itself to changing connectivity, to arbitrary large numbers of nodes.

5. Security.

   1. Both Sensors and Actuators carry sensitive information in an hostile environment.

Sensor-MAC (S-MAC)

S-MAC is a medium-access control (MAC) protocol designed for wireless sensor networks.

• Explores typical solutions also found in many other sensor MACs.

• Nodes periodically sleep, and sleep during other nodes transmissions.

  • Nearby nodes form virtual clusters to synchronize their wake-up and sleep periods.

• Trades energy efficiency for lower throughput and higher latency.

  • Message passing is used to reduce the contention latency and control overhead.