Architecture of Wireless Sensor Networks

The concept of wireless sensor networks is based on a simple equation:

Sensing + CPU + Radio = Thousands of potential applications

As soon as people understand the capabilities of a wireless sensor network, hundreds of applications spring to mind. It seems like a straightforward combination of modern technology.

WSN OSI layers

Transport layer: The function of this layer is to provide reliability and congestion avoidance where a lot of protocols designed to provide this function are either applied on the upstream (user to sink, ex: ESRT, STCP and DSTN), or downstream (sink to user, ex: PSFQ and GARUDA). These protocols use different mechanisms for loss detection ((ACK, NACK, and Sequence number)) and loss recovery ((End to End or Hop by Hop)) [4, 5]. This layer is specifically needed when a system is organised to access other networks. Providing a reliable hop by hop is more energy efficient than end to end and that is one of the reason why TCP is not suitable for WSN. Usually the link from sink to node is considered

as downstream link for multicast transmission and UDP traffic because of the limited memory and overhead avoiding. On the other hand from User to sink is considered as upstream link for mono-cast transmission and TCP or UDP traffic [1].

Data link layer: Responsible for multiplexing data streams, data frame detection, MAC,

and error control, ensure reliability of point–point or point– multipoint. Errors or unreliability

comes from 

MAC layer: Responsible for Channel access policies, scheduling, buffer management and error control. In WSN we need a MAC protocol to consider energy efficiency, reliability, low access delay and high throughput as a major priorities 

Physical Layer : Can provide an interface to transmit a stream of bits over physical medium. Responsible for frequency selection, carrier frequency generation, signal detection, Modulation and data encryption. IEEE 802.15.4: proposed as standard for low rate personal area and WSN with low: cost, complexity, power consumption, range of communication to maximize battery life. Use

CSMA/CA, support star and peer to peer topology. There are many versions of IEEE 802.15.4.

Application layer: Responsible for traffic management and provide software for different

applications that translate the data in an understandable form or send queries to obtain certain

information. Sensor networks deployed in various applications in different fields, for example;

military, medical, environment, agriculture fields [1, 7].