Wireless MAC Protocols for WSNs

Study Notes

Wireless MAC protocols are used in wireless communication networks to regulate access to the common medium.
Multiple nodes share a communication medium for transmitting their data packets.
The choice of MAC protocol has a direct bearing on the reliability and efficiency of network transmissions.

Nodes sense the medium before transmission to reduce collisions.
Two types of CSMA protocols:
- Non-persistent CSMA: Node transmits data immediately if medium is idle, otherwise waits for a certain amount of time before attempting to transmit again.
- Persistent CSMA: Node continuously senses the medium and transmits data immediately if medium is idle, otherwise waits for a random period of time before attempting to transmit again.
- p-persistent CSMA: Node transmits data with a probability p once the medium becomes idle, and delays transmission with a probability 1 − p.

Desirable aspects of WSNs: ability to communicate over a wireless link, mobile applications, flexible deployment of nodes.
Challenges of wireless communication: limited bandwidth, limited transmission range, poor packet delivery performance due to interference, attenuation, and multipath scattering.

Transmitter: represents one or more sensors that produce a message signal.
Channel: the medium through which the signal is transmitted.
Receiver: receives the transmitted signal and decodes it to retrieve the original message.

Analog signal is sampled and quantized by an analog-to-digital converter (ADC).
Quantized elements are mapped into corresponding binary symbols of length (r) from a codebook (C).
A codebook can be uniquely decoded if each sequence of symbols can be mapped back to a corresponding value in S = (s, s,..., s[n]).

The channel capacity can be estimated according to the Shannon-Hartley theorem.
The capacity of a channel to transmit a message without an error is given by the equation: C = B log2 (1 + S/N).

The baseband signal is transformed into a bandpass signal to transmit and receive signals with short antennas.

Wireless sensor networks operate in the license-free ISM spectrum.
A simple channel model ignores the effect of interference and considers the surrounding noise as the predominant factor that affects the transmitted signal.
Noise can be modeled as an additive white Gaussian noise (AWGN) that has a constant spectral density over the entire operating spectrum and a normal amplitude distribution.