wireless sensor network

Medium Access Control Protocols
For wireless Sensor Network
By:
Ahmed Shamel Noori
supervised by:
Dr.En. Muhammed Najm Al-Salam
3/10/2017

Outline
 Introduction
 Background
 Performance Requirements
 MAC Protocols :
 Contention-based
 Schedule-based
 Hybrid
 Conclusion

Introduction
Wireless Sensor Networks are typically composed of a large number
of low-cost, low-power, multifunctional wireless devices deployed
over a geographical area in an ad hoc fashion and without careful
planning. sensing devices are resource constrained and therefore are
only capable of a limited amount of processing and communication.

The choice of the medium access control protocol is the major determining factor in
WSN performance
the MAC protocol functionalities are provided by the lower sublayer of the data link
layer (DLL). The higher sublayer of the DLL is referred as the logical link control
(LLC) layer.
Background

The MAC sublayer resides directly above the physical layer. It supports the
following basic functions:
• Framing – Define the frame format and perform data encapsulation and
decapsulation for communication between devices.
• Reliability – Ensure successful transmission between devices. (by sending acknowledgement
(ACK) messages and retransmissions when necessary).
• Flow Control – Prevent frame loss (swamping) through overloaded recipient buffers.
• Error Control – Use error detection or error correction codes to control the
amount of errors present in frames delivered to upper layers.
• Medium Access – Control which devices participate in communication at any time. Medium
access becomes a main function of wireless MAC protocols since broadcasts easily cause data
corruption through collisions.

Why do we need MAC?
● Wireless channel is a shared medium.
● Radios transmitting in the same frequency band interfere with each other.
The role of Medium Access Control
● Controls when and how each node can transmit in the wireless channel.
● Solves the contention and collision.

Performance Requirements
In trying to determine the performance requirements of MAC protocols, the scope
of research has been very broad, issues such as :
 Delay: refers to the amount of time spent by a data packet in the MAC layer
before it is transmitted successfully.
 Throughput: typically defined as the rate at which messages are serviced by a
communication system. It is usually measured either in messages per second or
bits per second.
 Robustness: defined as a combination of reliability, availability, and
dependability requirements, reflects the degree of the protocol insensitivity to
errors and misinformation.

Performance Requirements
 Scalability: refers to the ability of a communications system to meet its
performance characteristics regardless of the size of the network or the number
of competing nodes.
 Stability: refers to the ability of a communications system to handle
fluctuations of the traffic load over sustained periods of time.
 Fairness: A MAC protocol is considered to be fair if it allocates channel
capacity evenly among the competing communicating nodes without unduly
reducing the network throughput.
 Energy: a sensor node is equipped with one or more integrated sensors,
embedded processors with limited capability, and short-range radio
communication ability.

Major Sources of Energy Wastes
● Control Packet Overhead
E.g., RTS/CTS
● Collision (RTS: Request To Send)
Retransmission (CTS: Clear To Send)
● Overhearing
The receiver of a packet is not the intended receiver of that packet
● Idle Listing
Listening to possible traffic that is not sent

Type of Multiple Access protocol

Fixed-Assignment Protocols ( Channelized protocol) In fixed-assignment strategies,
each node is allocated a predetermined fixed amount of the channel resources. his
category include frequency-division multiple access (FDMA), time-division multiple
access (TDMA), and code-division multiple access (CDMA).
Random Assignment Protocols In fixed-assignment schemes, each communicating
node is assigned a frequency band in FDMA systems or a time slot in TDMA systems.
This assignment is (static)
The ALOHA protocol (dynamic), also referred to as pure ALOHA, was one of the first
such media access protocols. improve the performance of pure ALOHA lead to the
development of several schemes, including carrier sense multiple access (CSMA),
carrier-sense multiple access with collision detection (CSMA/CD), and carrier-sense
multiple access with collision avoidance (CSMA/CA)
Type of Multiple Access protocol

 Every node senses the carrier before transmitting
 If the carrier is not clear, the node defers transmission for a specified period
Otherwise, transmits.
 While transmitting, the sender is listening to carrier and sender stops
transmitting if collision has been detected.
CSMA/CD (carrier sense multiple access/ collision detection)

Classification of MAC Protocols

 Contention-based protocols ( S-MAC , B-MAC )
1. S-MAC
 Basic Idea
Trades energy efficiency for lower throughput and higher latency
 Main Components
Periodic Listen and Sleep
Collision Avoidance
Overhearing Avoidance
Message Passing

Periodic Listen and Sleep
● Nodes periodically sleep
● Turn off radio when sleeping
● Reduce duty cycle to ~10%
● Trades energy efficiency for lower throughput and higher latency

Collision Avoidance
● Similar to IEEE 802.11
● Virtual Carrier Sense
● NAV (Network Allocation Vector)
● Physical Carrier Sense
● Four-way(steps) handshake
● RTS/CTS/DATA/ACK

Overhearing Avoidance
● The Basic Idea a node can go to sleep whenever its neighbor is talking with
another node.
● Who should sleep?
The immediate neighbors of sender and receiver.
● How do they know when to sleep?
By overhearing RTS or CTS.
● How long should they sleep?
NAV (Network Allocation Vector).

Message Passing
How to transmit a long message?
● Transmit it as a single long packet (Easy to be corrupted).
● Transmit as many independent packets (Higher Control Overhead & Longer Delay).
● Divide into fragments, but transmit all in burst.

2. B-MAC
 B-MAC is implemented in TinyOS.
 Major Feature: reconfigurable.
● Above B-MAC, one can implement an RTS-CTS scheme or a TDMS like
scheduling protocol.
 A small core of media access functionality
● arbitration, reliability, low power communication

2. B-MAC (Overview)
● Channel Arbitration
Clear Channel Assessment (CCA) & Backoffs
● Reliability
Link-layer acknowledgment
● Low Power Communication
Low Power Listening (LPL)

 Schedule-based MAC Protocols for WSNs
(Traditional TDMA MAC Protocols)
● Divide time into cycles
A cycle consists of several slots
● Advantages
Collision-Free, Low Idle Listing and Overhearing.
● Disadvantages
Synchronization, Low Channel Utilization
● An important Issue
Slot Assignment Strategy

Slot Assignment Strategy
A node should own a slot different from its one-hop and two-hop neighbors

 Hybrid (TDMA/CSMA)
● Z-MAC
● Funneling-MAC

1. Z-MAC (Zebra MAC)
● A hybrid TDMA and CSMA MAC scheme
● The main feature is its adaptability to the level of contention in the network
● under low contention, it behaves like CSMA, and
● under high contention, like TDMA

Funneling-MAC
● The authors propose a localized, sink-oriented funneling-MAC
● Localized
Using local TDMA scheduling in the intensity region only
● Sink-oriented
Manage TDMA scheduling
Compute and maintain the depth of the intensity region

Conclusion
● MAC Protocols in WSNs
● Energy Efficiency and Other Concerns
● Contention-based/Schedule-based/Hybrid

wireless sensor network

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