Wireless networks - Lecture 35: MAC protocols for WSN

Wireless networks - Lecture 35: MAC protocols for WSN. The main topics covered in this chapter include: challenges in WSNs; attributes of MAC protocol; overview of MAC protocols; energy efficiency in MAC; proposed routing protocol; QoS framework; network monitoring and management;...

Wireless Networks

Lecture 35 MAC Protocols for WSN Part II

Dr Ghalib A Shah

Outlines

 Challenges in WSNs.

 Attributes of MAC Protocol

 Overview of MAC protocols

 Energy Efficiency in MAC

 Proposed Routing Protocol

Last Lecture

 Introduction to WSN

 Applications of WSN

 Factors Influencing Performance of WSN

► Power consumption, fault tolerance, scalability, topology,

cost

 Architecture and Communication Protocols

 Network Monitoring and Management

 How to integrate WSNs into NGWI ?

Simulation for Sensor Networks

Simulation provides :

 Controlled , Reproducible testing environment

 Cost – effective alternative

 Means to explore and improve design space

TinyOS

 The role of any operating system (OS) is to promote

development of reliable application software by

providing a convenient and safe abstraction of

hardware resources

 Wireless sensor networks (WSNs) are embedded but

general-purpose, supporting a variety of applications, incorporating heterogeneous components, and capable

of rapid deployment in new environments

 An open-source development environment

► A programming language and model (NesC)

 TOSSIM for simulating TinyOS

 TinyDB for Sensor DB in TinyOS

► Scalability and adaptivity

• Number of nodes changes overtime

► Latency

► Fairness

► Throughput

► Bandwidth utilization

 Contention-based protocols

► CSMA — Carrier Sense Multiple Access

• Ethernet

• Not enough for wireless (collision at receiver)

► MACA — Multiple Access w/ Collision Avoidance

• RTS/CTS for hidden terminal problem

• RTS/CTS/DATA

Overview of MAC protocols

Hidden terminal: A is hidden from C’s CS

Overview of MAC Protocols

 Contention-based protocols (contd.)

► MACAW — improved over MACA

• RTS/CTS/DATA/ACK

• Fast error recovery at link layer

► IEEE 802.11 Distributed Coordination Function

• Largely based on MACAW

 Protocols from voice communication area

► TDMA — low duty cycle, energy efficient

► FDMA — each channel has different frequency

► CDMA — frequency hopping or direct sequence

Energy Efficiency in MAC Design

 Energy is primary concern in sensor networks

 What causes energy waste?

► Collisions

► Control packet overhead

► Overhearing unnecessary traffic

► Overemitting

► Long idle time

• bursty traffic in sensor-net apps

• Idle listening consumes 50—100% of the power for receiving (Stemm97, Kasten)

Dominant in sensor nets

Energy Efficiency in MAC Design

 TDMA vs contention-based protocols

► TDMA can easily avoid or reduce energy waste from

all above sources

► Contention protocols needs to work hard in all

directions

► TDMA has limited scalability and adaptivity

• Hard to dynamically change frame size or slot assignment when new nodes join

• Restrict direct communication within a cluster

► Contention protocols easily accommodate node

changes and support multi-hop communications

► Nodes are free to choose their listen/sleep schedule

► Requirement : neighboring nodes synchronize together

► Exchange schedules periodically (SYNC packet)

S-MAC: Coordinated Sleeping (1)

Frame Schedule Maintenance

1 Choosing a schedule

• Listen to the medium for at least SP

• Nothing heard, choose a schedule

• Broadcast a SYNC packet (should contend for medium)

1 Following a schedule

• Receives a schedule before choosing/announcing

• Follows the schedule

• Broadcast a SYNC packet

1 Adopting multiple schedules

• Receives a schedule after choosing/announcing

S-MAC: Coordinated Sleeping (2)

Maintaining Synchronization

 Clock drifts – not a major concern (listen time = 0.5s

– 105 times longer than typical drift rates)

 Need to mitigate long term drifts – schedule updating

using SYNC packet (sender ID, its next scheduled sleep time – relative);

 Listen is split into 2 parts – for SYNC and RTS/CTS

 Once RTS/CTS is established, data sent in sleep

interval

Receiver

Listen

Sleep for SYNC for RTS for CTS

S-MAC: Coordinated Sleeping (3)

Adaptive Lis tening – Low­duty cycle to active 

mode

* Overhearing nodes – wakeup at the end of the

current transmission (duration field in RTS/CTS)ListenR ListenON

RTS Sender

Drawbacks of S-MAC

 Active (Listen) interval – long enough to handle

to highest expected load

► If message rate is less – energy is still wasted in

idle-listening

 S­MAC fixed duty cycle – is  NOT OPTIMAL

 High Latency

Normal

S-MAC

T-MAC: Preliminaries

 Adaptive duty cycle:

 A node is in active mode until no activation event

occurs for time TA

► Periodic frame timer event, receive, carrier sense, send-done,

knowledge of other transmissions being ended

Active Active Active

T-MAC: Choosing TA

 Requirement: a node should not sleep while its

neighbors are communicating, potential next receiver

Dynamic Sensor-MAC (DSMAC)

 TMAC improves the latency in SMAC at cost of complexity.

 DSMAC provides simple solution to static duty cycle.

 All nodes start with same duty cycle.

 If one-hop latency is observed higher by receiver, it doubles its

duty cycle

 Nodes share their one-hop latency values with neighbors during

SYNC period.

 The transmitter also doubles its duty cycle if the destination

reported higher one-hop latency.

 This change will not affect the schedule of other neighbors.

DSMAC Schduling

Traffic-Adaptive MAC (TRAMA)

 Time is divided into random-access and scheduled-access

 the node calculates the number of slots for which it will have the

highest priority among two-hop neighbors

 The node announces the slots it will use as well as the intended

receivers for these slots with a schedule packet.

 the node announces the slots for which it has the highest priority

but it will not use

 The schedule packet indicates the intended receivers using a

bitmap whose length is equal to the number of its neighbors

 Advantages  

► Higher percentage of sleep time and less collision

probability are achieved, as compared to based protocols

CSMA-► Since the intended receivers are indicated by a

bitmap, less communication is performed for the multicast and broadcast types of communication patterns, compared to other protocols

► Transmission slots are set to be seven times longer

than the random-access period This means that

DMAC

 Supports convergecast communication model,

 Data-aggregation tree is formed from sources to sink

node

 It is an improved slotted ALOHA algorithm

 Slots are allotted according to the level of tree from leaf

to root

 It incurs low latency but no collision avoidance for

nodes at same level

A minimum period u consists

of one packet tx and rx.

Wakeup period in three is skewed depending on depth d so du is the wakeup time Node at higher layer will be in rx state when lower layer nodes are in

tx state

Contention-Free MAC protocols for Wireless

Sensor Networks

 Asynchronous Slot Assignment

► Each node locally discretizes its local time.

► The number of slots in a time frame, called the frame size and denoted by

, is set to 2 2.

► Having the same frame size at all nodes ensures that overlapping time

slots remain the same 

ASAND – Basic Approach

ASAND – Conflict Reporting

w v

u

 The 2-hop neighbors u and v are unaware that they have selected

conflicting time slots (their transmissions collide on w).

 Having observed a collision in its local time t, node w transmits at time t+ , creating a spurious conflict with both u and v.

 This is called conflict reporting essentially reduces a conflict

between hidden terminals to a conflict between neighbor nodes.

 After t+ , u and v will be forced to select new slots

 Challenges in WSNs.

 Attributes of MAC Protocol

 Overview of MAC protocols

 Energy Efficiency in MAC

 Proposed Routing Protocol