Medium Access Control in Wireless Sensor Networks

USC/ISI Technical Report ISI-TR-580, October 2003 Wei Ye and John Heidemann

Outline

Introduction Scheduled Protocols Contention-based Protocols S-MAC Performance Summary

Introduction

The role of medium access control (MAC) Controls when and how each node can

transmit in the wireless channel Why do we need MAC?

Wireless channel is a shared medium Radios transmitting in the same

frequency band interfere with each other – collisions

MAC Attributes and Trade-offs

Collision avoidance basic task of all MAC protocols

Energy efficiency important for sensor network

Scalability and adaptivity a good MAC protocol should

accommodate the changes in size, density, and topology

MAC Attributes and Trade-offs Channel utilization

how well the entire bandwidth of the channel is utilized in communications

Latency Refers to the delay from when a sender has a

packet to send until the packet is successfully received by the receiver

Throughput often measured in bits or bytes per second

Fairness

Energy Efficiency in MAC Protocols

Collision major problem in contention protocols,

but is generally not a problem in scheduled protocols

Overhearing when a node receives packets that are

destined to other nodes Control packet overhead

Energy Efficiency in MAC Protocols

Idle listening Is a dominant factor of radio energy

consumption often 50~100% of energy required for

receiving Idle:Receiving:Transmission

Stemm and Katz : 1:1.05:1.4 Wavelan card : 1:2:2.5 Mica2 mote : 1:1:1.41

Classification of MAC Protocols According to the underlying

mechanism for collision avoidance, MAC protocols can be broadly divided into two groups Scheduled-based Protocols

Scheduled nodes onto different sub-channel Ex: TDMA FDMA CDMA

Contention-based Protocols Nodes compete in probabilistic coordination Ex: ALOHA CSMA

Scheduled Protocols: TDMA

TDMA divides the channel into N time slots N slots comprises a frame, which repeats

cyclically Typically, mobile nodes communicate only

with the base station (low-duty-cycle)

Scheduled Protocols: TDMA

Disadvantages Requires nodes to form clusters Inter-cluster communications and

interference need to be handled by other approaches, such as FDMA or CDMA

limited scalability and adaptivity

Example : LEACH TDMA organizes nodes into cluster

hierarchies Cluster head is rotated among nodes

within a cluster depending on their remaining energy levels

Nodes in cluster only talks to head Cluster heads talk to base station

over a long-range levels

Example : Bluetooth

Designed for personal area networks (PAN) with target nodes as battery-powered PDAs, cell phones, and laptops

Organizes nodes into clusters, called piconets

Inter-cluster communication uses Frequency-hopping CDMA

Example : Bluetooth

Master use polling to decide which slave to transmit a special TDMA without pre-assigned slots

Each piconet has a master and up to 7 active slaves lack of scalability

Multiple connected piconets form a scatternet difficult to handle inter-cluster communications

Contention-based Protocols

A common channel is shared by all nodes and it is allocated on-demand

Advantages 1. scale more easily across changes in

node density or traffic load 2. more flexible as topologies change (no

need to form cluster) 3. do not require fine-grained time

synchronizations as in TDMA protocols

Contention-based Protocols

Disadvantages inefficient usage of energy node listen at all times and collisions and

contention for the media can waste energy

Overcoming this disadvantage is required if contention-based protocols are to be applied to long-lived sensor networks

Example : CSMA (Carrier Sense Multiple Access)

Listening (carrier sense) before transmitting

Send immediately if channel is idle Backoff if channel is busy non-persistent, 1-persistent and p-

persistentIn p-persistent CSMA, a node transmits with probability p if the medium is idle, and with probability (1-p) to back off and restart carrier sense

Hidden Terminal Problem

Example : CSMA / CA (collision avoidance)

Establish a brief handshake between sender and receiver before transmits

ba c

RTS

CTS

CTS

Request to send

Clear to send

Still have problem, but greatly reduced (short)

Contention Protocols: MACA and MACAW

MACA Based on CSMA/CA Add duration field in RTS/CTS informing

other node about their back-off time MACAW

Improved over MACA RTS/CTS/DATA/ACK Fast error recovery at link layer

Power save (PS) mode in IEEE 802.11 DCF

Assumption: all nodes are synchronized and can hear each other (single hop)

Nodes in PS mode periodically listen for beacons & ATIMs (ad hoc traffic indication messages)

Power save (PS) mode in IEEE 802.11 DCF

Beacon: timing and physical layer parameters All nodes participate in periodic beacon

generation One node periodically broadcasts a beacon

ATIM: tell nodes in PS mode to stay awake for Rx ATIM follows a beacon sent/received Unicast ATIM needs acknowledgement Broadcast ATIM wakes up all nodes — no ACK

Power save (PS) mode in IEEE 802.11 DCF

Power save (PS) mode in IEEE 802.11 DCF

Problems for multi-hop Clock synchronization Neighbor discovery

Tseng et al. proposed a resolution do not synchronize Listen intervals of two nodes periodically overlap Disadvantages

1.control overhead 2.longer delay

Contention Protocols: Piconet

Develop by Bennett et al. not the same piconet in Bluetooth use 1-persistent CSMA protocol each node sleeps autonomously beacon their ID when wake up Sending node needs to listen for

receiver’s beacon first

Contention Protocols: PAMAS (Power Aware Multi-Access with Signalling ) Proposed by Singh and Raghavendra Try to reduce the overhearing problem but not idle

listening Improve energy efficiency from MACA Use two channels, one for data and one for control Node who try to transmit probe in the control channel If any neighbor answers the probe, the node will go

back to sleep Disadvantage

1. need two radio system 2. does not reduce idle listening

Case Study : S-MAC

Basic Scheme is similar to 802.11 PS mode without assuming single-hop

Major components in S-MAC Periodic listen and sleep Collision avoidance Overhearing avoidance Massage passing

Scheduling

Establish low-duty-cycle operation About 1-10%

Every node are free to choose their own listen/sleep schedules

Periodically broadcast each schedule for few listen/sleep frame in a SYNC packet (synchronization)

Scheduling Encourages neighboring nodes to adopt

identical schedules When first configures, it listen for a sync period

and adopts the first schedule it hears Periodically perform neighbor discovery,

listening for an entire frame, to discovery node on different schedules

the listen period is significantly longer than clock error or drift (loose sync)

Data Transmission Collision avoidance is simmilar to IEEE

802.11 DCF Contention only happens at a

receiver’s listen interval Unicast packet: CSMA + RTS-CTS-

DATA-ACK (MACAW) Broadcast packet: CSMA only Put duration field in each packet

ease the overhearing problem

Message Passing Problem:

Sensor networks in-network processing requires entire message

Solution: Long message is fragmented & sent in burst

Only one RTS and CTS packet are used to reserve medium for entire message (by duration field)

Several fragments and ACKs (with duration field) Fragment-level error recovery

ACK for each fragment extend Tx time and re-transmit immediately

Other nodes sleep for whole message time

Adaptive listening

Problem a -> b -> c

solution

listen listen

b->c

listen listen

a->b

listen

b->c

a->b

Overall multi-hop latency can be reduced by at least half

Performance Measure with Mica motes

Performance Only one message in the network at a time

Summary

This paper reviews MAC protocols for wireless sensor networks

It described both scheduled and contention-based MAC protocols

Finally, it presented S-MAC as an example