MAC Protocols for
mobile wireless sensor
networks
Luís Bernardo
Miguel Pereira
Francisco Ganhão
Rodolfo Oliveira
Rui Dinis
Paulo Pinto
tele1.dee.fct.unl.pt
July 5, 2010
Ciência 2010
OUTLINE
Motivation
MAC layer
PHY layer
Conclusions
Outline Motivation
MAC layer
PHY layer
Conclusion
Motivation
Contradictory objectives:
Critical
infrastructure
protection with
wireless sensor
networks
Maximize WSN lifetime (minimize energy consumption)
Have controlled packet delay and throughput
Support mobile and fixed battery powered nodes
Outline Motivation
MAC layer
PHY layer
Conclusion
Motivation
Layered approach: 6lowPAN, ROLL, 802.15.4, …
Application
Cross-layer interfaces to handle
Objective:
hardware/energy limitations
Save Energy and meet the application’s
MAC layer (Multimode MAC protocols)
Transport communication
requirements
Routing
Adapt operation to application requirements
e.g. VehiclePHY
tracking
vs.
Environment
layer (PC H-ARQ / MPD receivers)
MAC
monitoring
PHY
Outline Motivation
Reduce energy lost with
collisions/interference satisfying app.
requirements
MAC layer
PHY layer
Conclusion
MAC - Motivation
A Wireless Sensor Network (WSN) mobility scenario
Mobile nodes moving through static WSN islands
Static nodes (single radio) - battery must be saved
Mobile nodes - external energy resources
High throughput needed during a short connection
Standard WSN Medium Access Control (MAC) protocol
do not handle the set of requirements mentioned before
Outline Motivation
MAC layer
PHY layer
Conclusion
MAC - Motivation
Medium Access Control (MAC) protocols save Energy by turning the radio
off
Asynchronous MAC protocols (e.g. B-MAC; X-MAC)
Low Power Listening bind the receiver and sender using a large
preamble
Advantages
Nodes run independent asynchronous duty-cyles - good for
mobility
Energy efficient to bursty traffic
Disadvantages
Limited throughput and high delay for more than one sender
Outline Motivation
MAC layer
PHY layer
Conclusion
MAC - Motivation
Synchronous MAC protocols (e.g. S-MAC, LL-MAC, Z-MAC, 802.15.4)
Contention protocols using Carrier Sense Multiple Access (e.g. SMAC)
Scheduled protocols using Time Division Multiple Access (e.g. LLMAC)
Use hybrid approach (e.g. Z-MAC)
Support both: CSMA and TDMA - changes to TDMA fallback
during load peaks, maximizing the throughput
Advantages
High throughput available for peak periods
Outline Motivation
MAC layer
PHY layer
Conclusion
MAC - Motivation
Disadvantages
High energy consumption even for idle periods
Synchronized duty-cyles - bad for mobility
CSMA requires SYNC frame before communication
TDMA requires an additional slot allocation
algorithm
High mobility requires high SYNC rates to keep track
from the neighbors
Outline Motivation
MAC layer
PHY layer
Conclusion
MAC - Conceptual Idea
Goal
Have a low energy asynchronous mode
Have a synchronous mode high throughput in the presence of
mobile asynchronous nodes
Allow shorter connection times than other hybrid protocols
Maximize throughput for mobile nodes in the neighborhood of
synchronous nodes
We propose the Mobile Multimode Hybrid MAC (MMH-MAC)
Asynchronous and Synchronous modes
Outline Motivation
MAC layer
PHY layer
Conclusion
Asynchronous Mode
Goal
Minimize the idle energy consumption
MMH-MAC asynchronous mode uses
Preamble sampling approach similar X-MAC protocol
Two techniques to minimize the interference between synchronous
and asynchronous nodes
It uses Low Power Listening mechanism
Sender sends a sequence of short preambles with duration up to
2*Tduty_cycle before the data frames
Unicast receivers may send and Early Preamble ACK
Outline Motivation
MAC layer
PHY layer
Conclusion
Asynchronous Mode
Passive interference mitigation
Alignment of the asynchronous active time with the
public slot of the last visited synchronous node
Preamble overhead is reduced due to the immediate
reception of an early PACK
Active interference mitigation
Improved Shut-up mechanism
Outline Motivation
MAC layer
PHY layer
Conclusion
Synchronous Mode
Slotted scheme - Nodes runs a synchronized duty-cycle period.
11 slots with fixed duration of 100ms each
Slots are subdivided in ten 10ms subslots
Public Slot (slot 0)
Shared by all the nodes, it’s used for broadcast traffic and
casual unicast traffic
Unicast traffic is acknowledged and run a contention based
protocol
First 50 ms reserved for MAC signaling (SYNC frames)
Outline Motivation
MAC layer
PHY layer
Conclusion
Synchronous Mode
Private slot (slot 1-10)
Reserved slots for unicast traffic between two nodes
Collision free environment
Traffic is acknowledged
After 25ms of inactivity nodes go into sleep
SYNC frames are used to:
Maintain inter-node duty-cycle synchronization
Broadcast private slot allocation
As beacons to detect neighborhood changes (above an RSSI
value)
Outline Motivation
MAC layer
PHY layer
Conclusion
Synchronous Mode
MMH-MAC mobility handling features
Multiple SYNC frames can be transmitted per dutycycle
Normal SYNC frames are transmitted in a
random subslot of public slot 0
Other SYNC frames are sent when an
asynchronous node is detected
A neighbor SYNC table is kept that measures link
stability allowing cluster formations
Outline Motivation
MAC layer
PHY layer
Conclusion
Synchronization Process
Goal
Guarantee that all neighbors follow the same duty-cycle schedule
(synchronous and asynchronous nodes)
If all nodes are asynchronous
Packet Hello is sent
preceded by a sequence
of preambles
Request/Ok exchange
identifies the neighbors
and reserves private slots
SYNC defines the initial
synchronization reference
Outline Motivation
MAC layer
PHY layer
Conclusion
Synchronization Process
If at least one node is synchronous, neighbor nodes follow the
existing duty cycle
Passive approach (classical)
Where M node waits for the SYNC
packet
Active approach (new)
M sends preambles to trigger the
Shup-Up mechanism in one active
slot in one of its neighbors
Wait for the SYNC to proceed with
the synchronization
First empty slot or idle
dedicated slot
Next public slot
Outline Motivation
MAC layer
PHY layer
Conclusion
Synchronization Process
Performance
Depends on the number of active private slots
more active slots = less time a node takes to listen to M preambles
more active slots = more time until finding an idle slot
MMH-MAC proposes the use of listening private slots mechanism
The node turns on the radio for 10 ms when the slot is free
Each listening slot costs 1% of duty-cycle
Depends on the preamble starting slot
Slot 0 is the optimal case
Outline Motivation
MAC layer
PHY layer
Conclusion
MAC - Results
We use TOSSIM simulator
Run MMH-MAC nesC code
Added the mobility support
Additional meters measure active time/sleep time/tx time/receive time
Simulated scenario
21 static nodes in synchronous mode organized in 6 static clusters
Each dedicated slot has CBR traffic (10 packets/sec and 35
bytes/packet)
Each static node sends one SYNC per duty-cycle (1,1s minimum value)
Energy estimation: Xbow Telos B current consumption
Outline Motivation
MAC layer
PHY layer
Conclusion
MAC - Results
Simulated scenario
A mobile node moves randomly on the scattered WSN
Connects 120 times to the islands with a variable
connection time
We evaluate three scenarios [WCNC’2010]
Passive syncronization
Active synchronization without listening slots
Active synchronization with one listening slot (slot 6)
Outline Motivation
MAC layer
PHY layer
Conclusion
MAC - Results
Time to synchronize
As function of the
number of
allocated
dedicated slots
Outline Motivation
MAC layer
PHY layer
Conclusion
MAC - Results
Throughput
As function of the
connection
duration time
Outline Motivation
MAC layer
PHY layer
Conclusion
MAC - Conclusions
MMH-MAC significantly reduces the time to an
asynchronous node to start communicating to a
synchronous node and vice versa
Minimize the interference between asynchronous and
synchronous nodes
We implement the code on TinyOS and we made short
tests on real nodes
We are implementing a mixed TelosB / SunSPOT
scenario
Outline Motivation
MAC layer
PHY layer
Conclusion
PHY - Motivation
Classical WSN PHY (e.g. 802.15.4) limit energy
efficiency
Packets involved in collisions/interference are lost
Low complexity H-ARQ may improve energy
efficiency
WSN applications with hard constraints on:
Delay
Bitrate
Outline Motivation
MAC layer
PHY layer
Conclusion
PHY - Motivation
Using an H-ARQ scheme enhances the throughput,
compared to a conventional ARQ scheme;
Energy could be saved on subsequent retransmissions;
Depending on the distance and the nodes density:
Circuit’s energy consumption ≥ expended energy
transmission.
Outline Motivation
MAC layer
PHY layer
Conclusion
PHY - Objectives
Analyze the Energy per useful packet (EPUP):
Diversity Combining (DC) H-ARQ technique;
Conventional ARQ (C-ARQ);
Obtain the optimal EPUP for a TDMA access mode
considering:
Delay constraints
Throughput constraints.
Outline Motivation
MAC layer
PHY layer
Conclusion
PHY - System Overview
Assumptions:
Synchronous TDMA MAC slot on a flat fading scenario;
Additive White Gaussian Noise channel (AWGN);
Slots of equal length, each equivalent to a packet of M
bits;
A receiver, holds up to R transmissions of a failed
packet;
After R transmissions, it gives up.
Outline Motivation
MAC layer
PHY layer
Conclusion
PHY - System Overview
Receiver
Characterization for DC
H-ARQ:
Linear Bit
Combination;
Enhancement of the
bit reception.
Outline Motivation
MAC layer
.
.
.
+
PHY layer
Conclusion
PHY - System Overview
Energy Analysis
EPUP – Energy per useful packet
E[N] – Expected number of retransmissions
Ep – Energy per Packet(d, Eb)
QR+1 – Probability of packet failure after R transmissions
Outline Motivation
MAC layer
PHY layer
Conclusion
PHY - System Overview
System Optimization - minimize EPUP,
subject to:
A minimum goodput Smin ;
A maximum delay Dmax ;
A minimum success probability.
Outline Motivation
MAC layer
PHY layer
Conclusion
PHY - Performance
C-ARQ vs. DC H-ARQ [ICCCN’2010a]:
Analytical and simulated results with the ns-2
simulator;
Simulation characteristics:
Packet size of M=256 bits;
8 Wireless Terminals;
Distances ranging between d=10m and 100m;
Retransmissions up to R=10.
Outline Motivation
MAC layer
PHY layer
Conclusion
PHY - Performance
EPUP in
function of
d and
Eb/N0.
Outline Motivation
MAC layer
PHY layer
Conclusion
PHY - Performance
Success Probability
Outline Motivation
MAC layer
Delay
PHY layer
Conclusion
PHY - Conclusions
DC H-ARQ can extend the battery of a Wireless
Terminal, compared to a conventional TDMA
ARQ scheme.
Longer distances;
Re-transmission tolerance.
Future Work:
MultiPacket Detection schemes [Globecom’07,
TWC09, ICCCN’2010b]
Outline Motivation
MAC layer
PHY layer
Conclusion
PHY – MPD vs DC H-ARQ
Delay
Outline Motivation
Throughput
MAC layer
PHY layer
Conclusion
Conclusions & Future Work
MAC layer approaches adapt radio sleep times and
synchronization to the application/routing requirements
PHY layer reduce transmission power, or
synchronization requirements, by using DC H-ARQ or
MPD
Future Work:
Continue to combine MAC and PHY approaches to
improve energy efficiency
Outline Motivation
MAC layer
PHY layer
Conclusion
Thank you for your attention
Q&A