Wireless Ad Hoc & Sensor Networks
(Wireless Sensor Networks – Part II)WS 2009/2010 WS 2009/2010
Prof. Dr. Dieter Hogrefe/Prof. Dr. Xiaoming Fu Dr. Omar Alfandi
Outline
• MAC protocols
• Naming and Addressing
• Time Synchronisation
Summary
• Summary
2
MAC protocols
• Medium access in WSNs is difficult mainly because of
– Impossible (or very difficult) to send and receive at the same time – Interference situation on the receiver is important, but can bedifferent from the observed situation by the sender – High error rates for signalling packetsHigh error rates for signalling packets
• Requirements
– High throughput, low overhead, low error ratesg g p , , – Energy efficiency, handle switched off devices
MAC protocols – Energy Problems
• Recall: Transceiver consumes a significant share of
energy
– Sending is costly; Receiving costs are often almost the same – Idling is cheaper, but about as expensive as receiving
Sleeping costs almost nothing but results in a “deaf” node – Sleeping costs almost nothing, but results in a deaf node
• Derived energy problems regarding the MAC protocol
– Collision: Waste of effort when two packets collideCollision: Waste of effort when two packets collide
– Overhearing: Waste of effort when receiving a packet that was directed at another destination
– Protocol overhead: Waste of effort due to MAC-related overhead – Idle listening: Waste of effort when waiting for incoming packets,
but nobody is sending but nobody is sending
MAC protocols – Main Options
Wireless medium access Centralised Distributed Centralised Distributed Schedule-based Contention-based Schedule-based Contention-based Fixed assignment Demand assignment Fixed assignment Demand assignment g g g g 5MAC protocols – Centralised Medium Access
• Idea: A central station controls, when a node may
access the medium
– Example: Polling, centralised computation and assignment of time slots (TDMA)
• Simple and efficient, but burden to the central stationp ,
• Not feasible for large WSNs, but if network is divided into
smaller groups, this approach can be useful
– e.g. compare Bluetooth piconets
Î Usually, only distributed medium access is considered!
6
MAC protocols – Distributed Medium Access
• Schedule-based protocols
– TDMA component provides a schedule regulating which
ti i t d hi h t hi h ti
participant may used which resource at which time
• Typical resource : frequency band (with given CDMA-code) • Implicit idle listening avoidance mechanismp g
– Schedule can be fixed or computed on-demand
• Sometimes mixed
C lli i O h i d Idl li t i i
– Collisions, Overhearing and Idle listening are no issues – BUT: time-synchronisation is needed
• Contention based protocols
• Contention-based protocols
– Risk of colliding packet is deliberately taken
– Mechanisms to avoid/reduce collisions required (Î often random)q ( )
7
MAC protocols – Main Options
Wireless medium access Centralised Distributed Centralised Distributed Schedule-based Contention-based Schedule-based Contention-based Fixed assignment Demand assignment Fixed assignment Demand assignment g g g g 8
Contention-based MAC protocols I
• Basic options
– ALOHA Î not good in most cases – Listen before talk (CSMA)
• BUT: sender is not knowing what is going on at the receiver Î might destroy packets despite listeningg y p p g
• Besides, receiver needs possibility to inform possible
senders in its neighbourhood upcoming transmission
Î “shut them up” for this duration
– Recall:
Hidden Terminal Problem • Hidden Terminal Problem
• Exposed Terminal Problem A B C D
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Contention-based MAC protocols II
• Receiver informs potential interferers
– While a reception is on-going
• By sending out a signal indicating the reception (Busy tone protocol) • Problem: cannot use the same channel on which the reception
takes place Î used separate channel for signalling
– Before a reception is on-going
• Can use the same channel
• Receiver itself needs to be informed • Receiver itself needs to be informed
10
Contention-based MAC protocols – S-MAC I
• Problem
– In WSNs most of the time nothing happens, i.e. there is only a
l d t t
low data rate
– For low data rates, MACA’s idle listening is unsuitable
• Idea: use ‘rendez-vous’ mechanism
• Idea: use rendez-vous mechanism
– turn off nodes and ensure that neighbouring nodes turn on simultaneously to allow packet exchange
• Proposal of this approach: S-MAC
– S-MAC = Sensor-MAC– S-MAC is energy efficient and provides collision avoidance and overhearing
Contention-based MAC protocols – S-MAC II
Listen Listen Sleep Listen period SYNCH RTS CTS Listen period
• Only in the ‘listen periods’
data will be exchanged
S di & i iWake-up period
Period – Sending & receiving
• SYNCH
• Synchronisation of the sleep schedule by
Duty-Cyle = Listen period length/
• Synchronisation of the sleep schedule by exchanging schedule tables with neighbours • Forming of virtual clusters
Listen period length/ Wakeup period length
Î “synchronised islands”
• Data transfer
P f RTS/CTS
• Perform RTS/CTS • Transfer data, ACK
• Not affected nodes change to sleep mode
Virtual clusters
• Not affected nodes change to sleep mode
Contention-based MAC protocols – T-MAC
• In S-MAC the ‘listen period’ is of fixed length
• Problem
– What happens when there is no traffic? Î nodes have to stay awake unnecessarily
Idea
• Idea
– Prematurely go back to sleep mode when there is no traffic for a certain time (timeout)( ) Î adaptive duty cycle
• Implementation of this idea: T-MAC
– Timeout-MACAs S MAC but uses timeout to reduce listen period – As S-MAC, but uses timeout to reduce listen period
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Contention-based MAC protocols – Preamble Sampling
• Until now, periodic sleep was used to synchronise the
sleep and wake up phases of the nodes
• Alternative: Don’t try to explicitly synchronise nodes
– Regularly sample the medium to check for activity, rest of thetime change to sleep mode time change to sleep mode
• Use long preambles to ensure that receiver stays awake
to catch the actual packet
to catch the actual packet
– e.g. WiseMACSender WUP Data
Wake-Up Preamble Sender Receiver ACK Rx Tx Sleep Period Wake Up Wake Up 14
Contention-based MAC protocols – B-MAC I
• B-MAC combines several of the mentioned approaches
– Tries to provide practically relevant solutions
• Clear Channel Assessment
– Adapts to noise floor by sampling channel when it assumed to be free
be free
– Samples are exponentially averaged Î result used in gain control
– For actual assessment when sending a packet, look at five channel samples Î channel is assumed as free, when even a single sample is significantly below noise
single sample is significantly below noise
– Optional: Random back-off, if channel is found busy
• Optional: Immediate link layer ACKs for received packets
p
y
p
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Contention-based MAC protocols – B-MAC II
• Low Power Listening
– Preamble sampling
– Uses the clear channel assessment techniques to decide whether there is packet arriving when node wakes up – Timeout puts node back to sleep if no packet arrivedTimeout puts node back to sleep if no packet arrived
• B-MAC does not have
– Synchronisationy – RTS/CTS
• BUT:
– This results in simpler and slimmer implementation – Clean and simple interface
• Currently, B-MAC is considered as default MAC protocol
Contention-based MAC protocols – PAMAS I
• Idea: Combine busy tone with RTS/CTS
– Results in detailed overhearing avoidance, but does not address idle listening
idle listening
– Uses separate data and control channels
• Approach of this idea: PAMAS
P A M lti A ith Si lli
– Power Aware Multi-Access with Signalling
• Procedure
– Node A transmits RTS on control channel; no channel sensing; g – Node B receives RTS, sends CTS on control channel if it can
receive; Node B does not know about ongoing transmissions – B sends busy tone as it starts to receive dataB sends busy tone as it starts to receive data
Control Channel AÎ BRTS Data Channel CTS BÎ A Busy tone sent by B D t
Data Channel Data
A Î B
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Contention-based MAC protocols – PAMAS II
• Already ongoing transmission
– Suppose Node C is in A’s neighbourhood is receiving a packet h A i iti t RTS
when A initiates RTS – Procedure
• A sends RTS to B
B
• C is sending busy tone (while receiving data) • B sends CTS to A
? C
A
• B sends CTS to A • CTS and busy tone collide
Î A does not receive CTS and thus, does not send any data
A
Control Channel AÎ BRTS Data Channel
CTS BÎ A Busy tone sent by C
Data Channel
No data!
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MAC protocols – Main Options
Wireless medium access Centralised Distributed Centralised Distributed Schedule-based Contention-based Schedule-based Contention-based Fixed assignment Demand assignment Fixed assignment Demand assignment g g g g
Schedule-based MAC protocols – LEACH I
• LEACH: Low-Energy Adaptive Clustering Hierarchy
• Given:
– Dense network of sensor nodes reporting to a central sink – Each node can reach the sink directly
• Idea: Group nodes into “clusters”, each controlled by a
clusterhead (CH)
About 5% of nodes become CH (scenario dependent) – About 5% of nodes become CH (scenario dependent) – Role of CH rotates to share the burden
– Clusterhead organisesg
• CDMA code for all members of the cluster • TDMA schedule that is used within the cluster
Schedule-based MAC protocols – LEACH II
Fixed length round
Setup Phase Steady-state Phase
Fixed-length round … … Advertisement phase Cluster setup phase Broadcast schedule Timeslot 1 Timeslot 2 Timeslot n … Self-election of clusterheads CH compete with CSMA Members compete with CSMA
• Setup phase
– CHs advertise themselves Î other nodes join the CH with the strongest signal; broadcast schedule is distributed by the CH
of clusterheads
strongest signal; broadcast schedule is distributed by the CH
• Steady-state phase
– CH collects and aggregates data from all cluster members – CH reports aggregated data to sink using CDMA
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IEEE 802.15.4 I
• IEEE Standard for low-rate WPAN applications
• Goals
– Low-to-medium bit ratesLow to medium bit rates
– moderate delays without too strict guarantee requirements – low energy consumption
• Physical layer
Physical layer
– 20 kbps over 1 channel @ 868-868.6 MHz – 40 kbps over 10 channels @ 905 – 928 MHz
250 kbps over 16 channels @ 2 4 GHz – 250 kbps over 16 channels @ 2.4 GHz
• MAC protocol
– Single channel at any time
C bi t ti b d d h d l b d h
– Combines contention-based and schedule-based schemes
– Slotted CSMA-CA protocol is used for the contention access period Î random delays
Different roles: PAN coordinator coordinator device – Different roles: PAN coordinator, coordinator, device
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IEEE 802.15.4 II
• Star networks
– Devices are associated with coordinators forming a PAN, identified by a PAN identifier
by a PAN identifier
• Coordinator
– Management of the devices, address assignments, generation of beacons (PAN id outstanding frames etc )
beacons (PAN id, outstanding frames, etc.) – Talks to devices and peer coordinators
• Beacon-mode super frame structure
Coordinator Device Beacon
– Guaranteed time slots (GTS) assigned to devices upon request
Active period Inactive period ACK
Data request Active period Inactive period
Data ACK
Beacon Contention access period
Guaranteed
time slots (GTS) ACK
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Further Protocols
• MAC protocols for WSNs are one of the most active
research areas
• A plenty of other additional protocols exist in literature:
• Examples
– STEM
– Mediation device protocol Many CSMA variants – Many CSMA variants
– Protocols for multi-hop reservations – …
• This section has barely scratched the surface
Outline
• MAC protocols
• Naming and Addressing
• Naming and Addressing
• Time Synchronisation
Summary
• Summary
25
Names vs. Addresses
• Name: denote/refer to “things”
– Nodes, networks, data, transactions – Often, but not always, unique
• Addresses: Information needed to find these things
St t dd IP dd MAC dd
– Street address, IP address, MAC address – Often, but not always, unique
– Addresses are often hierarchical organised e g routingAddresses are often hierarchical organised, e.g. routing
• Services to map between names and addresses
– e.g. DNSg
• Sometimes, same data serves as name and address
– e.g. IP addresses26
Address management issues
• Address allocation
– Assignment of an address to an entity from a given address pool
• Address deallocation
• Address deallocation
– Once an address is no longer in use, return it to the address pool
• Because of limited pool size
G f l b t d ll ti
• Graceful vs. abrupt deallocation
• Address representation
– Address format
• Conflict detection and resolution
– Due to merge of two networks or multiple assignment of the same address
assignment of the same address
• Binding
– Mapping between addresses used by different protocol layers
• e.g. IP addresses to MAC addresses by the ARP protocol
Distributed address assignment
• Option 1: Assignment of networkwide addresses
– Let every node randomly pick an address Î risk of duplicate addresses
• Option 2: Assignment of locally unique addresses
– Node communicate only in the local neighbourhood with unique address – Same address can be reused in different neighbourhoodsg
Î fewer bits for address representation required Î save energy • Option 3: Repair any observed conflicts
– Temporarily pick random address from a dedicated pool and a proposed p y p p p p fixed address
– Send an address request to the proposed address using temporary address – If address reply arrives, proposed address exists
– Collisions in temporary address unlikely because it just exists briefly
• Option 4: Similar to Option 3, but use a neighbour that already has a fixed address to perform requests
Content-based address assignment
• Recall: Paradigm change from id-centric to data-centric
– Describe content itself, not the involved nodes
• Example: Low level naming mechanism
• Example: Low-level-naming mechanism
– A sink indicates interest for a certain data/event by disseminating an ‘Interest message’ EQ = equal NE = not equal LT = lower than Interest message
• Interest message consists of a set of attributes to describe desired data
– Format: <attribute, value, operation>
GT = greater than LE = lower than or equal GE = greater than or equal EQ_ANY = matches anything Format: attribute, value, operation
e.g. <temperature, 20°C, GT>
– Source nodes that receive the ‘interest message’ check whether the interest matches the locally available data
IS = literal attribute
– Interest cache is built by intermediate nodes to forward request more quickly to the corresponding nodes
– Used in Direct Diffusion routing protocol (see routing section)g p ( g )
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Geographic addressing
• Often users do not only want to express queries by the type or
modality of the data, but rather by region or location
– Specify a region and let the network figure outSpecify a region and let the network figure out which sensors are appropriate
– Can be regarded as special case of content-based addresses of content based addresses
• There are many ways to specify a region
– Single point
Ci l h b i i i d di
– Circle or sphere by giving centre point and radius – Rectangle by given two corner points
– Polygon by given a list of pointsyg y g – etc.
• If location of a sensor is known, geographic routing schemes
can be applied
can be applied
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Outline
• MAC protocols
• Naming and Addressing
• Naming and Addressing
• Time Synchronisation
Summary
• Summary
31
Time Synchronisation – why is it needed?
• Time plays an important role in the operation of WSNs, since
these are supposed to observe and interact with physical
phenomena
phenomena
• Also for applications and protocols an exact time is required
– e.g. TDMA Î coordination of wakeup phases
• Example
– An acoustic wave front is generated by a sound source in a distance, hitting an array of acoustic sensor nodes
– d is known a priory; x must be estimated from the time differences of the arrival of the signal – Angle Θ can be calculated as followsg x
• Θ = arcsin x/d
ÎSmall errors in time
synchronisation leads to y Θ significant biased estimates !!! d
Time Synchronisation – how to improve?
• Two options two improve the reliability of the estimate
– Option 1: Keep sensor clocks as tightly synchronised as possible using dedicated ‘time synchronisation algorithms’
using dedicated time synchronisation algorithms Î This option is focused in this section
– Option 2: Combine readings of multiple sensors to “average out” ti ti
estimation errors
• Because WSNs have a direct coupling to the physical world,
their notion of time should be related to the ‘physical time’
their notion of time should be related to the physical time
– One second on the sensor’s clock should be close to one second in real time Î often Coordinated Universal Time (UTC) is used as time scale
time scale
• Other concept: logical time (Lamport) in which only the
relative ordering of events count, not their relation in real time
g
,
33
Time Synchronisation – node’s clock
• Often a ‘hardware clock’ is present
– An oscillator generates pulses at a fixed nominal frequency – A counter register is incremented after a fixed number of
pulses
• Only the register content is available to the node’s softwarey g • Time resolution is defined by the time between to increments
– Node i’s counter register at real time t is denoted as Hi(t)
• A local ‘software clock‘ can be derived as follows
– Li(t) := ΘiHi(t) + φi with Θias drift rate and φias phase shift
Ti h i ti l ith d l k dj t t
– Time synchronisation algorithms can do clock adjustment by modifying Θiand φi, but not the counter register
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Time Synchronisation – clocks precision
• Types of time synchronisation
– External synchronisation
• Synchronisation with external real time scale like UTC
– Internal synchronisation
• No external timescale, nodes must agree on common timeNo external timescale, nodes must agree on common time
• Sources of inaccuracies
– Nodes are switched on at random times Î drift rate (Θi) is random
– Oscillators have random deviations from nominal frequency (drift/ skew)Î cheaper oscillators have larger deviation (drift/ skew) Î cheaper oscillators have larger deviation – Oscillator frequency depends on time (oscillator aging) and
environment (temperature, pressure, supply voltage, etc.)
Time Synchronisation – protocol classification
• Time synchronisation protocols can be classified as
– Physical vs. logical time
– External vs. internal synchronisation – Global vs. local algorithms
Absolute vs relative time – Absolute vs. relative time
– Hardware vs. software-based mechanisms – A priori vs. a posteriori synchronisationp p y
• Post-facto synchronisation allows time-synchronisation on demand
– Deterministic vs. stochastic precision bounds
L l l k d t di i li Î id b k d j ?
Time Synchronisation – performance metrics
• Metrics
– Precision
• Maximum synchronisation error for deterministic algorithms
• Mean error/error variance for stochastic algorithms
– Energy costs
• Number of packets amount of computation requiredNumber of packets, amount of computation, required synchronisation frequency
– Memory requirements
Hi t f h i ti k t
• History of synchronisation packets Î longer history, more accuracy
– Fault tolerance
• What happens when a node dies?
37
Time Synchronisation – building blocks
• Time synchronisation protocols can be decomposed in
four fundamental building blocks:
– Resynchronisation event detection block
• When to trigger a time synchronisation round? Periodically? After external event?y
– Remote clock estimation block
• Acquire clock values from remote nodes/ remote clocks
Cl k ti bl k
– Clock correction block
• Compute adjustments for own local clock based on estimated clocks of the other nodes
– Synchronisation mesh setup block
• Determine which nodes synchronise with each other in a multihop network
network
38
Time Synchronisation – WSN constraints
• Scalability
– Algorithms must scale to large networks of unreliable nodes
• Precision
Precision
– Precision requirements can be diverse: from ms to seconds
• Extra hardware
– Use of extra hardware such as GPS receivers is mostly not an – Use of extra hardware, such as GPS receivers, is mostly not an
option due to extra costs and additional energy consumption
• Mobility
– Degree of mobility is low – Degree of mobility is low
• Packet delivery delay
– Mostly there are no fixed upper bound for packet delivery delay
P
ti
d l
• Propagation delay
– Propagation delay between neighbouring nodes is neglect able
• Configuration
– Manual configuration of single nodes is not an option
39
Sender/receiver time synchronisation
• Receiver synchronises to the sender’s clock by
exchanging packets with the sender
Classical protocol: network time protocol (NTP) – Classical protocol: network time protocol (NTP)• Example: Lightweight time synchronisation protocol (LTP)
– Synchronise clocks of a WSN to the clocks held by a subset of y ycertain reference nodes (e.g. equipped with GPS receiver) – Considers only phase shifts, but drift rate correction
Two components – Two components
• Pair-wise synchronisation: synchronises two neighbouring nodes • Networkwide synchronisation: construction of a spanning tree
from the reference node to all nodes from the reference node to all nodes
• There are similar protocols like LTP which differ in
– Method of spanning tree construction; how and when to make p gtimestamps; How to achieve post-facto synchronisation
Receiver/receiver time synchronisation
• Receiver of a time-stamped packets synchronise among
each other, but not with the sender
• Example: Reference broadcast synchronisation (RBS)
– RBS consists of two componentsS h i ti f i ithi i l b d t d i
• Synchronisation of receivers within a single broadcast domain • A scheme for relating timestamps between different nodes in
different domains
– RBS does not adjust the nodes’ local clocks, instead a conversion table is constructed including parameters to convert the clock values for each peer in a broadcast domainp
41
Time Synchronisation – summary
• Time synchronisation is important in WSN for protocols as
well as applications
• Using GPS receivers is typically not an option, thus
time-synchronisation protocols are required
• Post-facto synchronisation allows time-synchronisation on
demand, hence permanent updating is not required
Î save energy
Î save energy
ÎBUT: Just a very brief introduction to time synchronisation
ÎBUT: Just a very brief introduction to time-synchronisation
ÎLot of other protocols, optimisation opportunities and issues to solve
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Outline
• MAC protocols
• Naming and Addressing
• Naming and Addressing
• Time Synchronisation
• Summary
Summary
• There were several MAC protocols proposed that can be
used in WSNs Î however, still a huge research area
• For the naming and addressing, there is a paradigm shift
from address-centric to data-centric approaches in
WSNs
WSNs
• Time Synchronisation in WSN is crucial because they
interact with physical phenomena; also sleeping phases
interact with physical phenomena; also sleeping-phases
needs to be synchronised
