Schedule and Contention based MAC protocols

SCHEDULE & CONTENTION
BASED MAC PROTOCOLS
Darwin Nesakumar A, M.E, (P.hD)
Assistant Professor
Department of ECE
R.M.K. Engineering College

Agenda
• Review of previous session
• Mediation Device Protocol
• Scheduled based protocols in WSN
• S-MACS
• Contention based protocols in WSN
• PAMAS Protocol
• Low duty Cycle and Wake up Concepts
• Quizzes
2Schedule based protocolsFriday, 28August 2020

Review of previous session
• Joinmyquiz.com

Question
What is needed in schedule based MACs
Frequency Synchronization
Time Synchronization
Code Synchronization
Schedule based protocols 4Friday, 28August 2020

Answer for the Question
Time Synchronization

Question
Risk of packet collisions occur in
Schedule based MACs
Contention based MACs

Contention based MACs

Question
The two options to shut up senders are to inform
potential interferers while a reception is on-going
and before a reception is on-going
TRUE
FALSE

TRUE

Question
Clusterhead uses FDMA
TRUE
FALSE

FALSE

Question
Role of clusterhead is _____________ to share
the burden
Distributed
Rotated
Converged

Rotated

Question
What is the percentage of nodes to become
clusterhead?

5%

Question
Inter cluster uses
TDMA
CDMA

CDMA

Question
Which transmission is used in advertisement phase?
Unicast
Multicast
Broadcast

Broadcast

Question
TDMA is being used for
Intra Cluster
Inter Cluster

Intra Cluster

Question
LEACH stands for

Low-energy Adaptive Clustering Hierarchy

Question
What are the two phases in LEACH Protocol?

Set up phase
Steady State Phase

IMPORTANT POINTS TO BE REMEMBER
MAC Protocols
• MAC – Medium Access Control
• They coordinate the times where a
number of nodes access a shared
communication medium.
• Main requirement – Energy efficiency
• Energy waste due to – Overhead,
Overhearing, Collisions and idle
listening
• Switch the transceiver into Sleep
Mode – Energy saving approach
• There are trade offs between energy
expenditure and delay, throughput
• MAC is first protocol above the Physical
layer (PHY)
• Fundamental task is to regulate the
access of number of nodes to a shared
medium
• Few traditional criteria are delay,
throughput, fairness
• Energy conservation is an issue in MAC
• MAC is apart of Data Link Layer (DLL) –
OSI reference model
• MAC protocol determines for a node the
points in time when it accesses the
medium to try to transmit a data,
control, or management packet to
another node (unicast) or to a set of
nodes (multicast, broadcast). 26Schedule based protocolsFriday, 28August 2020

MAC Protocols
• MAC is a part of Data Link Layer (DLL)
• DLL Responsibilities are
• Error Control – used to ensure
correctness of transmission and to
take appropriate actions in case of
transmission errors
• Flow control - regulates the rate of
transmission to protect a slow receiver
from being overwhelmed with data
• Main approach to conserve energy –
Put nodes in sleep state whenever
possible
• Low duty cycle, Wake up concepts
• Two types of MAC Protocols are
Contention based and schedule
based
• Contention based – It is a
communication protocol for
operating wireless
telecommunication equipment that
allows many users to use same radio
channel without pre coordination
• Schedule based - A schedule exists,
regulating which participant may use
which resource at which time

SCHEDULE- VS. CONTENTION-BASED MAC
• Schedule-based MAC
– A schedule exists, regulating which participant may use which resource
at which time (TDMA component)
– Typical resource: frequency band in a given physical space (with a given
code, CDMA)
– Schedule can be fixed or computed on demand
• Usually: mixed – difference fixed/on demand is one of time scales
– Usually, collisions, overhearing, idle listening no issues
– Needed: time synchronization!
• Contention-based protocols
– Risk of colliding packets is deliberately taken
– Hope: coordination overhead can be saved, resulting in overall improved
efficiency
– Mechanisms to handle/reduce probability/impact of collisions required
– Usually, randomization used somehow

LEACH Protocol
• Given: Dense network of nodes,
reporting to a central sink, each node
can reach sink directly
• Group of nodes – Cluster, controlled
by clusterhead
• Clusterheads organize CDMA code for
all member transmissions, TDMA
schedule to be used within a cluster
• CHs collect & aggregate data from all
cluster members, aggregates the data
from members and forwards to sink
using CDMA
• About 5% of nodes become clusterhead,
Role of clusterhead is rotated to share
the burden
• Two phases – Set up phase and steady
state phase
• Set up phase – Election of Clusterhead,
advertisement of Clusterhead to the
members via broadcasting
• Setup phase – CH assigns TDMA for
members, CDMA for transmitting the
data to sink
• TDMA – Intra Cluster
• CDMA – Inter Cluster

LEACH Protocol
• Steady State Phase : CHs collect &
aggregate data from all cluster
members, report aggregated data to
sink using CDMA
• The cluster-head is maintained when
data is transmitted between nodes.
CH is responsible for creating and
maintaining a TDMA schedule; all the
other nodes of a cluster are member
nodes.
• To all member nodes, TDMA slots are
assigned, which can be used to
exchange data between the member
and the clusterhead.
• With the exception of their time slots,
the members can spend their time in
sleep state.
• The clusterhead aggregates the data of
its members and transmits it to the sink
node or to other nodes for further
relaying.
• Since the sink is often far away, the
clusterhead must spend significant
energy for this transmission.
• For a member, it is typically much
cheaper to reach the clusterhead than
to transmit directly to the sink
• There is no peer-to-peer
communication.

LEACH Protocol
• Advantages
• Increases the lifetime of the network, Even drain of energy
• Distributed, no global knowledge required
• Energy saving due to aggregation by CHs
• TDMA- Nodes assigned with time slot for transmission and let them sleep at
all other times.
• Transmission schedules achieve no collisions occur at receivers and hence
no special mechanisms are needed to avoid hidden-terminal situations.

LEACH Protocol
• The setup and maintenance of
schedules involves signaling traffic.
• If a TDMA variant is employed, time is
divided into comparably small slots
• Such schedules are not easily adapted
to different load situations on small
timescales.
• The schedule of a node (and possibly
those of its neighbors) may require a
significant amount of memory, which
is a scarce resource in several sensor
node designs.
Disadvantages
• LEACH assumes all nodes can transmit
with enough power to reach BS if
necessary (e.g., elected as CHs)
• Each node should support both TDMA &
CDMA
• Need to do time synchronization
• Nodes use single-hop communication
• LEACH would not be able to cover large
geographical areas of some square miles
or more, because a clusterhead two miles
away from the sink likely does not have
enough energy to reach the sink at all.

SCHEDULE-BASED MAC
PROTOCOLS

THE MEDIATION
DEVICE PROTOCOL
Friday, 28August 2020 Schedule based protocols 34

 It allows each node in a WSN to go into sleep mode periodically and to
wake up only for short times to receive packets from neighbor nodes.
 There is no global time reference, each node has its own sleeping
schedule, and does not take care of its neighbors sleep schedules.
To receive packets after wake up period
Upon each periodic wakeup, a node transmits a short query beacon,
indicating its node address and its willingness to accept packets from
other nodes.
The node stays awake for some short time following the query beacon, to
open up a window for incoming packets. If no packet is received during
this window, the node goes back into sleep mode.
THE MEDIATION DEVICE PROTOCOL

Question
Upon each periodic wakeup, a node transmits a _______
query beacon, indicating its node address and its
willingness to accept packets from other nodes.
Short
Medium
Long

Short

To transmit packets after wake up period
 When a node wants to transmit a packet to a neighbor, it has to
synchronize with it. One option would be to have the sender actively
waiting for query beacon, but this wastes considerable energy for
synchronization purposes only.
 The dynamic synchronization approach achieves this synchronization
without requiring the transmitter to be awake permanently to detect the
destinations query beacon.
 To achieve this, a mediation device (MD) is used. We first discuss the
case where the mediation device is not energy constrained and can be
active all the time.

Question
What is the purpose of Mediation device?

Dynamic synchronization

• Because of its full duty cycle, the mediation device can receive the query beacons from
all nodes in its vicinity and learn their wakeup period
Protocol
• Sender A sends RTS to MD
• MD stores this information
• Receiver B sends query to MD
• MD tells reciever B when to
wake up
• B sends CTS to A (now in sync)
• A sends data
• B acknowledges
• B returns to old timing

Advantages
 It does not require any time synchronization between the
nodes, only the mediation device has to learn the periods of
the nodes.
 The protocol is asymmetric in the sense that most of the
energy burden is shifted to the mediation device, which so
far is assumed to be power unconstrained.

Question
Mediation device is power constrained.
TRUE
FALSE

FALSE

Disadvantages
 The nodes transmit their query beacons without checking for ongoing
transmissions
 If the node density is sufficiently low, this collision probability can be low
too. However, in case of higher node densities or unwanted
synchronization between the nodes, the number of collisions can be
significant.
 A possible solution to this is the following: When the MD registers
collisions, it might start to emit a dedicated reschedule control frame to
all colliding nodes.

Question
When the MD registers collisions, it might start to
emit a dedicated reschedule control frame to ________
nodes.
All
Neighboring
Colliding

Colliding

 All colliding nodes can hear this frame as long as the MD repeats it often
enough.
 Reception of this frame causes each node to randomly pick a new period
from a certain interval [a, b] indicated in the reschedule frame.
 If the MD continues to perceive collisions, it can enlarge the interval
accordingly
Disadvantages

The mediation device is energy unconstrained, which does not
conform to the idea of a “simply thrown out” wireless sensor network
There are sufficient mediation devices to cover all nodes. The
distributed mediation device protocol deals with these problems in a
probabilistic manner.
Drawbacks

Question
Mediation device is energy _____________
Constrained
Unconstrained

Unconstrained

S-MAC PROTOCOL

S-MAC PROTOCOL
• S-MAC stands for Sensor – Medium Access Control
• S-MAC protocol provides mechanisms to avoid/ to bypass idle
listening, collisions, and overhearing. It does not require two
different channels.
• S-MAC adopts a periodic wakeup scheme, that is, each node
alternates between a fixed-length listen period and a fixed-length
sleep period according to its schedule.
• The listen period of S-MAC can be used to receive and transmit
packets.
• S-MAC attempts to coordinate the schedules of neighboring nodes
such that their listen periods start at the same time.

Question
Which of the following are true ?
1. S-MAC protocol provides mechanisms to avoid/ to
bypass idle listening, collisions, and overhearing.
2. S-MAC adopts a periodic wakeup scheme
3. S-MAC attempts to coordinate the schedules of neighboring
nodes

All are true

Question
S – MAC requires two different channels
channel
TRUE
FALSE

FALSE

S-MAC PROTOCOL
• A node x’s listen period is subdivided into three different phases
• Synchronization Phase
• RTS Phase
• CTS Phase

S-MAC PROTOCOL
• It is the first phase of S-MAC Protocol
• In this phase node x accepts SYNCH packets from its
neighbors.
• In these packets, the neighbors describe their own schedule and
x stores their schedule in a table (the schedule table).
• Node x’s SYNCH phase is subdivided into time slots and x’s
neighbors contend according to a CSMA scheme, that is, each
neighbor y wishing to transmit a SYNCH packet picks one of the
time slots randomly and starts to transmit if no signal was
received in any of the previous slots.

S-MAC PROTOCOL
• In the other case, y goes back into sleep mode and waits for x’s
next wakeup.
• In the other direction, since x knows a neighbor y’s schedule, x
can wake at appropriate times and send its own SYNCH packet to
y (in broadcast mode).
• It is not required that x broadcasts its schedule in every of y’s
wakeup periods. However, for reasons of time synchronization
and to allow new nodes to learn their local network topology, x
should send SYNCH packets periodically. The according period is
called synchronization period.

S-MAC PROTOCOL
• RTS Phase
• In this second phase x listens for RTS packets from neighboring
nodes.
• In S-MAC, the RTS/CTS handshake is used to reduce collisions of
data packets due to hidden-terminal situations.
• Again, interested neighbors contend in this phase according to a
CSMA scheme with additional backoff.

Question
In S-MAC, the __________________is used to reduce
collisions of data packets due to hidden-terminal
situations.
RTS/CTS handshake
RTS/CTS Milkshake
RTS/CTS legshake

RTS/CTS handshake

S-MAC PROTOCOL
• CTS Phase
• In the third phase node x transmits a CTS packet if an RTS packet
was received in the previous phase. After this, the packet
exchange continues, extending into x’s nominal sleep time.
• In general, when competing for the medium, the nodes use the
RTS/CTS handshake, including the virtual carrier-sense
mechanism, whereby a node maintains a NAV variable.
• The NAV (Network Allocation Vector – Virtual Carrier Sensing)
mechanism can be readily used to switch off the node during
ongoing transmissions to avoid overhearing.
• When transmitting in a broadcast mode (for example SYNCH
packets), the RTS and CTS packets are dropped and the nodes
use CSMA with backoff.
•

S-MAC PROTOCOL
• CTS Phase
• If we can arrange that the schedules of node x and its neighbors
are synchronized, node x and all its neighbors wake up at the
same time and x can reach all of them with a single SYNCH
packet.
• The S-MAC protocol allows neighboring nodes to agree on the
same schedule and to create virtual clusters.
• The clustering structure refers solely to the exchange of schedules;
the transfer of data packets is not influenced by virtual clustering.

Question
What are the three phases of listen period?

Synchronization Phase
RTS Phase
CTS Phase

• MACA’s idle listening is particularly unsuitable if average data rate
is low
• Most of the time, nothing happens
• Idea: Switch nodes off, ensure that neighboring nodes turn on
simultaneously to allow packet exchange
S-MAC PROTOCOL
• Only in these active periods, packet
exchanges happen
• Need to also exchange wakeup
schedule between neighbors
• When awake, essentially perform
RTS/CTS
• Use SYNCH, RTS, CTS phases
Wakeup period
Active period
Sleep period
For SYNCH For RTS For CTS

S-MAC Synchronized islands
• Nodes try to pick up schedule synchronization from neighboring nodes
• If no neighbor found, nodes pick some schedule to start with
• If additional nodes join, some node might learn about two different
schedules from different nodes
– “Synchronized islands”
• To bridge this gap, it has to follow both schemes
Time
A A A A
C C C C
A
B B B B
D D D
A
C
B
D
E E E EE E E

S-MAC PROTOCOL
• Given: Many radio channels, superframes of known length (not
necessarily in phase, but still time synchronization required!)
• Goal: Set up directional links between neighboring nodes
– Link: radio channel + time slot at both sender and receiver
– Free of collisions at receiver
– Channel picked randomly, slot is searched greedily until a
collision-free slot is found
• Receivers sleep and only wake up in their assigned time slots, once
per superframe
• In effect: a local construction of a schedule

Timeout-MAC (T-MAC)
• In S-MAC, active period is of constant
length
• What if no traffic actually happens?
• Nodes stay awake needlessly long
• Idea: Prematurely go back to sleep
mode when no traffic has happened
for a certain time (=timeout) ! T-MAC
• Adaptive duty cycle!
• One ensuing problem: Early sleeping
• C wants to send to D, but is
hindered by transmission A! B
A B C D
CTS
May not
send
Timeout,
go back to
sleep as
nothing
happened

B-MAC
• Clear Channel Assessment
– Adapts to noise floor by sampling channel when it is assumed to
be free
– Samples are exponentially averaged, result used in gain control
– For actual assessment when sending a packet, look at five
channel samples – channel is free if even a single one of them is
significantly below noise
– Optional: random backoff if channel is found busy
• Optional: Immediate link layer acknowledgements for received
packets

B-MAC II
• Low Power Listening (= preamble sampling)
– Uses the clear channel assessment techniques to decide whether
there is a packet arriving when node wakes up
– Timeout puts node back to sleep if no packet arrived
• B-MAC does not have
– Synchronization
– RTS/CTS
– Results in simpler, leaner implementation
– Clean and simple interface
• Currently: Often considered as the default WSN MAC protocol

CONTENTION-BASED
PROTOCOLS

CONTENTION-BASED PROTOCOLS
 If only one neighbor tries its luck, the packet goes through the channel.
 If two or more neighbors try their luck, these have to compete with each
other and in unlucky cases, for example, due to hidden-terminal
situations, a collision might occur, wasting energy for both transmitter
and receiver.
 Two important contention based protocols: (slotted) ALOHA and CSMA,
along with mechanisms to solve the hidden-terminal problem.
 In the following sections, we discuss variations of these protocols with
the goal to conserve energy. As opposed to some of the contention-
based protocols having a periodic wakeup scheme
 The protocols described in this section have no idle listening avoidance
and make no restrictions as to when a node can receive a packet. 75Schedule based protocolsFriday, 28August 2020

The PAMAS protocol (Power Aware Multi-
access with Signaling)
 Originally designed for ad hoc networks.
 It provides a detailed overhearing avoidance mechanism while it does not
consider the idle listening problem.
 The protocol combines the busy-tone solution and RTS/CTS handshake
similar to the MACA protocol
 A distinctive feature of PAMAS is that it uses two channels:
 Data channel (while the data channel is reserved for data packets)
 Control channel. (All the signaling packets (RTS, CTS, busy tones) are
transmitted on the control channel.

PAMAS PROTOCOL
• Let us consider an idle node x to which a new packet destined to a
neighboring node y arrives.
• First, x sends an RTS packet on the control channel without doing any
carrier sensing. This packet carries both x’s and y’s MAC addresses.
• If y receives this packet, it answers with a CTS packet if y does not know
of any ongoing transmission in its vicinity. Upon receiving the CTS, x starts
to transmit the packet to y on the data channel.
• When y starts to receive the data, it sends out a busy-tone packet on
the control channel.
• If x fails to receive a CTS packet within some time window, it enters the
backoff mode, where a binary exponential backoff scheme is used (i.e., the
backoff time is uniformly chosen from a time interval that is doubled after
each failure to receive a CTS).

PAMAS PROTOCOL
• Now, let us look at the nodes receiving x’s RTS packet on the control
channel. There is the intended receiver y and there are other nodes; let z
be one of them.
• If z is currently receiving a packet, it reacts by sending a busy-tone packet,
which overlaps with y’s CTS at node x and effectively destroys the CTS.
Therefore, x cannot start transmission and z’s packet reception is not
disturbed.
• Since the busy-tone packet is longer than the CTS, we can be sure that the
CTS is really destroyed.
• We consider the intended receiver y. If y knows about an ongoing
transmission in its vicinity, it suppresses its CTS, causing x to back off.

PAMAS PROTOCOL
• Node y can obtain this knowledge by either sensing the data channel or
by checking whether there was some noise on the control channel
immediately after receiving the RTS. This noise can be an RTS or CTS of
another node colliding at y.
• In the other case, y answers with a CTS packet and starts to send out a
busy-tone packet as soon as x’s transmission has started.
• Furthermore, y sends out busy-tone packets each time it receives some
noise or a valid packet on the control channel, to prevent its neighborhood
from any activities.

PAMAS - Power Aware Multi access with
Signaling
• Idea: Combine busy tone with RTS/CTS
– Results in detailed overhearing avoidance, does not address idle listening
– Uses separate data and control channels
• Procedure
– Node A transmits RTS on control channel, does not sense channel
– Node B receives RTS, sends CTS on control channel if it can receive and does not
know about ongoing transmissions
– B sends busy tone as it starts to receive data
Time
Control
channel
Data
channel
RTS
A ! B
CTS
B ! A
Data
A ! B
Busy tone
sent by B

PAMAS – Already ongoing transmission
• Suppose a node C in vicinity of A is already receiving a packet when A
initiates RTS
• Procedure
– A sends RTS to B
– C is sending busy tone (as it receives data)
– CTS and busy tone collide, A receives no CTS, does not send data
A
B
C
?
Time
Control
channel
Data
channel
RTS
A ! B
CTS
B ! A
No data!
Busy tone by C
Similarly:Ongoing
transmission near B destroys
RTS by busy tone

PAMAS - Power Aware Multi access with signaling
Node X Node y
RTS
CTS
BusyTone
• First, x sends an RTS packet on the control
channel without doing any carrier sensing.
This packet carries both x’s and y’s MAC
addresses
• If y receives this packet, it answers with a
CTS packet if y does not know of any
ongoing transmission in its vicinity.
• Upon receiving the CTS, x starts to
transmit the packet to y on the data
channel. When y starts to receive the data,
it sends out a busy-tone packet on the
control channel.
• If x fails to receive a CTS packet within
some time window, it enters the backoff
mode, where a binary exponential backoff
scheme is used (i.e., the backoff time is
uniformly chosen from a time interval that
is doubled after each failure to receive a
CTS)

Node X Node y
RTS
CTS
BusyTone
 If y knows about an ongoing transmission in
its vicinity, it with a CTS packet and starts to
send out a busy-tone packet as soon as x’s
transmission has started. suppresses its CTS,
causing x to back off.
 Node y can obtain this knowledge by either
sensing the data channel or by checking
whether there was some noise on the control
channel immediately after receiving the RTS.
 This noise can be an RTS or CTS of another
node colliding at y. In the other case, y
answers
 Furthermore, y sends out busy-tone packets
each time it receives some noise or a valid
packet on the control channel, to prevent its
neighborhood from any activities.

When can a node put its transceivers
(control and data) into sleep mode?
Any time a node knows that it cannot transmit or receive packets because
some other node in its vicinity is already doing so.
This decision is easy if a node x knows about the length of an ongoing
transmission, for example from overhearing the RTS or CTS packets or the
header of the data packets on the data channel.
However, often this length is unknown to x, for example, because these
packets are corrupted or a foreign data transmission cycle starts when x is
just sleeping.
Additional procedures are needed to resolve this

Suppose that x wakes up and finds the data channel busy. There are two
cases to distinguish:
Case 1
Either x has no own packet to send or x wants to transmit. In the first
case, x desires to go back into sleep mode and to wake up exactly when
the ongoing transmission ends to be able to receive an immediately
following packet.
Waking up at the earliest possible time has the advantage of avoiding
unwanted delays.
However, since x may not have overheard the RTS, CTS, or data packet
header belonging to the ongoing transmission, it runs a probing protocol
on the control channel to inquire the length of the ongoing packet.

• Case 2
• x wakes up during an ongoing transmission and wants to transmit
a packet. Therefore, x has not only to take care of ongoing
transmissions but also of ongoing receptions in its vicinity.
• To find the time for the next wakeup, x runs the described
probing protocol for the set of transmitters, giving a time t when
the longest ongoing transmission ends.

LOW DUTY CYCLE
PROTOCOLS AND
WAKEUP CONCEPTS

LOW DUTY CYCLE PROTOCOLS
AND WAKEUP CONCEPTS
• Low duty cycle protocols try to avoid spending (much) time in the
idle state and to reduce the communication activities of a sensor
node to a minimum. In an ideal case, the sleep state is left only
when a node is about to transmit or receive packets.
• Periodic wake up scheme
• In this approach, nodes spend most of their time in the sleep mode
and wake up periodically to receive packets from other nodes.
• Specifically, a node A listens onto the channel during its listen
period and goes back into sleep mode when no other node takes
the opportunity to direct a packet to A.

AND WAKEUP CONCEPTS
• Method 1
• Transmitter B must acquire knowledge about A’s listen periods to
send its packet at the right time, this task corresponds to a
rendezvous.
• Node A transmit a short beacon at the beginning of its listen period
to indicate its willingness to receive packets.

AND WAKEUP CONCEPTS
• Method 2
• This method is to let node B send frequent request packets until
one of them hits A’s listen period and is really answered by A

AND WAKEUP CONCEPTS
• Method 1 – B knows A’s listen time and send packets at right time
• Method 2 – B sends RTS to A’s listen period untill A answers (B
does not know A’s listen time)
• However, in either case, node A only receives packets during its
listen period.
• If node A itself wants to transmit packets, it must acquire the
target’s listen period.
• A whole cycle consisting of sleep period and listen period is also
called a wakeup period.
• The ratio of the listen period length to the wakeup period length is
also called the node’s duty cycle.

AND WAKEUP CONCEPTS
By choosing a small duty cycle, the transceiver is in sleep mode most of
the time, avoiding idle listening and conserving energy.
By choosing a small duty cycle, the traffic directed from neighboring
nodes to a given node concentrates on a small time window (the listen
period) and in heavy load situations significant competition can occur.
Choosing a long sleep period (Small duty cycle) induces a significant
per-hop latency, since a prospective transmitter node has to wait an
average of half a sleep period before the receiver can accept packets.
In the multihop case, the per-hop latencies add up and create
significant end-to-end latencies. Sleep phases should not be too short
lest the start-up costs outweigh the benefits.

Question
What are the three periods available in periodic
wakeup scheme?

Wake up period
Sleep Period
Listening Period

Question
For small duty cycle, the sleep time of the node is
very long.
TRUE
FALSE

TRUE

Question
Sleep phase should be short or long ?

Short

Question
The ratio of the listen period length to the wakeup period length is
also called

Node’s duty cycle

Demand Assignment Protocols
• Resources are allocated on a short term basis
• Centralized and distributed versions are possible
• Central
– Nodes request a reource (e.g. time slot) from a central server
– Waits for ACK and then transmits
– Polling by central station is possible
– Central server to be switced on always
– Central node requires a lot of energy
– Central node may be rotated (LEACH) 101Schedule based protocolsFriday, 28August 2020

Distributed Demand Assignment
Protocols
• Token passing (IEEE 802.4) may be used
• Token is passed among stations in a logical ring
• Ring management needed
– Include/exclude nodes from the ring
– Correct lost tokens
• In WSNs, maintenance is difficult
– Channel errors
– Node xceiver must be switched on all the time (energy ..) due to
variable token delivery times

Random Access Protocols
• Fully distributed
• ALOHA (more on this later)
• CSMA based
– Listen to the medium
– If idle, xmit
– If busy, wait (p persistent, non-persistent etc.)
• RTS/CTS based on MACAW protocol (more later)

Main options Wireless medium access
Centralized
Distributed
Contention-
based
Schedule-
based
Fixed
assignment
Demand
assignment
Contention-basedSchedule-based
Fixed
assignment
Demand
assignment

Centralized medium access
• Idea: Have a central station control when a node may access the
medium
– Example: Polling, centralized computation of TDMA schedules
– Advantage: Simple, quite efficient (e.g., no collisions), burdens
the central station
• Not directly feasible for non-trivial wireless network sizes
• But: Can be quite useful when network is somehow divided into
smaller groups
– Clusters, in each cluster medium access can be controlled
centrally – compare Bluetooth piconets, for example
! Usually, distributed medium access is considered

Preamble Sampling
• So far: Periodic sleeping supported by some means to synchronize wake
up of nodes to ensure rendez-vous between sender and receiver
• Alternative option: Don’t try to explicitly synchronize nodes
– Have receiver sleep and only periodically sample the channel
• Use long preambles to ensure that receiver stays awake to catch actual
packet
– Example: WiseMAC
Check
channel
Check
channel
Check
channel
Check
channel
Start transmission:
Long preamble Actual packet
Stay awake!

• Sensor Protocols for Information via Negotiation (SPIN)
• A Negotiation-Based Protocols for Disseminating Information in
Wireless Sensor Networks.
• Dissemination is the process of distributing individual sensor
observations to the whole network, treating all sensors as sink
nodes
• Replicate complete view of the environment
• Enhance fault tolerance
• Broadcast critical piece of information
SPIN -Sensor Protocols for Information
via Negotiation

• Flooding is the classic approach for dissemination
• Source node sends data to all neighbors
• Receiving node stores and sends data to all its neighbors
• Disseminate data quickly
• Deficiencies
• Implosion
• Overlap
• Resource blindness
SPIN

• Negotiation
• Before transmitting data, nodes negotiate with each other to
overcome implosion and overlap
• Only useful information will be transferred
• Observed data must be described by meta-data
• Resource adaptation
• Each sensor node has resource manager
• Applications probe manager before transmitting or processing
data
• Sensors may reduce certain activities when energy is low
SPIN

• SPIN : A three-stage handshake protocol for point-to-point media
• ADV – data advertisement
• Node that has data to share can advertise this by transmitting
an ADV with meta-data attached
• REQ – request for data
• Node sends a request when it wishes to receive some actual
data
• DATA – data message
• Contain actual sensor data with a meta-data header
• Usually much bigger than ADV or REQ messages
SPIN (cont.)

SPIN Protocol

THANK YOU

Schedule and Contention based MAC protocols

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