Wireless BAN requires efficient and sensible use of the available energy resources • Sensors in a network have limited amount of energy and need to save power to maximize lifetime • Implanted devices inside body need critical handling to save power.

1. WBAN MAC Issues
Kyung Sup Kwak
Inha University UWB-ITRC
kskwak@inha.ac.kr

2. Outline
Slide 2
• Introduction
• WBAN considerations
• Major sources of energy waste
• MAC classification tree
• Power saving mechanisms
• Major MAC approaches
• Pros and Cons of some MAC protocols
• Conclusion

3. Introduction
Slide 3
• Wireless BAN requires efficient and sensible
use of the available energy resources
• Sensors in a network have limited amount of
energy and need to save power to maximize
lifetime
• Implanted devices inside body need critical
handling to save power.

4. WBAN Considerations
• WBAN devices are limited for in/on body BAN
– small network size
• There is one Coordinator that acts as overall controller
• Not all devices have data to send all the time
– Idle nodes
• Only few of them may have data at same time
– Less traffic
• Communication flows
• Communication Reasons
– Devices have data to send
• Emergency data etc.
– Coordinator might need specific data
• Data on-demand
Slide 4
– WBAN Coordinator WBAN Device
– WBAN Device WBAN Coordinator

5. Fully function device (FFD)
Reduced function device (RFD)
WBAN Coordinator
In-body On-body Out-body
Two Basic roles:
1. End device (RFD or FFD)
2. Coordinator (FFD)
WBAN Considerations: Topology
* WBAN uses a hybrid topology
3-5 meter radius
Star Topology for in-body
Slide 5

6. Major Sources of Energy Waste
Slide 6
• Energy waste in WBAN
1.Collision
2.Idle Listening
3.Overhearing
4.Over-emitting
5.Others

7. Purely
Contention-Based
Protocols
Contention-based
with reservation
Contention-based
with scheduling
Sender-Initiated
Protocols
Receiver-Initiated
Protocols
Synchronous
Protocols
Asynchronous
Protocols
Single-Channel
Protocols
Multichannel
Protocols
MACAW
FAMA
RI-BTMA
MACA-BI
MARCH
D-PRMA
CATA
HRMA
SRMA/PA
FPRP
RI-BTMA
MACA-BI
MARCH
MACA/PR
RTMAC
Directiona
l Antennas
MMAC
MCSMA
PCM
RBAR
Centralized
Protocols
CDMA based FDMA based
MAC Classification Tree
MAC Protocols
Distributed
Protocols
Schedule based
Protocols
Other (Hybrid)
MAC Protocols
Schedule based
Protocols
Contention based
Protocols
Fixed
assignment
Demand
assignment
Contention based
Protocols
Slide 7
* The classification is more complex then it looks

8. Power Saving Mechanisms
Slide 8
• The Power saving mechanisms can be broadly
classified into[2]:
– Adaptive duty cycling protocols
• Power off radio when not actively transmitting and
receiving packet
– Wakeup on-demand protocols

9. Power Saving Mechanisms
Slide 9
• Related works are [3]:
– IEEE802.11
• High energy consumption when the nodes are in the idle mode
– CSMA
• To improve the energy consumption by avoiding overhearing among
neighboring nodes
– TDMA
• No contention-introduced overhead and collisions
• Not easy to manage the inter-cluster communication and interference
• Not easy to dynamically change its frame length and time slot
assignment

10. Power Saving Mechanisms
Slide 10
• The two major methods for WBAN power saving mechanism
can be:
– TDMA
– Contention based
• TDMA vs. Contention-based protocols
Table 1: Comparison between TDMA and Contention based protocols
TDMA Contention-based
TDMA can easily avoid or reduce energy waste from all
major sources [collision, idle listening etc.]
Contention protocols needs to work
hard in all directions
TDMA has limited scalability and adaptability
–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
Good for in-body communication Good for on/out body communication

11. Power Saving Mechanisms: Sleep-wake-up
Slide 11
• Components in nodes are switched off.
• Need proper wake-up methods
• Question is: “When should a device switch to low power mode
and for how long?”
– An efficient power saving protocol must answer this question.
• Sleep mode power consumption is much less than idle power
consumption
• Using information about traffic in the network, we can make
better decisions about how frequently to wake up.
• Objective: Reducing energy consumption through a careful
scheduling of wakeup times.

12. Power Saving Mechanisms: Sleep-wake-up
• One way towards lower power consumption is
to turn off all unused components
– This hampers working of whole network
• Sleep mode power consumption is much less
than idle power consumption
[1]
Table 2: Power characteristics for a Mica2 Mote Sensor [1]
Slide 12

13. Power Saving Mechanisms: Wake-up
Protocols
Slide 13
• Types of Wake-up protocols [1]:
– Synchronous
• When nodes enter sleep mode, they schedule a timer to
wake up at a pre-determined time
• Examples: IEEE 802.11 PSM, S-MAC
– Out-Of-Band (OOB)
• A sleeping node can be woken at any time via an out-of-
band channel
• Examples: STEM, PicoRadio
– Hybrid
• Synchronous plus Out-Of-Band

14. Out-Of-Band Protocol
Slide 14
• Use a busy tone (BT) channel to wake up neighbors
– BT is broadcast on the channel for specified duration
– No information is encoded in the BT
– Serves as binary signaling mechanism to neighbors
• Advantage
– Only have to detect energy on channel rather than decode packet
• Simple hardware
• Small detection time
– No need to handle collisions
• Disadvantage
– BT may awake all the nodes in the neighborhood
• Possible Solution
– Two Radios: One for Data & one for BT

15. Power Saving Mechanisms:
Major MAC Approaches
Slide 15

16. Major MAC Approaches
Slide 16
• The Three most suitable MAC approaches in power
saving are:
– Channel Polling (Low power listening)
– Scheduled contention
– TDMA – contention free /cluster-based

17. Channel Polling (Low power listening)
Slide 17
• A node wakes up for a very small amount of
time.
• It checks the channel for any activity without
receiving data
– If channel idle it goes back to sleep otherwise stays
awake to receive data
– Performed regularly but not synchronised among
nodes

18. Scheduled Contention
Slide 18
• Nodes periodically wake up in unison
• Contend for access to channel
– Receiver listen to brief contention periods while senders
contend
• Only nodes participating in data transfer remain awake
after contention periods while others can sleep
• Schedule coordinated transmission and listen periods
• Nodes adopt common schedule
• Synchronising with periodic control messages

19. TDMA
Slide 19
• Cluster of nodes is formed under a cluster head or
Coordinator
• Communication takes place between nodes and
Coordinator.
• Node uses time slots allocated by the Coordinator
• Only one node is allowed to transmit in a slot
• Synchronization is maintained in the network,
provided by Coordinator
• Peer to peer communication is not allowed

20. Slide 20
Table 3: Comparison of MAC approaches [4]
Comparison of MAC approaches
Channel polling
(Low power listening)
Scheduled
contention
TDMA
(Contention free /cluster-based )
~ 10 times less expensive than listening
for full contention period
Listening for full contention
period
Low duty cycle
Asynchronous Synchronous Synchronous -
Fine grained time synchronisation
Sensitive to tuning for neighbourhood
size and traffic rate
Sensitive to clock drift Very sensitive to clock drift
Poor performance when traffic rates
vary greatly.
(optimised for known periodic traffic)
Improved performance with
traffic increase
Limited throughput and number of
active nodes
Receiver and polling efficiency is gained
at the much greater cost of senders
similar cost incurred by sender
and receiver
Require clustering >>cost incurred
more on Cluster head
challenging to adapt low power listening
directly to newer radios like 802.15.4
(preamble size limited)
Scalable, adaptive and flexible Limited scalability and adaptability
to changes on number of nodes

21. Pros and Cons of Some MAC Protocols
Slide 21
Table 4: Pros and Cons of MAC protocols [4]
Channel Polling Protocols Scheduled Contention TDMA
WiseMAC BMAC TICER/RICER S-MAC PMAC T-MAC FLAMA LEACH HEED
Scalable Flexible
Compared
to S-MAC
Considerable
reduction of power
consumption if wake
up period is optimal
Loosely
synchronised
Loose time
synchronization
Loosely
synchronised
Better end to
end reliability
and energy
saving
compared to
S-MAC
Distributed ,no global
knowledge
required
Energy
efficient
Support
mobility and
adaptive to
traffic load
Better packet
delivery rates and
power
conservation
Effect of channel
fading on rendezvous
schemes is major
High transmission
latency
Adaptation to changes
might be slow
Better delay-All
queued packets are
sent on one listen
state in a burst
Smaller delays,
improved
energy saving
and reliability
compared to
TRAMA
Extra overhead for
dynamic clustering
Scalable
Low delay Better throughput
and
Latency
Semi synchronous
schemes yield
substantial power
savings under weak
fading conditions
Low throughput Higher throughput
under heavier traffic
Reasonable
throughput
prolonged
network
lifetime

22. MAC Protocol Comparison
Slide 22
Table 5: Comparison of MAC protocols [6]
Algorithm Hidden/
exposed nodes
Channel 802.11
compatible
Antannae
direction
Additional
H/W
Pros Cons
CSMA (mainly) Hidden Single No Omni No Simplicity Hidden nodes
MACA/ MACAW (mainly) Exposed Single No Omni No Simplicity Exposed nodes
802.11/ 802.11e Both Single Yes Omni No Simplicity; Easy to implement;
Prevalent in reality. QoS support.
Hidden/exposed nodes;
Problematic sensing range.
Differentiated
Distributed
Coordination
Function (DDCF)
Both Single Yes Omni No QoS support. Same as 802.11.
Received-Based
AutoRate (RBAR)
Both Single Yes Omni No Rate adaptive; Improve
throughput over 802.11.
Computation overhead.
Mobile point
coordinator-MAC
( MPC-MAC)
Both Single Yes Omni No Implement PCF in ad hoc
networks; QoS support.
Bottleneck problem; Single
node failure problem;
Overhead.
CA-CDMA Both Multi Need modifications Omni Yes Access control based on the
estimation of channel condition;
No contention between
data/control packets.
Complicated
hardware/software;
Overhead; Exclusive control
channel.
Bidirectional
Multi-channel
MAC (Bi-MCMAC)
Both Multi Need modifications Omni Yes Improve throughput over
802.11; No contention between
data/control packets.
Require additional hardware;
Exclusive control channel.

23. Comparison of Some Classical Protocols
Slide 23
• S-MAC : Listen-sleep
• T-MAC : Activation event
• WiseMAC : Preamble Sampling

24. S-MAC, T-MAC, WiseMAC
• S-MAC
– Main goal –reduce power consumption
– Three major components:
• Periodic sleep-listen
• Collision and overhearing avoidance
• Message passing
– Each node goes to sleep for some time,
and then wakes up and listens to see if any other node wants to talk to it.
– During Sleep it turn off its radio
– Comments:
• Energy waste caused by idle listening is reduced by sleep schedules.
• Sleep and listen periods are predefined and constant which decreases
the efficiency of the algorithm under variable traffic load.
Slide 24

25. S-MAC, T-MAC, WiseMAC
• T-MAC
– Main goal –improvement over S-MAC under variable traffic
– Listen period ends when no activation event has occurred for a time threshold
TA.
– Reduce idle listening by transmitting all messages in bursts of variable length,
and sleeping between bursts.
– times out on hearing nothing.
– Comments:
• Gives better result under variable load.
• Suffers from early sleeping problem –node goes to sleep when a neighbor
still has messages for it..
Slide 25

26. S-MAC, T-MAC, WiseMAC
• WiseMAC
– All nodes defined to have two communication channels.
• Data channel uses TDMA
• Control channel uses CSMA
– Preamble sampling used to decrease idle listening time.
– Nodes sample the medium periodically to see if any data is going to arrive.
– Comments:
• Dynamic preamble length adjustment results in better performance.
• Conflict when one node starts to send the preamble to a node that is already receiving
another node’s transmission where the preamble sender is not within range.
• Hidden terminal problem
Slide 26

27. Other MACs
Slide 27
• Some other protocols such as TRAMA, SHFIT also faces
various problems
• This makes them not suitable for WBAN specially for in-body
communication.
• TRAMA
– Higher percentage of sleep time and less collision probability is achieved
compared to CSMA based protocols. Without considering the
• transmissions and receptions, the duty cycle is at least 12.5 %, which is a
• considerably high value.
– Increased idle listening caused by listening to all slots before sending.
• SHIFT
– Very low latency is achieved with many traffic sources.
– System-wide time synchronization is needed for slotted contention
windows.

28. Further Comparisons
Slide 28
Table 6: Comparison of MAC protocols [7]
Time Sync
Needed
Comm.
Pattern
Support
Type Adaptivity to
Changes
S-MAC/T-
MAC/DSMA
C
No All CSMA Good
WiseMAC No All np-CSMA Good
TRAMA Yes All TDMA/CSMA Good
SIFT No All CSMA/CA Good
DMAC Yes Convergecast TDMA/slotted
ALOHA
Week

29. Conclusion
Slide 29
• Various kinds of power saving mechanisms for MAC
protocols are applied for sensor network applications.
• All the mechanisms have a common design objective - to
maximize network lifetime.
• A single method cannot satisfy the requirements for
WBAN.
• Hence we recommend that a hybrid approach is most
suitable in the diverse scenario of WBAN.
• TDMA based protocols are suitable for in-body scenario
and CSMA based protocols are suitable for on/out body
operations.

30. References
Slide 30
[1] Miller, M. J. and Vaidya, N. H., Power Save Mechanisms for Multi-Hop Wireless
Networks. In Proceedings of the First international Conference on Broadband Networks,
BROADNETS, 2004.
[2] Vivek Jain, Ratnabali Biswas, Dharma P. Agrawal: Energy-Efficient and Reliable
Medium Access in Sensor Networks. WOWMOM 2007: 1-8.
[3]http://nesl.ee.ucla.edu/courses/ee206a/2002s/student_presentations/SP03_QianHuang.ppt
[4] Hind Chebbo, Literature Review of Energy Efficient MAC in WSN/BAN, IEEE, May
2008.
[5] M. J. Miller and N. H. Vaidya, “A MAC Protocol to Reduce Sensor Network Energy
Consumption Using a Wakeup Radio”, IEEE Transactions on Mobile Computing, 4, 3,
May/June 2005.
[6] Wu, M., A Survey of MAC Protocols in Ad Hoc Networks, University of Texas at Dallas.
[7] Demirkol I, Ersoy C, Alagoz F., MAC protocols for wireless sensor networks: A survey.
IEEE Commun 2006;44(4):115-21

31. The End
Slide 31
Thank You