1. Medium Access Control and WPAN Technologies
Wireless Sensor Networks and Internet of Things
Chaiporn Jaikaeo
Department of Computer Engineering Kasetsart University
Materials taken from lecture slides by Karl and Willig
Cliparts taken from openclipart.org
Last updated:

2. Overview Principal options and difficulties Contention-based protocols
Schedule-based protocols
Wireless Personal Area Networks Technologies

3. Difficulties
Medium access in wireless networks is difficult, mainly because of
Half-duplex communication
High error rates
Requirements
As usual: high throughput, low overhead, low error rates, …
Additionally: energy-efficient, handle switched off devices!

4. Energy-Efficient MAC: Requirements
Recall
Transmissions are costly
Receiving about as expensive as transmitting
Idling can be cheaper but is still expensive
Energy problems
Collisions
Overhearing
Idle listening
Protocol overhead
Always wanted: Low complexity solution

5. Wireless medium access
Main Options
Wireless medium access
Centralized
Distributed
Schedule- based
Contention- based
Schedule- based
Contention- based
Fixed assignment
Demand assignment
Fixed assignment
Demand assignment

6. Centralized Medium Access
A central station controls when a node may access the medium
E.g., Polling, computing TDMA schedules
Advantage: Simple, efficient
Not directly feasible for non-trivial wireless network sizes
But: Can be quite useful when network is somehow divided into smaller groups
Distributed approach still preferable

7. Schedule- vs. Contention-Based
Schedule-based protocols
FDMA, TDMA, CDMA
Schedule can be fixed or computed on demand
Usually mixed
Collisions, overhearing, idle listening no issues
Time synchronization needed
Contention-based protocols
Hope: coordination overhead can be saved
Mechanisms to handle/reduce probability/impact of collisions required
Randomization used somehow

8. Overview Principal options and difficulties Contention-based protocols
Schedule-based protocols
Wireless Personal Area Networks Technologies

9. Distributed, Contention-Based MAC
Basic ideas
Receivers need to tell surrounding nodes to shut up
Listen before talk (CSMA)
Suffers from sender not knowing what is going on at receiver
Hidden terminal scenario:
Also: recall exposed terminal scenario A B C D

10. How To Shut Up Senders Inform potential interferers during reception
Cannot use the same channel
So use a different one
Busy tone protocol
Inform potential interferers before reception
Can use same channel
Receiver itself needs to be informed, by sender, about impending transmission
Potential interferers need to be aware of such information, need to store it

11. MACA Multiple Access with Collision Avoidance
Sender B issues Request to Send (RTS)
Receiver C agrees with Clear to Send (CTS)
Potential interferers learns from RTS/CTS
B sends, C acks
Used in IEEE

12. Virtual Carrier SensingB C D
RTS
CTS
NAV
NAV
Data
ACK
NAV
 Network Allocation Vector (Virtual Carrier Sensing)

13. Problems Solved?
RTS/CTS helps, but do not solve hidden/exposed terminal problems

14. MACA Problem: Idle listening
Need to sense carrier for RTS or CTS packets
Simple sleeping will break the protocol
IEEE solution
Idea: Nodes that have data buffered for receivers send traffic indicators at prearranged points in time
ATIM - Announcement Traffic Indication Message
Receivers need to wake up at these points, but can sleep otherwise
ATIM – Announcement Traffic Indicator Message

15. Sensor-MAC (S-MAC) MACA unsuitable if average data rate is low
Most of the time, nothing happens
Idea: Switch off, ensure that neighboring nodes turn on simultaneously to allow packet exchange
Need to also exchange wakeup schedule between neighbors
When awake, perform RTS/CTS

16. Schedule Assignment Synchronizer Follower Listen for a mount of time
If hear no SYNC, select its own SYNC
Broadcasts its SYNC immediately
Follower
Listen for amount of time
Hear SYNC from A, follow A’s SYNC
Rebroadcasts SYNC after random delay td
Listen A
Listen for SYNC
Go to sleep after time t
Sleep
Broadcasts B
Listen
Go to sleep after time t- td
Sleep td
Broadcasts

17. S-MAC Synchronized Islands
Nodes learn schedule from other nodes
Some node might learn about two different schedules from different nodes
“Synchronized islands”
To bridge this gap, it has to follow both schemes A A A A A A B B B B B E E E E E E E C D C C C C
Time D D D

18. Preamble Sampling
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: B-MAC, WiseMAC, LoRa
Start transmission:
Das gibt eine SEHR schöne Übungsaufgabe!
Long preamble
Actual packet
Check channel
Stay awake!

19. B-MAC Very simple MAC protocol Employs
Clear Channel Assessment (CCA) and backoffs for channel arbitration
Link-layer acknowledgement for reliability
Low-power listening (LPL)
I.e., preamble sampling
Currently: Often considered as the default WSN MAC protocol

20. B-MAC B-MAC does not have Synchronization RTS/CTS
Results in simpler, leaner implementation
Clean and simple interface

21. Clear Channel Assessment
"Carrier Sensing" in wireless networks
Thresholding CCA algorithm
Outlier detection CCA algorithm

22. Overview Principal options and difficulties Contention-based protocols
Schedule-based protocols
Wireless Personal Area Networks Technologies

23. LEACH Low-Energy Adaptive Clustering Hierarchy Assumptions
Dense network of nodes
Direct communication with central sink
Time synchronization
Idea: Group nodes into “clusters”
Each cluster controlled by clusterhead
About 5% of nodes become clusterhead (depends on scenario)
Role of clusterhead is rotated

24. LEACH Clusterhead Each CH organizes In steady state operation
CDMA code for its cluster
TDMA schedule to be used within a cluster
In steady state operation
CHs collect & aggregate data from all cluster members
Report aggregated data to sink using CDMA

25. LEACH rounds

26. Scheduled-Access Period
TRAMA
Traffic Adaptive Medium Access Protocol
Assume nodes are time synchronized
Time divided into cycles, divided into
Random access period
Scheduled access period
time cycle
Random Access Period
Scheduled-Access Period
Exchange and learn two-hop neighbors
Exchange schedules
Used by winning nodes to transmit data

27. TRAMA – Adaptive Election
How to decide which slot (in scheduled access period) a node can use?
For node id x and time slot t, compute p = h (x  t)
h is a global hash function
Compute p for next k time slots for itself and all two-hop neighbors
Node uses those time slots for which it has the highest priority
t = 0
t = 1
t = 2
t=3
t = 4
t = 5 A 14 23 9 56 3 26 B 33 64 8 12 44 6 C 53 18 57 2

28. Overview Principal options and difficulties Contention-based protocols
Schedule-based protocols
Wireless Personal Area Networks Technologies

29. IEEE 802.15.4 IEEE standard for low-rate WPAN (LR-WPAN) applications
Low-to-medium bit rates
Moderate delays without too strict requirements
Low energy consumption
Physical layer
20 kbps over MHz
40 kbps over – 928 MHz
250 kbps over GHz
MAC protocol
Single channel at any one time
Combines contention-based and schedule-based schemes
Asymmetric: nodes can assume different roles

30. 802.15.4 PHY Overview Operating frequency bands 868MHz / 915MHz PHY
Channel 0
Channels 1-10
868MHz / 915MHz
PHY
2 MHz
868.3 MHz
902 MHz
928 MHz
2.4 GHz
PHY
Channels 11-26
5 MHz
2.4 GHz
GHz

31. 802.15.4 Device Classes Full function device (FFD)
Any topology
Network coordinator capable
Talks to any other device
Reduced function device (RFD)
Limited to star topology
Cannot become a network coordinator
Talks only to a network coordinator
Very simple implementation

32. Network Topologies

33. 802.15.4 Beaconed Mode Superframe structure
GTS assigned to devices upon request

34. 802.15.4 GTS Data Transfer Device  coordinator Coordinator  device
If having allocated GTS, wake up and send
Otherwise, send during CAP
Using slotted CSMA
Coordinator  device
If having allocated GTS, wake up and receive
Otherwise, see picture

35. IEEE 802.15.4 Adopters ZigBee Nest (acquired by Google)
Requires battery life of at least two years be certified
Applications: Industrial control, embedded sensing, home automation
ZigBee RF4CE (Radio Frequency for Consumer Electronics)
Nest (acquired by Google)
Learning thermostats, Smoke and CO alarms
WiFi- and ZigBee-enabled

36. Bluetooth Smart Formally Bluetooth Low Energy (BLE)
Part of Bluetooth 4.0 Specification
Based on Nokia's Wibree technology
First smartphones to support  iPhone 4S
Now supported by most recent smartphones

37. Bluetooth: Classic vs. Smart
Source: Bluetooth SIG

38. Bluetooth Compatibility

39. Bluetooth Smart: Device Roles
Central device
Serves as a hub to one or more peripheral devices
Two central devices cannot directly communicate
Similar to IEEE 's FFD
Peripheral device
Must be connected to a central device
Two peripheral devices cannot directly communicate
Similar to IEEE 's RFD

40. ANT / ANT+ / NIKE+ Primarily used for fitness monitoring devices
open access multicast wireless sensor network
NIKE+
Proprietary protocols on 2.4 GHz band
Nike.com

41. WiFi/ZigBee/Bluetooth Coexistence
They all employ 2.4 GHz spectrum
WiFi vs. Zigbee
WiFi vs. Bluetooth

42. Summary
Many different ideas exist for medium access control in MANET/WSN
Comparing their performance and suitability is difficult
Especially, clearly identifying interdependencies between MAC protocol and other layers/applications is difficult
Which is the best MAC for which application?
Nonetheless, certain “common use cases” exist
IEEE DCF for MANET
IEEE for some early “commercial” WSN variants
B-MAC for WSN research not focusing on MAC