1. Medium Access Control Protocols Lecture 7 (Lecture material contributed by K. Langendoen(TUDelft) and W. Ye(USC/ISI)) September 23, EENG 460a / CPSC 436 / ENAS 960 Networked Embedded Systems & Sensor Networks
Andreas Savvides
Office: AKW 212
Tel
Course Website

2. Protocol stack Data link layer: mapping network packets  radio frames
OSI
Network
Layer 3
Data link layer:
mapping network packets  radio frames
transmission and reception of frames over the air
error control
security (encryption)
Data Link
Layer 2
MAC Protocol
Physical
Layer 1

3. Medium Access Control
Control access to the shared medium (radio channel)
avoid interference between transmissions
mitigate effects of collisions (retransmit)
Approaches
contention-based: no coordination
schedule-based: central authority (access point)
proactive vs. reactive

4. Collision-based MAC protocols
ALOHA :
packet radio networks
send when ready
18-35% channel utilization
CSMA (Carrier Sense Multiple Access):
“listen before talk”
50-80% channel utilization

5. Hidden terminal problemC cs
Time cs
Carrier sense at sender
may not prevent
collision at receiver
DATA
DATA

6. CSMA/CA: Collision AvoidanceB C
MACA:
Request To Send
Clear To Send
DATA
MACAW (Wireless)
additional ACK cs
Time
RTS
CTS
Blocked
DATA
MACA: Medium Access with Collision Avoidance [Phil Karn, 1990]
ACK

7. Exposed terminal problemC D cs
Time
RTS
Parallel CSMA transfers
are synchronized by
CSMA/CA
Collision avoidance can
be too restrictive!
CTS
Blocked
DATA
ACK

8. IEEE 802.11 Operation infrastructure mode (access point) ad-hoc mode
Power save mechanism; not for multi-hop networks
Protocol
carrier sense
collision avoidance (optional)

9. IEEE 802.11 Network Allocation Vector (NAV) collision avoidance
RTS
DATA
Contention Window
Sender
Receiver
Others
NAV(RTS)
NAV(CTS)
SIFS
DIFS
CTS
ACK
Network Allocation Vector (NAV)
collision avoidance
overhearing avoidance: other nodes may sleep

10. Schedule-based MAC protocols
Communication is scheduled in advance
no contention
no overhearing
support for delay-bound traffic (voice)
Time-Division Multiple Access
time is divided into slotted frames
access point broadcasts schedule
coordination between cells required

11. TDMA Typical WLAN setup no direct communication between nodesCP
Frame n
Frame n+2
Frame n+1
downlink
uplink
Typical WLAN setup
no direct communication between nodes
access point broadcast Traffic Control (TC) map
(new) nodes signal needs in Contention Period (CP)

12. Requirements for Sensor Networks
Handle scarce resources
CPU: 1 – 10 MHz
memory: 2 – 4 KB RAM
radio: ~100 Kbps
energy: small batteries
Unattended operation
plug & play, robustness
long lifetime

13. Sensor Node Energy Roadmap (DARPA)
10,000
1,000
100 10 1
Deployed (5W)
-Rehosting to Low Power COTS
(10x)
-Simple Power Management
-Algorithm Optimization
-System-On-Chip
-Adv Power Management
-Algorithms
(50x)
PAC/C Baseline (.5W)
Average Power (mW)
(50 mW)
(1mW)
software does it!
[Srivastava:2002]

14. Energy consumption (mW)25 20 15 10 5
Sleep
5 MHz
1 MHz
LED
Light
Transmit
Receive
Standby
Compass
Accelerometer
Transceiver
Processor
Sensors
LED
[Hoesel:2004]

15. Energy-Efficient MAC Design
Power save (PS) mode in IEEE DCF
Assumption: all nodes are synchronized and can hear each other (single hop)
Nodes in PS mode periodically listen for beacons & ATIMs (ad hoc traffic indication messages)
Beacon: timing and physical layer parameters
All nodes participate in periodic beacon generation
ATIM: tell nodes in PS mode to stay awake for Rx
ATIM follows a beacon sent/received
Unicast ATIM needs acknowledgement
Broadcast ATIM wakes up all nodes — no ACK

16. Energy-Efficient MAC Design
Unicast example of PS mode in DCF

17. Communication patterns
local gossip
WSN applications:
local collaboration when detecting a physical phenomenon
periodic reporting to sink
Characteristics
low data rates
small messages
fluctuations (in time and space)
convergecast
<1000 bps
~25 bytes
[Kulkarni:2004]

18. Design guidelines switch radio off when possible (duty cycle)
AND, minimize number of switches
low complexity (memory footprint)
trade off performance for energy
optimize for traffic patterns

19. Energy-efficient medium access control
Performance/Cost trade-off
latency
throughput
fairness
energy consumption
Organizational/Flexibility trade-off
contention-based
schedule-based

20. Sources of overhead
idle listening (to handle potentially incoming messages)
collisions (wasted resources at sender and receivers)
overhearing (communication between neighbors)
protocol overhead (headers and signaling)
traffic fluctuations (overprovisioning and/or collapse)
scalability/mobility (additional provisions)

21. Contention-based vs. Schedule-based
source of overhead
performance
(latency, throughput, fairness)
cost
(energy-efficiency)
idle listening C
collisions
overhearing
protocol overhead
C,S
traffic fluctuations
scalability/mobility S

22. Energy-efficient MAC protocols
WSN-specific protocols
starting from 2000 (1 paper)
exponential growth (2004, 16+ papers)
Classification (up to May 2004, 20 papers)
the number of channels used
the degree of organization between nodes
the way in which a node is notified of an incoming msg

23. Protocol classification
Channels
Organization
Notification
2000
SMACS [34]
FDMA
frames
schedule
2001
PACT [28]
single
PicoRadio [10]
CDMA+tone
random
wakeup
2002
STEM [33]
data+ctrl
Preamble sampling [6]
listening
Arisha [2]
S-MAC [36]
slots
PCM [18]
Low Power Listening [13]

24. Protocol classification
2003
Sift [17]
single
random
listening
EMACs [15]
frames
schedule
T-MAC [5]
slots
TRAMA [30]
WiseMAC [7]
2004
BMA [24]
Miller [27]
data+tone
wakeup+list
DMAC [26]
SS-TDMA [23]
LMAC [14]
B-MAC [29]

25. Latency Fairness Energy Case Study: S-MAC
S-MAC — by Ye, Heidemann and Estrin
Tradeoffs
Major components in S-MAC
Periodic listen and sleep
Collision avoidance
Overhearing avoidance
Massage passing
Latency
Fairness
Energy
We call our protocol Sensor-MAC or S-MAC. These are the major tradeoffs in S-MAC. We sacrifice the latency and fairness to gain better energy efficiency.
S-MAC includes the following major components : periodic listen and sleep, collision avoidance, overhearing avoidance, and message passing.
I’m going to talk about each of them in details.

26. Latency Energy Coordinated Sleeping
Problem: Idle listening consumes significant energy
Solution: Periodic listen and sleep
sleep
listen
Turn off radio when sleeping
Reduce duty cycle to ~ 10% (120ms on/1.2s off)
As we said, idle listening is a big problem. Listening consumes significant amount of energy.
Our solution is to put nodes into periodic sleep state. After sleeping for some time, each node wakes up and listens to see if anyone wants to talk to it. If yes, it will stay awake. If no, it will go to sleep again. During the sleep time, the node turns off its radio. In our implementation , we have reduced the node duty cycle to about 10%, which is listening for 200 milliseconds and sleeping for 2 seconds.
The major tradeoff here is the latency vs. energy savings. Latency is increased due to the periodic sleep.
Latency
Energy

27. Coordinated Sleeping Schedules can differ Node 1 Node 2 Schedule 1
listen
Prefer neighboring nodes have same schedule
— easy broadcast & low control overhead
Schedule 2
Schedule 1
Before nodes perform periodic listen and sleep, they need to choose a schedule about when to listen and when to sleep.
This figure shows that even if two nodes have different schedules, they can still talk to each other as long as they know each others’ schedules. For example, if node 1 wants to talk to node 2, it just wait until node 2 is listening. However, we prefer neighboring nodes to have the same schedule, so that it’s easy to do broadcast and the control overhead is low.
But in a large network, we cannot guarantee that all nodes follow the same schedule. For example, in this figure, there are two different schedules on each side. The node on the border will follow both schedules. When it broadcasts a packet, it needs to do it twice, first for nodes on schedule 1 and then for those on schedule 2.
Border nodes:
two schedules or
broadcast twice

28. Coordinated Sleeping Schedule Synchronization
New node tries to follow an existing schedule
Remember neighbors’ schedules
— to know when to send to them
Each node broadcasts its schedule every few periods of sleeping and listening
Re-sync when receiving a schedule update
Periodic neighbor discovery
Keep awake in a full sync interval over long periods
Here are some procedures for synchronization on schedules.
Nodes need to remember their neighbors’ schedules so that they know when to send to each other.
Each node periodically broadcasts its schedule and re-synchronizes on a neighbor’s schedule when receiving an update. This prevents long-term clock drift.
The schedule packets also serve as beacons for new nodes to join a neighborhood.

29. Coordinated Sleeping t1 t2 Adaptive listening
Reduce multi-hop latency due to periodic sleep
Wake up for a short period of time at end of each transmission 1 2 3 4
RTS
CTS
CTS t1 t2
listen
listen
Reduce latency by at least half

30. Collision Avoidance S-MAC is based on contention
Similar to IEEE ad hoc mode (DCF)
Physical and virtual carrier sense
Randomized backoff time
RTS/CTS for hidden terminal problem
RTS/CTS/DATA/ACK sequence
The second component in S-MAC is collision avoidance. If multiple senders want to talk to the same receiver, they need to avoid collisions. We argue that contention-based protocols have better scalability in node density than TDMA protocols, and S-MAC is contention based. The collision avoidance procedure is similar to that in ad hoc mode. That is, RTS/CTS/DATA/ACK sequence.

31. Overhearing Avoidance
Problem: Receive packets destined to others
Solution: Sleep when neighbors talk
Basic idea from PAMAS (Singh, Raghavendra 1998)
But we only use in-channel signaling
Who should sleep?
All immediate neighbors of sender and receiver
How long to sleep?
The duration field in each packet informs other nodes the sleep interval
The third component in S-MAC is overhearing avoidance. Receiving packets destined to other nodes is a waste of energy. The basic solution is to put a node into sleep when its neighbors are talking. This idea is from PAMAS. PAMAS uses a second control channel to achieve the goal. In our solution, we only use in-channel signaling.
To appropriately put nodes into sleep, we need to answer two questions.
The first is, who should sleep. The short answer is, all immediate neighbors of the sender and receiver should go to sleep.
The second question is, how long for them to sleep. In S-MAC, each packet has a duration field, which is the remaining time that is needed for current transmission. If a node receives any packet from its neighbor, it will learn from the duration field about how long it should sleep.

32. Energy Fairness Msg-level latency Message Passing
Problem: Sensor net in-network processing requires entire message
Solution: Don’t interleave different messages
Long message is fragmented & sent in burst
RTS/CTS reserve medium for entire message
Fragment-level error recovery — ACK
— extend Tx time and re-transmit immediately
Other nodes sleep for whole message time
The last component in S-MAC is message passing. It is motivated by the in-network data processing, which requires efficient transmission of a meaningful unit of message, which can be quite long.
Our approach is to fragment a long message into short ones, and transmit them in burst. The key is that do not interleave the transmission of different messages, since the receiver cannot start data processing if only partial of a message is received. The RTS and CTS reserve medium for the entire message. The receiver will send ACK for each received fragment. If an ACK is not received, the sender will extend transmission time and immediately re-transmit current fragment.
Other nodes will sleep for long time until the whole message transmission is done.
The major tradeoff here is, the fairness vs. energy and message-level latency. It’s unfair for a node with a short message to wait for a long transmission even if there are errors in the middle.
Energy savings is obtained by putting nodes into sleep for long time. Message-level latency can be reduced by not interleaving different messages and by the fast retransmission of erroneous fragments.
Fairness
Energy
Msg-level latency

33. Implementation on Testbed Nodes
Platform
Mica Motes (UC Berkeley)
8-bit CPU at 4MHz,
128KB flash, 4KB RAM
20Kbps radio at 433MHz
TinyOS: event-driven
Configurable S-MAC options
Low duty cycle with adaptive listen
Low duty cycle without adaptive listen
Fully active mode (no periodic sleeping)

34. Experiments: two-hop network
Topology and measured energy consumption on source nodes 2 4 6 8 10
200
400
600
800
1000
1200
1400
1600
1800
Average energy consumption in the source nodes
Message inter-arrival period (second)
Energy consumption (mJ)
like protocol
without sleep
Overhearing
avoidance
S-MAC w/o adaptive listen
Source 1
Source 2
Sink 1
Sink 2
S-MAC consumes much less energy than like protocol w/o sleeping
At heavy load, overhearing avoidance is the major factor in energy savings
At light load, periodic sleeping plays the key role
We used a simple topology in our experiments. It’s a two-hop network with 2 source nodes and two sinks.
In each test, there are 10 messages generated on each source node. Each message has 10 fragments, and each fragment has 40 bytes. We measure the total energy consumption of each node for sending this fixed amount of data.
This is the measured energy consumption on the source nodes. The X-axis indicates the traffic load. It’s denoted by the message inter-arrival time in seconds. For example, the point 2 means that every 2 seconds, there will be a message generated on each source node. So a small value indicates a heavy traffic load. The Y-axis is the energy consumption of the radio in milli-joule.
The red line is the result of the like protocol without sleeping. The blue line is the message passing plus overhearing avoidance. The black one is the complete S-MAC which incorporates the periodic listen and sleep.
We can see that in all cases, the blue line and black line outperform the always listening MAC. When traffic load is high, the blue line and the black line are about the same. In this case, there are few chances to go to periodic sleep, and the energy savings is mainly due to the overhearing avoidance. When traffic becomes lighter, the periodic sleep plays a key role, and makes S-MAC much better than the always-listening MAC.

35. Energy Consumption over Multi-Hops
Ten-hop linear network at different traffic load
3 configurations of S-MAC 2 4 6 8 10 5 15 20 25 30
Message inter-arrival period (S)
Energy consumption (J)
10% duty cycle without adaptive listen
No sleep cycles
10% duty cycle with adaptive listen
Energy consumption at different traffic load
At light traffic load, periodic sleeping has significant energy savings over fully active mode
Adaptive listen saves more at heavy load by reducing latency

36. Latency as Hops Increase
Adaptive listen significantly reduces latency causes by periodic sleeping 2 4 6 8 10 12
Latency under lowest traffic load 2 4 6 8 10 12
Latency under highest traffic load
10% duty cycle without
adaptive listen
10% duty cycle without
adaptive listen
Average message latency (S)
Average message latency (S)
10% duty cycle with
adaptive listen
10% duty cycle with adaptive listen
No sleep cycles
No sleep cycles
Number of hops
Number of hops

37. Throughput as Hops Increase
Adaptive listen significantly increases throughput 2 4 6 8 10 20 40 60 80
100
120
140
160
180
200
220
Effective data throughput under highest traffic load
Using less time to pass the same amount of data
No sleep cycles
Effective data throughput (Byte/S)
10% duty cycle
with adaptive listen
10% duty cycle without adaptive listen
Number of hops

38. Combined Energy and Throughput
Energy-time cost on passing 1-byte data from source to sink 2 4 6 8 10
0.5 1
1.5
2.5 3
Periodic sleeping provides excellent performance at light traffic load
With adaptive listening, S-MAC achieves about the same performance as no-sleep mode at heavy load
No sleep cycles
Energy-time product per byte (J*S/byte)
10% duty cycle without
adaptive listen
10% duty cycle with adaptive listen
Message inter-arrival period (S)

39. IEEE 802.15.4 MAC Protocol Based on an IEEE standard for WPAN
Goal: Ultra-low cost, low power radios
Support multiple configurations (e.g point-to-point, groups, ad-hoc etc)
CSMA-CA based protocol
Each packet can be individually acknowledged
Key features
Three types of node functionalities
PAN Coordinator, Coordinator and Device
Two device types
FFD – Full Function Device
RFD – Reduced Function Device

40. Frequencies and Data Rates
BAND COVERAGE DATA RATE # OF CHANNEL(S)
2.4 GHz ISM Worldwide kbps
868 MHz Europe kbps
915 MHz ISM Americas kbps
See class website for more information about Zigbee
More abut MAC protocols on the next lecture

41. Paper Reading
[Elson02] Fine-Grained Network Time Synchronization using Reference Broadcasts, Jeremy Elson, Lewis Girod and Deborah Estrin In Proceedings of the Fifth Symposium on Operating Systems Design and Implementation (OSDI 2002), Boston, MA. December UCLA Technical Report  
You should all read this paper closely before lecture 9!