1. An Energy-Efficient MAC Protocol for Wireless Sensor Networks
Wei Ye1, John Heidemann1, Deborah Estrin2
1USC Information Sciences Institute
2UCLA and USC/ISI
Ladies and gentlemen,
It’s my pleasure to have this opportunity to present our work here: an energy- efficient MAC protocol for wireless sensor networks.
IEEE INFOCOM 2002

2. Introduction Wireless sensor network Sensor-net applications
Special ad hoc wireless network
Large number of nodes w/ sensors & actuators
Battery-powered nodes energy efficiency
Unplanned deployment self-organization
Node density & topology change robustness
Sensor-net applications
Nodes cooperate for a common task
In-network data processing
Wireless sensor network is a special type of ad hoc wireless network, having large number of nodes equipped with sensors and actuators. Most nodes are likely to be battery-powered. So energy-efficiency is one of the most important issues in system design. We expect to deploy the sensor-net in an environment without careful planning. So the nodes need to do self-configuration and self-organization. In sensor-net, the node density and topology change over time, which requires sensor-net protocols to be scalable and robust.
Sensor-net applications are also different with that in traditional networks. In sensor-net, nodes cooperate for a common task. So fairness among individual nodes is not very important. Sensor-net nodes will do some in-network data processing instead of simply forwarding the packets.
Our MAC protocol design is based on the observations of these special properties of sensor-net.
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3. Medium Access Control in Sensor Nets
Important attributes of MAC protocols
Collision avoidance
Energy efficiency
Scalability in node density
Latency
Fairness
Throughput
Bandwidth utilization
Primary
Secondary
Now we look at the important attributes of a MAC protocol in the context of sensor networks.
The first is collision avoidance. It’s the basic task of medium access control. The second is energy efficiency, which significantly affects the network lifetime. And then the scalability in node density, latency, fairness, throughput and bandwidth utilization. In our point of view, the first three ones are the most important attributes and will be the primary concern in our protocol design. We’re going to make tradeoffs on secondary attributes to improve the performance of the primary ones.
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4. Energy Efficiency in MAC
Major sources of energy waste
Idle listening
Energy consumption of typical WLAN cards
idle:receive — 1:1.05 to 1:2 (Stemm 1997)
Example: directed diffusion (Intanagonwiwat 2000)
0.02
0.04
0.06
0.08
0.1
0.12
0.14 50
100
150
200
250
300
Average Dissipated Energy
(Joules/Node/Received Event)
Network Size
Diffusion
Omniscient Multicast
Flooding
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.016
0.018
Over always-listening MAC
Over energy-aware MAC
Since one of our main goals is to achieve energy efficiency, we need to investigate what causes the energy waste.
The first source is idle listening, meaning a node keeps listening just to receive possible packets. In idle state, the radio still consumes significant amount of energy. Some researchers have shown that on some typical wireless LAN cards, the ratio of energy consumption of idle and receive is from 1 to 1.05 to 1 to 2.
Another example shows that the energy efficiency in MAC significantly affect the performance of other protocols. Here’s the energy consumption of directed diffusion running over different MACs. Directed diffusion is another piece of work in our research group. In the left figure, it uses an always-listening MAC. There’s no benefits of energy savings in directed diffusion, since energy consumption is dominated by idle listening. If running over an energy-aware MAC, diffusion has much better performance than other protocols.
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5. Energy Efficiency in MAC
Major sources of energy waste (cont.)
Idle listening
Long idle time when no sensing event happens
Dominant in sensor nets
Collisions
Control overhead
Overhearing
We try to reduce energy consumption from all above sources
Combine benefits of TDMA + contention protocols
Common to all wireless networks
As we’ve seen, idle listening is a dominant factor of energy waste in sensor networks, because nodes are in idle state for long time if no sensing event happens.
There are other sources of energy waste, which are common to all wireless networks. They are: collisions, control overhead and overhearing. Overhearing means receiving packets destined to other nodes.
In our MAC design, we’re trying to reduce energy consumption from all these sources, and we try to combine the benefits of both TDMA and contention protocols.
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6. Sensor-MAC (S-MAC) Design
Tradeoffs
Latency
Fairness
Energy
Major components in S-MAC
Periodic listen and sleep
Collision avoidance
Overhearing avoidance
Message passing
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.
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7. Periodic Listen and Sleep
Problem: Idle listening consumes significant energy
Solution: Periodic listen and sleep
sleep
listen
Turn off radio when sleeping
Reduce duty cycle to ~ 10% (200ms on/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
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8. Periodic Listen and Sleep
Schedules can differ
Node 1
Node 2
sleep
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
broadcast twice
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9. Periodic Listen and Sleep
Schedule Synchronization
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
Schedule packets also serve as beacons for new nodes to join a neighborhood
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.
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10. Collision Avoidance Problem: Multiple senders want to talk
Options: Contention vs. TDMA
Solution: 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.
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11. 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.
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12. 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
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13. Msg Passing vs. 802.11 fragmentation
S-MAC message passing
RTS 21
...
Data 19
ACK 18
CTS 20
Data 17
ACK 16
Data 1
ACK 0
Fragmentation in IEEE
No indication of entire time — other nodes keep listening
If ACK is not received, give up Tx — fairness
Here is an example of message passing. The RTS reserves time for CTS and all subsequent data and ACK packets. The CTS reserves time for all subsequent data and ACK packets, and so as each data and ACK packet. This way, if a neighbor receives any of these packets, it knows how long it should sleep.
As a comparison, we also look at the fragmentation scheme in The RTS reserves time for CTS and the first data and ACK. The first data and ACK reserves time for the second data and ACK, and so on so forth. If an ACK is not received, the sender has to give up transmission and a new contention will start to give other nodes a chance to send. Clearly, this is the promotion of fairness. But other nodes need to keep monitoring the channel, which means no sleep for them. In comparison, message passing is more energy efficient.
RTS 3
...
Data 3
ACK 2
CTS 2
Data 1
ACK 0
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14. Implementation on Testbed Nodes
Platform
Motes (UC Berkeley)
8-bit CPU at 4MHz,
8KB flash, 512B RAM
916MHz radio
TinyOS: event-driven
We have implemented S-MAC on Motes developed by UC Berkeley, which has a microcontroller and a radio at 916MHz. It runs an event-driven TinyOS.
We have compared three MAC modules. The first is an like protocol without sleeping. The second module includes message passing and overhearing avoidance. The last one is the complete S-MAC, which is basically module 2 plus the periodic listen and sleep.
Compared MAC modules
IEEE like protocol w/o sleeping
Message passing with overhearing avoidance
S-MAC (2 + periodic listen/sleep)
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15. Experiments Topology and measured energy consumption on source nodes
Sink 1
Sink 2
Each source node sends 10 messages
— Each message has 400B in 10 fragments
Measure total energy over time to send all messages
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.
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16. Conclusions
S-MAC offers significant energy efficiency over always-listening MAC protocols
Future Plans
Measurement of throughput and latency
Throughput reduces due to latency, contention, control overhead and channel noise
Experiments on large testbeds
~100 Motes, ~30 embedded PCs w/ MoteNIC
URL:
Thank You!
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