1. Wireless Sensor Networks COE 499 Medium Access Control
Tarek Sheltami
KFUPM
CCSE
COE
12/30/2018

2. Outline Traditional MAC protocols Asynchronous sleep techniques
Sleep-scheduled techniques
Contention free protocols
12/30/2018

3. Traditional MAC protocols
ALOHA protocol
CSMA
Hidden terminal problem
Exposed terminal problem
(a) hidden node (b) exposed node
12/30/2018

4. Traditional MAC protocols
ALOHA protocol
CSMA
Hidden terminal problem
Exposed terminal problem
(a) hidden node (b) exposed node
12/30/2018

5. Traditional MAC protocols..
MACA
RTS
CTS
IEEE (CSMA/CA)
Can operate in infrastructure or ad hoc modes
Includes two mechanisms: DCF and PCF
In DCF mode, a sender first checks the medium, if it is busy it waits for DIFS time before transmitting
The receiver of the message sends an ACK upon successful reception after SIFS time
Nodes which overhear RTS/CTS messages record the duration of the entire corresponding DATA-ACK exchange in their NAV (network allocation vector) and defer access during this duration
12/30/2018

6. Traditional MAC protocols..
IEEE (CSMA/CA)..
An exponential backoff is used in case of:
the medium is sensed busy
after each retransmission (in case an ACK is not received)
after a successful transmission
In PCF mode, a central access point coordinates medium access by polling the other nodes for data periodically
It is particularly useful for real-time applications because it can be used to guarantee worst-case delay bounds
12/30/2018

7. Traditional MAC protocols..
IEEE MAC (LR-WPAN)
low-rate wireless personal area networks is used mostly in star topology
A superframe is defined by a periodic beacon signal sent by the PAN coordinator
Within the superframe there is an active phase for communication between nodes and the PAN coordinator and an inactive phase
The active period has 16 slots that consist of three parts: the beacon, a contention access period (CAP), and a collision-free period (CFP) that allows for the allocation of guaranteed time slots (GTS)
12/30/2018

8. Traditional MAC protocols..
IEEE MAC (LR-WPAN)..
Nodes which communicate only on guaranteed time slots can remain asleep and need only wake-up just before their assigned GTS slots
The communication during CAP is a simple CSMA-CA algorithm, which allows for a small backoff period to reduce idle listening energy consumption
12/30/2018

9. Energy efficiency in MAC protocols
Power management in IEEE
Nodes inform the access point (AP) when they wish to enter sleep mode so that any messages for them can be buffered at the AP
The nodes periodically wake-up to check for these buffered messages
Energy savings are thus provided at the expense of lower throughput and higher latency
Power aware medium-access with signaling (PAMAS)
An extension of the MACA technique, where the RTS/CTS signaling is carried out on a separate radio channel from the data exchange
Nodes go to sleep whenever they overhear a neighbor transmitting to another node, or if they determine through the control channel RTS/CTS signaling that one of their neighbors is receiving
12/30/2018

10. Energy efficiency in MAC protocols
Power aware medium-access with signaling (PAMAS)
The duration of the sleep mode is set to the length of the ongoing transmissions indicated by the control signals received on the secondary channel
If a transmission is started while a node is in sleep mode, upon wake-up the node sends probe signals to determine the duration of the ongoing transmission and how long it can go back to sleep
There can be considerable energy wastage in the idle reception mode (i.e. the condition when a node has no packets to send and there is no activity on the channel)
12/30/2018

11. Asynchronous sleep techniques
Secondary wake-up radio
One low power Tx/Rx always on and main Tx/Rx always off
If the wake-up radio of a node receives a wake-up signal from another node, it responds by waking up the primary radio to begin receiving
This ensures that the primary radio is active only when the node has data to send or receive
Low power listening/preamble sampling
The receivers periodically wake-up to sense the channel
If no activity is found, they go back to sleep
If a node wishes to transmit, it sends a preamble signal prior to packet transmission
Upon detecting such a preamble, the receiving node will change to a fully active receive mode
12/30/2018

12. Asynchronous sleep techniques..
Low power listening/preamble sampling..
The wake-up signal could potentially be sent over a high-level packet interface
A more efficient approach is to implement this directly in the physical layer – thus the wake-up signal may be no more than a long RF pulse
The detecting node then only checks for the radio energy on the channel to determine whether the signal is present
This scheme will also potentially wake-up all possible receivers in a given transmitter’s neighborhood
12/30/2018

13. Asynchronous sleep techniques..
Transmitter/receiver-initiated cycle receptions (TICER/RICER)
Similar to lower-power listening/preamble sampling
In the transmitter-initiated cycle receiver technique, the receiver node wakes up periodically to monitor the channel for signals from the sender (which is a wake-up request to send (RTS) signal)
The sender sends a sequence of such RTS signals followed by a short time when it monitors the channel
When the receiver detects an RTS, it responds right away with a CTS signal
If the sender detects a CTS signal in response to its RTS, it begins transmission of the packet
12/30/2018

14. Asynchronous sleep techniques..
Transmitter/receiver-initiated cycle receptions (TICER/RICER)..
In the receiver-initiated cycle receiver technique a receiving node periodically wakes up to execute a three phase monitor–send wake-up beacon–monitor sequence
A source that wishes to transmit wakes up and stays in a monitoring state
When it hears a wake-up beacon from a receiver, it begins transmission of the data
The receiver in a monitor state that sees the start of a data packet remains on until the packet reception is completed
12/30/2018

15. Asynchronous sleep techniques..
Reconfigurable MAC protocol (B-MAC)
Low-power listening (LPL), which implements the preamble-based wake-up technique described above, to permit nodes to have sleep as the default mode, helping to conserve energy. Different channel sampling durations and preamble durations can be selected by the higher layers.
Clear channel assessment (CCA), which determines whether the channel is busy or not by examining multiple adjacent samples and using an appropriate outlier detection technique. If CCA is disabled, a scheduling protocol may be implemented above B-MAC. If it is enabled, the backoff duration (in case a busy channel is detected) may be selected by the higher layer. CCA is used for low-power listening.
Acknowledgements (ACK): If acknowledgements are enabled, a response is sent immediately after receiving any unicast packet.
12/30/2018

16. Asynchronous sleep techniques..
Reconfigurable MAC protocol (B-MAC)
12/30/2018

17. Sleep-scheduled techniques
Sensor MAC (S-MAC)
Designed specifically for WSN
All node wake up and go to sleep simultaneously
During initialization, nodes remain awake and wait a random period to listen for a message providing the sleep–listen schedule of one of their neighbors
If they don’t receive such a message, they become synchronizer nodes, picking their own schedules and broadcasting them to their neighbors
Nodes that hear a neighbor’s schedule adopt that schedule and are called follower nodes
Some boundary nodes may need to either adopt multiple schedules
The nodes periodically transmit these schedules to accommodate any new nodes joining the network
Sleep schedules are not followed during data transmission
12/30/2018

18. Sleep-scheduled techniques
Sensor MAC (S-MAC)..
S-MAC come at the expense of potentially significant sleep latency: a packet travelling across the network will need to pause (every few hops, depending on the settings) during the sleep period of intermediate nodes
12/30/2018

19. Sleep-scheduled techniques..
Timeout MAC (T-MAC)
Very similar to S-MAC, however, The length of each cycle is kept constant, but the end of the active period is determined dynamically by the use of a timeout mechanism
If a receiver does not receive any messages (data or control) during the timeout interval, it goes to sleep
If it receives such a message, the timer starts afresh after the reception of the message
This renewal mechanism allows for easy adaptation to spatio-temporal variations in traffic
The basic T-MAC scheme suffers from the so-called early sleep problem, which can reduce throughput
When a node has to be silent due to contention in a given cycle, it is unable to send any message to its intended receiver to interrupt its timeout
When the sender can send after the end of the contention period, the intended receiver is already in sleep mode
12/30/2018

20. Sleep-scheduled techniques..
Data-gathering MAC (D-MAC)
Proposed to overcome the data-forwarding interruption problem encounter by S-MAC and T-MAC
Applies only to flows on a predetermined data-gathering tree going up from the various network nodes to a common sink
Cycles are aligned so that a node at level k is in the receiving mode when the node below it on the tree at level k+1 is transmitting
To deal with contention and interference, D-MAC also includes optional components referred to as data prediction and the use of more-to-send (MTS) packets
D-MAC in itself is not a general purpose MAC as it applies only to one-way data-gathering trees
12/30/2018

21. Sleep-scheduled techniques..
Data-gathering MAC (D-MAC)..
12/30/2018

22. Sleep-scheduled techniques..
Delay-efficient sleep scheduling (DESS)
Each node picks a unique slot out of k slots to use as a reception slot and publishes this to its neighbors
If node i wishes to transmit to another node j in any cycle, wakes-up at j’s active slot during the cycle to transmit
On a multi-hop path, the delay that a packet will encounter at each hop is then purely a function of the reception slot times of the corresponding two nodes
For a packet to be sent from node i to its adjacent node j, which have reception slots xi,, xj respectively, the delay is given as xi,- xj mod k, and if xi,≠ xj then it is equal to k
The path delay is the sum of all the per-hop delays along a given path
12/30/2018

23. Sleep-scheduled techniques..
Asynchronous sleep schedules
Objective is to design independent sleep–wake schedules for individual nodes that guarantee that the wake-up intervals for neighbors overlap
The design must have a number of active slots that is at least the square root of the total number of slots in the cycle
The wake-up schedule function (WSF) design problem is related to the theory of combinatorial block designs
(T, k, m) symmetric block design is equivalent to a WSF symmetric design that has T slots, with k active slots such that m of them overlap between any two of them
It is shown that if p is a power of a prime, there exists a (p2 +p+1, p+1, 1) design
12/30/2018

24. Sleep-scheduled techniques..
Asynchronous sleep schedules..
A (7,3,1) design for slotted asynchronous wake-up (P2+P+1, P+1, 1)
12/30/2018

25. Contention-free protocols
Stationary MAC and Startup (SMACS)
Each node need only maintain local synchronization
During the starting phase, each node decides on a common communication slot with a neighboring node through handshaking on a common control channel
Each link also utilizes a unique randomly chosen frequency or CDMA frequency hopping code
It is assumed that there are sufficiently many frequencies/codes (no contention)
The slot is then used periodically, once each cycle, for communication between the two nodes.
12/30/2018

26. Contention-free protocols
BFS/DFS-based scheduling
BFS, each node gets contiguous time slots
DFS, each node does not have contiguous slots, but the slots from each sensor source to the sink are contiguous, ensuring that intermediate node buffers are not filled up during data-gathering.
12/30/2018

27. Contention-free protocols..
Reservation-based synchronized MAC (ReSync)
Each node choose a unique time slot of k and publishes this to its neighbors
Traffic-adaptive medium access (TRAMA)
A distributed TDMA technique that allows for flexible and dynamic scheduling of time slots
Time epochs are divided into a set of short signaling slots, followed by a set of longer transmission slots
Neighbor protocol (NP)
Schedule exchange protocol (SEP)
Adaptive election algorithm (AEA)
12/30/2018