Department of Electrical and Computer Engineering

S-MAC cont’d
• Adaptive listening – To resolve sleep latency – Potential delay on every hop! • Basic idea – Let the nodes who overhear its neighbor’s transmission (ideally RTS or CTS) go to sleep and wake up for a short period at the endAC cont’d
• Additional collision avoidance – Record transmission time as NAV (Network Allocation Vector) – Check if NAV = 0 before transmission attempt – NAV = 0 indicates ongoing transmission is over
• Flat topology – Virtual cluster comprises nodes with common schedule – No coordination from cluster head
Accommodate both schedules
Schedule 1
Schedule 2
• General view on MAC protocol in WSN – Requirements – Problems • MAC protocols for WSN • Conclusion
General Purposes of MAC Protocol
• MAC protocol is to ensure that the channel can be accessed by multiple users, dealing with the situation of interference. • Has a direct bearing on how reliably and efficiently data can be transmitted
S-MAC cont’d
• Synchronization period – Nodes exchange schedules by sending SYNC packets to immediate neighbors at their scheduled listen time
S-MAC cont’d
S-MAC cont’d
• Cons: – Sleep latency – Active time must be long enough to handle expected highest load, inefficient when load is lower. • Essentially S-MAC trades energy with latency
overhearing
Tx
data
Rx
overhearing
Traffic Pattern
• Local broadcast – Schedule exchange/update between neighbors – Omni-directional transmission is desired • Nodes to sink report – Payload and signaling – In favor of directional transmission
S-MAC cont’d
– New schedule received by node A • Discard current schedule if node A has no other neighbor or • Adopt both schedules if node A has other neighbors Accommodate both schedules
S-MAC cont’d
• Coordinating sleeping – listen for a fixed amount of time • Following the existing schedule or • Establishing new schedule if no schedule exists and announcing new schedule by SYNC
Requirement of WSN
• Application-level performance is the goal as opposed to per-node fairness • Long battery life • Delay – Surveillance, Low traffic, Regular update – Emergency, Quick response, Bursty heavy load
T-MAC cont’d
Active Sleep Active Sleep Active Sleep
TA Frame 1
TA Frame 2
TA Frame 3
• An active period ends when no activation event has occurred for a time TA
S-MAC cont’d
• Message passing – Divide long messages into small fragments – Transmit fragments in a burst through reserved channel
S-MAC cont’d
• Features: – Loose synchronized due to large scale of intervals, no need for precise synchronization – Virtual cluster – Adaptive listen to reduce sleep delay – Message passing
T-MAC (Timeout-MAC)
• Minimize idle listening • Using timeout to be adaptive to traffic during wakeup period • Transmitting all messages in burst of variable length • RTS/CTS provides both collision avoidance and reliable transmission
T-MAC cont’d
• Definition of activation event: – The firing of a periodic frame timer (the beginning) – The reception of any data on the radio – The sensing of traffic (during collision) – The end-of-transmission of a node’s own data packet or acknowledgement – End of neighbor’s transmission (knowledge from prior overhearing)
Classification
• Basic idea – Control the active time of radio • Control the times of on-off switches • Scheduling: time-slotted system, wake-up by schedule, clock shift can be disastrous • LPL (Low Power Listening): preamble sampling, wake-up tone
Overhearing
• Receiving packets that are not destined to the node • Interception, waste of energy in receiving, error responding will cause potential collision
Requirement of WSN cont’d
• Adaptive to changing topology – Nodes die out – Mobile nodes • Applicable with limited computing capability
MAC Protocol for WSN
T-MAC cont’d
• Node successfully transmitting 3 packets
T-MAC cont’d
• Determining TA – RTS starts transmission, the TA should be at least long enough to hear the CTS – TA > RTS + CTS + Turn_around_Time
Frame
S-MAC cont’d
Wake-up sleep
Frame
– Total frame length is limited by latency requirement, buffer space, and active time – Active time depends on message rate; fragmentation is used while transmitting large messages
Energy Efficient MAC Protocol
for Wireless Sensor Network