Medium Access Control and WPAN Technologies 01204525 Wireless Sensor Networks and Internet of Things Chaiporn Ja Jaikaeo (c (chaiporn.j@ku.ac.th) Department of f Computer Engineering Kasetsart University Materials taken from lecture slides by Karl and Willig Last updated: 2018-11-17 Cliparts taken from openclipart.org
Overview • Principal options and difficulties • Contention-based protocols • Schedule-based protocols • Wireless Personal Area Networks Technologies 2
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! 3
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 4
Main Options Wireless medium access Centralized Distributed Schedule- Contention- Schedule- Contention- based based based based Fixed Demand Fixed Demand assignment assignment assignment assignment 5
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 6
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 7
Overview • Principal options and difficulties • Contention-based protocols • Schedule-based protocols • Wireless Personal Area Networks Technologies 8
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 Also: recall scenario: exposed terminal scenario A B C D 9
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 10
MACA A B C D • M ultiple A ccess with C ollision A voidance RTS • Sender B issues Request to Send (RTS) CTS • Receiver C agrees with Clear to Send ( CTS ) Data • Potential interferers learns NAV indicates from RTS/CTS busy medium NAV indicates • B sends, C acks busy medium • Used in IEEE 802.11 Ack 11
Virtual Carrier Sensing A B C D RTS CTS NAV Data NAV ACK NAV → Network Allocation Vector (Virtual Carrier Sensing) 12
Problems Solved? • RTS/CTS helps, but do not solve hidden/exposed terminal problems A B C D A B C D RTS RTS RTS CTS CTS RTS RTS Data Data CTS CTS Data Ack 13
MACA Problem: Idle listening • Need to sense carrier for RTS or CTS packets ◦ Simple sleeping will break the protocol • IEEE 802.11 solution ◦ Idea: Nodes that have data buffered for receivers send traffic indicators at prearranged points in time ◦ ATIM - A nnouncement T raffic I ndication M essage ◦ Receivers need to wake up at these points, but can sleep otherwise 14
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 Active period wakeup schedule Wakeup period between neighbors ◦ When awake, Sleep period perform RTS/CTS For SYNCH For RTS For CTS 15
Schedule Assignment • Synchronizer ◦ Listen for a mount of time Listen ◦ If hear no SYNC, select its A Sleep Go to sleep after time t Listen for SYNC own SYNC ◦ Broadcasts its SYNC immediately Broadcasts • Follower ◦ Listen for amount of time ◦ Hear SYNC from A, follow Listen A’s SYNC B Go to sleep after time t- t d Sleep t d ◦ Rebroadcasts SYNC after random delay t d Broadcasts 16
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 C C C C D Time D D D 17
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: Long preamble Actual packet Check Check Check Check channel channel channel channel Stay awake! 18
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 19
B-MAC • B-MAC does not have ◦ Synchronization ◦ RTS/CTS ◦ Results in simpler, leaner implementation ◦ Clean and simple interface 20
Clear Channel Assessment • "Carrier Sensing" in wireless networks Thresholding CCA algorithm Outlier detection CCA algorithm 21
Overview • Principal options and difficulties • Contention-based protocols • Schedule-based protocols • Wireless Personal Area Networks Technologies 22
LEACH • L ow- E nergy A daptive C lustering H ierarchy • 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 23
LEACH Clusterhead • Each CH organizes ◦ 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 24
LEACH rounds Fixed-length round ……… .. ……… .. Setup phase Steady-state phase Time slot Time slot Time slot Time slot … .. … .. … .. 1 2 n 1 Advertisement phase Cluster setup phase Broadcast schedule Clusterheads Members compete with compete Self-election of CSMA with CSMA clusterheads 25
TRAMA • Tr affic A daptive M edium A ccess 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 • Used by winning nodes to transmit data neighbors • Exchange schedules 26
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 6 33 57 2 27
Overview • Principal options and difficulties • Contention-based protocols • Schedule-based protocols • Wireless Personal Area Networks Technologies 28
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 1 channel @ 868-868.6 MHz ◦ 40 kbps over 10 channels @ 905 – 928 MHz ◦ 250 kbps over 16 channels @ 2.4 GHz • MAC protocol ◦ Single channel at any one time ◦ Combines contention-based and schedule-based schemes ◦ Asymmetric: nodes can assume different roles 29
802.15.4 PHY Overview • Operating frequency bands Channel 0 Channels 1-10 2 MHz 868MHz / 915MHz PHY 868.3 MHz 902 MHz 928 MHz 2.4 GHz PHY Channels 11-26 5 MHz 2.4 GHz 2.4835 GHz 30
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 31
802.15.4 Network Topologies 32
802.15.4 Beaconed Mode • Superframe structure Active period Inactive period Contention Guaranteed time access slots (GTS) period Beacon • GTS assigned to devices upon request 33
Recommend
More recommend
