and an d Im Impl plementa ementation tion (01 0120 20442 - PowerPoint PPT Presentation

Net etwork work Ke Kerne nel l Ar Archi chitect tectures ures and an d Im Impl plementa ementation tion (01 0120 20442 4423) ) ) Me Medi dium um Ac Acce cess ss Con ontrol trol and WP an WPAN AN Te Tech chnologies nologies Chaiporn Jaikaeo chaiporn.j@ku.ac.th Department of Computer Engineering Kasetsart University Materials taken from lecture slides by Karl and Willig

Overv Ov rvie iew Pr Prin incipal ipal optio ions ns and nd dif iffic icult ulties ies  Contention-based protocols  Schedule-based protocols  Wireless Personal Area Networks  Technologies 2

Di Diffi fficulties culties 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

Re Requirements quirements fo for r En Ener ergy gy-Ef Effic ficient ient MA MAC C Prot otoco ocols ls 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 Mai n Opt Optio ions ns Wireless medium access Centralized Distributed Schedule- Contention- Schedule- Contention- based based based based Fixed Demand Fixed Demand assignment assignment assignment assignment 5

Cent ntralize ralized d Me Medi dium um Acc 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

Sch chedule dule- vs. . Cont ntention ention-Based Based Sch chedul ule-bas based ed protocols  FDMA, TDMA, CDMA  Schedule can be fixed or computed on demand  Usually mixed  Collisions, overhearing, idle listening no issues  Time synchronization needed  Con ontenti ention on-bas based ed protocols  Hope: coordination overhead can be saved  Mechanisms to handle/reduce probability/impact of  collisions required Randomization used somehow  7

Overv Ov rvie iew Principal options and difficulties  Cont ntention ntion-bas based ed proto tocol cols  Schedule-based protocols  Wireless Personal Area Networks  Technologies 8

Di Dist stribute ributed, d, Con onte tenti ntion on-Based Based MAC Basic ideas   Receivers need to tell surrounding nodes to shut up  Listen before talk (CSMA) A) Suffers from send nder not knowing what is going on  at re receiver ver Hidden terminal Also: recall scenario: exposed terminal scenario A B C D 9

Ho How w To To Shu hut t Up S Up Send nders rs Inform potential interferers during reception  Cannot use the same channel  So use a different one  Busy tone e protocol  Inform potential interferers be befo fore 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

MA MACA CA A B C D Multiple Access with  Collision Avoidance RTS Sender B issues  Request est to Send (RTS) S) CTS Receiver C agrees with  Clear r to Send nd ( CTS CTS ) Data Potential interferers  NAV indicates learns from RTS/CTS busy medium NAV indicates B sends, C acks busy medium  Used in IEE EEE E 802.11  Ack 11

Virt Vi rtual ual Car arri rier r Sens nsing ing A B C D RTS CTS NAV Data NAV ACK NAV  Network Allocation Vector (Virtual Carrier Sensing) 12

Pro roblems blems Solv lved? d? 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 CTS Data CTS Data Ack 13

MAC MACA A Pro roble blem: m: Id Idle le li liste tening ning 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 traff ffic c indica cato tors rs at prearranged points in time  ATIM - Announcement Traffic Indication Message  Receivers need to wake up at these points, but can sleep otherwise 14

Sens nsor or-MAC MAC (S (S-MAC 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  wakeup schedule Active period between neighbors Wakeup period When awake,  perform RTS/CTS Sleep period For SYNCH For RTS For CTS 15

Sch chedule dule As Assig ignment nment Synch chroniz ronizer  Listen for a mount  Listen of time A If hear no SYNC, Sleep Go to sleep after time t Listen for SYNC  select its own SYNC Broadcasts its  SYNC immediately Broadcasts Follower  Listen for amount  Listen of time B Go to sleep after time t- t d Sleep Hear SYNC from A, t d  follow A’s SYNC Rebroadcasts SYNC Broadcasts  after random delay t d 16

S-MAC MAC Syn ynchronized chronized Is Isla lands nds 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

Pre reamble amble Sam ampl pling ing Alternative option: Don’t try to explicitly  synchronize nodes Have receiver sleep and only periodically sample the  channel Use lon ong preamb mbles es to ensure that receiver stays  awake to catch actual packet Example: B-MAC, WiseMAC  Start transmission: Long preamble Actual packet Check Check Check Check channel channel channel channel Stay awake! 18

B-MAC 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 defa faul ult t WSN N  MAC AC protocol 19

B-MAC MAC B-MAC does not have   Synchronization  RTS/CTS  Results in simpler, leaner implementation  Clean and simple interface 20

Cle lear ar Cha hanne nnel l As Assessment ment "Carrier Sensing" in wireless networks  Thresholding CCA algorithm Outlier detection CCA algorithm 21

Contik ntiki LP LPL and LPP L and LPP Low-Power Listening (LPL)   Also known as ContikiMAC  Similar to B-MAC, but allowing packet-based MAC such as IEEE 802.15.4 Low-Power Probing (LPP)   Receivers periodically broadcast a probe  Sender listens for probes from receivers before transmitting 22

Ov Overv rvie iew Principal options and difficulties  Contention-based protocols  Schedu edule le-bas based ed proto toco cols ls  Wireless Personal Area Networks  Technologies 23

LE LEAC ACH Low-Energy Adaptive Clustering Hierarchy  Assumptions  Dense network of nodes  Direct communication with central sink  Time synchronization  Idea: Group nodes into “ cl clusters rs ”  Each cluster controlled by cluster terhe head ad  About 5% of nodes become clusterhead (depends on  scenario) Role of clusterhead is rotated  24

LEAC LE ACH H Clu luste terhead rhead 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 25

LEAC LE ACH H ro round unds s 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 26

TRAMA TR AMA Tr Traffic Adaptive Medium Access 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 neighbors transmit data • Exchange schedules 27

TR TRAMA AMA – Ada Adapt ptiv ive El Election ction 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 28