Network Kernel Architectures and Implementation ( ) ) Medium Access Control and WPAN Technologies Chaiporn Jaikaeo Department of Computer Engineering Kasetsart University Materials taken from lecture slides by Karl and Willig

2 Overview Principal options and difficulties Principal options and difficulties Contention-based protocols Contention-based protocols Schedule-based protocols Schedule-based protocols Wireless Personal Area Networks Technologies Wireless Personal Area Networks Technologies

3 Difficulties Medium access in wireless networks is difficult, mainly because of Medium access in wireless networks is difficult, mainly because of  Half-duplex communication  High error rates Requirements Requirements  As usual: high throughput, low overhead, low error rates, …  Additionally: energy-efficient, handle switched off devices!

4 Requirements for Energy-Efficient MAC Protocols Recall Recall  Transmissions are costly  Receiving about as expensive as transmitting  Idling can be cheaper but is still expensive Energy problems Energy problems  Collisions  Overhearing  Idle listening  Protocol overhead Always wanted: Low complexity solution Always wanted: Low complexity solution

5 Main Options Wireless medium access CentralizedDistributed Contention- based Schedule- based Fixed assignment Demand assignment Contention- based Schedule- based Fixed assignment Demand assignment

6 Centralized Medium Access A central station controls when a node may access the medium 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 Not directly feasible for non-trivial wireless network sizes But: Can be quite useful when network is somehow divided into smaller groups But: Can be quite useful when network is somehow divided into smaller groups Distributed approach still preferable Distributed approach still preferable

7 Schedule- vs. Contention-Based Schedule-based protocols 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 Contention-based protocols  Hope: coordination overhead can be saved  Mechanisms to handle/reduce probability/impact of collisions required  Randomization used somehow

8 Overview Principal options and difficulties Principal options and difficulties Contention-based protocols Contention-based protocols Schedule-based protocols Schedule-based protocols Wireless Personal Area Networks Technologies Wireless Personal Area Networks Technologies

9 A Distributed, Contention-Based MAC Basic ideas 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 BC D Hidden terminal scenario: Also: recall exposed terminal scenario

10 How To Shut Up Senders Inform potential interferers during reception Inform potential interferers during reception  Cannot use the same channel  So use a different one  Busy tone protocol Inform potential interferers before reception 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

11 MACA Multiple Access with Collision Avoidance Multiple Access with Collision Avoidance Sender B issues Request to Send (RTS) Sender B issues Request to Send (RTS) Receiver C agrees with Clear to Send (CTS) Receiver C agrees with Clear to Send (CTS) Potential interferers learns from RTS/CTS Potential interferers learns from RTS/CTS B sends, C acks B sends, C acks Used in IEEE Used in IEEE

12 Virtual Carrier Sensing RTS CTS Data ACK ABCD NAV NAV NAV  Network Allocation Vector (Virtual Carrier Sensing)

13 Problems Solved? RTS/CTS helps, but do not solve hidden/exposed terminal problems RTS/CTS helps, but do not solve hidden/exposed terminal problems

14 MACA Problem: Idle listening Need to sense carrier for RTS or CTS packets Need to sense carrier for RTS or CTS packets  Simple sleeping will break the protocol IEEE solution IEEE solution  Idea: Nodes that have data buffered for receivers send traffic indicators at prearranged points in time  ATIM - Announcement Traffic Indication Message  Receivers need to wake up at these points, but can sleep otherwise

15 Sensor-MAC (S-MAC) MACA unsuitable if average data rate is low 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 Idea: Switch off, ensure that neighboring nodes turn on simultaneously to allow packet exchange  Need to also exchange wakeup schedule between neighbors  When awake, perform RTS/CTS

16 Listen for SYNC tdtd Schedule Assignment Synchronizer   Listen for a mount of time   If hear no SYNC, select its own SYNC   Broadcasts its SYNC immediately Follower   Listen for amount of time   Hear SYNC from A, follow A’s SYNC   Rebroadcasts SYNC after random delay t d Sleep Listen Go to sleep after time t Sleep Listen Broadcasts A B Go to sleep after time t- t d

17 S-MAC Synchronized Islands Nodes learn schedule from other nodes Nodes learn schedule from other nodes Some node might learn about two different schedules from different nodes Some node might learn about two different schedules from different nodes  “Synchronized islands” To bridge this gap, it has to follow both schemes To bridge this gap, it has to follow both schemes Time AAAA CCCC A BBBB DDD A C B D E EEE EEE

18 Preamble Sampling Alternative option: Don’t try to explicitly synchronize nodes 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 Use long preambles to ensure that receiver stays awake to catch actual packet  Example: B-MAC, WiseMAC Check channel Start transmission: Long preambleActual packet Stay awake!

19 B-MAC Very simple MAC protocol Very simple MAC protocol Employs 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 Currently: Often considered as the default WSN MAC protocol

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

21 Clear Channel Assessment "Carrier Sensing" in wireless networks "Carrier Sensing" in wireless networks Thresholding CCA algorithm Outlier detection CCA algorithm

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

23 Overview Principal options and difficulties Principal options and difficulties Contention-based protocols Contention-based protocols Schedule-based protocols Schedule-based protocols Wireless Personal Area Networks Technologies Wireless Personal Area Networks Technologies

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

25 LEACH Clusterhead Each CH organizes Each CH organizes  CDMA code for its cluster  TDMA schedule to be used within a cluster In steady state operation In steady state operation  CHs collect & aggregate data from all cluster members  Report aggregated data to sink using CDMA

26 LEACH rounds

27 TRAMA Traffic Adaptive Medium Access Protocol Traffic Adaptive Medium Access Protocol Assume nodes are time synchronized Assume nodes are time synchronized Time divided into cycles, divided into Time divided into cycles, divided into  Random access period  Scheduled access period Random Access Period Scheduled-Access Period time cycle Exchange and learn two-hop neighborsExchange and learn two-hop neighbors Exchange schedulesExchange schedules Used by winning nodes to transmit dataUsed by winning nodes to transmit data

28 TRAMA – Adaptive Election How to decide which slot (in scheduled access period) a node can use? 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 B C

29 Overview Principal options and difficulties Principal options and difficulties Contention-based protocols Contention-based protocols Schedule-based protocols Schedule-based protocols Wireless Personal Area Networks Technologies Wireless Personal Area Networks Technologies

30 IEEE IEEE standard for low-rate WPAN (LR-WPAN) applications 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 Physical layer  20 kbps over MHz  40 kbps over – 928 MHz  250 kbps over GHz MAC protocol MAC protocol  Single channel at any one time  Combines contention-based and schedule-based schemes  Asymmetric: nodes can assume different roles

31 868MHz / 915MHz PHY 2.4 GHz MHz Channel 0 Channels 1-10 Channels GHz 928 MHz902 MHz 5 MHz 2 MHz 2.4 GHz PHY PHY Overview Operating frequency bands Operating frequency bands

Device Classes Full function device (FFD) Full function device (FFD)  Any topology  Network coordinator capable  Talks to any other device Reduced function device (RFD) Reduced function device (RFD)  Limited to star topology  Cannot become a network coordinator  Talks only to a network coordinator  Very simple implementation

Network Topologies

Beaconed Mode Superframe structure Superframe structure GTS assigned to devices upon request GTS assigned to devices upon request

GTS Data Transfer Device  coordinator Device  coordinator  If having allocated GTS, wake up and send  Otherwise, send during CAP  Using slotted CSMA Coordinator  device Coordinator  device  If having allocated GTS, wake up and receive  Otherwise, see picture

36 IEEE Adopters ZigBee ZigBee  Requires battery life of at least two years be certified  Applications: Industrial control, embedded sensing, home automation  ZigBee RF4CE (Radio Frequency for Consumer Electronics) Nest (acquired by Google) Nest (acquired by Google)  Learning thermostats, Smoke and CO alarms  WiFi- and ZigBee-enabled

37 Bluetooth Smart Formally Bluetooth Low Energy (BLE) Formally Bluetooth Low Energy (BLE)  Part of Bluetooth 4.0 Specification Based on Nokia's Wibree technology Based on Nokia's Wibree technology First smartphones to support  iPhone 4S First smartphones to support  iPhone 4S  Now supported by most recent smartphones

38 Bluetooth: Classic vs. Smart Source: Bluetooth SIG

39 Bluetooth Compatibility

40 Bluetooth Smart: Device Roles Central device Central device  Serves as a hub to one or more peripheral devices  Two central devices cannot directly communicate  Similar to IEEE 's FFD Peripheral device Peripheral device  Must be connected to a central device  Two peripheral devices cannot directly communicate  Similar to IEEE 's RFD

41 ANT / ANT+ / NIKE+ Primarily used for fitness monitoring devices Primarily used for fitness monitoring devices ANT / ANT+ ANT / ANT+  open access multicast wireless sensor network NIKE+ NIKE+  Proprietary protocols on 2.4 GHz band Nike.com

42 WiFi/ZigBee/Bluetooth Coexistence They all employ 2.4 GHz spectrum They all employ 2.4 GHz spectrum WiFi vs. ZigbeeWiFi vs. Bluetooth

43 Summary Many different ideas exist for medium access control in MANET/WSN Many different ideas exist for medium access control in MANET/WSN Comparing their performance and suitability is difficult Comparing their performance and suitability is difficult Especially, clearly identifying interdependencies between MAC protocol and other layers/applications is difficult Especially, clearly identifying interdependencies between MAC protocol and other layers/applications is difficult  Which is the best MAC for which application? Nonetheless, certain “common use cases” exist Nonetheless, certain “common use cases” exist  IEEE DCF for MANET  IEEE for some early “commercial” WSN variants  B-MAC for WSN research not focusing on MAC