Medium Access Control and WPAN Technologies 01204525 Wireless Sensor Networks and Internet of Things Chaiporn Jaikaeo (chaiporn.j@ku.ac.th) Department of Computer Engineering Kasetsart University Materials taken from lecture slides by Karl and Willig Cliparts taken from openclipart.org Last updated: 2018-11-17

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

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!

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

Wireless medium access Main Options Wireless medium access Centralized Distributed Schedule- based Contention- based Schedule- based Contention- based Fixed assignment Demand assignment Fixed assignment Demand assignment

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

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

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

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 scenario: Also: recall exposed terminal scenario A B C D

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

MACA Multiple Access with Collision Avoidance Sender B issues Request to Send (RTS) Receiver C agrees with Clear to Send (CTS) Potential interferers learns from RTS/CTS B sends, C acks Used in IEEE 802.11

Virtual Carrier Sensing B C D RTS CTS NAV NAV Data ACK NAV  Network Allocation Vector (Virtual Carrier Sensing)

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

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 - Announcement Traffic Indication Message Receivers need to wake up at these points, but can sleep otherwise ATIM – Announcement Traffic Indicator Message

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 wakeup schedule between neighbors When awake, perform RTS/CTS

Schedule Assignment Synchronizer Follower 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 td Listen A Listen for SYNC Go to sleep after time t Sleep Broadcasts B Listen Go to sleep after time t- td Sleep td Broadcasts

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 D C C C C Time D D D

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: Das gibt eine SEHR schöne Übungsaufgabe! Long preamble Actual packet Check channel Stay awake!

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

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

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

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

LEACH Low-Energy Adaptive Clustering Hierarchy 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

LEACH Clusterhead Each CH organizes In steady state operation 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

LEACH rounds

Scheduled-Access Period TRAMA 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 neighbors Exchange schedules Used by winning nodes to transmit data

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 57 2

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

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

802.15.4 PHY Overview Operating frequency bands 868MHz / 915MHz PHY Channel 0 Channels 1-10 868MHz / 915MHz PHY 2 MHz 868.3 MHz 902 MHz 928 MHz 2.4 GHz PHY Channels 11-26 5 MHz 2.4 GHz 2.4835 GHz

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

802.15.4 Network Topologies

802.15.4 Beaconed Mode Superframe structure GTS assigned to devices upon request

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

IEEE 802.15.4 Adopters ZigBee Nest (acquired by Google) 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) Learning thermostats, Smoke and CO alarms WiFi- and ZigBee-enabled https://nest.com

Bluetooth Smart Formally Bluetooth Low Energy (BLE) Part of Bluetooth 4.0 Specification Based on Nokia's Wibree technology First smartphones to support  iPhone 4S Now supported by most recent smartphones http://redbearlab.com/blenano/

Bluetooth: Classic vs. Smart Source: Bluetooth SIG

Bluetooth Compatibility http://blog.laptopmag.com/just-what-is-bluetooth-4-0-anyway

Bluetooth Smart: Device Roles Central device Serves as a hub to one or more peripheral devices Two central devices cannot directly communicate Similar to IEEE 802.15.4's FFD Peripheral device Must be connected to a central device Two peripheral devices cannot directly communicate Similar to IEEE 802.15.4's RFD

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

WiFi/ZigBee/Bluetooth Coexistence They all employ 2.4 GHz spectrum WiFi vs. Zigbee WiFi vs. Bluetooth http://www.digikey.com/en/articles/techzone/2011/aug/comparing-low-power-wireless-technologies

Summary Many different ideas exist for medium access control in MANET/WSN Comparing their performance and suitability 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 IEEE 802.11 DCF for MANET IEEE 802.15.4 for some early “commercial” WSN variants B-MAC for WSN research not focusing on MAC