EL2745 Principles of Wireless Sensor Networks

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1 EL2745 Principles of Wireless Sensor Networks Lecture 5 Stockholm, February 2, 2012 Carlo Fischione Royal Institute of Technology - KTH Stockholm, Sweden

2 Course content Part 1 Ø Ø Ø Ø Ø Ø Lec 1: Introduction Lec 2: Programming Lec 3: Sensor modelling Lec 4: Physical layer Lec 5: Mac layer Lec 6: Routing Part 2 Ø Ø Ø Ø Lec 7: Estimation Lec 8: Detection Lec 9: Positioning and localization Lec 10: Time synchronization Part 3 Ø Lec 11: Networked control systems 1 Ø Lec 12: Networked control systems 2 Ø Ø Lec 13: Security Lec 14: Summary

3 Previous lecture Process Controller Application Presentation Session Transport Routing MAC Phy How information is successfully transmitted and received over the wireless channel?

4 Today s lecture Process Controller Application Presentation Session Transport Routing MAC Phy When a node gets the right to transmit? What it the mechanism to get such a right?

5 Today s learning goals What is the Medium Access Control (MAC)? What are the options to design MACs? What is the MAC of IEEE ?

6 Outline Definition and classification of MACs The IEEE protocol

7 Medium Access Control - MAC MAC: mechanism for controlling when sending a packet and when listening for a packet MAC is one of the major component for energy expenditure Especially, idly waiting to receive packets wastes huge amounts of energy Typical power consumption of a node We study MAC schemes that are Suitable for WSNs Emphasize energy-efficient operation

8 Requirements for energy-efficient MAC protocols Remember Transmissions are costly Receiving about as expensive as transmitting Idle listening is still expensive Energy problems Collisions wasted effort when two packets collide Overhearing waste effort in receiving a packet destined for another node Idle listening sitting idly and trying to receive when nobody is sending Protocol overhead

9 Main options Wireless medium access Centralized Distributed Schedulebased Contentionbased Schedulebased Contentionbased Fixed assignment Demand assignment Fixed assignment Demand assignment

10 Centralized MAC A central station controls when a node may access the medium Ø Example: Polling, centralized computation of Time Division Multiple Access (TDMA) schedules Ø Simple and quite efficient (e.g., no collisions), but burdens the central station Not feasible for non-trivial wireless network sizes Can be quite useful when network is somehow divided into smaller clusters Ø In each cluster medium access can be controlled centrally

11 Schedule-based MAC Schedule- vs. contention-based MACs Ø A schedule regulates which participant may use which resource at which time, TDMA component Ø Schedule can be fixed or computed on demand Usually: mixed difference fixed/on demand is one of time scales Ø Collisions, overhearing, idle listening no issues Ø Time synchronization needed Contention-based protocols Ø Based on random techniques Ø Risk of colliding packets is deliberately taken Ø Hope: coordination overhead can be saved, resulting in overall improved efficiency Ø Mechanisms to handle/reduce probability/impact of collisions required

12 Distributed, contention-based MAC Hidden terminal scenario: A B C D Listen before talk (Carrier Sense Multiple Access, CSMA) suffers from sender not knowing what is going on at receiver, might destroy packets despite first listening for a clear channel Hidden terminal problem: a transmitter does not see another transmitter Exposed terminal problem: a transmitter senses another transmitter, which is not causing collisions at the receiver

13 Slotted ALHOA The slotted ALHOA is a random protocol working on top of TDMA Nodes synchronized If receiver sends a no acknowledgement packet, the transmitter tries to send again a packet after a random time p( probability that a node has a packet Probability of successful packet transmission: p(1 p) n 1

14 Outline Definition and classification of MACs The IEEE protocol

15 IEEE protocol architecture Now we study the MAC of the standard IEEE IEEE is the de-facto reference standard for low data rate and low power WSNs Characteristics: Ø low data rate for ad hoc self-organizing network of inexpensive fixed, portable and moving devices Ø high network flexibility Ø very low power consumption Ø low cost

16 The IEEE protocol Application Presentation Session Transport IEEE specifies two layers Ø Physical layer 2.4Ghz global, 250Kbps 915MHz America, 40Kbps 868MHz Europe, 20Kbps Ø Medium Access Control (MAC) layer Routing MAC Phy IEEE does not specify the routing

17 IEEE networks IEEE network composed of Ø full-function device (FFD) Ø reduced-function device (RFD). A network includes at least one FFD The FFD can operate in three modes: Ø a personal area network (PAN) coordinator Ø a coordinator Ø a device An FFD can talk to RFDs or FFDs RFD can only talk to an FFD

18 IEEE network topologies 3 types of topologies Ø star topology Ø peer-to-peer topology Ø cluster tree

19 Cluster-tree topology

20 IEEE physical layer Frequency bands: GHz Ghz, global, 16 channels, 250Kbps MHz, America, 10 channels, 40Kbps MHz, Europe, 1 channel, 20Kbps Features of the PHY layer Ø activation and deactivation of the radio transceiver Ø energy detection (ED) Ø link quality indication (LQI) Ø clear channel assessment (CCA) Ø transmitting and receiving packets across the wireless channel Ø dynamic channel selection by a scanning a list of channels in search of beacon, ED, LQI, and channel switching

21 IEEE physical layer

22 Physical layer data unit The SFD indicates the end of the SHR and the start of the packet data PHR: PHY header PHY payload < 128 byte

23 IEEE MAC The MAC provides two services: Ø data service Ø management service MAC features: beacon management, channel access, GTS management, frame validation, acknowledged frame delivery, association and disassociation.

24 Superframes Superframe structure: Ø format defined by the PAN coordinator Ø bounded by network beacons Ø divided into 16 equally sized slots Beacons Ø synchronize the attached devices, identify the PAN and describe the structure of superframes Ø sent in the first slot of each superframe Ø turned off if a coordinator does not use the superframe structure Superframe portions: active and an inactive Ø inactive portion: a node does not interact with its PAN and may enter a low-power mode Ø active portion: contention access period (CAP) and contention free period (CFP) Ø Any device wishing to communicate during the CAP shall compete with other devices using a slotted CSMA/CA mechanism Ø The CFP contains guaranteed time slots (GTSs).

25 IEEE CSMA-CA A Carrier Sense Multiple Access/ Collision Avoidance (CSMA/CA) algorithm is implemented at the MAC layer If a superframe structure is used in the PAN, then slotted CSMA-CA is used in the CAP period If beacons are not used in the PAN or a beacon cannot be located in a beacon-enabled network, unslotted CSMA-CA is used

26 CSMA-CA Each device has 3 variables: NB, CW and BE. NB: number of times the CSMA/CA algorithm was required to backoff while attempting the current transmission. Ø It is initialized to 0 before every new transmission. BE: backoff exponent Ø how many backoff periods a device shall wait before attempting to assess the channel. CW: contention window length (used for slotted CSMA-CA), Ø Is the number of backoff periods that need to be clear of activity before the transmission can start. Ø It is initialized to 2 before each transmission attempt and reset to 2 each time the channel is assessed to be busy.

27 CSMA/CA Flow diagram to Transmit a packet with CSMA/CA in Slotted and Unslotted modalities

28 Guarantee Time Slot (GTS) Allocation The GTSs always appears at the end of the active superframe starting at a slot boundary immediately following the CAP. The PAN coordinator may allocate up to 7 GTSs. A GTS can occupy more than one slot period. SO <15. If SO=15, the superframe will not be active anymore after the beacon BO < 15. If BO=15, the superframe is ignored

29 Guaranteed Time Slots A GTS allows a device to operate within a portion of the superframe that is dedicated exclusively to it A device attempts to allocate and use a GTS only if it is tracking the beacons GTS allocation: Ø The management of the GTSs is undertaken by the PAN coordinator only. Ø A GTS is used only for communications between the PAN coordinator and a device. Ø The GTS direction is specified as either transmit or receive. Ø A single GTS can extend over one or more superframe slots.

30 Uplink MAC: beacon and non beacon-enabled Communication to a coordinator in a beacon-enabled network Communication to a coordinator in non-beacon-enabled network

31 Downlink MAC: Beacon and nonbeacon-enabled From a coordinator in a beacon-enabled PAN From a coordinator in a nonbeacon-enabled PAN

32 Conclusions Application Presentation Session Transport Routing MAC Phy We have seen a MAC classification, Ø TDMA, ALHOA, CSMA Seen in detail the prince protocol for WSNs, IEEE Identifying interdependencies between MAC protocol and other layers/applications is difficult Ø Which is the best MAC for which application? Ø Need of a MAC engine that optimally selects the best MAC for given conditions

33 Next lecture Now that we know how nodes get the right to access the wireless medium, we would like to see how a message is routed over possible paths Routing protocols How a node decides to route a message? What are the mechanisms to get such a decision?

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