Wireless Sensor Networks

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1 Wireless Sensor Networks Office Hours: Tuesday 3 5 Main Building, second floor Credits: 6

2 The IEEE

3 Protocol Stack Time Synchronization Energy Management Distributed Processing Application Layer Network Layer MAC Layer PHY Layer

4 Outline 1. Random Access 2. IEEE MAC

5 Outline 1. Random Access 2. IEEE MAC

6 Random Access: ALOHA and CSMA ALOHA S- ALOHA CSMA CSMA/CA CSMA/CA with RTS/CTS

7 Random Access: ALOHA Whenever a node has a frame in the data link layer buffer, it sends it. Collisions (i.e., partial or total overlap between data blocks transmitted by separate sources) may occur, with probability P colla In an interference-limited environment, frame losses (i.e., events of data blocks not captured by the receiver because of C/I below capture threshold) may occur, with probability P lossa < P colla. A B A B A B A B ; Z Y wanted transmission undesired transmission A Y A Z Z time time

8 Random Access: Slotted ALOHA Time is slotted. Whenever a node has a frame put in the data link layer buffer, it sends it starting from instant when next slot initiates. Collisions (i.e., total overlap between data blocks transmitted by separate sources) may occur, with probability P collsa In an interference-limited environment, frame losses (i.e., events of data blocks not captured by the receiver because of C/I below capture threshold) may occur, with probability P losssa < P collsa. A B A B A B A B ; Z Y wanted transmission undesired transmission A Y A Z Z time time

9 Random Access: CSMA 1-persistent CSMA Whenever a node has a frame put in the data link layer buffer, it starts sensing the channel. If the channel is free, it sends the frame; if not, it waits until it becomes free and then transmits. Collisions may occur if two nodes stop sensing the carrier simultaneously, with probability P collc P collc can not be reduced to zero unless collision resolution algorithms are used. In an interference-limited environment, data block losses may occur, with probability P lossc < P collc. A B A B A B A B ; Z Y wanted transmission undesired transmission busy A time Y A Z Z time

10 Random Access: CSMA/CA non-persistent CSMA Whenever a node has a packet put in the data link layer buffer, it starts sensing the channel. If the channel is free, it sends the frame; if not, a backoff phase (i.e., transmission is deferred by a random delay) is initiated; at its end, the node repeats the procedure. Collisions may occur with probability P collcc. Large backoffs can reduce collisions deliberately (P collcc can be reduced to zero). In an interference-limited environment, data block losses may occur, with probability P losscc < P collcc. A B A B A B A B ; Z Y wanted transmission undesired transmission busy A A time Y A Z A Z Z time A random delay A random delay Z random delay

11 Random Access: CSMA Hidden Terminal Problem Even if backoff could solve collision problems in a perfect broadcast channel (every node is within range of all others), hidden nodes might cause collisions in an environment where transmission ranges do not cover the whole network. An hidden node (Z) is a node, willing to transmit to a given terminal (Y), which does not sense the transmission from a given source (A) to an other node (B) because received power (from A) is too small, but the node receiving both data blocks (B) Is within range of both transmitting nodes (A and Z) and detects a collision. A B A B A B A B ; Z Y R R = Range wanted transmission undesired transmission busy A A time Y A Z A Z Hidden terminal time

12 Random Access: CSMA with RTS/CTS Whenever a node has a packet put in the data link layer buffer, it starts sensing the carrier. When it detects the channel is free, a backoff phase is initiated; at its end, the node sends a short RTS packet informing everyone within its range to avoid transmission for a packet time. The receiver responds with a short CTS packet allowing transmission and informing everyone within its range to avoid transmission for a packet time. Upon reception of the CTS, the source node broadcasts the packet. All nodes within range of receiver know that have to wait before transmitting a RTS. A B A B A B A B ; Z Y R R = Range wanted transmission undesired transmission busy A RTS B CTS time Y A Z B Z Hidden terminal Y time

13 Random Access: CSMA Exposed Terminal Problem If a node (A) willing to transmit a packet to a given sink (B), broadcasts the RTS, all other nodes within its range will refrain from transmitting. On the other hand, there might be a node (Y) willing to transmit to a sink (Z) which would not detect a collision because it is not within range of the interferer (A), which refrains from transmitting though its transmission would not be a problem because its (Y s) range does not include the other link s sink (B). CSMA would work better in this case. CSMA/CA with RTS/CTS reduces throughput. A B A B A B R R = Range Exposed terminal A B ; Y Z wanted transmission undesired transmission busy A RTS B CTS time Y A Z B Y Z time

14 Random Access: CSMA with CTS/RTS RTS / CTS mode is a powerful tool but must be used with care, The energy consumption increases (transmission of control packets) Exposed terminal node is generated

15 Outline 1. Random Access 2. IEEE MAC

16 MAC Layer MAC Layer is responsible for: Supporting PAN formation (association/disassociation) Managing the access to the channel (CSMA/CA) Providing a reliable link (FCS & ACK) IEEE Coordinator

17 IEEE MAC Beacon-Enabled Mode Superframe Beacon Beacon CAP (slotted) CSMA/CA CFP Maximum 7 GTSs (Guaranteed Time Slots) t Non Beacon-Enabled Mode Only CAP, with (unslotted) CSMA/CA

18 IEEE Beacon-Enabled Mode Beacon Beacon CAP CFP Non active Period Superframe Duration (SD) Beacon Interval (BI) SD = 960* T S * 2 SO (SO= Superframe Order) BI = 960 * T S * 2 BO (BO = Beacon Order) where T S (symbol period)=16μs. 0 < SO < BO < 14 SO=0 SD = 15 ms SO=1 SD = 30 ms

19 Beacon-Enabled Mode: slotted CSMA/CA Locate backoff period boundary NB=0, CW=2 BE=BEmin - NB (number of backoff) counts the number of times the node tries to access the channel - BE (backoff exponent)used to derive the backoff time - CW used so that two sensing phases are performed before the transmission Delay for random (0, 2 BE -1) backoff periods Perfom CCA Channel idle? N CW=2, NB=NB+1, BE=min(BE+1, BEmax) Y CW=CW-1 N NB>NBmax? CW=0? N Y Y Failure Success

20 Slotted CSMA/CA: An example 2nd backoff stage Idle Delay for a random number of backoff periods [0 2 BEmin -1] Sensing for CW backoff periods t BE min BE BE max BE is increased by 1 at each backoff stage BE min =[0 3] (3 default value), BE max = 5, NB max = [0 5] (4 default value), CW=2 Backoff Period = T = 320 μs Sensing duration (at PHY) = 128 μs

21 Non Beacon-Enabled Mode: unslotted CSMA/CA NB=0, BE=BEmin Delay for random (2 BE -1) unit backoff periods - NB counts the number of times the node tries to access the channel - BE used to derive the backoff time Perform sensing Channel Idle? Y N NB=NB+1, BE=min(BE+1,BEmax) N NB>NBmax? Y Failure Success

22 Unslotted CSMA/CA: An Example Packet generation Transmission Delay for a random number of backoff periods [0 2 BE -1] t BE min BE BE max BE is increased by 1 at each backoff stage BE min =[0 3] (3 default value), BE max = 5, NB max = [0 5] (4 default value), CW=1 Backoff Period = T = 320 μs Sensing duration (at PHY) = 128 μs

23 CCA Modes Mode 1: Energy above the threshold (Energy Detection) Mode 2: Carrier Sensing Only Mode 3: Carrier Sensing with ED

24 IEEE Addressing The MAC layer of IEEE provides two address modes: 64-bit IEEE address more than 128 billion devices 16-bit short logical address more than devices connected to one Coordinator The 64-bit IEEE address is unique and prevents address conflicts.

25 IEEE Network Devices A Network (PAN) is managed by a PAN Coordinator. RFD (Reduced Function Device) Reduced functionalities Battery powered No forwarding Can communicate only with a FFD FFD (Full Function Device) All functionalities are implemented Forwarding Can play the role of Coordinator

26 IEEE Network Topologies Mesh Star Tree PAN Coordinator Full Function Device Reduced Function Device

27 IEEE Data Exchange Beacon-Enabled Net Non Beacon-Enabled Net

28 IEEE Data Exchange Beacon-Enabled Net Non Beacon-Enabled Net

29 Frame Format Data Frame PHY Payload (PSDU) Max 127 bytes PHY Layer Protocol Data Unit (PPDU)

30 Frame Format Data Frame

31 Frame Format Acknowledge Frame

32 Frame Format Beacon Frame

33 Frame Format MAC Command Frame

34 Wireless Sensor Networks

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