Lower Layers PART1: IEEE and the ZOLERTIA Z1 Radio

Transcription

1 Slide 1 Lower Layers PART1: IEEE and the ZOLERTIA Z1 Radio Jacques Tiberghien Kris Steenhaut Remark: all numerical data refer to the parameters defined in IEEE for 32.5 Kbytes/s transmission in the 2.4 GHz frequency band. 1

2 Slide 2 IEEE MAC Overview Star Topology PAN Coordinator Master/slave Full function device Communications flow Reduced function device 2 Slide 2 Joe Dvorak, Motorola 9/27/05 The IEEE supports different kinds of network topologies and also networks that have some very simple nodes with reduced functionality. The simplest supported topology is the star topology, where a full function node controls many reduced function nodes in a master/slave organisation. Of course, some of the slaves may be overqualified full function nodes.

3 Slide 3 IEEE MAC Overview Peer-Peer Topology Point to point Full function device Cluster tree Communications flow 3 Slide 3 Joe Dvorak, Motorola 9/27/05 At the other end of the complexity spectrum, one can consider peer to peer networks with interconnected full function nodes using routing algorithms to determine the optimal path between nodes.

4 Slide 4 IEEE MAC Overview Combined Topology Clustered stars - for example, cluster nodes exist between rooms of a hotel and each room has a star network for control. Full function device Communications flow Reduced function device 4 Slide 4 Joe Dvorak, Motorola 9/27/05 Of course a combination of the two previous topologies is possible: several full functional nodes constitute a peer to peer network, but some of these nodes are the master of a master slave network, with reduced function nodes.

5 Slide Frame Format Frame Payload (<=102 bytes) Address Information (0 to 20 bytes) Data Sequence Number (1 byte) Frame Control Field (2 bytes) Frame Check Sequence (2 bytes) MAC Protocol Data Unit (6 to 127 bytes) Frame Length (1 byte) Start of Frame Delimiter (1 byte) Preamble (4 bytes) PHY Protocol Data Unit (12 to 133 bytes) 5 The standard defines frames whose length can vary between 12 and 133 bytes. The first 6 bytes of each frame belong to the physical layer: the first 4 bytes are used to synchronize the receiver clock and the 5 th to announce the end of the synchronization phase. The 6 th gives the length of the subsequent part of the frame (MAC protocol Data Unit). This length does not include the length field itself and has a maximum value of 127. The most significant bit of the length field is always 0. All subsequent bytes form the MPDU. The first two bytes of the MPDU are the Frame control field, which defines the format and the meaning of the other parts of the MPDU. The third byte is a data sequence number that can be used to identify successive frames in a connection oriented link. The data sequence number is followed by an optional address field that can have a length of max. 20 bytes, as determined by the frame control field. The two last bytes of the MPDU are the Frame Check Sequence computed over the entire MPDU (not the Frame Length).

6 Slide Frame Control Field Frame Type (3 bits) Security Enabled (1 bit) Frame Pending (1 bit) Ack Request (1 bit) Intra Pan (1 bit) Reserved (2 bits) Destination Addressing Mode (2 bits) Frame Version (2 bits) Source Addressing Mode (2 bits) 6 Bits 0,1 and 2 give the type of the frame (beacon = 000, data = 001, ack = 010, MAC command = 011, other values to be defined). Bit 3 is used to enable or disable optional security features. Bit 4 is set in beacon systems when more frames are waiting to be transmitted Bit 5 tells the receiver that this frame should be acknowledged. Bit 6 gives the format of the Personal Area Network identifiers, if used. Bits 10 and 11 give the format of the destination address (00 = no addresses, 10 = 16 bit, 11 = 64 bit) Bits 12 and 13 give the applicable version of the standard (00 = , 01 = ) Bits 14 and 15 give the format of the source address (00 = no addresses, 10 = 16 bit, 11 = 64 bit)

7 Slide Beacon Frame Format Beacon Payload Pending Address Fields GTS Fields Superframe specifications Source Address Information (4 or 10 bytes) Beacon Sequence Number (1 byte) Frame Control Field (xxxxxxxxxxxxx000) Frame Check Sequence (2 bytes) 7

8 Slide Data Frame Format Frame Payload (<=102 bytes) Address Information (0 to 20 bytes) Data Sequence Number (1 byte) Frame Control Field (xxxxxxxxxxxxx001) Frame Check Sequence (2 bytes) 8

9 Slide Ack Frame Format Frame Check Sequence (2 bytes) Data Sequence Number (1 byte) Frame Control Field (xxxxxxxxxxxxx010) 9

10 Slide Command Frame Format Command Payload Command Type (1 byte) Address Information (0 to 20 bytes) Data Sequence Number (1 byte) Frame Control Field (xxxxxxxxxxxxx011) Frame Check Sequence (2 bytes) 10

11 Slide 11 Z1 Radio Architecture Rx 128 bytes FIFO Bytes/s Tx 128 bytes FIFO CC2420 µcontroller Tx: FL + MPDU (-FCS) in FIFO FIFO underflow detection FCS calculation & insertion PHY header addition Encoding & modulation Sending ack frames Retransmitting frames Rx: Receiver synchronization Discarding PHY header CCA, RSSI and LQI evaluation Discarding corrupted frames Demodulation & decoding Address based frame selection Generation of ACK frames FL+MPDU-FCS+RSSI+LQI in FIFO Higher level protocols Mac functions not supported by CC2420 (Frame Length, Radio Duty Cycle, ) 11 The Z1 motes use the cc2420 radio. The transfer of data between the computer in the mote and the radio is done through two 128 byte fifo stores. To transmit a frame, the computer loads the length of the frame (FL) and its contents, without FCS, in the FIFO buffer. The transmitter adds the 5 bytes of the physical header and the two bytes of the FCS, encodes everything and transmits the frame. If requested, the transmitter keeps the contents of the FIFO buffer and retransmits repeatedly the frame. The receiver checks for incoming radio signals (Clear Channel Assessment), decodes the received signal, computes the RSSI and LQI figures and checks the FCS field. If, according to the FCS, the frame is corrupt, it is discarded. Otherwise, the destination address in the received frame is compared with the address of the receiver and if they match, the frame is pushed in the receiver FIFO. In this FIFO the two bytes of the FCS are replaced by the values of RSSI and LQI computed while the frame was being received. When a frame with the Ack request bit on is received, the receiver instructs directly the transmitter to send an Ack frame (and to stop retransmitting the frame kept in the transmitter fifo).

12 Slide 12 Transmitter byte encoding Bytes/s Symbol delimiter Each byte is split into two four bit symbols. Symbol encoding Symbols/s Chips/s Pseudo-random Sequences (16 different sequences of 32 bits, called chips ) Modulator Each chip is encoded by half a period of a sine 500 KHz phase modulated sine waves in quadrature. 12 The transmitter uses spread spectrum technology for transmitting data. It works at a fixed speed of bytes per second. Each byte is split into two 4 bit entities, called symbols. Each symbol is encoded by means of a predetermined sequence of 32 bits. This results in a transmitted bit rate of 2 Mb/s. By transmitting 8 times more bits as there is information to be transmitted, one uses a lot of bandwidth to transmit a small quantity of information, meaning that, according to the theorem of Shannon, the signal to noise ratio of the radio channel can be pretty bad. The 32 bits used to encode a symbol are called chips.

13 Slide 13 IEEE symbol-to-chip mapping Symbol Chip sequence (C0, C1, C2,, C31) This table gives for all 16 possible symbols, the corresponding chip sequence.

14 Slide 14 Symbol decoding (7 chips instead of 32 are shown) This slide shows a simplified (7 instead of 32 chips) example of symbol recognition. The Hamming distance between the received symbol encoded as and the 16 possible correct symbols is computed. The symbol is identified as the one with the smallest hamming distance (in this case the symbol encoded as ).

15 Slide 15 Frame assembler Receiver byte decoding Contents of received frame. Symbol correlator Frame delimiter LQI Pseudo-random Sequences (16 different sequences of 32 bits, called chips ) Demodulator RSSI, CCA 500 KHz phase modulated sine waves in quadrature. 15 The receiver evaluates the level of incoming radio energy. This is used to generate the Clear Channel Assessment binary information and the RSSI figure associated with each received frame. Next the incoming chips are correlated with the 16 pseudo random sequences representing the 16 possible symbols and the symbol with the highest correlation is selected. The LQI figure is the value of the correlation and gives an indication of the quality of the digital radio link.

16 Slide GHz unlicenced band: WiFi overlapping channels For WiFi non overlapping channels should be chosen The 2.4 GHz unlicenced band is very crowded as many appliances use this band for wireless communications and microwave ovens use it for heating. To help in using it without too much interferences it has been subdivided in 14 overlapping channels. When an application is set up, all participating devices should choose a channel, which is preferably not used by other devices in the neighboorhood. To avoid interferences between overlapping channels, it is recommended to use only a subset of the available channels. In practice only channels 1,6,11,and 14 are used for b, 1,5,9 and 13 for g while n, which requires more bandwidth, uses channel 3 and 11.

17 Slide GHz unlicenced band: or WiFi US WiFi EU For Wireless Sensor and Actuator Networks channel 26 is preferred, provided that no WiFi in channel 13 is being used. Channels that are quite safe in Europe are 15, 16, 21, 22, 25 and 26.

18 Slide 18 CC2420 Radio Tx output: software selectable between 0dBm and 24dBm. Rx input: Minimum input signal = - 94 dbm. Maximum input signal = + 10 dbm. Received Signal Strength Indicator (RSSI) : Average of the signal power at input of receiver Measured over 8 last received symbols (128 µs). Range : from -100 to 0 dbm Offset : - 45 dbm Error : +/- 6 db Linearity : +/- 3 db 18 The output level of the CC2420 transmitter can be adjusted between 0 and -24 dbm. The receiver can work properly with input signals between +10 dbm and -94 dbm.

19 Slide 19 CC2420 Radio Clear Channel Assessment (CCA) : measured over 128 µs 3 conditions can be selected: - Received signal below predefined threshold. - Not receiving an IEEE frame. - Both above conditions simultaneously. Link Quality Indication (LQI): Average of the correlation values obtained when the 8 first symbols are identified. Excellent : 110 Minimal : 55 19

20 Slide 20 LQI vs RSSI LQI Based upon 1000 packets RSSI (dbm) RSSI < -90 dbm : LQI drops quickly RSSI > - 90 dbm: poor correlation 20

21 Slide 21 Address Field Handling The IEEE MAC frame has: Provisions for sender and/or receiver addresses Provisions for 16 (MIME) or 64 bit (6LowPan) addresses Receiver Address = all F for broadcast frames CC2420 receiver can filter frames in function of addresses: leave in Rx FIFO only - frames from specified sender - frames with specified receiver address - broadcast frames 21

22 Slide 22 Long distance range 150 PDR(%) Whip Antenna, 0 dbm RSSI av. PDR 0 Distance(m) RSSI(dBm) In an open field, a range of 160 m is still reliable 22

23 Slide 23 Bushes -15 ± 3 db 23

24 Slide 24 Rain -3 ± 1 db 24

25 Slide 25 Reproducibility -50, Distance (m) -55,0 Whip Antenna, -10 dbm -60,0-65,0 aug/03 aug/06-70,0-75,0 RSSI (dbm) Same measurements differ by 5 db on different days 25

26 Slide 26 Lower Layers PART 2: Radio Duty Cycling (RDC) and MAC protocols in Contiki Jacques Tiberghien Kris Steenhaut 26

27 Slide 27 Contiki RDC & MAC main protocols NullMAC CSMA NullRDC LPP X-MAC ContikiMAC 27 Contiki uses a MAC layer with underneath, above the physical layer, a Radio Duty Cycle layer. For the MAC layer, two options are available: - CSMA : this is the default option, it can retransmit a frame when a collision is detected by the RDC layer. - NullMAC : this MAC layer is just an empty interface between the upper layers and the RDC layer. For the RDC layer, 5 different options are available: - ContikiMAC: this is the default option, heavily optimized for IEEE frames and the CC2420 radio. - X-MAC : Older RDC protocol, with less stringent timing constraints than ContikiMAC, but often less performing from a power point of view. - LPP : Low Power Probing is a simple, receiver initiated, RDC protocol that can be useful to understand the principles of radio-duty-cycling. - NullRDC : is an alternative RDC protocol that leaves the radio always on The MAC and RDC drivers are selected by means of the contiki-conf.h file.

28 Slide 28 Contiki CSMA MAC driver (simplified) contiki/core/net/mac/csma.c Send packet Sending thread Iterate over queues broadcast? YES Dest = n queue n full? NO YES queue i empty? NO send packet p from queue i send YES drop Add to queue n Increment tx count packet p NO Ack? Return NO tx>max Random delay YES YES remove packet p from queue i Directly sends (through the RDC layer) broadcast packets Puts unicast packets in a queue associated with each neighbor. Drops packets when queue is full Whenever possible, sends (through the RDC layer) the first packet of each queue Removes packet from queue when packet is acknowledged When a packet is not acknowledged or if a collision is detected, wait a random time growing linearly with the number of retransmissions and try to retransmit the packet. The linear growth stops after 3 retransmissions, and the packet is dropped. This maximum of 3 retransmissions refers to the number of times a frame is given to the RDC layer for transmission. Some RDC layers transmit themselves a frame many times.

29 Slide 29 Contiki NULLMAC driver contiki/core/net/mac/nullmac.c Send packet send Return No acknowledgments! Directly transmits all packets This set of functions implements the same functions as the CSMA driver, but the functions don t perform any task except the blind transmission of packets

30 Slide 30 Contiki LPP contiki/core/net/mac/lpp.c Sender wants to send to X Sender Rx on, Tx off Tx on Data Packet t Receiver X Receiver wakes up (periodically scheduled) Probe (broadcasted) Ack The sender spends energy while listening possibly a full sleep period before it is allowed to send The receiver has to broadcast its probe as it doesn t now who is waiting to transmit a packet. 30 This is an implementation of the LPP (Low-Power Probing) MAC protocol. LPP is a power-saving MAC protocol that works by broadcasting a probe packet each time the radio is turned on. If another node wants to transmit a packet, it can do so after hearing the probe. To send a packet, the sending node turns on its radio to listen for probe packets.

31 Slide 31 Contiki XMAC (simplified) contiki/core/net/mac/xmac.c Sender wants to send to x Sender Probe with address x Receiver x Receiver wakes up (periodically scheduled) Ack Data Packet t A random delay is inserted between probes to allow other candidate senders to try their chance. 31 The sender, when it needs to send a packet, sends repeatedly a short probe that contains the destination address, leaving a random time interval between probes to allow other candidate senders to try their chance. When the intended receiver wakes up, it answers with an Ack and stays awake. When the sender receives that Ack, it sends its frame. When the receiver has stored the frame it sends an Ack and returns to sleep.

32 Slide 32 Contiki RDC MAC Sending a frame with destination address Sender t ~0.5 ms Receiver ~125 ms ~125 ms The receiver wakes up briefly (< 1 ms) a few times per second for checking radio activity by means of two CCAs. The transmitter sends repeatedly its frame. When the receiver detects radio activity, it remains awake. After receiving a complete frame the receiver sends an Ack and returns to sleep. After receiving an Ack the sender returns to sleep. 32 The contikimac protocol implements an original radio duty cycling protocol. Most of the time the receiver sleeps, but a few times per second (default value = 8) the receiver wakes up to perform two consecutive CCAs. If these CCAs observe no radio-activity above a predetermined threshold the receiver returns to sleep. The transmitter, when it has to transmit a frame, transmits that frame repeatedly, for a duration longer than the interval between two pairs of CCAs. When a CCA detects radio-activity, the receiver stays awake and waits till it has received an entire and correct frame. When that has happened, the receiver requests the transmission of an Ack and returns to sleep and

33 Slide 33 ContikiMAC Broadcasting a frame Sender t Receiver The receiver wakes up briefly (< 1 ms) a few times per second for checking radio activity by means of two CCAs. The transmitter sends repeatedly its frame, during a period that exceeds the interval between receiver wake-ups. When the receiver detects radio activity, it remains awake. After receiving a complete frame the receiver returns to sleep. 33 As broadcast frames can not be acknowledged (the ACKs would collide with each other) the transmitter retransmits the frame for the whole duration of a receiver wake-up cycle.

34 Slide 34 Sender Contiki RDC MAC Detecting Radio Activity by CCA t i Data frame Data frame t s t a t Receiver Ack t r t c t r t d t i = interval between data frame retransmissions. t r = time required for a stable Clear Channel Assessment. t c = time interval between the 2 CCAs performed at a wake-up. t a = time between receiving a frame and sending the Ack. t d = time required for recognizing an Ack. t s = transmission time of shortest frame 34 Why is it necessary to perform two consecutive CCAs? Between retransmissions, the sender waits a predefined time tj. If a CCA would be performed during that interval, no radio-activity would be detected, while the transmitter is attempting to get a frame through! Therefore contikimac uses two consecutive CCAs, separated by a time interval tc larger than tj.

35 Slide 35 Contiki RDC MAC Timing requirements t i t s t a Sender Data frame Data frame t Receiver Ack t r t c t r t d t i < t c (no frame should go undetected by 2 successive CCAs). t s > t c + 2t r (shortest frame should be detectable). t a + t d < t i (Ack should preempt retransmission of frame). t a + t d < t i < t c < t c + 2t r < t s 35 Some other constraints apply to the timing of the CCAs: the time tc between the two consecutive CCAs should be shorter than the duration of the shortest possible frame, otherwise transmission of such a frame could go undetected. To avoid collisions between a retransmitted frame and an Ack for the just transmitted frame, the time necessary to generate the Ack on the receiver side and to recognize it at the sender side should be smaller than the time tj between successive retransmissions.

36 Slide 36 Sender Receiver Contiki RDC MAC Figures for IEEE and CC2420 t i Data frame Data frame Ack t s t a t t r t c t a = 12 symbols = 192 µs (IEEE ) t d = 10 symbols = 160 µs (IEEE ) t r = 12 symbols = 192 µs (CC2420). t s > 736 µs or frame length > 23 bytes, all included. Contiki choices: Receiver wake-up period = 125 ms. t i = 400 µs. t c = 500 µs. t s = 884 µs (padding may be required in case of short addresses) t r t d 36

37 Slide 37 Poweroptimization Fast Sleep: too long frame too long silence no frame header Transmission Phase-Lock Introduced with ContikiMAC Can be used with other RDC protocols 37

38 Slide 38 Contiki RDC MAC Fast Sleep optimizations : too long frame Frame detection by CCA is not reliable: radio activity does not imply that somebody is sending a frame to this receiver! Sender Receiver Radio Noise awake t max frame length = 4.5 ms Receiver wake-up due to noise detected by CCA t Return to sleep 38 When a CCA detects radio-activity, this does not imply that an IEEE frame is being received. It could result from any variety of radio transmission or noise. As the duration of an IEEE frame can not exceed 4.5 ms, the receiver returns to sleep when it did not yet observe any period of radio-silence 4.5 ms after waking up.

39 Slide 39 Contiki RDC MAC Fast Sleep optimizations : too long silence Frame detection by CCA is not reliable: radio activity does not imply that somebody is sending a frame to this receiver! Sender Radio activity Radio activity t Receiver awake t i Radio silence Return to sleep Receiver wake-up due to radio activity detected by CCA 39 When data frames are being transmitted repeatedly, the time interval between successive transmissions is defined by the standard. When the receiver, after being woken-up by a CCA observes a radio-silence longer than the allowed interval between successive frames, it returns to sleep.

40 Slide 40 Contiki RDC MAC Fast Sleep optimizations : no frame header Frame detection by CCA is not reliable: radio activity does not imply that somebody is sending a frame to this receiver! Sender Radio activity Radio activity t Receiver awake t i Radio silence No correct frame header detected! Return to sleep Receiver wake-up due to radio activity detected by CCA 40 After a silence of the correct duration, an IEEE frame should be received. If the header of the frame being received doesn t obey the standards rules, the receiver returns to sleep.

41 Slide 41 Contiki RDC MAC Transmission Phase-Lock From a received Ack, the sender can deduce the phase of the wake-up cycle of a specific receiver and memorize it. Before phase locking Sender t Receiver 125 ms 125 ms Sender After phase locking t Receiver 125 ms 41 To optimize the power at the sender side, it is desirable to minimize the number of retransmissions. When a transmitter receives an Ack, it knows that the receiver was awake just before the transmission of the acknowledged frame. As the wake-ups occur strictly at a fixed rhythm, the transmitter can set up a table giving for each destination the optimal time to start transmitting a frame (phase locking the transmitter and receiver)