Zig Bee Primer IEEE 802 15 4 radio
Zig. Bee Primer § IEEE 802. 15. 4 radio § Line-powered routers, low power edge nodes § Just like Wi. Fi… § …Why not use Wi. Fi? § Single frequency – why? § Edge nodes can wake up and TX whenever they want. Routers must be ready. § Claim: Single-frequency narrow-band communication is unreliable
Consider one path E ~ 1/R; P ~ 1/R 2 metal Now received energy depends on both distances. At some distances, the two waves will interfere constructively, and at others they interfere destructively. These nulls depend on wavelength and the relative distances
R -2 -30 d. Bm -60 d. Bm The reflective surface is 10, 000 m away -90 d. Bm 1 m 100 m 1 km
R -2 -30 d. Bm -60 d. Bm The reflecting surface is 100 m away -90 d. Bm 1 m 100 m 1 km
-30 d. Bm The reflecting surface is 10 m away -90 d. Bm 1 m 100 m 1 km
R -2 -30 d. Bm -60 d. Bm The reflecting surface is 1 m away R-4 -90 d. Bm 1 m 100 m 1 km
-50 d. Bm -70 d. Bm -90 d. Bm The reflecting surface is 2 m away from one node and 8 m from the other 1 m 100 m 1 km
-50 d. Bm Add a second reflecting surface 10 m away from both antennas (like a metal ceiling) and it changes again -90 d. Bm 1 m 100 m 1 km
Multipath Fading ch. 11
Multipath Fading 100% reliability ch. 11 ch. 12
Multipath Fading ch. 11 ch. 12 ch. 19 ch. 20 ch. 13 ch. 14 ch. 21 ch. 22 ch. 15 ch. 16 ch. 23 ch. 24 ch. 17 ch. 18 ch. 25 ch. 26
Average RX power vs. Distance -40 -50 -60 PR [d. Bm] -70 -80 -90 -100 0 20 40 Distance [meters] 60
Path Stability by 802. 15. 4 Channel 802. 11 bg Channels 802. 15. 4 Channels 2. 480 GHz 2. 405 GHz Each dot is the 15 minute average
Power-optimal communication • Assume all motes share a network-wide synchronized sense of time, accurate to ~1 ms • For an optimally efficient network, mote A will only be awake when mote B needs to talk A A wakes up and listens B B transmits B receives ACK A transmits ACK Expected packet start time Worst case A/B clock skew
Packet transmission and acknowledgement Mote Current Radio TX startup Packet TX Radio TX/RX turnaround (2011): 15 m. C ACK RX (2008): 50 m. C Charge cost (2003): 300 m. C
Timing – imperfect synchronization (latest possible transmitter) A B CCA: RX startup, listen, RX->TX RX startup Tg Tg Transmit Packet: Preamble, SS, Headers, Payload, MIC, CRC RX packet Tcrypto Verify CRC RX startup or Tx->Rx Verify MAC MIC Tg ACK RX ACK Calculate ACK MIC+CRC Transmit ACK RX->TX Expected first bit of preamble • TCCA = 0. 512 ms to be standards compliant – Worst case is a receive slot followed by a transmit slot to a different partner, as radio will be finishing up the ACK TX just as it needs to look for a clear channel, so – TCCA = TTX->RX + Tchannel assessment + TRX->TX = 0. 192 ms + 0. 128 ms + 0. 192 ms • Tpacket = 4. 256 ms for a maximum length packet – Preamble+SS+packet = 4+1+128 B = 133 B = 1064 bits 4. 256 ms @ 250 kbps • • Tcrypto is the total time to verify packet MIC and create ACK MIC Tg. ACK is the tolerance to variation in Tcrypto and/or mote B’s turnaround time from RX to TX TACK is a function of the ACK length. It is likely to be just under 1 ms. Tslot = TCCA+2*Tg+Tpacket+Tcrypto+Tg. ACK+TACK = 0. 512+2+4. 256+1+0. 1+1 = 9 ms
Fundamental platform-specific energy requirements • Packet energy & packet rate determine power – (QTX + QRX )/ Tcycle – E. g. (60 u. C + 40 u. C) /10 s = 10 u. A
Scheduled Communication Slots • Mote A can listen more often than mote B transmits • Since both are time synchronized, a different radio frequency can be used at each wakeup • Time sync information transmitted in both directions with every packet A B TX, A ACK B Ch 3 Ch 4 Ch 5 Ch 6 Ch 7 Ch 8
Idle listen (no packet exchanged) Mote Current Radio RX startup Empty receive (2011): 5 m. C (2008): 27 m. C Charge cost (2003): 70 m. C
Latency reduction • Energy cost of latency reduction is easy to calculate: – Qlisten / Tlisten – E. g. 20 u. C/1 s = 20 u. A • Low-cost “virtual on” capability • Latency vs. power tradeoff can vary by mote, time of day, recent traffic, etc.
Multi-hop routing • Global time synchronization allows sequential ordering of links in a “superframe” • Measured average latency over many hops is Tframe/2 G T 2, ch y A T 1, ch x B Superframe
TSMP Foundations • Time Synchronization – Reliability – Power – Sensor • Reliability – Frequency diversity • Multi-path fading, interference – Spatial diversity • True mesh (multiple paths at each hop) – Temporal diversity • Secure link-layer ACK • Power – Turning radios off is easy
Link 53 37 A link defines a timeslot and channel offset.
Graph 17 47 53 37 41 43 A graph is a collection of connections and the links belonging to them A graph contains both routing and capacity Similarities to ATM circuits, flow labels, and Cisco tag switching (MPLS)
Superframe – repeating TDMA schedule • Color indicates TX slot for mote of that color – Wake up: RX? TX? – Use time+channel offset to calculate RF frequency (pseudo-random) • Similar to GSM, but with channel hopping Time Channel offset One Slot
High throughput – multiple hops • Use multiple channels • Only need 2 time slots • Throughput is 50 packets/second – independent of n – Limited by worst PDR (not combination) Odd slots Even slots Odd slots
Link = (Time Slot, Channel Offset) D One Slot Time Chan. offset A B A C D A B B A B C E F B E B F • All of B’s transmit links are dedicated and won’t collide – B has twice as much bandwidth to A as to C – B can broadcast to E and F, or use that link of a unicast to one or the other • D and C share a link for transmitting to A. A backoff algorithm is needed in case of collisions.
Time Synchronization • Timestamps available to application – RMS <0. 1 ms with Wireless HART – RMS <0. 01 ms with Smart. Mesh IP
Standards • Industrial Standards based on TSMP – Wireless HART, 2007 (IEC 62591, 2010) – ISA 100. 11 A, 2009, 2010 – WIA-PA • IPSO Alliance, 2008 – IP for Smart Objects – Created to bring IPv 6 to Zigbee SE 2. 0 • IEEE 802. 15. 4 E, 2012 – TSCH - time synchronized channel hopping – Rolled into 802. 15. 4 -2015, 2015 • IETF – 6 Lo. WPAN, Co. AP – RFC 6550, 2012 • RPL – routing protocol for low-power lossey networks – 6 Ti. SCH, 2013 • Minimal – simplest IPv 6 on TSCH configuration • 6 top – peer-to-peer link negotiation • security
Zigbee Smart Energy 1. 0, 2. 0 UDP 6 Lo. WPAN Stupid-MAC – single channel, powered routers 802. 15. 4 ISA 100 Wireless HART Zigbee Pro 2007 Zigbee 2006 Zigbee 2004 Various 15. 4 stacks Co. AP Wi-SUN UDP/TCP RPL (central) 6 Lo. WPAN (central) 6 Ti. SCH ? 802. 15. 4 E 802. 15. 4 G
Multi-platform, multi-OS interoperability (2012) open. WSN. berkeley. edu Dust Networks Huron (Dust Oski) u. C/OS-II UCB, GINA (MSP, Atmel ‘ 231) Open. OS • Open. WSN • Smart. Mesh. IP • Open. WSN http Co. AP UDP LBNL Jennic JN 5148 Free. RTOS xyz TCP 6 Lo. WPAN, RPL 802. 15. 4 E - TSCH PLC Wi. Fi 2. 4 GHz 802. 15. 4 PLC Wi. Fi
Scavenging Wireless HART Products ü Battery ü Vibration ü Battery ü 4 -20 m. A loop ü Solar ü Battery ü 4 -20 m. A loop ü Thermal
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