The IEEE 802.15.4 Standard and the ZigBee Specifications Course T-110.5111 (Computer Networks II – Advanced Topics) Lecture about Wireless Personal Area Networks Mario Di Francesco Department of Computer Science and Engineering, Aalto University Department of Computer Science and Engineering, University of Texas at Arlington October 2, 2013 The IEEE 802.15.4 Standard IEEE 802.15.4 and ZigBee 2/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Architecture and objectives Upper layers Network layer IEEE 802.2 LLC Other LLC Data link layer SSCS IEEE 802.15.4 MAC IEEE 802.15.4 IEEE 802.15.4 Physical layer 868/915 MHz PHY 2400 MHz PHY Architecture Objectives two physical (PHY) layer low-rate MAC layer low-power ZigBee for the upper layers low-complexity IEEE 802.15.4 and ZigBee 3/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Components Full Function Device Reduced Function Device (FFD) (RFD) Implements the entire standard Implements a reduced portion of the standard Coordinator manages (part of) the cannot be a (PAN) network coordinator PAN coordinator only communicates with manages the whole PAN FFDs (unique in the network) (Regular) Device communicates with FFDs and/or RFDs IEEE 802.15.4 and ZigBee 4/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Topology Peer-to-peer Star FFD RFD PAN C Coordinator C C neighboring nodes can all messages flow through communicate directly the center (hub) of the star only available to FFDs IEEE 802.15.4 and ZigBee 5/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Radio and modulation (1 of 2) Two distinct physical layers PHY 868/915 MHz PHY 2400 MHz Shared features direct sequence spread spectrum (DSSS) ISM (Industrial, Scientific and Medical) bands IEEE 802.15.4 and ZigBee 6/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Radio and modulation (2 of 2) PHY 868/915 MHz PHY 2400 MHz 2 MHz 5 MHz Channel 0 Channels 1-10 Channels 11-26 f (MHz) f (MHz) 868.0 868.6 902.0 928.0 2400.0 2483.5 868 MHz (Europe) 16 channels 1 channel (20 kbps) 250 kbps bandwidth 915 MHz (USA) orthogonal encoding 8 channel (40 kbps) (1 sym = 4 bits) differential encoding O-QPSK modulation (1 sym = 1 bit) BPSK encoding IEEE 802.15.4 and ZigBee 7/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Format of the PHY frame PHY Protocol Data Unit (PPDU) Synchronization Header PHY Header Start-of-frame Preamble Frame length PHY Service Data Unit (PSDU) delimiter 4 bytes 1 byte 1 byte ≤ 127 bytes Header Payload synchronization preamble is the same as the MSDU delimiter of the PHY frame maximum size of 127 bytes frame length IEEE 802.15.4 and ZigBee 8/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Available primitives Transceiver modes Link Quality Indication (LQI) RX_ON active in receive mode “quality” of received frames TX_ON active SNR, ED, or both in transmit mode Clear Channel Assessment TRX_OFF inactive (CCA) (idle mode) Different modes Channel Selection 1. energy above threshold 2. carrier sense only Energy Detection (ED) 3. combination of 1 and 2 IEEE 802.15.4 and ZigBee 9/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Addressing modes PAN address PANs can be co-located 16 bits chosen by the PAN coordinator Device address 64-bit IEEE Extended Unique Identifier (EUI-64) – 24-bit Organizationally Unique Identifier (OUI) – 40 bits assigned by the manufacturer 16-bit short address – assigned by the PAN coordinator during association Overhead reduction flag in the frame control field IEEE 802.15.4 and ZigBee 10/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Format of the MAC frame MAC Protocol Data Unit (MPDU) MAC Header MAC Service Data Unit (MSDU) MAC Footer Frame Sequence Frame check Addressing fields Payload control number sequence 2 bytes 1 byte ≤ 20 bytes Variable 2 bytes Header Frame payload frame control Footer sequence number frame check sequence addressing fields (FCS) ITU-T CRC-16 IEEE 802.15.4 and ZigBee 11/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Frame types Beacon frame synchronization and management of the PAN – list of devices with pending messages – superframe parameters Acknowledgment frame MAC payload MAC command command identifier (1 byte) command payload IEEE 802.15.4 and ZigBee 12/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Channel access methods MAC Non-beacon enabled Beacon enabled Superframe Structure Contention based Contention based Contention free Unslotted CSMA-CA Slotted CSMA-CA Reserved time slot IEEE 802.15.4 and ZigBee 13/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Superframe structure Active Beacon Beacon GTS GTS Inactive 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 CAP CFP SD = aBaseSuperFrameDuration*2 SO sym BI = aBaseSuperFrameDuration*2 BO sym IEEE 802.15.4 and ZigBee 14/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Active period Contention Access Period (CAP) always present in the superframe immediately follows the beacon slotted CSMA-CA protocol Contention Free Period (CFP) optional contiguous slots at the end of the superframe without CSMA-CA All transactions end within the CAP (CFP) IEEE 802.15.4 and ZigBee 15/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Superframe parameters Beacon interval BI = aBaseSuperFrameDuration· 2BO sym interval between subsequent beacons 0 ≤ BO ≤ 14, if BO = 15 no beacons Superframe duration SD = aBaseSuperFrameDuration· 2SO sym duration of the active part 0 ≤ SO ≤ BO ≤ 14, if SO = 15 only active period (no duty-cycle) aBaseSuperFrameDuration = 960 sym ≈ 32 µs (2.4 GHz PHY) IEEE 802.15.4 and ZigBee 16/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Synchronization Tracking mode the device gets the first beacon then activates the transceiver before the subsequent one Non tracking mode the device only gets a single beacon it has to reactivate the transceiver for at most aBaseSuperframeDuration· (2BO + 1) sym Orphaned device does not detect beacons for aMaxLostBeacons (4) superframes IEEE 802.15.4 and ZigBee 17/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture GTS management Features of GTSs unidirectional at most 7, all in the CFP each spanning one or more contiguous slots GTS allocation managed by the PAN coordinator – the device requests a GTS to the PAN coordinator – the PAN coordinator decides whether to assign it or not advertised in the GTS parameters of the superframe not always possible – no GTS available – cannot reduce the size of the CAP further IEEE 802.15.4 and ZigBee 18/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Frame spacing Frames need to be separated by an Inter Frame Space (IFS) Long frame Another frame LIFS Short frame Another frame SIFS if pframe ≤ aMaxSIFSFrameSize (18) bytes then SIFS (Short IFS) ≥ aMinSIFSPeriod (12) sym if pframe > aMaxSIFSFrameSize bytes then LIFS (Long IFS) ≥ aMinLIFSPeriod (40) sym IEEE 802.15.4 and ZigBee 19/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture The CSMA-CA algorithm Common features wait before transmitting without RTS/CTS Two variants slotted (beacon enabled mode CAP) unslotted (non-beacon enabled mode) Features backoff period slot of 20 sym (6= superframe slot) slotted variant aligns rx/tx to backoff periods IEEE 802.15.4 and ZigBee 20/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Initialization CSMA-CA Parameters NB =0 NB number of backoffs (i.e., backoff attempts) CW =2 CW contention window BE backoff exponent macMinBE = 3 (default) Yes Battery Life BE =min(2, Extension? macMinBE ) Battery Life Extension No power saving mode BE =macMinBE IEEE 802.15.4 and ZigBee 21/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Main loop Delay for a random backoff period Slotted mode ∈ [0, 2 BE -1] waiting and CCAs Perform CCA are aligned to backoff on backoff period boundary periods two CCAs before tx Yes Channel idle? backoff timer stopped at the No end of the CAP and CW =2, NB =NB +1 BE =min( BE +1, CW =CW -1 reactivated at the beginning aMaxBE ) of the subsequent one No NB > No macMaxCSMA CW =0? Backoffs ? In both cases Yes Yes default max backoffs is 4 Failure Success IEEE 802.15.4 and ZigBee 22/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Channel access example Slotted CSMA-CA B C 12 13 14 15 0 1 2 Superframe Slot Packet CCA arrival aUnitBackoffPeriod Backoff Data A Backoff Backoff Backoff Data B Backoff timer paused IEEE 802.15.4 and ZigBee 23/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Communication reliability CRC (FCS) check CRC-16 computed over header and payload checked against the FCS Acks and retransmissions at most aMaxFrameRetries = 3 ack waiting time is macAckWaitDuration (54 sym) IEEE 802.15.4 and ZigBee 24/60 M. Di Francesco October 2, 2013 Aalto University T-110.5111 WPAN Lecture Acks and retransmissions Ack timing Frame Ack tack aUnitBackoffPeriod Frame Ack tack tack = aTurnAroundTime (unslotted) aTurnAroundTime ≤ tack ≤ aTurnAroundTime + aUnitBackoffPeriod (slotted) tack < SIFS < LIFS, at most aMaxFrameRetries = 3 IEEE 802.15.4 and ZigBee 25/60 M.