Introduction to IEEE LR-WPANs/ZigBee

Transcription

1 Introduction to IEEE LR-WPANs/ZigBee

2 Outline Introduction General Description Network topologies PHY Sublayer MAC Sublayer Superframe Structure Frame Structure PHY Specification 2450 MHz Mode 868/915 MHz Mode 2

3 Introduction

4 The IEEE 802 Family => Spanning Tree Bridge => Logical Link Control (LLC) Protocol => CSMA/CD Networks (Ethernet) MAC Protocol => Token Bus Networks MAC Protocol => Token Ring Networks MAC Protocol => Metropolitan Area Networks (MAN) => WLAN (wireless local area network) b => 2.4GHz Band; 11 Mbps; direct-sequence a => 5.0GHz Band; 54 Mbps; OFDM g => 2.4GHz Band; 54 Mbps; OFDM => WPAN (wireless personal area network) UWB (Ultra Wide Band) LR-WPAN (low rate wireless PAN) => WLL (wireless local loop) 4 LAN

5 Overview LR-WPANs stands for low-rate wireless personal area networks. Wireless personal area networks (WPANs) are used to convey information over relatively short distance. Unlike wireless local area networks (WLANs), connections effected via WPANs involve little or no infrastructure. This feature allows small, power-efficient, inexpensive solutions to be implemented for a wide range of devices. Typically operating in the personal operating space (POS) of 10m. 5

6 ZigBee & IEEE

7 ZigBee Membership ZigBee Alliance grows to over 90 members (August 16, 2004) Promoter Ember Honeywell Invensys Mitsubishi Electric Motorola Philips Samsung 7

8 Periodic data Sensors Intermittent data Light switch Repetitive, low-latency data Mouse Traffic Types The raw data rate will be high enough (maximum of 250 kb/s) to satisfy a set of simple needs such as interactive toys, but scalable down to the needs of sensor and automation needs (20 kb/s or below) for wireless communications. 8

9 General Description

10 General Description A LR-WPAN is a simple, low-cost communication network that allows wireless connectivity in applications with limited power and relaxed throughput requirements. Some of the characteristics of an LR-WPAN are: Over-the-air data rates of 250 kb/s, 40 kb/s, and 20 kb/s. Star or peer-to-peer operation Allocated 16 bit short or 64 bit extended addresses Allocation of guaranteed time slots (GTSs) Carrier sense multiple access with collision avoidance (CSMA-CA) channel access Fully acknowledged protocol for transfer reliability 10

11 Low power consumption Energy detection (ED) Link quality indication (LQI) General Description 16 channels in the 2450 MHz band, 10 channels in the 915 MHz band, and 1 channel in the 868 MHz band Two different device types can participate in an LR- WPAN network: Full-function device (FFD) Can talk to RFDs or other FFDs. Reduced-function device (RFD) Can only talk to an FFD. Intended for applications that are extremely simple. 11

12 Components of the IEEE WPAN The most basic component in the IEEE WPAN is the device. A device can be an RFD or an FFD. Two or more devices within a POS communicating on the same physical channel constitute a WPAN. A network shall include at least one FFD, operating as the PAN coordinator. An IEEE network is part of the WPAN family of standards. 12

13 Network Topologies Depending on the application requirements, the LR- WPAN may operate in either of two topologies: the star topology or the peer-to-peer topology. Each independent PAN will select a unique identifier. 13

14 Star Topology The communication is established between devices and a single central controller, called the PAN coordinator. A PAN coordinator is the primary controller of the PAN. The PAN coordinator may be mains powered, while the devices will most likely be battery powered. Applications that benefit from a star topology include home automation, personal computer (PC) peripherals, toys and games, and personal health care. 14

15 Star Network Formation After an FFD is activated for the first time, it may establish its own network and become the PAN coordinator. All star networks operate independently from all other star networks currently in operation. This is achieved by choosing a PAN identifier, which is not currently used by other network within the radio sphere of influence. Once the PAN identifier is chosen, the PAN coordinator can allow other devices to join its network; both FFDs and RFDs may join the network. 15

16 Peer-to-Peer Topology The peer-to-peer topology also has a PAN coordinator. Any device can communicate with any other device as long as they are in range of one another. Allows more complex network formations to be implemented, such as mesh networking topology. Applications such as industrial control and monitoring, wireless sensor networks, asset and inventory tracking, intelligent agriculture, and security would benefit from such a network topology. Can be ad hoc, self-organizing and self-healing. Allow multiple hops to route messages from any device to any other device on the network. 16

17 Peer-to-peer Network Formation Each device is capable of communicating with any other device within its radio sphere of influence. One device will be nominated as the PAN coordinator, for instance, by virtue of being the first device to communicate on the channel. An example of the use of the peer-to-peer communications topology is the cluster-tree. The cluster-tree network is a special case of a peer-to-peer network in which most devices are FFDs. An RFD may connect to a cluster tree network as a leave node at the end of a branch, because it may only associate with one FFD at a time. 17

18 Topology Models Star Mesh Cluster tree PAN Coordinator Full function device Reduced function device 18

19 LR-WPAN Device Architecture The upper layers consist of a network layer, which provides network configuration, manipulation, and message routing. an application layer provides the intended function of the device. LLC: logical link control. SSCS: service specific convergence sublayer. 19

20 PHY Sublayer The PHY provides two services The PHY data service The PHY management service interfacing to the physical layer management entity (PLME). The PHY data service enables the transmission and reception of PHY protocol data units (PPDUs) across the physical radio channel. The features of the PHY are activation and deactivation of the radio transceiver, ED, LQI, channel selection, clear channel assessment (CCA), and transmitting as well as receiving packets across the physical medium. 20

21 ZigBee Operating Bands 2.4 GHz PHY Channels MHz 2.4 GHz GHz 868MHz / 915MHz PHY Channel 0 Channels MHz MHz 902 MHz 928 MHz 21

22 Frequency Band and Data Rate Frequency Band Coverage Data # of Channels Rx Sensitivity Modulation 2.4 GHz ISM Worldwide 250 kbps dbm O_QPSK 868 MHz Europe 20 kbps 1-92 dbm BPSK 915 MHz ISM Americas 40 kbps dbm BPSK 22

23 MAC Sublayer The MAC sublayer provides two services: The MAC data service The MAC management service interfacing to the MAC sublayer management entity (MLME) service access point (SAP). The MAC data service enables the transmission and reception of MAC protocol data units (MPDUs) across the PHY data service. The features of the MAC sublayer are beacon management, channel access, GTS management, frame validation, acknowledged frame delivery, association, and disassociation. 23

24 Superframe Structure The LR-WPAN standard allows the optional use of a superframe structure. The format of the superframe is defined by the coordinator. The superframe is bounded by network beacons, is sent by the coordinator, and is divided into 16 equally sized slots. The beacon frame is transmitted in the first slot of each superframe. If a coordinator does not wish to use a superframe structure, it may turn off the beacon transmissions. The beacons are used to synchronize the attached devices, to identify the PAN, and to describe the structure of the superframes. 24

25 Superframe Structure without GTSs 25

26 Superframe Structure with GTSs 26

27 Frame Structure The LR-WPAN defines four frame structures A beacon frame, used by a coordinator to transmit beacons A data frame, used for all transfers of data An acknowledgement frame, used for confirming successful frame reception A MAC command frame, used for handling all MAC peer entity control transfers 27

28 Schematic View of the Beacon Frame 28

29 Schematic View of the Data Frame 29

30 Schematic View of the Acknowledgement Frame 30

31 Schematic View of the MAC Command Frame 31

32 Concept of Primitives 32

33 PHY Specification

34 Introduction The PHY is responsible for the following tasks: Activation and deactivation of the radio transceiver Energy detection (ED) within the current channel LQI for received packets CCA for CSMA-CA Channel frequency selection Data transmission and reception 34

35 Operating Frequency Range Frequency bands and data rates 35

36 Channel Assignments and Numbering A total of 27 channels, numbered 0 to 26, are available across the three frequency bands. Sixteen channels in the 2450 MHz band. Ten channels in the 915 MHz band. One channels in the 868 MHz band. The center frequency of these channels is defined as follows: 36

37 Receiver Sensitivity Definition 37

38 General Packet Format Each PPDU packet consists of the following basic components: A SHR (synchronization header), which allows a receiving device to synchronize and lock onto the bit stream. A PHR (PHY header), which contains frame length information. A variable length payload, which carriers the MAC sublayer frame. General packet format 38

39 Preamble field Packet Fields Used by the transceiver to obtain chip and symbol synchronization with an incoming message. Composed of 32 binary zeros. SFD (start-of-frame delimiter) field An 8 bit field indicating the end of the synchronization (preamble) field and the start of the packet data. Format of the SFD field 39

40 Frame length field Packet Fields 7 bits in length and specifies the total number of octets contained in the PSDU. PSDU field Has a variable length and carries the data of the PHY packet. For all packet types of length five octets or greater than seven octets, the PSDU contains the MAC sublayer frame (i.e., MPDU). 40

41 PHY Constants 41

42 PIB: PAN information base. PHY PIB Attributes 42

43 Data rate: 250 kb/s MHz PHY Specifications Modulation and spreading Employs a 16-ary quasi-orthogonal modulation technique. During each data symbol period, four information bits are used to select one of 16 nearly orthogonal pseudo-random noise (PN) sequences to be transmitted. The PN sequences for successive data symbols are concatenated. The aggregate chip sequence is modulated onto the carrier using offset quadrature phase-shift keying (O-QPSK) 43

44 2450 MHz PHY Specifications Reference modulator diagram Reference transmitter diagram 44

45 Symbol to Chip Mapping 45

46 O-QPSK modulation 2450 MHz PHY Specifications The chip sequences representing each data symbol are modulated onto the carrier using O-QPSK with half-sine pulse shaping. Pulse shape () p t t sin π 0 t 2Tc = 2Tc 0 otherwise 46

47 2450 MHz PHY Specifications Sample baseband chip sequences with pulse shaping Symbol rate The 2450 MHz PHY symbol rate shall be 62.5 ksymbol/s. Receiver sensitivity A compliant device shall be capable of achieving a sensitivity of -85 dbm or better. 47

48 868/915 MHz PHY Specifications 868/915 MHz band data rates 868 MHz: 20 kb/s. 915 MHz: 40 kb/s. Modulation and Spreading The 868/915 MHz PHY shall employ direct sequence spread spectrum (DSSS). The binary phase-shift keying (BPSK) is used for chip modulation. Differential encoding is used for data symbol encoding. 48

49 868/915 MHz PHY Specifications Reference modulator diagram Differential encoding Differential encoding is the modulo-2 addition (exclusive or) of a raw data bit. E = R E R E E n n n 1 n n n 1 is the raw data bit being encoded, is the corresponding differentially encoded bit, is the previous differentially encoded bit. 49

50 868/915 MHz PHY Specifications For each packet transmitted, R 1 is the first raw bit to be encoded and E 0 is assumed to be zero. Conversely, the decoding process, as performed at the receiver, can be described by: Rn = En En 1 For each packet received, E 1 is the first bit to be decoded, and E 0 is assumed to be zero. Bit-to-chip mapping Each input bit shall be mapped into a 15-chip PN sequence 50

51 868/915 MHz PHY Specifications BPSK modulation The chip sequences are modulated onto the carrier using BPSK with raised cosine pulse shaping (roll-off factor = 1). The chip rate is 300 kchip/s for the 868 MHz band and 600 kchip/s in the 915 MHz band. Pulse shape The raised cosine pulse shape (roll-off factor = 1) used to represent each baseband chip is described by () p t ( πt T ) ( πt T) sin / c cos / = πt/ T 1 4 t / T ( 2 2) c 51

52 Symbol rate 868/915 MHz PHY Specifications 868 MHz: 20 ksymbol/s 915 MHz: 40 ksymbol/s Receiver sensitivity A compliant device shall be capable of achieving a sensitivity of -92 dbm or better. 52

53 Receiver Architecture Over-Sampling Rate (n chip rate) RF A/D Coarse Synchronization Half-sine Matched Filter Packet Detection Fine Syn. and/or Start of Data Down Sampling to Chip Rate Data Stream Detection (Sym. Rate) OQPSK Demodulation (Sym. Rate) Despreading to (Sym. Rate) 53

54 Despreading and Demodulation C I1 C I2 C I3 C I4 C I5 C I6 C I F i n d + + M a x i m u m C Q1 C Q2 C Q3 C Q4 C Q5 C Q6 C Q16 54

55 CSMA/CA Algorithm The CSMA/CA algorithm shall be used before the transmission of data or MAC command frames transmitted within the CAP, and shall not be used for the transmission of beacon frames, acknowledgment frames or data frames transmitted in the CFP. NB is the number of times the CSMA/CA algorithm was required to backoff. CW defines the number of backoff periods that need to be clear of channel activity. BE is related to how many backoff periods a device shall wait before assess a channel. *backoff = 20 symbols 55

56 CSMA-CA NB=0,CW=2 Battery life extension? Y BE=lesser of (2,macMinBE) Slotted N BE=macMinBE Locate backoff period boundary Delay for random (2 BE 1) unit backoff periods Performance CCA on backoff period boundary Channel idle? Y N CW=2,NB=NB+1, BE=min(BE+1,aMaxBE) CW=CW-1 N NB>macMaxCS MABackoff? CW=O? N Y Failure 56 Y Success

57 CSMA-CA NB=0, BE=macMinBE Unslotted Delay for random (2 BE 1) unit backoff periods Perform CCA Channel idle? Y N NB=NB+1, BE=min(BE+1,aMaxBE) N NB>macMaxCS MABackoffs? 57 Y Failure Success