Yao Liang (IUPUI, Indianapolis USA) The IEEE 802.15.4 standard Credits to: Yao Liang (IUPUI, Indianapolis USA)

Wireless Simplified Stack 802.15.4

Principal options and difficulties Medium access in wireless networks is difficult mainly because of Impossible (or very difficult) to send and receive at the same time Interference situation at receiver is what counts for transmission success, but can be very different from what sender can observe High error rates compound the issues Requirement As usual: high throughput, low overhead, low error rates, … Additionally: energy-efficient, handle switched off devices!

Requirements for energy-efficient MAC protocols Recall Transmissions are costly Receiving about as expensive as transmitting Idling can be cheaper but is still expensive Energy problems Collisions and high BERs – wasted effort when two packets collide or corrupted packet Overhearing – waste effort in receiving a packet destined for another node Idle listening – sitting idly and trying to receive when nobody is sending Protocol overhead Always nice: Low complexity solution

Schedule- vs. contention-based MACs Schedule-based MAC A schedule exists, regulating which participant may use which resource at which time (TDMA component) Schedule can be fixed or computed on demand Usually: mixed – difference fixed/on demand is one of time scales Usually, collisions, overhearing, idle listening no issues Needed: time synchronization! Contention-based MAC Risk of colliding packets is deliberately taken Hope: coordination overhead can be saved, resulting in overall improved efficiency Mechanisms to handle/reduce probability/impact of collisions required Usually, randomization used somehow

www.IEEE802.org Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs) Submission Title: [IEEE 802.15.4 Tutorial] Date Submitted: [4 January, 2003] Source: [Jose Gutierrez] Company: [Eaton Corporation] Address: [4201 North 27th Street, Milwaukee WI. 53216] Voice:[(414) 449-6525], FAX: [(414) 449-6131], E-Mail:[josegutierrez@eaton.com] Re: [IEEE 802.15.4 Overview; Doc. IEEE 802.15-01/358r0, TG4-Overview; Doc IEEE 802.15-01/509r0] Abstract: [This presentation provides a tutorial on the 802.15.4 draft standard.] Purpose: [] Notice: This document has been prepared to assist the IEEE P802.15. It is offered as a basis for discussion and is not binding on the contributing individual(s) or organization(s). The material in this document is subject to change in form and content after further study. The contributor(s) reserve(s) the right to add, amend or withdraw material contained herein. Release: The contributor acknowledges and accepts that this contribution becomes the property of IEEE and may be made publicly available by P802.15.

802.15.4 Applications Space Home Networking Automotive Networks Industrial Networks Interactive Toys Remote Metering

802.15.4 Applications Topology Cable replacement - Last meter connectivity Virtual Wire Wireless Hub Stick-On Sensor Mobility Ease of installation

Some needs in the sensor networks Thousands of sensors in a small space  Wireless but wireless implies Low Power! and low power implies Limited Range. Of course all of these is viable if a Low Cost transceiver is required

Solution: LR-WPAN Technology! By means of IEEE 802.15.4

802.15.4 General Characteristics Data rates of 250 kb/s, 40 kb/s and 20 kb/s. Star or Peer-to-Peer operation. Support for low latency devices. CSMA-CA/TDMA channel access. Dynamic device addressing. Fully handshaked protocol for transfer reliability. Low power consumption. Frequency Bands of Operation 16 channels in the 2.4GHz ISM band 10 channels in the 915MHz ISM band 1 channel in the European 868MHz band.

802.15.4 Architecture Upper Layers IEEE 802.2 LLC Other LLC IEEE 802.15.4 MAC IEEE 802.15.4 IEEE 802.15.4 868/915 MHz 2400 MHz PHY PHY

Operating Frequency Bands IEEE 802.15.4 PHY Overview Operating Frequency Bands Channel 0 Channels 1-10 868MHz / 915MHz PHY 2 MHz 868.3 MHz 902 MHz 928 MHz 2.4 GHz PHY Channels 11-26 5 MHz 2.4 GHz 2.4835 GHz

IEEE 802.15.4 PHY Packet Structure PHY Packet Fields Preamble (32 bits) – synchronization Start of Packet Delimiter (8 bits) PHY Header (8 bits) – PSDU length PSDU (0 to 1016 bits) – Data field Start of Packet Delimiter PHY Header PHY Service Data Unit (PSDU) Preamble 6 Octets 0-127 Octets

Mappatura bit a simbolo IEEE 802.15.4: PHY Layer Input Bit Output signal Symbol to chip Bit to symbol Modulation PHY Frequency Channel numbering Spreading Data parameters Chip rate Modulation Bit rate Symbol rate Mappatura bit a simbolo 800/915 MHz 868-870 MHz 300 kchip/s BPSK 20 kb/s 20 kbaud Binary 902- 928 MHz Da 1 a 10 600 kchip/s 40 kb/s 40 kbaud 2.4 GHz 2.4-2.4835 GHz Da 11 a 26 2.0 Mchip/s O-QPSK 250 kb/s 62.5 kbaud 16-ary Orthogonal 16 channels, 5 MHz each

IEEE 802.15.4 PHY Primitives PHY Data Service PHY Management Service PD-DATA – exchange data packets between MAC and PHY PHY Management Service PLME-CCA – clear channel assessment PLME-ED - energy detection PLME-GET / -SET– retrieve/set PHY PIB parameters PLME-TRX-ENABLE – enable/disable transceiver

Simple but flexible protocol IEEE 802.15.4 MAC Overview Design Drivers Extremely low cost Ease of implementation Reliable data transfer Short range operation Very low power consumption Simple but flexible protocol

Typical Network Topologies IEEE 802.15.4 MAC Overview Typical Network Topologies An example network topology in the area of home (living room) control. A set-top-box is acting as the master with a remote, TV, DVD, lamp and curtains enumerated on the network. This has no functionality since the slaves can only talk to the master. What the consumer actually wants is to be able to control the TV, DVD, lamp and curtains using the remote. In this case there needs to be some virtual peer-to-peer links between the remote and the other devices on the network. The mechanism of creating these links is known as pairing.

IEEE 802.15.4 MAC Overview Device Classes Full function device (FFD) Any topology Network coordinator capable Talks to any other device Reduced function device (RFD) Limited to star topology Cannot become a network coordinator Talks only to a network coordinator Very simple implementation

IEEE 802.15.4 MAC Overview Star Topology PAN Coordinator Master/slave Full function device Communications flow Reduced function device

IEEE 802.15.4 MAC Overview Peer-Peer Topology Point to point Cluster tree Full function device Communications flow

IEEE 802.15.4 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. Communications flow Full function device Reduced function device

IEEE 802.15.4 MAC Overview Addressing All devices have IEEE addresses Short addresses can be allocated Addressing modes: Network + device identifier (star) Source/destination identifier (peer-peer)

General Frame Structure IEEE 802.15.4 MAC Overview General Frame Structure 4 Types of MAC Frames: Data Frame Beacon Frame Acknowledgment Frame MAC Command Frame

Optional Superframe Structure IEEE 802.15.4 MAC Overview Optional Superframe Structure GTS 2 GTS 1 Contention Access Period Contention Free Period 15ms * 2n where 0  n  14 Network beacon Transmitted by network coordinator. Contains network information, frame structure and notification of pending node messages. Beacon extension period Space reserved for beacon growth due to pending node messages Contention period Access by any node using CSMA-CA Guaranteed Time Slot Reserved for nodes requiring guaranteed bandwidth [n = 0].

IEEE 802.15.4 MAC Overview Traffic Types Periodic data Application defined rate (e.g. sensors) Intermittent data Application/external stimulus defined rate (e.g. light switch) Repetitive low latency data Allocation of time slots (e.g. mouse)

IEEE 802.15.4 MAC Overview Originator Recipient MAC Data Service MCPS-DATA.request Channel access Data frame Originator Recipient Acknowledgement (if requested) MCPS-DATA.indication MCPS-DATA.confirm

IEEE 802.15.4 PHY Overview MAC Primitives MAC Data Service MCPS-DATA – exchange data packets between MAC and PHY MAC Management Service MLME-ASSOCIATE/DISASSOCIATE – network association MLME-SYNC / SYNC-LOSS - device synchronization MLME-SCAN - scan radio channels MLME-GET / -SET– retrieve/set MAC PIB parameters MLME-START / BEACON-NOTIFY – beacon management MLME-POLL - beaconless synchronization MLME-GTS - GTS management MLME-ORPHAN - orphan device management MLME-RX-ENABLE - enabling/disabling of radio system

A numerical example Adopting beacon-enabled networks; Data transfer protocols (e.g. towards PANC); Maximum bandwidth 250 kb/s = 62.5 ksym/s (16-ary coding, 1sym = 4 bits); Maximum number of GTS (Guaranteed Time Slots) = 7. Only part of the MAC superframe can be allocated to real-time services; the Superframe Order (SO) enlarges or reduces the distance between two consecutive beacons. AP= 16 (slots) * 960 (baseslotduration) * 2SO цs 17/04/2017

Transfer of large data sets (1) Suppose you want to transmit 1 picture (P2P), use the lowest resolution (80 * 64 pel): is 1.6 kBytes Maximum MAC MSDU (payload) is 102 bytes, i.e. 16 MAC frames each resulting in 132 bytes = 264 sym at the PHY layer; The average amount of time to transmit the data in CSMA is (BE=2, default) w/o taking into account traffic and different sources of overheads: 16 * [(1.5 (avg BT) * BP) + 2 * SP (CCA) + 264 ] sym/ 62.5 Ksym/s = 80 ms (optimistic); In free access: 16 * 264 sym / 62.5 Ksym/s = 68 ms (but pay attention at the reservation cost). The optimistic time to send the picture in CSMA is not bad but no guarantees are given to operate at 12 fps. Accurate performance studies on the medium must be done in order to estimate the traffic volume (and the caused delay) as a function of the time. In the CFP you pay the reservation cost which is dependent on the Superframe Order.

Transfer of large data sets (2) To fit the transmission into 6 slots of CAP we have to use SO = 4: 960 цs * 24 * 6 > 80 ms; If we want to use the GTSs: we have an overhead of 1 superframe + minimum CAP (440 symbols) = 16 * 960 * 24 цs + 7 ms = 100 ms (maximum) !!!!! 2 examples show how to set the Superframe Order so to fit the transaction into the CAP or the CFP. Reminder: the longer the superframe period, the more distant the beacons approaching several minutes. From a networking point of view, distant beacons generate problems concerning time synchronization and bandwidth dynamically allocated to different stations.