Transcript Teleinformatique
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
or corrupted packet
Overhearing
node – wasted effort when two packets collide – waste effort in receiving a packet destined for another
Idle listening
sending – sitting idly and trying to receive when nobody is –
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:[[email protected]]
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 Mobility Stick-On Sensor 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 transceiver is required Low Cost
By means of
Solution:
LR-WPAN Technology!
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
868/915 MHz PHY IEEE 802.15.4
2400 MHz PHY
IEEE 802.15.4 PHY Overview Operating Frequency Bands 868MHz / 915MHz PHY
Channel 0 868.3 MHz
2.4 GHz PHY
2.4 GHz Channels 1-10 902 MHz 2 MHz 928 MHz Channels 11-26 5 MHz 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
Preamble Start of Packet Delimiter
6 Octets
PHY Header PHY Service Data Unit (PSDU)
0-127 Octets
IEEE 802.15.4: PHY Layer
Input Bit Bit to symbol Symbol to chip Modulation Output signal
PHY Frequency
800/915 MHz 2.4 GHz 868-870 MHz 902- 928 MHz 2.4-2.4835 GHz
Channel numbering
0 Da 1 a 10 Da 11 a 26
Spreading Chip rate Modulation
300 kchip/s 600 kchip/s 2.0 Mchip/s BPSK BPSK O-QPSK
Bit rate
20 kb/s 40 kb/s 250 kb/s
Data parameters Symbol rate
20 kbaud 40 kbaud 62.5 kbaud
Mappatura bit a simbolo
Binary Binary 16-ary Orthogonal 16 channels, 5 MHz each
IEEE 802.15.4 PHY Primitives PHY Data 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
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
IEEE 802.15.4 MAC Overview Typical Network Topologies
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 Full function device Reduced function device Master/slave Communications flow
IEEE 802.15.4 MAC Overview Peer-Peer Topology
Point to point Full function device Cluster tree 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)
IEEE 802.15.4 MAC Overview General Frame Structure
Synch. Header (SHR) PHY Header (PHR) MAC Header (MHR) Payload MAC Service Data Unit (MSDU) MAC Protocol Data Unit (MPDU) PHY Service Data Unit (PSDU) MAC Footer (MFR) 4 Types of MAC Frames: • Data Frame • Beacon Frame • Acknowledgment Frame • MAC Command Frame
Network beacon Beacon extension period Contention period Guaranteed Time Slot
IEEE 802.15.4 MAC Overview Optional Superframe Structure
Contention Access Period GTS 2 GTS 1 Contention Free Period 15ms * 2 n where 0 n 14 Transmitted by network coordinator. Contains network information, frame structure and notification of pending node messages.
Space reserved for beacon growth due to pending node messages Access by any node using CSMA-CA 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 MAC Data Service
Originator MAC
MCPS-DATA.request
Channel access
Recipient
Data frame Acknowledgement (if requested)
MAC
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.
AP= 16 (slots) * 960 (baseslotduration) * 2 SO цs
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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).
Transfer of large data sets (2) • To fit the transmission into 6 slots of CAP we have to use SO = 4: – 960 цs * 2
4
* 6 > 80 ms; • If we want to use the GTSs: – we have an overhead of 1 superframe + minimum CAP (440 symbols) = 16 * 960 * 2
4