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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
Project: IEEE P802.15 Working Group for Wireless Personal Area Networks (WPANs)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.
www.IEEE802.org
• Home Networking
• Automotive Networks
• Industrial Networks
• Interactive Toys
• Remote Metering
802.15.4 Applications Space
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.
IEEE 802.15.4 MAC
Upper Layers
IEEE 802.2 LLC Other LLC
IEEE 802.15.4
2400 MHz
PHY
IEEE 802.15.4
868/915 MHz
PHY
802.15.4 Architecture
IEEE 802.15.4 PHY OverviewOperating Frequency Bands
868MHz / 915MHz PHY
2.4 GHz
868.3 MHz
Channel 0 Channels 1-10
Channels 11-26
2.4835 GHz
928 MHz902 MHz
5 MHz
2 MHz
2.4 GHz PHY
IEEE 802.15.4 PHY Packet Structure
PreambleStart ofPacket
Delimiter
PHYHeader
PHY ServiceData Unit (PSDU)
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
6 Octets 0-127 Octets
IEEE 802.15.4: PHY Layer
Bit to symbolSymbol to
chipModulation
Input Bit Output signal
PHY FrequencyChannel
numbering
Spreading Data parameters
Chip rate ModulationBit rate
Symbol rate
Mappatura bit a simbolo
800/915 MHz
868-870 MHz
0300
kchip/sBPSK
20 kb/s
20 kbaud Binary
902- 928 MHz
Da 1 a 10600
kchip/sBPSK
40 kb/s
40 kbaud Binary
2.4 GHz2.4-2.4835
GHzDa 11 a 26
2.0 Mchip/s
O-QPSK250 kb/s
62.5 kbaud16-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
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 OverviewDesign Drivers
IEEE 802.15.4 MAC OverviewTypical Network Topologies
• 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 OverviewDevice Classes
Full function device
Reduced function device
Communications flow
Master/slave
PANCoordinator
IEEE 802.15.4 MAC OverviewStar Topology
Full function device Communications flow
Point to point Cluster tree
IEEE 802.15.4 MAC OverviewPeer-Peer Topology
Full function device
Reduced function device
Communications flow
Clustered stars - for example,cluster nodes exist between roomsof a hotel and each room has a star network for control.
IEEE 802.15.4 MAC OverviewCombined Topology
• 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 OverviewAddressing
IEEE 802.15.4 MAC OverviewGeneral Frame Structure
Payload
PH
Y L
ayer
MA
CLa
yer
MAC Header(MHR)
MAC Footer(MFR)
MAC Protocol Data Unit (MPDU)
MAC Service Data Unit(MSDU)
PHY Header(PHR)
Synch. Header(SHR)
PHY Service Data Unit (PSDU)
4 Types of MAC Frames:
• Data Frame
• Beacon Frame
• Acknowledgment Frame
• MAC Command Frame
15ms * 2n
where 0 n 14
Network beacon
Contention period
Beacon extensionperiod
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
GTS 2 GTS 1
GuaranteedTime Slot
Reserved for nodes requiring guaranteed bandwidth [n = 0].
IEEE 802.15.4 MAC OverviewOptional Superframe Structure
Contention Access Period Contention Free Period
• 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 OverviewTraffic Types
OriginatorMAC
RecipientMAC
MCPS-DATA.request
Data frame
MCPS-DATA.confirmMCPS-DATA.indication
Acknowledgement(if requested)
Channelaccess
IEEE 802.15.4 MAC OverviewMAC Data Service
Orig
inat
orR
ecipient
IEEE 802.15.4 PHY OverviewMAC 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
21/04/23 29/31
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) * 2SO цs
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 * 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) !!!!!