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3. Towards A Standards-Based Stack
Thomas Watteyne @ EDERC 2010
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IEEE
IETF
Protocol Stack
Application
OpenADR, XML
Transport
TCP, UDP
routing
IETF RPL
IETF 6LoWPAN
MAC
IEEE 802.15.4E
PHY
IEEE 802.15.4-2006
Thomas Watteyne @ EDERC 2010
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Section 3 - Overview
3. Towards A Standards-Based Stack
3.1
3.2
3.3
3.4
3.5
IEEE 802.15.4E
IETF 6LoWPAN
IETF RPL
OpenWSN
Conclusions
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3.1 IEEE802.15.4E
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15.4-2006, 15.4 PHY, 15.4 MAC, 15.4E?
•
IEEE 802 LAN/MAN Standards Committee
–
–
•
IEEE 802.15 Working Group for WPAN
–
–
–
•
wireless Personal Area Network
standardizes a.o. 802.15.1 (Bluetooth), 802.15.4
http://www.ieee802.org/15/
IEEE 802.15 WPAN Task Group 4
–
–
–
–
–
–
•
standardizes a.o. 802.3 (Ethernet) 802.11
http://www.ieee802.org/
low data rate solution with multi-month to multi-year battery life, very low complexity
operating in an unlicensed, international frequency band
sensors, interactive toys, smart badges, remote controls, and home automation, etc.
first standard in 2003, updated in 2006
standardizes both PHY and MAC
http://www.ieee802.org/15/pub/TG4.html
IEEE 802.15 WPAN Task Group 4e
–
–
–
–
–
define a MAC amendment to the existing standard 802.15.4-2006
enhance and add functionality to the 802.15.4-2006 MAC to better support the industrial markets
uses 802.15.4-2006 PHY
draft standard on 03/08/2010, integrated in next revision of the 802.15.4 standard (exp. 2011)
http://www.ieee802.org/15/pub/TG4e.html
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IEEE802.15.4 – Overview
• Emphasis of IEEE 802.15.4 is:
– low-cost, low-speed ubiquitous communication between nearby devices with little to
no underlying infrastructure
– basic framework assumes 10-meter communications area @ 250 kbit/s
– lower transfer rates of 20, 40 and 100 kbit/s are now considered too
– to meet embedded constraints, several PHY layers are available
• Key technology features are:
–
–
–
–
–
real-time suitability by reservation of guaranteed time slots
collision avoidance through CSMA/CA
integrated support for secure communications (128-bit AES encryption)
power management functions such as link quality and energy detection
16 channels in ISM bands for operation, i.e. 868-868.8 MHz (Europe), 902-928 MHz
(North America), 2400-2483.5 MHz (worldwide)
– star and mesh topologies can theoretically be built
– support for low-latencies and dynamic device addressing
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IEEE802.15.4 – PHY Layer
• The 2006 revision of the standard defines 4 PHY layers:
–
–
–
–
868/915 MHz DSSS with binary phase shift keying (BPSK)
868/915 MHz DSSS with offset quadrature phase shift keying (OQPSK)
2450 MHz DSSS with offset quadrature phase shift keying (OQPSK)
868/915 MHz PSSS, i.e. combination of binary keying and amplitude shift keying
• The 2007 IEEE 802.15.4a version includes 2 PHY layers more:
– Chirp Spread Spectrum (CSS) @ 2450 MHz ISM
– Direct Sequence Ultra-wideband (UWB) @ < 1GHz, 3-5GHz, 6-10 GHz
• Beyond these PHYs at the three bands, there are:
– IEEE 802.15.4c for 314-316, 430-434 and 779-787MHz bands in China
– IEEE 802.15.4d for 950-956MHz band in Japan
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IEEE802.15.4 – PHY Layer
•
binary phase shift keying (BPSK)
•
quadrature phase shift keying (QPSK)
•
offset quadrature phase shift keying (OQPSK)
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IEEE802.15.4 – 2.4GHz PHY
•
•
•
•
•
O-QPSK, 250 kb/s, 62.5 ksymbol/s
Direct Sequence Spread Spectrum
Max PSDU = 127B
Turnaround: TX-RX ≤ RX-TX ≤ 192us
ED over 8 symbol periods
DSSS: 4 bits of information = 32 chips
(raw data rate of 2Mbps)
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IEEE802.15.4 – MAC Layer
• Some key attributes:
–
–
–
–
CSMA/CA channel access
manages access to the physical channel and network beaconing
controls frame validation, guarantees time slots, handles node associations
offers hook points for secure services
• In more details:
– networks which are not using beaconing mechanisms utilize an un-slotted variation
which is based on the listening of the medium, leveraged by a random exponential
backoff algorithm (acknowledgments do not adhere to this discipline)
– confirmation messages may be optional under certain circumstances, in which case a
success assumption is made; timeout-based retransmission can be performed a
number of times
– due to the maximization of battery life, the protocols tend to favor methods
implementing periodic checks for pending messages, the intensity of which depends
on application needs
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IEEE802.15.4 – MAC Layer
• There are two general channel access methods:
• Non-Beacon Network:
– simple, traditional multiple access system used in simple peer networks
– standard CSMA conflict resolution
– positive acknowledgement for successfully received packets
• Beacon-Enabled Network
– can be used in beacon-request mode without superframes
– superframe structure - network coordinator transmits beacons at predetermined
intervals
– dedicated bandwidth and low latency
– low power consumption mode for coordinator
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IEEE802.15.4 – MAC Layer
• Super-Frame Structure for Beacon-Enabled Mode:
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IEEE802.15.4 – Packet Format
4B of data
(all 0’s)
0x7A
synchronization header
0-127
physical header
16-bit
CRC
beacon, ACK,
DATA, command
MAC header
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IEEE802.15.4 – 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
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IEEE802.15.4 – Star Topology
PAN
Coordinator
Master/Slave
Full function device
Communications flow
Reduced function device
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IEEE802.15.4 – P2P Topology
Cluster tree
Point to point
Full function device
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Communications flow
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IEEE802.15.4 – Combined Topology
Clustered stars - for example,
cluster nodes exist between rooms
of a hotel and each room has a
star network for control.
Full function device
Reduced function device
Communications flow
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IEEE802.15.4 Scenario
• First node makes sure it is alone, scans for a “good” frequency and
transmits beacons.
• New node scans (active or passive) and hears beacon. Sends an
association request (indirect response). Tracks beacon periodically.
• Upstream data transmitted in CAP using CSMA/CA. If downstream data,
coordinator set pending data field.
• Device can ask to (dis)allocate a GTS to the coordinator. GTS slots are
announced in beacon, CSMA is not used in GTS slot.
• Secondary coordinators to create a generalized star topology.
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IEEE802.15.4 - Problems
• Powered Routers
– router nodes have their radio on all the time
– if battery-powered: 2400mAh AA pack @ 81mA (CC2420) -> 29h of lifetime
– assumption: mains powered
• Single channel operation
– WiFi-like: one channel for the whole network
– suffers from external interference (WiFi, Bluetooth)
– suffers from persistent multipath fading (especially indoors)
• Topologies
– works great in star topologies
– e.g. multiple switches connected to a single lamp
– extended star topologies are hard to manage
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IEEE802.14.4E - TSCH
• Time Synchronized Channel
Hopping
• cut time into slots
• have nodes follow a common
schedule
B
D
A
C
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IEEE802.14.4E - TSCH
• The channel offset is translated
to frequency using a translation
function
• This insures that successive
packets sent over a same link
are sent over all
frequency = (absolute slot number + channel offset)%16
frequencies
superframe ASN channel offset frequency
– iff the superframe length and
number if frequencies are
mutually prime
1
8
1
9
2
18
1
3
3
28
1
12
4
38
1
6
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IEEE802.14.4e - TSCH
A
B
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Thomas Watteyne @ EDERC 2010
TsPacketWaitTime
TsRxOffset
Watchdog_TXACK
TsTxAckDelay
Stopping_time
Startup_time
SETTING_
CHANNEL
STAR
TED
STARTING
2
1
RXDATA
7
TXACK
WAIT_TXACK
8
9
Guard_time_large
STOPPING
STOPPED
10
11
Guard_time_small
Watchdog_TXDATA
TsAckWaitTime
Watchdog_TXACK+Guard_time_small
TsRxAckDelay
TsTxOffset
Startup_time+Guard_time_large
SETTING_
CHANNEL
STARTING
1
STARTED
2
WAIT_RX
ACK
TXDATA
4
5
RXACK
6
STOPPING
10
SLOT_TIME
>TsRxOffset+TsPacketWaitTime+TsTxAckDelay+Watchdog_TXACK+Stopping_time
STOPPED
11
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IEEE802.15.4e – Synchronization
clocks drift
Periodic realignment
(10ppm typical)
(within a clock tick)
∆t
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Improved Reliability
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Improved Connectivity
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Improved Throughput
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Improved Energy Consumption
• 2ms maximum desynchronization
• 20ppm relative drift
• Resynchronization every
100 seconds (10ms slots)
0.010% idle duty cycle
• 25mA when mote is
active
• 2400mAh batteries
(AA batteries)
lifetime of 109 years
(>> shelf-life)
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Improved Throughput
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IEEE 802.15.4e - TSCH
visible light
sensor
IR light
sensor
antenna
humidity
sensor
CC2420
radio
•
•
•
•
•
TelosB mote
TinyOS operating system
30ms time slots
19kB ROM / 3kB RAM
10kbps over 14 hops
MSP430
microcontroller
1
2
3
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3.2 IETF 6LoWPAN
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IPv4 vs. IPv6
• Internet Protocol v4 (IPv4):
–
–
–
–
IPv4 (RFC 791) originates from 1981
upper layer protocols responsible for end-to-end reliability
works over almost any layer 2 network and with many routing protocols
addressing is being pushed to extremes by Internet growth
• Internet Protocol v6 (IPv6):
– IPv6 (RFC 2460) is the next generation of the Internet Protocol
– complete redesign on IP addressing: hierarchical 128-bit address with decoupled host
identifier; stateless auto-configuration; etc
– simple routing and address management
– majority of traffic not yet IPv6 but most PC operating systems already have IPv6,
governments are starting to require IPv6, most routers already have IPv6 support
– IPv6 transition is coming slowly but quietly
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IPv4 vs. IPv6
• IPv4 ...
... versus IPv6 addressing:
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IPv4 vs. IPv6
Monday, September 26, 2011
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IP headers
IPv4 header [RFC791], 1981
IPv6 header [RFC791], 1998
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IETF 6LoWPAN - Overview
• Key properties:
– IP for very low power embedded devices
– IETF Standard for IPv6 over IEEE 802.15.4: RFC4944, to be obsolete by IPHC
– 80% compression of headers
– IPv6 40-byte header -> 2 bytes (best case)
– UDP 8-byte header -> 4 bytes
– end-to-end Internet integration
– fragmentation (1260 byte IPv6 frame -> 127 byte 802.15.4 frames)
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Header Compaction
Not compacted
Well-known value
Value inferred from
IEEE802.15.4 header
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Internet Integration
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3.3 IETF RPL
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IETF ROLL - Overview
• Routing Over Low-Power and Lossy Networks (ROLL):
–
–
–
–
IETF information discussion started in 2008
Finalizing “RPL: IPv6 Routing Protocol for Low power and Lossy Networks”
website: http://tools.ietf.org/wg/roll
list: http://www.ietf.org/mail-archive/web/roll/current/threads.html
• Since WSNs are application specific, 4 scenarios are dealt with:
–
–
–
–
building applications:
home applications:
industrial applications:
urban applications:
draft-ietf-roll-building-routing-reqs
draft-ietf-roll-home-routing-reqs
RFC 5673
RFC 5548
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IETF ROLL – RPL
• Adopted as a working document by IETF ROLL on August 3, 2009
• Close integration with IPv6/6LoWPAN
– DAG Information Option (DIO)
– Destination Advertisement Option (DAO)
• Core operation:
– build a Directed Acyclic Graph (DAG) onto the connectivity graph of the
network, directed toward a DAG root
– each node has at least one DAG parent; nodes send inward traffic to their
DAG parent
– nodes announce their presence to the DAG root using Destination
Advertisement
– Source routing is used for outward traffic
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IETF ROLL – RPL
• Constraint Based Routing
– finding the shortest path according to some metrics satisfying a set of
constraints
• Objective Code Point (OCP) included in DIO:
– The set of metrics used within the DAG
e.g. Expected Transmission Count (ETX)
– The objective functions used to determine the least cost constrained paths
in order to optimize the DAG
e.g. minimize ETX
– The function used to compute DAG Depth
e.g. DAG Depth is equivalent to ETX
– The functions used to construct derived metrics for propagation within a
DIO
e.g. additive
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wsn.eecs.erkeley.edu
IETF ROLL – RPL
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wsn.eecs.erkeley.edu
IETF ROLL – RPL
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IETF ROLL – RPL
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3.4 OpenWSN
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Charter
The OpenWSN project serves as a repository
for open-source implementations of protocol
stacks based on Internet of Things standards,
using a variety of hardware and software
platforms.
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Open Source
• http://openwsn.berkeley.edu/
• Source code repository:
Subversion with public check
out
• Documentation: wiki
• Project management: Timeline
& Roadmap
• Bug reporting: ticketing system
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Hardware/Software Platforms
• Hardware:
– TelosB (2004)
MSP430f1611 (16-bit, 8MHz,
10kB RAM, 48kB ROM)
CC2420
– Jennic JN5140 (2009)
32-bit, 32MHz, 128kB RAM,
• Software:
128kB ROM
– TinyOS
15.4 radio with RF ToF engine
– Contiki
– Atmel RAVEN Stick (2009)
AT90USB1287 (8-bit, 16MHz,
– FreeRTOS
8kB RAM, 128kB ROM),
– uC-OS II/III
AT86RF230
– no-OS
Thomas Watteyne @ EDERC 2010
– GINA 2.0 (2009)
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Slot Organization
//Slot Durations
enum {
Startup_time
Guard_time_large
Guard_time_small
Watchdog_TXDATA
Watchdog_TXACK
Stopping_time
TsRxOffset
TsTxOffset
TsPacketWaitTime
TsRxAckDelay
TsTxAckDelay
TsAckWaitTime
//SLOT_TIME
SLOT_TIME
= 114, //32kHz ticks = 3.479ms
= 33, //32kHz ticks = 1.007ms
= 16, //32kHz ticks = 0.488ms
= 393, //32kHz ticks = 11.993ms
= 213, //32kHz ticks = 6.500ms
= 17, //32kHz ticks = 0.519ms
= Startup_time,
= Startup_time+Guard_time_large,
= Watchdog_TXDATA+Guard_time_large
= 1, //go in reception mode immediately
= TsRxAckDelay+Guard_time_small,
= Watchdog_TXACK+Guard_time_small
>= TsRxOffset+TsPacketWaitTime+TsTxAckDelay+Watchdog_TXACK+Stopping_time
= 983, //ticks = 29.999ms
};
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Full Debugging Environment
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48kB
Memory Usage
21960 B
30788 B
RPL
802.15.4E
10kB
8828 B
55
OS
ROM
2920B
Thomas Watteyne @ EDERC 2010
1352 B
1568 B
RAM
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Next Step
actuation
OpenADR
server
sensor.network.com
data
collection
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3.5 Conclusions
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Conclusions
• Lifting barriers to adoption
–
–
–
–
–
Robust wireless communication through channel hopping
A fully standards-based solution
Internet integration provides ease of use
Aggressive duty cycling provides <1% duty cycle
Being implemented on the low-end of today’s motes
• openwsn.berkeley.edu
The Internet
Thomas Watteyne @ EDERC 2010