Standards Certification Education & Training Publishing Conferences & Exhibits Time Synchronized Meshing K. Pister Prof. EECS, UC Berkeley Founder & CTO, Dust Networks This document is provided strictly for the purpose of gathering information leading to the development of an ISA standard, recommended practice or technical report. Copies may be reproduced and distributed, in whole or in part, but only for the following purposes: Review of and comment on the ISA-SP100 draft proposal Submission to ISA-SP100 Committee Informing and educating others about the ISA-SP100 draft standard development process.
This document is provided strictly for the purpose of gathering information leading to the development of an ISA standard, recommended practice or technical report. Copies may be reproduced and distributed, in whole or in part, but only for the following purposes:Review of and comment on the ISA-SP100 draft proposalSubmission to ISA-SP100 CommitteeInforming and educating others about the ISA-SP100 draft standard development process.
2 Sept. 11th, 2006
Goals
• Non-fanaticism– TDMA & CSMA– Centralized & decentralized management– Efficient use of powered infrastructure when available
• Conceptually and practically simple• 802.15.4 MAC w/ extensions• Provide framework to approach limits of the radio
– 16x250kbps, ~1ms packets
•
3 Sept. 11th, 2006
Statement of Religious Alignment
• Time synchronization is required– Application– Low power– Multi-channel
• Multi-channel is required– Reliability– Bandwidth, scale
4 Sept. 11th, 2006
802.15.4 Slot and superframe timing
• Slot length– When SO = 0 60 symbols 0.96ms
• Active superframe duration– 16 slots 15.36ms when SO=0
• Superframe duration– 15.36ms * 2BO ; BO = 0..14– Up to 4 minutes (> 250,000 slots)
16|17|18|19|20|21|22|23|24|25|26|27|28|…
Semi-active
Channel-hopping
5 Sept. 11th, 2006
Timeslots and Frames
• Each mote-to-mote communication happens within a scheduled timeslot
• All timeslots are contained within a frame
• Frames repeat in time• Multiple frames can operate
– 10ms / PDR per hop– Statistical, but well modeled
• Scale– > 1,000 nodes per gateway channel
12 Sept. 11th, 2006
Industrial Automation Use Cases
Monitoring
Diagnostics
Configuration
Handheld
Peer to PeerPhase II
Simulation of a 250 node network(courtesy Bob Karschnia)
13 Sept. 11th, 2006
Multiple graphs Multiple frames
BA
CA
BA
Time
Cha
nnel
BA
CA
BA
BA
BA
CA
BC
CB
CA
BA
BA
BC
CB
A
CB
Cycle M of red frame Cycle M+1
14 Sept. 11th, 2006
Frames overlayed
BA
CB CA
BA
Time
Cha
nnel
BC
BA
CA
BA
BA
BA
CA
CA
BA
CB
BA
A
CB
• Cell collisions can be avoided by time or channel partitioning• Intentional scheduling collisions are resolved by packet priority and graph priority
15 Sept. 11th, 2006
Subnetworks: single-hop, low latency
G
CB
E
AF
H
Black superframe• All motes• 1,000 slots (10 seconds)• Data, Health reports up• Control info downRed superframe• Mote F is light switch• Mote A is light• 1 slot, ~10ms latencyBlue superframe• Mote H is temp sensor• Mote B is HVAC control point• 100 slots, ~1second latency
Motes A and B are likely poweredAll frames on all the timeAll other motes run at <100uA
16 Sept. 11th, 2006
Subnetworks 2: reliable multi-hop control
G
CB
E
AFH
Black superframe• All motes• 10s period• Data, Health reports up• Control info downRed superframe• ~2s latency• Mote H is industrial process sensor• Mote A is industrial process controller
Both frames on all the timeAll motes run at <100uA
17 Sept. 11th, 2006
Subnetworks 3: query/response & log upload
Black superframe• All motes• Data, Health reports up• Control info downRed superframe• Query/response from A to G• 50 slots (0.5 second)• Mean round-trip latency < 1sBlue superframe• Mote H sends a log file• 2 slots, 1 payload delivered to A per cycle• ~80kbps
Red & Blue frames are only on occasionallyAll motes run at <100uA under “normal” conditionsZero collisions, zero lost packets
A
CB
E
GFH
Without black graph
18 Sept. 11th, 2006
Subnetworks 4, et ceteraG->E E->C
C->AA
CB
E
GH
H->B H->C
C->A B->A
Red frame:1 packet delivered from G to D every other slot
Blue frame:1 packet delivered from H to A every slot
P
RQ
S
D
F
Z
Y
X
W
W->X X->Y
Y->Z
H->B H->C
C->A B->A
Gold frame:1 packet delivered from W to Z every other slot
Green frame:1 packet delivered from S to P every slot
19 Sept. 11th, 2006
Many Knobs to Turn
• Trade performance and power– Sample & reporting rate– Latency– High bandwidth connections
• Tradeoffs can vary with– Time– Location– Events
• Use power intelligently if you’ve got it– Highest performance with powered infrastructure
20 Sept. 11th, 2006
Communication Abstraction
• Packets flow along independent digraphs• Digraphs/frames have independent periods• Energy of atomic operations is known, (and can be
• Some motes powered– Hybrid: Sleepy Slotted Aloha
• Routers powered, leaf nodes minimum power• Point-to-point• Star networks
• Compare to CSMA approach for– Latency– Scalability– Power consumption
25 Sept. 11th, 2006
Moving forward
• Radio RX current going down– QIDLE < 10uC
– listening every slot < 1mA– 100ms latency/hop 100uA current
• Embedded microprocessor capabilities scaling at least 10x– 32 bit cores, > 1MB flash, >128kB RAM, 100MHz– Lower current!
• Our standard should embrace these changes
26 Sept. 11th, 2006
27 Sept. 11th, 2006
Scalability: Outdoor Test Network
Interferer(PosA)
10
SmartMeshManager
2
Smart Mes hManager
1
SmartMeshManager 9.5
4.5
8
4.5
OPEN SPACE DEPLOYMENT
Netw ork deployed at 1 Thayer Road, Santa Cruz , CA, on roughpasture. Modif ications to Implementation plan due to deploymen t f itinto " thin and hou rglass" shape of s ite (no minimum plan distancescompromised) . All measurements given in meters, and accurate to
w ithin +/-25 cent imeter s (gopher hole off se ts ).
• Average power consumption, non-congested– IRX * (2ms/10ms) = 0.2 IRX
• Average latency, non-congested– 10ms/hop
• Relative to Aloha w/ 80B ACKed payloads– Payload goodput = ~150kbps * e-2 = – Average Current = IRX – Non-congested Latency 5ms/hop
• Relative to Aloha w/ 10B ACKed payloads
30 Sept. 11th, 2006
Duty Cycling
• Slotted aloha– Any fractional slot duty cycle possible with varying frame length
– Use x links in a y slot frame to get = x/y duty cycle– Current decreases proportional to – Latency increases as 1/– 10% slot duty cycle 100ms latency per hop.– Radio duty cycle is still lower, i.e. 2% (=10% slot duty cycle * 20% radio duty
cycle in slot)
• Aloha– “chunky-ness” of the duty cycle will set latency– Typical approaches (e.g. Millennial) use long sleep intervals, e.g. 6 seconds
on, 54 seconds off to get 10% duty cycle– Latency is tens of seconds, radio duty cycle is same as overall duty cycle,
=10%
31 Sept. 11th, 2006
Powered routers
32 Sept. 11th, 2006
Point to point links
• Use scheduled communication, e.g. one Tx and one RX link in a two slot frame– Available guaranteed bandwidth
– 50 slots/sec in each direction– =50 payloads/sec = 4kB/s = 32kbps full duplex
– E.g. 5 downstream slots/sec– Average query/response latency = 120ms– End-point radio duty cycle = 5% * 20% = 1%
34 Sept. 11th, 2006
Variable slot length• Slot length will be a variable number of 1/1024ths of a
second, hereafter referred to as milliseconds for convenience.
• Expected values for slot length are 8-20ms.• Single slot length networks
– All slots, frames, and motes in a network will use the same slot size, chosen by the first mote in the network (e.g. the gateway)
– There will be no provision for changing the slot length of an existing network without restarting the network. Hence, a “slow” mote (e.g. without hardware crypto) would not be able to join a fast network.
– Problem: Rob won’t go for it for “bad customer experience”• Multi- slot length networks
– Different frames could have different slot lengths– Different paths could have different slot lengths. Manager
blocks out the appropriate amount of time for each link.• Dual slot length
– 8-10ms slot length– M2135 motes can only handle even-numbered slots
• TCCA = 0.512ms to be standards compliant– Worst case is a receive slot followed by a transmit slot to a different partner, as radio will be finishing up the ACK TX
just as it needs to look for a clear channel, so– TCCA = TTX->RX + Tchannel assessment + TRX->TX = 0.192ms + 0.128ms + 0.192ms– With gold24, we believe we can do a faster turnaround, so we’d get 0.228 instead of 0.512
• Tpacket = 4.256ms for a maximum length packet– Preamble+SS+packet = 4+1+128B = 133B = 1064 bits 4.256ms @ 250kbps
• Tcrypto needs to be chosen. For gold24 it will be about 0.25 or 0.5 ms. For the cc2420 it appears to be a bit slower – maybe 0.5 to 1 ms.
• TgACK needs to be chosen. It is the tolerance to variation in Tcrypto and/or mote B’s turnaround time from RX to TX
• TACK is a function of the ACK length. It is likely to be just under 1ms.• Tslot = TCCA+2*Tg+Tpacket+Tcrypto+TgACK+TACK = 0.512+2+4.256+1+0.1+1 = 9ms
Tcrypto
37 Sept. 11th, 2006
Late TX, early neighbor TX next slot
X
First bit of late transmitter shows up at +X relative to network-wide clock. That late transmitter performed a CCA starting 178us earlier.The early transmitter in the next slot wakes up early enough to perform a CCA and get the first bit of its preamble out at –X relative to network-wide clock.The last bit of the late transmitter is done before the first sniff of the early CCA has taken place.
Expected first bit of preamble
X
TX, RX ACK (late)
TX, RX ACK (early)
Expected first bit of preambleCCA = 178us
Preamble+SS, 160us
38 Sept. 11th, 2006
Tcrypto includes TgACK and all CRC, crypto, and radio turnaround times. It’s the time from the last bit of the packet to the first bit of the preamble of the ACK.
• Periodic traffic• Time Division Multiplexing assigned to slots in frames• Dedicated access and Quality of Service
– Deterministic latency– Bandwidth assignment– Configurable latency – Transport and resource priority
• Connectivity– One-to-one– One-to-many
44 Sept. 11th, 2006
TSMP Shared Services
• Used for burst traffic– Provides pool of available slots as needed
– Low latency alarms– High-speed on-demand file transfer
• Slotted Aloha assigned to slots in frames– Time slots can be configured to be shared– MAC level ACK detects collisions– Exponential back off algorithm– Transport and resource priority
• Connectivity– Many-to-one– Many-to-many
45 Sept. 11th, 2006
TSMP Network Management
• Unified resource allocation• Dynamic:
– Adapts to changing RF environment – global response to local changes
– Robust against network device failures– Responds to application resource requests and provides QOS
• Optimized: allocation of resources across the network • Flexible: Network management and device
interoperability do not require the standardization of how resources are allocated– Innovations can be added after the standard is released– Specialized network managers can target vertical markets
• Secure: Critical functions are removed from physically unsecured locations
