TinyOS Radio Stacks

1
TinyOS Radio Stacks

• Joe Polastre, Phil Buonadonna, etc
• Monday, October 20 2003

2
Radio Stacks

• Mica (Old)
• Mica (MicaHighSpeedRadio)
• Mica2 (CC1000)
• Future?

3
Typical Radio Stack
Generic Comm
AM
Packet Layer
New Radios (CC2.4K)
Byte Layer
Radio Control
Mica2
Bit Layer
4
Mica (Old)
Generic Comm
AM
CRCPacketObj
SecDedRadioByte
RFM
5
Mica (MicaHighSpeedRadio)
Generic Comm
CSMA Preamble Detect Start Sym Detect
AM
RadioCRCPacket
Encoding
Captures radio timing and syncs receiver to sender
MicaHighSpeedRadioM
ChannelMon
RadioTiming
SlavePin
SpiByteFifo
SecDedEncoding
RandomLFSR
6
Mica2 (CC1000)
Wires the control and data paths Implementation
hidden from app
Generic Comm
AM
CSMA Encoding Data Preamble Detect Synchronization
Control (Freq,Power,etc)
CC1000RadioC
CC1000Control
CC1000RadioIntM
SpiByteFifo
RandomLFSR
ADC
7
Mica2 Radio Stack

• Phil B

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Radio Packets
9
Mica
10
Mica2
Switch to TX Mode
Switch to RX Mode
MAC Delay
Preamble
Sync
Packet Transmission
1-128
18
2
36
56
250ms
250ms
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Comparison
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Mica2 Preamble

• CC1000 must sync its clock to incoming signal
• With manchester encoding, 17 sync bytes are
  required for longest range
• an extra byte is added for offset in receiving

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Strength

• Mica
• sends two strength bytes to measure high
  amplitude via RSSI pin
• Mica2
• with FSK, both 0 and 1 contain energy
• measure energy at any point during packet
  reception, no need for extra bytes

14
Synchronous ACKs

• Mica
• Switch from Transmit to Receive mode instantly.
• No need to resynchronize ack sender.
• Mica2
• Switch from Transmit to Receive mode in 250ms
• CC1K requires that each receiver lock to the
  incoming signal (18 bytes 7.5ms)
• Almost as expensive as sending a packet!

15
CSMA
16
Mica

• Hardware sets automatic gain control (AGC) using
  a capacitor in ASK mode
• Channel Monitor looks for 1 bits on the radio
  while waiting to send
• start symbol/preamble detection

17
Mica2 Release

• Set a fixed noise floor threshold
• On transmission
• Take RSSI reading
• If reading gt floor, transmit on next byte
  boundary from radio
• With a few months in the lab, what happens?

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Collisions Oh My!
Channel Clear
1
2
4
3
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CC1000 Radio MAC
Radio RSSI sampled at 5kHz
Current MAC
Incoming Packet
Adjusted Threshold
Adjusted Threshold with Averaging
Channel Considered Clear
Periodic Errors
Channel Busy during Xmit
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CC1000 Radio MAC
Radio RSSI sampled at 5kHz
Median over 5 sample window
Average over 5 sample window
Max over 5 sample window
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Mica2 Joe

• Use calibrated ADC
• Adjust noise floor
• After each packet transmission, sample RSSI and
  add to 10 sample FIFO queue
• Select median(Q) and add to EWMA
• On transmit
• Take lt5 RSSI samples
• If one reading gt floor, transmit on next byte
  boundary from radio
• Downside can take up to 10 packets for the noise
  floor to settle

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Comparison
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Results
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Results
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Brief Low Power Operation

• Mica
• Snooze Node is either on or off
• Radio Can detect channel activity in
  microseconds
• Radio startup time is negligible (10s of ms)
• Mica2
• HPLPowerManagement check the state of the mote
  and shut down if appropriate
• Radio Currently can detect channel activity in
  7.5ms (18 bytes)
• Using RSSI and MAC noise floor estimation, can
  reduce this time to 1ms.
• Radio start up time is up to 2500ms!

26
ACKs Revisited

• If we can now reliably detect channel activity,
  can we do ACKs?
• Answer Hopefully.
• How?
• By switching modes and sending data (or pulse
  sending data), the receiver can detect this
  signal
• Think amplitude shift keying
• May be able to detect hidden terminals too
• If ACK doesnt have pattern but instead is
  constant energy, then someone didnt detect the
  original transmitter and transmitted over him
• Can be done in 4-5 bytes and doesnt require the
  receiver to synchronize!
• but it hasnt been tried yet.

27
Open Questions

• Allow Application to control MAC backoff?
• Is TOS_Msg the right networking abstraction for
  building sensor network apps and services?
• OSI? S-MAC?
• What does a stack look like where you can change
  MAC layers, security models, and encodings? Mica
  and Mica2 stacks are close, but not quite there

28
webs.cs.berkeley.edu

• tinyos-1.x/beta/CC1000Radio/