David Culler, Jason Hill, Robert Szewczyk, and Alec Woo

1
TinyOS Programming Boot Camp (V) Communication

• David Culler, Jason Hill, Robert Szewczyk, and
  Alec Woo
• U.C. Berkeley
• 2/9/2001

2
Outline

• Media Access Control
• Ad Hoc Routing Mechanism
• Hands on Exercise

3
Mote Characteristics

• Hardware single channel RF radio
• Nodes contend for the same wireless channel
• Traffic is likely
• Periodic nature of sensor networks
  applications
• Correlated detection of common events
• Collision detection mechanism is not available
• Carrier Sense Multiple Access (CSMA)

4
CSMA in TOS Framework

• Channel capacity 25 packet/s
• High amount of traffic due to
• High node density
• High transmission rate of each node
• Application layer lacks information about the
  channel
• Place CSMA at RADIO_BYTE level in between
  application layer and RFM radio component

5
CSMA Design

• Basic mechanism
• Listen on the channel before transmit
• Only transmit if channel is idle
• If the channel is busy, wait until channel
  becomes idle again
• Correlated periodic traffic may lead to repeated
  collisions
• Introduce random delay in MAC layer
• If channel becomes busy, MAC backpressures to
  application to drop further transmissions

6
CSMA in TOS Network Stack
Application
AM_SEND_MSG fails
AM
VAR(state) ! 0
PACKET
RADIO BYTE
Backpressure
Channel is busy
RFM
7
One CSMA Example

• In each RADIO_BYTE components
• Random listening period

Busy
Transmission Request
Listen for Random Period
Transmit
Idle
8
FOUR_B_RADIO_BYTE.c (RADIO_BYTE_INIT)

• 16 bit Linear Feedback Shift Register (LFSR) as
  Pseudo Random Number Generator
• Initialization
• char TOS_COMMAND(RADIO_BYTE_INIT)()
• TOS_CALL_COMMAND(RADIO_SUB_INIT)()
• VAR(wait_amount) 12
• VAR(shift_reg) 119 LOCAL_ADDR_BYTE_2
• return 1

Minimum number of bits to listen (Encoding
Dependent)
Seed for LFSR
9
FOUR_B_RADIO_BYTE.c(RADIO_BYTE_TX_BYTES)

• char TOS_COMMAND(RADIO_BYTE_TX_BYTES)(char data)
• if(VAR(state) 0)
• //if currently in idle mode, then switch over to
  transmit mode
• //and set state to waiting to transmit first
  byte.
• VAR(third) data
• VAR(state) 10
• return 1

Goes to CSMA
10
FOUR_B_RADIO_BYTE.c (RADIO_BYTE_RX_BIT_EVENT)

• else if(VAR(state) 10)
• if(data)
• VAR(waiting) 0
• else
• if (VAR(waiting) 11)
• bit (VAR(shift_reg) 0x2) gtgt 1
• bit ((VAR(shift_reg) 0x4000) gtgt 14)
• bit ((VAR(shift_reg) 0x8000) gtgt 15)
• VAR(shift_reg) gtgt1
• if (bit 0x1) VAR(shift_reg) 0x8000
• VAR(wait_amount) (VAR(shift_reg)
  0x3f)12
• if(VAR(waiting) gt VAR(wait_amount))
• Prepare for transmission

Channel is busy. Starts again
16 bit LFSR
Listening period ? 12,75
11
CSMA Evaluation
Channel Utilization 70
Throughput per node is fair
12
Outline

• Media Access Control
• Ad Hoc Routing Mechanism
• Hands on Exercise

13
Ad Hoc Routing Goal

• Nodes should self organize into a multi-hop
  network topology
• Each node can discover a route to deliver packets
  to the base station
• uni-directional towards the base station
• Simple, robust, local algorithm
• Works in an ad hoc, spontaneously changing
  network
• Assume bi-directional connectivity in general

14
A Simple Ad Hoc Routing Example (I)

• AM_ROUTE.c
• A route is simply knowing a neighboring node (or
  parent) that can send to the base station
• Base station broadcasts itself as a route
• Node gets the route and retransmits the broadcast
  with itself as the route

15
A Simple Ad Hoc Routing Example (II)

• Base station refreshes route with periodic
  broadcast
• Nodes expire the old route and wait for base
  station next route refreshment
• Nodes only broadcast once for each route update
  it gets
• Prevent rebroadcast of childrens broadcasts
• has an effect in lowering the number of messages
  in broadcasting to the entire network
• Broadcast becomes unidirectional (from BS
  outward)

16
Packet Forwarding

• Node receives a packet destined to it will
• forward it to the next hop
• Base station receives a packet destined to it
  will
• forwards it to the UART

17
Packet Format

• Defined in system/include/MSG.h
• define DATA_LENGTH 30
• struct MSG_VALS
• char addr
• char type
• char group
• char dataDATA_LENGTH
• Special Address (addr)
• Broadcast 0xff (TOS_BCAST_ADDR)
• Send to UART 0x7e (TOS_UART_ADDR)

Application Data Unit
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Routing Data Unit

• data0 number of hops from the base station
• Each node appends its address to data indexed
  by number of hops (data(int)data0)

BS (12)
Node 33
Node 56
Node 33s data unit
2
12
33
1
2
0
3
19
Ad Hoc Routing Code in AM_ROUTE.c
20
AM_ROUTE.c (AM_ROUTE_INIT) (Base Station)

• char TOS_COMMAND(AM_ROUTE_INIT)()
• //set beacon rate for route updates to be sent
• TOS_COMMAND(CONNECT_SUB_CLOCK_INIT)(255, 0x06)
• //route to base station is over UART.
• VAR(route) TOS_UART_ADDR
• VAR(set) 1
• VAR(place) 0
• VAR(data_buf).data0 1
• VAR(data_buf).data1 TOS_LOCAL_ADDRESS

Tick every 2 seconds
Set route to UART
BS is 1 hop away
Append my ID
21
AM_ROUTE.c (AM_ROUTE_INIT) (NOT Base Station)

• char TOS_COMMAND(AM_ROUTE_INIT)()
• //set rate of sampling
• TOS_COMMAND(CONNECT_SUB_CLOCK_INIT)(255, 0x03)
• VAR(set) 0
• VAR(route) 0
• VAR(count) 0
• return 1

Tick every 0.25 seconds
Route is not set initially
22
AM_ROUTE.c (AM_ROUTE_SUB_CLOCK)

• void TOS_EVENT(AM_ROUTE_SUB_CLOCK)()
• ifdef BASE_STATION
• TOS_CALL_COMMAND(ROUTE_SUB_SEND_MSG)(TOS_BCAST_AD
  DR,AM_MSG(ROUTE_UPDATE),VAR(buf))
• else
• if(VAR(set) gt 0) VAR(set) --
• endif

Base stations periodic broadcast
Nodes count down to expire its route
23
AM_ROUTE.c (ROUTE_UPDATE)

• TOS_MsgPtr TOS_MSG_EVENT(ROUTE_UPDATE)(TOS_MsgPtr
  msg)
• //if route hasn't already been set this
  period...
• if(VAR(set) 0)
• //record route
• VAR(route) data(int)data0
• VAR(set) 8
• //create an update packet to be sent out.
• data0
• data(int)data0 TOS_LOCAL_ADDRESS
• TOS_CALL_COMMAND(ROUTE_SUB_SEND_MSG)
• (TOS_BCAST_ADDR,AM_MSG(ROUTE_UPDATE),msg)

Set route to parents ID
Set to expire the route after 8 ticks
I am 1 more hop away from the BS
Append my ID to the buffer
Broadcast the message
24
AM_ROUTE.c (ROUTE_UPDATE cont.)

• TOS_MsgPtr TOS_MSG_EVENT(ROUTE_UPDATE)(TOS_MsgPtr
  msg)
• If (VAR(set) 0)
• else
• return msg

Only broadcast once for each new route
otherwise, do nothing.
25
Packet Forwarding Code in AM_ROUTE.c
26
AM_ROUTE.c (ROUTE_SUB_DATA_READY)

• Data source
• char TOS_EVENT(ROUTE_SUB_DATA_READY)(int data)
• if(VAR(route) ! 0 other App. Specific
  conditions)
• TOS_CALL_COMMAND(ROUTE_SUB_SEND_MSG)
• (VAR(route),AM_MSG(DATA_MSG),
• VAR(data_buf))

Send out a data packet if a node has a route.
27
AM_ROUTE.c (DATA_MSG)

• TOS_MsgPtrTOS_MSG_EVENT(DATA_MSG)
• (TOS_MsgPtr msg)
• if(VAR(route) ! 0)
• Application Specific Modification of the
• forwarding packet
• TOS_CALL_COMMAND(ROUTE_SUB_SEND_MSG)
• (VAR(route),AM_MSG(DATA_MSG),msg)

Node has a route
Forward message if you have a route.
28
AM_ROUTE.c (DATA_MSG)

• TOS_MsgPtrTOS_MSG_EVENT(DATA_MSG) (TOS_MsgPtr
  msg)
• if(VAR(route) ! 0)
• //update the packet.
• data5 data4
• data4 data3
• data3 data2
• data2 data1
• data1 TOS_LOCAL_ADDRESS
• TOS_CALL_COMMAND(ROUTE_SUB_SEND_MSG)
• (VAR(route),AM_MSG(DATA_MSG),msg)

Modify the forwarding packet to stores the hops
taken.
29
Outline

• Media Access Control
• Ad Hoc Routing Mechanism
• Hands on Exercise

30
Hands on Exercise (I)

• Run the apps/router.desc application
• Set it to your own LOCAL GROUP in Makefile
• Default is 0x13
• Goes to tools directory
• Run listen.c

31
Hands on Exercise (II)

• Modify ROUTE_UPDATE in AM_ROUTE.c
• GOAL pick route based on depth of the tree
• Pick the route with the smallest number of hops
  from the base station
• Can use the same data unit format
• data0 already tells you number of hops from the
  base station of the sender of the packet
• and the corresponding node id
  data(int)data0
• If a better route (smaller number of hops) is
  received, use that as the new route.
• You may need to introduce a new variable (e.g.
  myLevel) to remember your current depth