Analysis of cap of ieee 802 15 4 superframe
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Analysis of CAP of IEEE 802.15.4 Superframe. Iyappan Ramachandran University of Washington November 15, 2005. Model Assumptions. Beacon-enabled Star M nodes attached to a coordinator All nodes within the carrier sensing range of each other No inactive period in the superframe, i.e. BO=SO

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Analysis of cap of ieee 802 15 4 superframe

Analysis of CAP of IEEE 802.15.4 Superframe

Iyappan Ramachandran

University of Washington

November 15, 2005


Model assumptions
Model Assumptions

  • Beacon-enabled Star

    • M nodes attached to a coordinator

    • All nodes within the carrier sensing range of each other

  • No inactive period in the superframe, i.e. BO=SO

  • Contention access part (CAP) occupies active period fully

  • No acknowledgements

  • Poisson arrival of packets, i.e. probability p of packet arrival every backoff slot.

  • Packet length is fixed and equal to N backoff slots

  • No buffering at nodes

  • Only Uplink


Approximations to simplify analysis
Approximations to simplify analysis

  • Presence of beacons and beacon boundaries have negligible effect

  • Every node sees a probability pic that channel is idle in the first of two CCA backoff slots

    • Not slot-to-slot independence; probability that channel is idle in the second CCA backoff slot is pci|i

    • Independence for backoff slots separated by a backoff

  • Channel sees a probability ptn that a node begins transmission in any generic slot

  • Geometrically distributed backoff durations with same mean as original uniform distribution

  • Validity of approximations will be verified by simulations


Consequences of approximations
Consequences of approximations

  • CAP can be simply analyzed as non-persistent CSMA

  • Channel and nodes have been virtually decoupled

    • Each node can be analyzed independent of the others

  • Probability pkn that node will get out of kth backoff stage


Node state model see handout 1
Node state model (see handout #1)

  • Node stays in IDLE state with prob. (1-p) and goes to BO1 with prob. p

  • BO1  CS11 with prob. p1n

  • CS11CS12 with prob. pic and BO2 with prob. (1-pic)

  • CS12TX with prob. pi|ic and CS12 with prob. (1-pi|ic) …

  • … and so on

  • TXIDLE with prob. 1 after N backoff slots

  • CS51IDLE and CS52 IDLE with probabilities (1-pic) and (1-pi|ic) respectively


Channel state model handout 2
Channel state model (handout #2)

  • Channel stays in (IDLE, IDLE) state when no node begins transmission (prob.α=(1-pt|iin))

  • (IDLE, IDLE)SUCCESS when exactly one node transmits (prob. β=Mpt|iin(1-pt|iin)M-1)

  • (IDLE, IDLE)FAILURE when more than one node transmit (prob. δ=1-α-β)

  • Channel stays in SUCCESS/FAILURE state for N backoff slots

  • SUCCESS(IDLE,IDLE) and

    FAILURE(IDLE,IDLE) with probability 1


Calculation of channel throughput
Calculation of channel throughput

  • Approximations have led to virtual decoupling of nodes’ activities

    • Solve node state chain to find ptn in terms of pic (1)

    • Solve channel state chain to find pic in terms of ptn (2)

  • Solve (1) and (2) numerically to find picand ptn

  • Aggregate channel throughput, S is the fraction of time spent in SUCCESS state


Calculation of average power consumption
Calculation of average power consumption

  • Chipcon CC2420 radio for illustration (see handout #3)

    • Four energy states: shutdown, idle, transmit, receive

  • Included beacon receptions

  • Considered two cases

    • Stay in idle state if no packet is waiting  included idle-to-receive ramp-up for beacon reception and CCA

    • Shutdown node if no packet is waiting  included shutdown-idle-receive ramp-up for beacon reception and CCA


Simulations
Simulations

  • All simulations were run in NS-2; used IEEE 802.15.4 module developed by J. Zheng and M. J. Lee, CUNY

  • Same model assumptions, but NO approximations

  • No. of nodes, M=12; Packet length, N=10 backoff slots

  • BO=6  Beacon Interval=3072 backoff slots=0.983 sec; Beacon length=2 backoff slots



Conclusions
Conclusions

  • Analysis predicts very accurate throughput and power consumption estimates

  • Although shutting down has the ramp-up overhead time, it saves considerable energy at low traffic

  • Analysis can be extended

    • Easily to include acknowledgements

    • With some effort to include inactive part



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