1. doc.: IEEE 802.15- Submission 15 November 2005 Analysis of CAP of IEEE 802.15.4 Superframe Iyappan Ramachandran University of Washington November 15, 2005

2. doc.: IEEE 802.15- Submission 15 November 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 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

3. doc.: IEEE 802.15- Submission 15 November 2005 Approximations to simplify analysis Presence of beacons and beacon boundaries have negligible effect Every node sees a probability p i c 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 p c i|i –Independence for backoff slots separated by a backoff Channel sees a probability p t n 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

4. doc.: IEEE 802.15- Submission 15 November 2005 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 p k n that node will get out of k th backoff stage

5. doc.: IEEE 802.15- Submission 15 November 2005 Node state model (see handout #1) Node stays in IDLE state with prob. (1-p) and goes to BO 1 with prob. p BO 1  CS 11 with prob. p 1 n CS 11  CS 12 with prob. p i c and BO 2 with prob. (1-p i c ) CS 12  TX with prob. p i|i c and CS 12 with prob. (1-p i|i c ) … … and so on TX  IDLE with prob. 1 after N backoff slots CS 51  IDLE and CS 52  IDLE with probabilities (1-p i c ) and (1-p i|i c ) respectively

6. doc.: IEEE 802.15- Submission 15 November 2005 Channel state model (handout #2) Channel stays in (IDLE, IDLE) state when no node begins transmission (prob. α=(1-p t|ii n )) (IDLE, IDLE)  SUCCESS when exactly one node transmits (prob. β=Mp t|ii n (1-p t|ii n ) 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

7. doc.: IEEE 802.15- Submission 15 November 2005 Calculation of channel throughput Approximations have led to virtual decoupling of nodes’ activities –Solve node state chain to find p t n in terms of p i c  (1) –Solve channel state chain to find p i c in terms of p t n  (2) Solve (1) and (2) numerically to find p i c and p t n Aggregate channel throughput, S is the fraction of time spent in SUCCESS state

8. doc.: IEEE 802.15- Submission 15 November 2005 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

9. doc.: IEEE 802.15- Submission 15 November 2005 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

10. doc.: IEEE 802.15- Submission 15 November 2005 Simulations (cont.)

11. doc.: IEEE 802.15- Submission 15 November 2005 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

12. doc.: IEEE 802.15- Submission 15 November 2005 Thank you!