| Popis: |
Non-beacon mode operation is included in IEEE 802.1.5.4 due to low overhead, energy efficient transmission and no tight synchronization requirement as compared to the beacon-mode transmissions. We model non beacon mode operation of an 802.15.4 unslotted CSMA/CA using Markov chain analysis for low data rate unsaturated traffic. Our modeling is reasonably detailed to capture an exact 802.15.4 system. The modeling captures various network features in detail, such as channel access probability, packet collision, acknowledged transmissions, exponential backoff with retry limits. The packet generation process at the sensor nodes is considered as periodic. We thoroughly derive the packet delay distribution and study the system throughput and energy efficiency trade off which is often crucial for the low rate networks. Our exact Markov chain based analysis reveals that the service process of the protocol is close to an exponential distribution. Therefore, we approximate it by a single server queue with an exponential service time. We then analyze a D/M/1 queue with an unsaturated periodic traffic and show that it can be a good fit for the 802.15.4 system. Using D/M/1 queue model, we derive an end-to-end delay and analyze the delay performance. We also study a system with non-exponential backoff and compare the optimal network performance with the legacy exponential backoff system. Our simulation results corroborate with the theoretical analysis. Our analysis can be used as a system design tool where various network system parameters can be configured to meet a desired end-to-end delay, energy efficiency and throughput. |
