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.

Versioning file systems are widely used in modern computer systems as they provide system recovery and old data access functions by retaining previous file system snapshots. However, existing versioning file systems do not perform well with the emerging PCM (phase-change memory) storage, because they are optimized for hard disks. Specifically, a large amount of additional writes incurred by maintaining snapshot degrades the performance of PCM seriously as write operations are the performance bottleneck of PCM. This paper presents a novel versioning file system, designed for PCM, that reduces the writing overhead of a snapshot significantly. Unlike existing versioning file systems that incur cascade writes up to the file system root, our scheme breaks the recursive update chain at the immediate parent level. The proposed file system is implemented on Linux 2.6 as a prototype. Measurement studies with various I/O benchmarks show that the proposed file system improves the I/O throughput by 144 percent on average, compared to ZFS, a representative versioning file system.