Osamu Moriya

An Advanced CSMA/CA System for Wide Area Broadband Wireless Access

An advanced CSMA/CA (carrier sense multiple access with collision avoidance) system incorporating total precedence transmission of all CA and MAC frames (known as a full-flying synchronous CSMA/v-MCA system in Japan, where v- MCA means a variable multiple CA) was developed to minimize the time wasted for upstream channels. In this system, the CA procedures and the controls of the MAC frame transmission run independently of each other by adopting out-of-band signaling (OBS) in order to achieve flexible access control. However, in highly offered traffic, MAC frames, that have completed the CA procedure, queue to be transmitted, causing latency to increase rapidly. To solve this problem, the CA and MAC channels are properly distributed, and flow control is added to the CA procedure. In this study, the performance of this system was evaluated and confirmed using a theoretical calculation and a computer simulation. Maximum throughput of approximately 0.9 was observed regardless of network length (up to tens of kilometers) and physical transmission speed, maintaining the low latency that is characteristic of contention systems.

Performance Evaluation of Synchronous Variable-multiple Collision Avoidance Systems

To address the digital divide in developing countries, fixed wireless access (FWA) networks have the potential to quickly provide economical access over a wide area within a radius of tens of kilometers. The conventional synchronous variable-multiple collision avoidance (v-MCA) system, which is referred to as a non-precedence (NP) system, can be operated over a network of any size without the need to use frame-length restrictions. However, it has a potential drawback of rapid degradation of the throughput due to the intervention of the round trip time, which is proportional to the network length and upload bandwidth. The advanced synchronous v-MCA system incorporating total precedence (TP) transmission of frames provides high throughput regardless of network length and upload bandwidth. In this paper, after showing the medium access control mechanisms of the NP and TP systems, their theoretical calculation models are discussed in detail. Then, their system performances are evaluated and overlooked by comparing theoretical and simulated results. The TP system provides an ultimate maximum throughput performance regardless of network length and total upload bandwidth, while maintaining the low delay characteristics of a contention-based access scheme.