Performance Analysis of DCF- Two Way Handshake vs RTS/CTS During Train-Trackside Communication in CBTC based on WLAN802.11b

Abstract

Background:Wireless Local Area Network (WLAN) is used primarily in CBTC because of easy availability of commercial WLAN equipment. In present scenario, WLAN Medium Access Control (MAC) protocol is a well-known protocol which is used to satisfy real-time traffic and delay- sensitive applications. The bidirectional train-trackside communication is the fundamental key of train control in CBTC.Methods:DCF describes two basic techniques used for packet transmission: First technique is a Two Way Handshake (TWH) mechanism and another is Four Way Handshake (FWH) mechanisms. RTS/CTS FWH protocol specified by IEEE802.11b is introduced to rectify the Hidden Node Problem (HNP) encounters in TWH protocol. That is why the TWH mechanism of DCF technique suffers from higher average packet delay time when this protocol is applied to CBTC. DCF- Four Way Handshake (FWH), Request To Send (RTS) and Clear To Send (CTS) delay model is proposed to develop Communication Based Train Control (CBTC) system.Results:FWH is applied in CBTC to overcome the packet delay and throughput limitations of Two Way Handshake (TWH) mechanism of distributed coordination function (DCF) based technique. An experiment is designed to simulate and compare the performance of RTS/CTS delay model against TWH mechanism of DCF.Conclusion:It was found that the Average packet delay is slightly higher and throughput is lesser in RTS/CTS in comparison to TWH method. By comparing the performance of these two medium access mechanism in CBTC it was found that for multiple retransmissions with various data rates the RTS/CTS model had better packet delay time than TWH.