Wireless LANs; Protocols, Security and Deployment
Ajay Prasad · 2003
This thesis presents a broad range of work done in the field of Wireless Local Area Networks (WLANs). It proposes several novel schemes which performance are analyzed. The thesis first proposes an Automatic Repeat reQuest (ARQ) scheme named as Selective Repeat / Multi-Copy (SR/MC). The purpose of the proposed SR/MC ARQ scheme is to transmit Internet Protocol (IP) packets efficiently in WLANs. At first the proposed scheme fragments an IP packet and transmits the fragments in Selective Repeat (SR) mode until the last fragment is transmitted after that, if erroneous fragments are still left, it goes in Multi Copy (MC) mode. In MC mode multiple copies of erroneous fragments are transmitted cyclically. A numerical performance analysis of the proposed SR/MC ARQ scheme is done and all its parameters are optimized. Using the simulation results of the Physical layer the optimized values are then used to study the performance of the scheme in terms of throughput both in an Additive White Gaussian Noise (AWGN) channel and a flat Rayleigh fading channel. The numerical results show that the proposed SR/MC ARQ scheme gives an improvement of 8 dB when compared to the Selective Repeat + Stutter Scheme 2 (SR+ST 2) scheme, under flat Rayleigh fading channel using BCH(63,51,2) for the throughput of 0.9. The measurement results also show similar performance. The proposed SR/MC ARQ scheme was implemented and applied for patent by the author; it was used for Wireless Network Interface Card (WNIC), a proprietary WLAN, of Uniden Corporation, Tokyo, Japan. Next a novel Medium Access Control (MAC) protocol, named as Channel Sharing Protocol (CSP), is proposed. The proposed CSP uses tokens in the wireless medium to give fair access of the medium to a large number of users. The scheme uses a p-persistence algorithm to avoid collision. Simulations were performed to optimize the different parameters of the proposed CSP and to study its performance. A numerical study was also done for performance evaluation. There is a difference between the numerical and the simulation results due to the assumptions made during the numerical performance evaluation. Both the numerical and the simulation results show that the proposed CSP outperforms the Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) MAC protocol of the IEEE 802.11 WLANs. A detailed design of the proposed CSP for implementation in WNIC was done and it was applied for patent by the author. Today the WLANs focus mainly on the non real time data communication while the market is looking towards real time traffic. Bearing this in mind the IEEE 802.11, a WLAN standard, is working on MAC enhancements focusing on Quality of Service (QoS). Several QoS solutions for the IEEE 802.11 existed when the work presented in this thesis was started. These solutions are Blackburst, priority queuing, the Distributed Coordination Function (DCF) and the Point Coordination Function (PCF). The work presented in this thesis was started at the preliminary stages of the IEEE 802.11e standardization. The purpose of the work was to give a first idea of an acceptable solution. In this thesis the four solutions are presented and a qualitative analysis is done. IEEE 802.11e has accepted priority queuing, chosen as the best solution in the thesis, as one of the solutions for QoS. Security in a wireless medium, a medium accessible to all, is a major issue. The WLAN currently in use, IEEE 802.11, has been found to have several security flaws. Even before the security flaws were publicly known the IEEE 802.11 standardization committee had started working on security enhancements. The work presented in this thesis was done during the preliminary stage of the IEEE 802.11i, security enhancements of IEEE 802.11 MAC, standardization (then IEEE 802.11e). Some of the ideas were presented to the IEEE 802.11 standardization committee. The work started with proposing requirements for various environments where the WLANs were envisioned to be used. The environments studied in this thesis are enterprise, academic and public. The security requirements for these environments are proposed in this thesis. Security solutions for each environment are also proposed herein. A qualitative analysis of the proposed solutions is also given in this thesis. A common problem in all the solutions was access control; so as to solve this problem an access control protocol is proposed. Finally the security issues for the future wireless communications systems are examined. The standard reflects some of the ideas presented in the thesis. The access control protocol proposed in the thesis, applied for patent by the author, is very similar to the now standardized IEEE 802.11f, Inter Access Point Protocol (IAPP). System design and planning of the network for the IEEE 802.11 WLANs in different environments is a major issue. The IEEE 802.11 standard gives several options for optimum system design while the standard makes use of the unlicensed band. The unlicensed band can be used by any wireless system fulfilling the regulatory requirements; these systems will cause interference with the IEEE 802.11 WLANs at the same time IEEE 802.11 has limited the number of non-overlapping channels. A study on systems design and deployment of the WLAN network is required. In this thesis the issues related to the IEEE 802.11 system design and results of several deployment related critical issues like coverage, cell/frequency planning, interference, and data rate are examined. This work was used for system design and deployment of the ORiNOCO WLANs of Lucent Technologies. An automatic rate control scheme used by the ORiNOCO product was applied for patent by the author.