Dr. S. Srikanth
AU-KBC Research Centre,
MIT Campus,
Anna University, Chennai

Email: srikanth@au-kbc.org

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ABSTRACT:

The flexibility offered by wireless local area networks (WLANs) has been a major factor in the widespread deployment and popularity. A wireless local area network (WLAN) uses radio frequencies and provides all the features and benefits of traditional LAN technologies but without the limitations of a cable. A WLAN is a flexible data communications system implemented as an extension to or as an alternative for a wired LAN. Wireless LANs combine data connectivity with user mobility.

The IEEE 802.11 standard is the predominant standard for wireless LANs as seen by the various products which are popular in the marketplace. Any LAN application, network operating system or protocol including TCP/IP will run on 802.11 compliant WLANs as easily as they run over Ethernet. The IEEE 802.11 devices operate in the unlicensed ISM band and thus enable organisations to set up networks in a rapid manner.

This tutorial will focus on the following topics:

1. Overview of 802.11 Networks

   The focus of this lecture will be on the existing wireless scenario, advantages of WLANs, emergence of WLANs (specific to IEEE 802.11) as a complementary service to 3G networks and their major applications in offices,hotspots, and outdoor systems. The basic elements of a WLAN will be highlighted.

2. IEEE 802.11 specifications: 802.11, 802.11a, 802.11b, 802.11g

   The specifications given in the IEEE 802.11 standards will be the focus of the discussion. The evolution of the standard along with the various new features introduced shall be discussed.

3. Media Access Control Layer:
   This session will be focussed on the MAC issues related to 802.11.
   1. The challenges for the MAC layer in 802.11.

      Radio frequency links are subjected to interference and noise which are significant when the frequencies used are unlicensed ISM bands as in the case of IEEE 802.11 systems. It is also not possible to detect collisions in wireless networks as in the wired case which used carrier sense multiple access with collision detection (CSMA/CD). Collisions resulting from hidden nodes are hard to detect in wireless networks which use carrier sense multiple access with collision avoidance (CSMA/CA). Exposed nodes in the WLAN network may lead to inefficient usage of the resource. We plan to address all the above issues and the implemented solutions in IEEE 802.11 standard shall be discussed.

   2. MAC Access modes: Contention free access and contention based access

      The IEEE 802.11 MAC operates in two modes, the centralized or the Point Co-ordination Function (PCF) mode in which an access point co-ordinates the communication between stations and the Distributed Co-ordination Function (DCF) or ad-hoc mode in which all stations behave independently in a distributed fashion. In this session we plan to highlight and discuss the above two modes of operation.

   3. Carrier Sensing Functions: Physical and Virtual Carrier Sensing

      Carrier sensing is done by the stations which want to communicate with the other station. The IEEE 802.11 MAC specifies both physical and virtual carrier sensing as a means to avoid collisions. While physical carrier sensing is done through signal measurement, virtual carrier sensing id done using the Network Allocation Vector (NAV). In this part of the lecture, we plan to address the above features of IEEE 802.11.

4. QoS - IEEE 802.11e standard

   In this session we plan to address the MAC issues in the IEEE 802.11e standard to support traffic with quality of service (QoS) requirements. Adhoc networks do not provide support to traffic streams with guaranteed service, but the available bandwidth can be distributed based on the traffic category. The IEEE 802.11e standard provides enhanced mechanism to handle the various traffic categories. This mechanism is the Enhanced Distributed Co-ordination Function (EDCF). The standard also specifies the centralized mode of operation, the Hybrid Co-ordination Function (HCF) mode for QoS enhanced stations with a QoS enhanced access point. The focus of this part of the tutorial will be on the above two modes of operation of the IEEE 802.11e MAC.

5. Physical Layer
   1. Orthogonal Frequency Division Multiplexing (OFDM) and its application to 802.11a/g:

      In this part of the tutorial we plan to introduce the basics of OFDM. Discrete time implementation using IFFTs and FFTs, the role of Cyclic Prefix(CP) and channel equalization will be discussed. The associated problems like high Peak-to-Average-Power Ratio, the effect of frequency offset and timing offset will be explained and techniques for Timing offset, Frequency offset estimation, correction and tracking and channel estimation will be discussed. Pointers to advanced topics like OFDMA, MIMO OFDM, Adaptive modulation and space time coding will be put forth. Simulation results and details shall be presented. The various rate modes which are achieved in 802.11a shall be discussed.

   2. The IEEE 802.11b Physical Layer:

      The IEEE 802.11b Physical layer used spread spectrum as the means to achieve reliable transmissions. The parameters used shall be outlined and the important blocks in the transmitter and the receiver shall be discussed. Compatibility between the IEEE 802.11b and 802.11g shall be discussed.

BIOGRAPHY:

Dr S Srikanth obtained his BE degree form Anna University,Chennai, India in 1990, M A Sc. and Ph D from University of Victoria, Canada during 1993 and 1997 respectively. He worked in Harris Corporation, Calgary, Canada from 1997 to 1999. He was a UGC Visiting Professor at College of Engineering, Anna University from 1999 to 2000. From July 2000 onwards he is with AU-KBC Research Centre, Chennai, where he takes up R & D assignments in wireless area besides teaching. He is acting as consultant to various organizations. His research interests include multicarrier communications systems for wireless, powerline communications, space-time processing and coding for wireless systems, physical and medium access control layer techniques for wireless communications systems with emphasis on IEEE 802.11 based WLANs. He has many research papers to his credit. He has been awarded a Govt. of India Young Scientist Fellowship for a period of 3 years.