Project Employing QualNet
Quality of Service (QoS) Aware Medium Access Control (MAC) Protocols for Mobile Broadband Wireless Access (MBWA) Networks.
PhD student: Shehan Perera
Supervisor: Professor Harsha Sirisena
Co-supervisors: Professor Krys Pawlikowski, Dr Kishore Mehrotra
The MBWA MAC needs to be designed to address the need for very high bit rates, both in the uplink and downlink direction. Access and bandwidth allocation algorithms must cover hundreds of fixed and mobile terminals per channel. The downlink can be optimized relatively easily as it is point-to-multipoint but the uplink being multipoint-to-point needs to have a robust access control. A suitable contention resolution mechanism must be devised to prevent large access delays. Some terminals may be shared by multiple end users. A robust MAC protocol is required in order to maximize efficiency and utilization of network resources.
Mobile hosts with speeds of up to 250km/h should be accommodated. Fast mobility brings in problems of fading, ever changing multipath effects, Doppler spectrum effects which could cause errors in the reception leading to outage. Modulation schemes used will also play a big part in the performance of the network. Hence it is important to investigate the effects of the chosen modulation scheme on different MAC protocols. Since adaptive modulation schemes have been proposed for 802.16 where burst profiles are modified according to transmission conditions, MBWA will also need similar or even more robust schemes.
Scheduling and reservation management is an area where much research can be done. Since this part of the system is vendor specific it will be an important part of this research to device a robust bandwidth allocation system for MBWA.
As per MBWA draft requirements the MAC protocol must guarantee periodic access to the medium. Physical layer (PHY) resources dedicated to this function cannot impact system goodput capacity by more than 5%. The MAC will most likely be a contention and fixed access combined MAC. The contention access mechanism must not incur more than 15ms of system delay, excluding blocking state delays and the delay must remain constant regardless of the number of active mobiles.
QoS Issues
This is the all important issue in wireless networking today. The applications of the future will demand stringent band width, delay, jitter and packet loss rate levels. In order to provide these at a satisfactory level the MAC layer needs to be optimized and made flexible enough to communicate with the application layer down to the physical layer.
Several service classes have been defined in the 802.16 standard which are UGS, rtPs, nrtPS and BE. The 802.20 working group have also defined similar service classes. These classes are have some common features to the Diffserve classes ER, AF and BES and will need to be mapped to one another when packets need to traverse the internet to get to their destination.
System model
To evaluate the current 802.16 standard and then enhance it QualNet by Scalable Network Technologies has been chosen as the simulator. Its ease of use and simplicity in modifying and adding new code are the main advantages. Development of the 802.16 MAC is almost complete and now we are in the process of fine tuning its performance for VoIP applications. We are also looking at http web traffic and how to adapt the predefined service classes to work well with it.