RT-WLAN: a soft real-time extension to ORiNOCO Linux device driver

The current IEEE 802.11 wireless LAN (WLAN) systems are unable to support real-time applications because the underlying contention-based MAC (medium access control) protocol causes unpredictable delays. In this paper, we present the implementation details of a new RT-WLAN device driver module, which extends the original Linux device driver for the popular Agere ORiNOCO cards to support soft real-time communications. To our best knowledge, this is the first effort in providing real-time support in the WLAN environment at the device driver level. By shifting the design focus from the MAC layer, which is normally hard-coded in the NIC (network interface card), to the device driver level, which is between the system kernel and the MAC layer, our scheme has a clear advantage. Users can simply replace the original ORiNOCO driver with RT-WLAN, and then enjoy the benefits of real-time communications without having to change the NIC firmware or re-compile the Linux kernel. RT-WLAN uses two separate queues for real-time and non-real-time traffic. The real-time queue is served according to the EDF (earliest-deadline-first) policy, while the non-real-time queue is served in a FIFO (first-in-first-out) manner. Besides, an adaptive traffic smoother is implemented in RT-WLAN to regulate bursty non-real-time traffic before they are injected into the network, thus giving higher priority to in-progress real-time transmissions. Experimental results show that the desired real-time support and service differentiation among multiple real-time sessions are achieved by using RT-WLAN.