SoftMAC makes a transit to the Media Access Control (MAC) layer from hardware to software. Traditional network communication is classified into several layers which are dedicated for specific tasks. Higher layers tend to be implemented in software. Conversely, lower layers are likely to be carried out by hardware. However, in order to gain more flexibility and reduce the cost induced by hardware. Lower layers are considered to be implemented with increased software portion. SoftMAC plays an important role for fulfilling this objective.
For instance, the next generation of wireless communication system may involve Software Define Radio (SDR) that can accommodate various protocols concurrently. SDR supports different protocols by replacing some hardware components with software. The idea of SDR is initially developed by US military but is eager to commercialize due to its excellence.
Consequently, substantial effort has been dedicated to apply it onto the physical (PHY) layer of the SDR terminal.
Figure 1-1: Next generation wireless communication scenario
Figure 1-1 depicts the next generation wireless communication scenario. In the future, the mobile devices will be required adaptively changing from one network to another. The modulation, coding and data handling mechanisms may vary within different radio protocols.
SDR enables the mobile devices to roam through different environments without specific hardware requirement.
Besides, different devices also provide different channel access control mechanisms.
Studies for SDR encourage us to extend the concept onto the data link layer. The data link layer is usually composed of software and hardware. The commercial Wi-Fi modules are equipped with a chip to cooperate with driver. A subset of the MAC protocol is implemented in microcode and executed by the MAC chip. It manipulates time critical frames, such as beacon frame, request to send (RTS)/clear to send (CTS), ACK and so forth. These frames are required to fit into a time period with a short interframe space (SIFS) in between them.
Because of the proprietary concern, the driver and microcode are rarely released in source form. The research has been confined due to the non-transparency.
Therefore, the SoftMAC research platform has been developed by Michael Neufeld et al. in 2005 [1].The combination of various layer 2 protocols has been achieved by overriding the original MAC protocol. The study might have been far more interesting if they have included the original MAC protocol. There is a barrier in software implementation of the original protocol, since it requires the frames being sent in a timely fashion. Hyunseok Lee and Trevor Mudge point out the difficulty in their study [2]. In addition, a dual-processor platform has been proposed to meet the hard real-time constraint.
Recent researches in wireless networking have largely deployed the experiment environment by using commodity products. These products offer better cost-performance ratio. Furthermore there are abundant resources that can be acquired on the Internet, such as Linux wireless driver on linuxwireless [5]. The time consuming for developing the fundamental software is eliminated though. The researchers can therefore concentrate on the design of novel concepts. Besides, building up a real environment enables researchers to inspect the problems may occur as applying the design into reality.
Several attempts have been made to SoftMAC. Michael Neufeld et al. investigates a mechanism to ignore the time critical task, whilst Hyunseok Lee and Trevor Mudge exploit a supplemental processor to respond to the time critical frames. However, ignoring time critical task is not practical to infrastructural environment since IEEE 802.11 has been largely deployed to these places. For the compatibility, we have to adopt the standard IEEE 802.11 protocol. Although the supplemental processor proposed by Hyunseok Lee and Trevor Mudge can meet the time requirement, it cannot be carried out without specific hardware support.
Recently, however, real-time techniques are evolving from time to time. Processing ability is also increasing as time passes. The barrier, time constraint, may be able to break up with these advances.
In addition, even if the IEEE 802.11 has been commercialized for a long period, numerous evidences have been presented by previous research that it fails to guarantee the security. John Bellardo and Stefan Savage have reported the vulnerabilities of IEEE 802.11 [3].
Deny-of-Service is able to carry out by deauthentication since the message is not authenticated itself. Any other station can generate a deauthentication to a victim. Moreover, network allocation vector (NAV) offers another way to force other stations unable to transmit.
NAV is originally used by CTS/RTS mechanism to prevent collisions that induced by hidden terminal. Once the station receives RTS or CTS, further transmission can only be performed until the duration indicated by NAV expired. Nevertheless, they have argued that most of the commodity devices reset the NAV improperly. In general, the commodity products do not offer modification to microcode or driver. Therefore, their examination of NAV was done on ns simulator.
So far, however, there has been little discussion about how we can counter the time constraint without a specific hardware. The aim of this paper is to examine the mechanism that we can use for measuring the latency of the frames. It enables us to verify the software efficiency in timing perspective. To overcome the real-time constraint, a porting procedure will also be performed on a real-time embedded system. The system with real-time characteristic may be able to respond the message more rapidly.
The rest of this paper is structured as follows. In section 2, a comprehensive overview of IEEE 802.11 MAC layer will be given. Section 3 describes the concepts of the experimental platform. Section 4 explains how we achieved these concepts. Section 5 gives the result.
Section 6 concludes this paper.