Free Space Optics: A New Concept for Improving Wireless Local Area Network Security
...refore, being simultaneously provided with the advantages from both optical communication and wireless technique, free space optics (FSO) appears on the network stage. Contrary to the general thoughts, free space optics is hardly to be regarded as a novel technique in the world. Actually, it was first developed in 1970s (Acampora, 2002), and has been adopted in both outdoor (Virginia, 2003) and indoor (Gfeller and Bapst, 1979) applications. FSO bi-directionally transmits signals (data, voice, and video) in infrared region at rate of from 10Mbps to several Gbps. The application area of FSO could be similar to that of optical fibre communications instead of transmission medium. A simple FSO system consists of a couple of integrative transmitting/receiving equipments that operate at duplex (see figure 1). Figure 1. Basic operating principle of FSO system As can be seen above, after receiving information from other networks, the integrated transmitter will aim at the corresponding receiver, and launches the modulated optical signal toward it. Due to the divergence angle of laser beam, the receiving area of receiver should be designed many times larger than transmitter. ■ Network Architecture, Physical Equipment a) Topologies of FSO Before the network structure has been discussed, another issue should be considered. Where FSO systems can be used and implemented? Generally, FSO can be chosen at both outdoor and indoor communications. Correspondingly, their architectures are different from each other. In outdoor applications, the topology of FSO can be categorized in four kinds of structures (see figure 2). (A) Mesh (B) Star (C) Ring (with spurs) (D) Point-to-Point Figure 2. Topologies of FSO in outdoor application (reproduce from www.freespaceoptics.org) The above four structures are mesh, star, ring and point-to-point. Mesh topology (figure 2(A)) has the maximum fault tolerance, whereas the cost will ascends sharply according to the increasing of nodes. Star topology (figure 2(B)) leads all nodes to be connected to hub point (central point), and can be easily implemented. People just need to increase the hub points if they want to extend the network. The major drawback of star topology is the low fault tolerance due to the few hub points. Ring topology (figure 2(C)) actually is the closed loop of point-to-point topology (figure 2(D)). By utilizing token-ring protocol in ring topology, collision accident in network can be minimized. However, in both ring and point-to-point topology, the crisis of failure is averagely distributed on every link between two nodes (Kenyon, 2002). In outdoor applications, those four architectures enable people to flexibly adopt suitable solution according to different environment. Since mid 1990s, most FSO systems are designed to be able to cover distance from 100 to 500m (Killinger, 2002), and some commercial systems can operate up to 5,000m (Willebrand, 2002). The transmission distance is limited in FSO system due to several reasons, such as weather, divergence of laser beam, responsivity of detectors, etc. They will be discussed in next few sections. When it comes to indoor applications, the topology is simpler than outdoor’s. Basically, it can be classified into two major types: line-of-sight system and non-line-of-sight system, and each of them can be further divided into three groups: directed, non-directed and hybrid (Singh et al, 2002). Figure 3 illustrates those solutions. Directed Non-directed Hybrid Line of Sight Non-line of Sight Figure 3. Categories of indoor FSO system (reproduce from Singh et al, 2002) Line-of –sight system depends on the communication between fixed transmitter on the ceiling and receivers, and the signal will be directly received by receivers. Non-line-of-sight depends on the reflection of signal through the ceiling or walls. Moreover, the types of transmitter and receiver vary as directed, non-directed and hybrid modes according to different transmitting or receiving angles. Intuitively, directed line-of-sight system provides the minimum loss and maximum transmission rate, where as transmitter and receiver have to be precisely aligned. Non-directed non-line-of-sight system provides high barrier-tolerance when there are some objects between transmitter and receiver, however, the transmission loss will correspondingly increase. Some hybrid systems can provide compromise solutions (Kahn and Barry, 1997). b) Light Source and Detector Similar to general optical communication equipments, FSO equipment combines the electrical module and optical module. As the one of the kernel devices, light source and its operating wavelength are profoundly considered. The determination of operating wavelengths do not ascribe to a unique reason, but a synthesis of many factors. Basically, the wavelength that adopted by FSO system falls in two infrared regions, one is 780 nm to 850 nm and another is around 1550nm (fSONA®, 2001). It is mainly because of the emitting wavelength of the light source, such as laser diode (LD) and light emitting diode (LED). However, a key issue should be considered when designing light source, that is, the safety to human eyes, especially in indoor applications. Fortunately, laser equipments can be designed safe to eyes in both of these two regions (Sliney and Wolbarsht, 1980). Furthermore, the power of 1550nm laser may 10 to 100 times higher than that of 850nm laser. For example, The GaAsAl laser diode (operates 800-900nm) can reach 0.1W, while InGaAsP laser diode (1550nm) can reach even 10W (Killinger, 2002). Therefore, 1550nm is suitable for outdoor applications due to the high possibility of attenuation in transmission, Corresponding to short-wavelength (780-850nm) and long-wavelength region (1550nm), people should properly choose appropriate detectors. Detectors that operate at relatively low bandwidth are suitable for the former. Such detectors include Si-based PIN and Si-based APD detectors. For long-wavelength operation, InGaAs-based PIN or APD would be a wise solution (Bloom, 2003). ■ Security Issues in FSO a) Inherent Safeguards By introducing the natural features of light, the improvement of security in FSO systems is more distinct than other solutions in WLAN. Essentially, the security issue is mainly focused on outdoor applications because most of the interception behaviors are done outside of the building. Intuitively, the building itself is a secure barrier that can prevent any inbreaks toward indoor applications. However, in outdoor applications, even though FSO system are normally installed outside of the building, it still has some native strategies to enhance the security of communication. First of all, the place of installation is hard for interception. Normally, FSO system is placed far from ground (i.e. top of the building). To a certain extend, the intruder is hardly easy to approach that point (LightPointe®, 2002). Even they achieve that place, surveillance camera will tell people everything. Last but not least, if that behavior does not detected by camera, and the intruder successfully intercepts a piece of optical signal, the communication will be interrupted. Consecutively, the interruption of optical signal will result in the occurrence of error messages in system, and the enquiries for recovering proper information will repeatedly be generated between transmitter and receiver. Eventually, what the intruder gets is the error message and countless enquiries (Steege, 2002). For most access points in 802.11 applications, signal is always transmitted in forms of broadcasting. Therefore, intruders can easily listen to the traffic through some free or advance client tools as mentioned above (Johnson, 2002). FSO system operates in point-to-point way. In other words, for a certain transmitter, it is absolutely impossible to intercept any sig...