**2. Optical CDMA physical layer networks**

Due to the transparency increment in optical communications network components and systems, network management and maintenance have been faced additional security challenges . An evaluation on several existing physical security violates on optical communications network is presented in (Teixeira et al. 2008).There are four main threats that can be described in terms of how they affect the normal flow of information in the network, as shown in figure (1),they are: denial of service, interception, modification and creation . Table 1 summarized some of these attacks.


Table 1. Optical networks attack methods

The security services of a network have four fundamental objectives designed to protect the data and the network's resources (Fisch & White 2000). These objectives are:


In this chapter, ensuring confidentiality against eavesdropper interception strategies for optical CDMA aims to investigate the limitations and tradeoffs between security and performance .

There are various fiber optic tapping methods, of which fall into the following main categories (Oyster Optics 2008): splice (involves literally breaking the cable at some point and adding a splitter), splitter or coupler (involves bending the cable to a certain radius, which allows a small amount of the transmitted light to escape) and non-touching methods (passive and active), involve highly sensitive photo-detectors that capture the tiny amounts of light that emerge laterally from the glass fiber owing to a phenomenon known as Rayleigh scattering.

Fig. 1. Pattern of network attacks

Communication between authorized users in a network can be implemented by two approaches; point-to-point and broadcast. In the point-to-point, approach each user transmits to another specific one whereas in a broadcast approach users transmit in common to the medium accessible to all other users. Figure (2) shows a common topology found in point-to-point networks . Figure (3) shows two topologies established in broadcast networks .

Fig. 2. Point to point star topology

Fig. 3. Broadcast network topologies

Topology is an important architectural consideration, and different topologies have different security properties. Ring topologies allow attacks to be relatively easily localized, because of the structured interconnectivity of nodes. Star topologies make attack detection nominally easier than other topologies, because any propagating attacks are commonly received at many stations. Optical CDMA has many advantages such as sharing bandwidth, controlling and high security compared to other access technologies such as TDMA and WDMA. Recently, studies discovered that Optical CDMA systems suffer from weakness against eavesdropping and jamming attacks.

Figure (4) shows the possible positions, within the network, to tap a signal from the user. Therefore, when just a single user is active, optical CDMA system cannot guarantee physical layer security any more. In certain time, this situation can be existed even in a multiuser active optical CDMA network as reported in current theoretical analyses (Shake 2005a,2005b ).

Fig. 4. Places for an eavesdropper to attack and tap optical CDMA encoded pulses
