Architecture

In any subscriber access network, there is a central office (CO). The subscribers are connected to central office and all the communication takes place through it. The central office does all the management including metering and billing. The proposed architecture for fiber optic subscriber access network is shown in Fig.1.

 

Figure 1: Dual bus topology for SAN

In this architecture, the subscribers are partitioned into groups. Each group uses a common shared wavelength to communicate to the central office. The wavelength can be shared among the subscribers in the group using a suitable access protocol. An example of such a protocol is given in [3], and many more can be designed.

There are two fibers running from central office through all the subscriber groups. One fiber carries as many wavelengths as the groups for downstream transmission. The second fiber is used for upstream traffic. For providing the group the access to one of the wavelength in the fiber, use of wavelength add-drop multiplexers is proposed. The above topology is dual bus topology. One can use the add-drop multiplexers in ring configurations also as shown in Fig.2. In ring configuration, while a wavelength is dropped for downstream transmission, the same wavelength can be added for upstream transmission. The advantange of this configration is that only one fiber is needed unlike two fiber in dual bus topology.

 

Figure 2: Ring topology for SAN

After a wavelength has been dropped at add-drop multiplexer, it is broadcasted to all the subscribers in a group using star coupler. There is another parrallel broadcast network for collecting upstream traffic from all the subscribers.

In this network, the number of users which can be supported will be limited by split loss of star coupler and insertion loss of add-drop multiplexers. While WDM will be providing large bandwidth, to use it effectively the network need to support large number of users. This can be done by placing the optical amplifiers appropriately.