OA placement in proposed architecture

In the proposed dual bus architecture, one can place optical amplifier all along the bus wherever required or after the signal is dropped from OADM. Using the OA in the bus gives the advantage that a single amplifier can amplify multiple wavelength and hence less number of amplifier will be needed. While if SOA is placed in signal path after dropping from downstream or before adding the wavelength in upstream direction, it will amplify only one wavelength. Consequently, OA placement only in the bus has been considered. The basic algorithm which has been used is as follows.

Only one amplifier is placed at a time. Let us term each group as branch. First of all using computational analysis number of branches which can be supported without OAs are determined. The criterion for supporting a branch is that BER should be less than 10^-9. Let ibranches are supported without OAs. This means that if we want to place an OA, there are i+1 positions. Now the OA is placed in each of the i+1 positions and number of branches that can be supported are computed. Thereafter the OA is placed in the position which leads to maximum number of branches. In case if there are more than one position supporting same number of branches, the position leading to minimum BER in the worst affected branch is choosen.

Once the position of first OA is fixed, let the supportable number of branches be j. In that case j possible positions for placement of OA are there. This is with the assumption that more than one OA cannot be placed between two consecutive branches. In computational procedure, the next OA is placed at each of the position and supportable number of branches alongwith the maximum BER (denoted as BER for that OA position) among all the branches are computed. The position which gives maximum number of branches and thereafter minimum BER is choosen. This algorithm is followed till no more increase in number of branches is there.

The computational model considers three cases; network with a) unsaturated OAs, b) average gain saturated OAs and c) OAs with average gain saturation as well as gain fluctuations. The computational model is given in more details in [8,9] and has been adopted from [10].