# Evolutionary Strategy for Practical Design of Passive Optical Networks
## 3 Adop Topologies for Proposed Systems  
PON systems can use bus, ring, tree or a mix of these previous topologies [21]. This choice occurs at the network planning stage and each topology has different characteristics, being implemented according to the situation and requirements of each project [26]. Therefore, the appropriate topology choice depends on the project premises, such as redundancy requirement, service capillarity and how subscribers and potential subscribers will be geographically arranged.  
Among the basic PON topologies, Internet service providers (ISPs) typically adopt the tree topology [27]. This is mainly due to cost reduction and greater network connectivity for subscribers who lives in a certain region [22]. However, the wide range of splitter configuration and placement possibilities create an extensive list of tree-topology types using single or multiple stages. The way the optical splitters are arranged in the network defines whether the system is composed of a centralized topology (with a single splitter stage in the outside plant) or a distributed (also known as cascaded) topology (with multiple splitter stages in the outside plant) [26; 28]. In general, a centralized approach typically offers lower operational costs and is easier to access and to control for technicians. On the other hand, distributed topology brings faster return-on-investment, lower initial costs and lower fiber costs [28]. This leads to a typical benefits and disadvantages trade-off decision.  
It also should be highlighted that the filling rate of an OLT PON port has a direct impact on the number of OLT PON cards and the size of the OLT shelf. Consequently, it has a direct impact on the deployment cost. The way how fiber distribution hubs (FDHs) are distributed and connected in the ODN (e.g., same areas or areas with different fiber service appetites) may influence how PON cards are gradually activated or utilized. In other words, PON cards activations can be postponed or reduced based on how FDHs are inter-connected.  
Although our proposal can be easily adapted for different scenarios, this paper focuses on fiber-to-the-home (FTTH) deployment planning using GPON system. For this purpose, the following topologies were considered (Sections 3.1-3.4): (1) Centralized Topology--Type 1 (CTT1), (2) Centralized Topology--Type 2 (CTT2), (3) Distributed Topology--Type 1 (DTT1) and (4) Distributed Topology--Type 2 (DTT2). It is worth mentioning that the specifics of these hypothetical topologies are considered in order to represent a real GPON deployment using the two aforementioned approaches for tree topologies (known as centralized and distributed topologies). These topologies are commonly used by ISPs and basic concepts are described in [28]. Figure 1 shows the physical topology diagram for: (a) CTT1, (b) CTT2, (c) DTT1 and (d) DTT2, and Table 2 presents the summary of the adopted topologies in terms of topology type approach, number of splitter stages, network split ratio and splitter's placement.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Item** & [18] & [20] & [21] & [22] & **Our Proposal** \\ \hline Approach & ACO & Dijkstra and \(k\)-means & GA & ILP and & GA \\ Georeferenced real maps & – & X & – & – & X \\ Number of splitters possible positions (worst & 5 & N/A & N/A & 50 & 714 \\ available case) & 1033 & 40 & 120 & 1000 & 1544 \\ Number of ONUs (worst available case) & X & X & X & X & X \\ Maximum split ratio (per PON) & N/A & 64 & 128 & 64 & 64 \\ Elapsed time (min) (for worst available case) & 4.06 & N/A & N/A & N/A & 72.27 \\ \hline \end{tabular}
\end{table}
Table 1: Summary of the contributions of the papers available in the literature and the main characteristics of our proposal in this paper.  
Figure 1: Physical topology diagram for: (**a**) CTT1, (**b**) CTT2, (**c**) DTT1 and (**d**) DTT2.