 If we all agree that different deployments or different arrangements of the optical distribution network as we call for Fiber to the home exist, each has its own pros and cons, but in general since deploying Fiber from the central office to the customer premises on a link by link individual Fiber is not possible. It is not simply worth the investment. So the overall consensus is that the passive optical network is the way to go. So the passive optical network exists and it is increasingly being considered for future deployments. In this module, we shall quickly look at how the passive optical networks operate. It would just be a recap and then we'd look at the advantages and disadvantages because when passive optical network saves us the cost, it also comes with certain constraints and certain limitations. So first of all, passive optical networks are deployed as the peer to multi peer configuration. It means that a central office through a single feeder fiber talks to multiple ONUs. I mean one OLT talks to multiple ONUs. Since we are talking about PON, not specifically WDM PON, so it is mostly based on TDM. Now the TDM signals are distributed through optical signals in a tree like topology. A tree like topology is what we saw in the passive splitter. Of course, using such a simple mechanism has the inherent advantage that only few optical fiber cables are deployed in a certain geographical area. All the households which are closely placed or the adjacent to each other can benefit from one feeder fiber and then they only need to lay the fiber from the passive splitter to the door steps. That's obviously a very big advantage. Likewise, the central office equipment serves more than one ONUs. So all it has to do is have a single optical interface that in a time division multiplexed manner talks to multiple ONUs. Because the traffic gets aggregated from the user side and it gets split from the OLT side. Of course, since there are countable, very few active devices along the route end to end. So the overall supply, the main supply requirement is also limited to the end devices. So no intermediate device needs to be powered. This is an advantage because providing the mains power to an intermediate device is cumbersome. So with all these advantages, it looks like the passive optical networks are slam dunk winners. But this may not always be true. For instance, the capacity of fiber optic cable is going to be shared amongst the number of users. So it means as the number of users increase, it simply means that number of ONUs are increasing. So each additional deployment of an ONU causes the bandwidth degradation from other users perspective. Likewise, the optical fiber is carrying limited light power. So it means this optical power, which is coming from the OLT is now split or distributed between the output ports. There's a device known as optocoupler. So the passive optocoupler as in splitter, splits power, you can say equally to all the outgoing splits or the ports. So this results in lower power. So the signal to noise ratio as a measure would be affected. And then it has a direct bearing on the total distance that this fiber optic can provide an acceptable signal to noise ratio. So it means the overall length between the OLT and the ONU or the ONT would be limited, would be reduced. Now there's another flip side to it that on the upstream, if a certain user is malicious and simply inserts a beam, a high intensity beam of light that is through a laser device, it would result into a continuous engagement of the splitter. Because now the splitter from the ONU side to the OLT side is acting like an aggregator. So because of that, this one malicious user can disrupt the communication of other users. And lastly, since the traffic coming from the OLT downstream to the ONUs is being replicated on all the outgoing ports or the splits in the passive splitter. So it means security becomes a consideration because what a user gets is its own traffic and the traffic of all the other users. So any promiscuous user can actually have access to the traffic going to the other users. So some high end security mechanisms have to be put in place as well.