 Although, when we looked at different variants of passive optical networks, which service the exercise that is the user premises, we looked at Ethernet passive optical network as the most widely prevalent technology and since Ethernet is so much in the offing, so inherently some mechanism was thought which could actually utilize the presence of Ethernet as such. So, this essentially forms the principle and we look at the corresponding Ethernet operation that comes out of that particular principle. So, if you look at Ethernet, there's no denying whether we talk about the wired side that is 802.3, IEEE 802.3 or the wireless side that is 802.11, Ethernet is the de facto standard on the access side. Whether it is smartphones, computers, home side, offices or an enterprise, the traffic which is carried from the user side to the network side is encapsulated in an Ethernet frame. Ethernet frame has a well-known preamble header and a payload. So, this Ethernet frame is carried between the user device that is a user equipment such as a smartphone and a computer or any other device to the stub of that particular network and then from that particular stub that is the user equipment or the customer premises equipment to the central office it is also carried on internet and on the other side then there's another network that could be equally an Ethernet network. So, it is from the Ethernet on an IP network to an Ethernet network. So, essentially the requirement arises why not have the intermediate network or the intermediate link for that matter between the user equipment and the central office to be also based on Ethernet. So, essentially Ethernet is everywhere and it has monopolized the deployment of IP based network so much that someone actually thought about proposing a passive optical network purely for the Ethernet. So, here it goes. Ethernet once it is deployed in the customer premises is in a shared medium that is Ethernet is offered either through a hub and spoke topology in which it could be a mere hub a passive device or it could be a switched network in which a switch automatically learns the MAC addresses of all the interfaces aka computers which are connected to that particular switch. So, Ethernet frames are carried and shared between transmitted between different users on a LAN in essentially a broadcast manner. So, an Ethernet network is a broadcast medium when it comes to broadcast traffic. So, the traffic is isolated only in terms of MAC addresses for one-to-one communication. This is how Ethernet essentially works but when we want to transmit Ethernet frames between different ONTs and OLT we actually have to think about a mechanism which is which allows Ethernet frames to be sent onto a shared medium. For that again from the central office that is the OLT to the ONU or ONT side we have the downstream and on the other side we have the upstream. So, on the upstream the ONT actually transmits Ethernet frames individually towards the OLT by using a certain time slot. Now, in order to make sure that this time slot is not used by any other ONT some kind of scheduling like TDMA mechanism has to be there. So, yes indeed there is a mechanism known as request and grant mechanism by a famous protocol called MPCP protocol. We would look at it in due detail subsequently. So, this is how the upstream traffic is transmitted. The overall control of MPCP that manages traffic partitioning in a time division multiplexed manner from different ONTs actually is done by the OLT as the central entity. So, when this particular kind of mechanism is to be introduced then EPON comes up with a new kind of physical layer. Introducing a new physical layer means some additional kind of encapsulation or framing. So, not much changes when it comes to Ethernet frame. It is the same IEEE 802.3 frame structure that is used between the central office and the ONT but there is a slight introduction of a physical layer. So, that is something which is modified here. As far as the downstream traffic is concerned it can be any as if you recall we had the peer-to-peer relationship and peer-to-multipere relationship. So, it can be simply broadcast in the downstream but it can also be organized to make sure that each ONT does not receive traffic for meant for other ONTs. We would look at that in subsequent modules.