 In this module, we are going to look at the interaction that takes place between the OLT and the ONUs in the EPON architecture. Specifically, we'd look at the principle which governs it, the operation of the MPCP protocol, and then we'd look at the operation in a little more detail. Ethernet on first mile, as the working group tasked with the establishment of the Ethernet protocol deployment requirements in the last mile, was targeted to introduce a mechanism through which the OLT and the ONUs could communicate. For that, the MPCP protocol was introduced at the MAC layer, so this calls for slight modification to the operation of the MAC layer as such. The multipoint control protocol is nothing but a mechanism through which the upstream traffic, that is from the ONUs to the OLT, is managed and controlled by a message called the grant message. The grant message is assigned from the OLT to each ONU when it makes a specific request. MPCP as such is a mechanism that allows the EPON architecture which is essentially a peer-to-multipere architecture because we have one OLT and we have multiple ONUs to emulate a scenario through which each OLT and ONU interaction looks like a unique interaction. So each ONT has the MPCP part deployed in it, likewise the OLT has the MPCP part or the entity deployed within. Since the MPCP interactions are taking place between the OLT and the ONU MPCP entities on both sides, consequently a peer-to-peer architecture looks like an overlay. Now, when the operation starts, the ONT on the customer premises side first of all encaches data for the upstream transmission and then it makes a bandwidth allocation request. This allocation request is to get a grant message. So when the grant message is bestowed or given to the ONT, it readily transmits. So you may wonder that a little bit of caching delay is incorporated between the ONT to OLT upstream transmission. Consequently, the transmission takes place. This transmission only is restricted to the time slot which is given in the grant message. Utilizing this time slot, the ONU transmits at the maximum data rate. So using this method, each ONU in a round robin turn-by-turn fashion is able to transmit its user data towards the OLU that subsequently sends this to the internet.