 Since we are interested in understanding how exactly carrier aggregation works, it is important to look at the role of the user equipment that is the mobile terminal in carrier aggregation. After all, the carrier aggregation is going to be implemented on the wireless side. For that, we would look at the initial 4G devices which were taken up for deployment of carrier aggregation and then we are also going to look at the layered model. Initially, the mobile phones were considered for the deployment of carrier aggregation. However, there were certain constraints. The first one was the hardware complexity because carrier aggregation actually meant to have multiple MAC layer and the radio link control layer interfaces available on a single mobile phone. That's the hardware complexity. Then, there was a requirement for increased power which was to be transmitted because after all multiple carriers would be transmitted and received. The third one, of course, was the involvement of the Federal Commission on Communication Guidelines which says that the mobile phone need not transmit beyond a certain radio power which could be hazardous to the individual carrying the mobile phone. For that, initial deployment instead was considered on laptops. And if you recall, we had LTE advanced radio interface which was provided on laptops via the USB port. The carrier aggregation as the concept entails is the grouping of the resultant bit stream that is coming from multiple carriers. Each carrier is going to send its own bit stream over the MAC. Now, we know that at the data link layer, we have the logical link control that sits on top of the medium access control sublayer. In the context of wireless, it is the radio link control sublayer. The responsibility of the radio link control sublayer is to provide an aggregate bit rate which is obtained from all the carriers below it. So correspondingly, it means the RLC is going to talk to multiple carriers each coming from the physical layer and its corresponding MAC layer. And since we have up to five bands and we can have from 1.4 to 20 MHz bandwidth allocated on a single band and we could have up to five bands. Let us look at the overall diagram. Here, we are looking at the OSI protocol model. That is, we have the network layer on the top, we have the data link layer and then we have the physical layer. We see that we have one IP protocol layer implementation which sits on top of the packet data convergence protocol layer. Now this PDCP is actually responsible for aggregating all the traffic stream. The radio link control aka the logical link control is responsible for talking to multiple individual MAC layer implementations. Each intern has its own physical layer which is responsible for managing a certain carrier in a certain band. So at the physical layer you see, we have the LTE spectrum, we have up to five carriers. Each of these individual frequency is providing a bit stream to the MAC layer right to the radio link control. The radio link control manages it and passes it on to the packet data convergence protocol. So we see how exactly in a layered manner these protocols work together to provide an aggregated bit stream which is the hallmark of the LTEA.