 Now, we are going to look at the concept of quality of service with regards to how exactly the quality of service can be provisioned. For that, we have the concept of bearer. What are bearer channels? How do bearer channels provide bearer services? And what is the way to set up these bearer services? So bearer is essentially a service between different interfaces in the overall network architecture. Bearer allows a mechanism to exchange information between different network elements under certain constraints, that is, the exact requirements for bearer service are spelled out and bearer service is based on those requirements as constraints. These constraints actually mean if there is a definite bit rate requirement, if there is a certain quality of service class that needs to be implemented. Let us look at the overall network architecture and different bearers which are incorporated in the system architecture evolution. Here you can see we have the network elements starting from the user equipment on the customer premises side. We have the enode b, the serving gateway, the packet data network gateway and we have the extraneous or the external internet. Here we see that we have the interface between the user equipment and enode b and for that we have a radio bearer known as LTE and LTEA radio bearer also known as RAN radio access network bearer. Between enode b and the serving gateway we have the S1 bearer. If you recall, we looked at different interfaces starting from S1 right up to S9. Then we have the serving gateway and packet data network gateway connected through S5, S8 bearer. On top of this we have the evolved packet system. The evolved packet system actually consists of all these network elements and the UTRAN and EUTRAN in LTEA. So we have the EPS bearer, EPS bearer actually is opened between the user equipment and the packet gateway. Then on the top we have the end to end service bearer. So we have all these different kinds of bearers with their scope. Let's continue our discussion on these bearers one by one. Whenever a mobile or a user equipment enters into a certain network after necessary connection and registration, each user equipment is entitled to at least one basic bearer. It is known as the default bearer for providing IP connectivity. Please remember we are actually looking at these bearers because these bearers would be establishing the signaling and then carrying the data traffic which could be like voice or any other type of data. So it means by default at least one bearer is established known as a default bearer. In addition if there is a requirement other bearers can be established. Each bearer could have different quality of service requirements. So if we have multiple flows and if different flows correspond to the same quality of service requirements, these flows can be mapped onto same bearer because the bearer meets the quality of service requirements in terms of the bit rate and the priority level. Now regardless of whatever bearers we have seen, the bearers need to be set up. The process of the initiation of bearer setup starts at the user equipment. So when the user equipment starts an application layer activity, for instance if a client tries to contact a web server and establish a connection with it or if a voice call connection request is received at the call party or it's the other way the calling party is trying to establish a voice call connection. For that if there is a specific quality of service requirement which is going to be there because we are talking about interactive services like voice, voice over IP, video. So for that the user equipment and an application server which sits on the service provider or the operator side. The application server sits on the operator side. So the user equipment opens a connection and this is mediated or this is managed through the signaling system which is used in the overall engine architecture known as IP multimedia subsystem. So the user equipment and the application server are connected via the IP multimedia subsystem. Now it all depends on how subsequently the application server requests the evolved packet system bearer services. The point that is the most important and needs to be the take away point here is that when the user equipment opens up a connection with the application server the application server takes the responsibility to open respective bearers at different scopes. For instance between the user equipment to the E node B and between other network entities. So correspondingly the bearers are established for signaling with the lower layers including the radio bearer, the S1 bearer, S5, S8 bearer between the serving gateway and the packet data network gateway. The main point here is that the bearers are established to keep the load as much off the user equipment in terms of QS knowledge that is specific QS requirements and the configuration complexity. So what it means is that the user equipment has to be kept as light as possible in invoking these bearer channels so that the complexity stays on part of the network. So let's get back here once more. We see that we have the user equipment that basically opens the end to end service bearer with the application server. The application server correspondingly opens up lower layer bearers so that the user equipment is kept as much aloof and as much independent of the configuration and QS complexity as possible.