 Now it is also important to look at how the ADSL technology is implemented as in the network side. We know that the critical components of it from the operational point of view, that is some tasks are performed by DSLAM, others are performed by the splitter, then we have the corresponding transceiver units which perform modulation, demodulation. We looked at how the concentrator works on both on the uplink as well as the downlink. But if we were to provide ADSL service in a certain neighbourhood, then an ISP, Internet Service Provider, has to deploy a complete network architecture. The network architecture means the routers which might be used in there, the servers which might be necessary to provide the user subscription and billing services. For that we need to look at the network architecture. So in this module we look at the ADSL network architecture as a whole and then the specific, the brass, the broadband radio access service server. And then we look at the the access part and the core part in ADSL and the relationship between the network elements in there. So the ADSL network architecture comprises the core part as well as the access part. The access part is where the user resides, where the customer premises equipment is. The ADSL modem that your service provider provides you is in your home, in your residence or at your office. That's the access. The core is where the service provider actually is where the central office or the exchange is. Now DSLAM is a device that performs the aggregation, the splitting on the user traffic that is coming from the user side in a multiplexed manner and does the reverse of it for the downstream traffic. Now the DSLAM actually works and interacts with a lot of ADSL modems in the customer premises. So it is placed on the network side and we can think about many to one relationship for DSLAM where we have a single DSLAM as a multiplexer and then we have many ADSL modems each placed in a customer premises respectively. So the broadband remote access server is essentially a router. So it is placed between the access network and the core network. So what does it do? Since router mainly deals with the IP traffic meant for the internet. So it takes traffic from the DSLAM on behalf of the access network for which the DSLAM is there and it sends this traffic to the core IP network. So a router is actually a device that as the remote access server, it's a router that takes traffic from one side IP traffic that is from the access side via the DSLAM and sends it to the IP network. Now the brass actually performs aggregation of user sessions from access network and splits them to the customer premises side, towards the customer premises side. So what we can say is that although we haven't looked at the broadband remote access server in a figure that we shall shortly do but we have already started to understand it is doing something that a concentrator is meant to do. So a concentrator is only an aggregating service or aggregating facility that provides a many to one traffic convergence into a single router. Now this router is connected to the internet so all the traffic that leaves the access network has to pass through a single router. Now since this brass router is placed between the access side and the core side let's look at the core side as well. The core side is in effect responsible for providing billing, charging and IP address assignment traffic forwarding to the internet. So it comprises this application layer facilities like the proxy server and the AAA service, the access authentication and authorization. The AAA server you can call it as the registrar service that ensures the access. So it's an access registrar service. The AAA server is implemented in the form of a radius protocol based server. It is remote authentication dial-in service that is it was meant for dial-in users that is users with a dial-up modem but with the broadband technology the DSL radius is still compatible and it is still being used although it is being increasingly replaced by another extension of radius called the diameter protocol. So the radius since it is meant for AAA so it means it requires some transactions between the user side and the network side in the form of a client server manner. So the radius protocol was promoted and it was adopted by the ITF. The radius client is in the router side that is the brass router that represents all the ADSL modems which seek internet connectivity. So a single brass router radius client talks to the AAA server on behalf of all the user ADSL modems. Let's look at the diagram. It is going to make things much clearer. We see that we have starting from the leftmost side we have the ADSL modems. Each modem is placed in a customer premises residential corporate enterprise whatever it may be. Then all these ADSL modems are connected to the DSLAM that talks to all of them and gets their internet bound traffic and gives it to the concentrator or the aggregator. The concentrator or an aggregator is nothing but a service that concatenates or appends the user traffic in such a manner that it can be sent as IP packets to the broadband remote access server. Now this router brass is the stub router or the last mile router that represents the authentication and authorization details of all the ADSL modems on the customer side for this side of the network and then negotiates with the registrar the AAA registrar server and the radius server and the registrar server. This is how the complete end-to-end internet connectivity is realized in the form of complete network architecture the ADSL as we see and use today.