How Medium Type (B-type) ISP Node works?

B-Node Diagram ( Medium ISP Node)
Click on the router or switch to view larger image Step1: This diagram is for a dial-up user. When the user dials the ISP number, he is connected to the Remote Access Server(RAS) through the telephone exchange.
Step2: The RAS get the username and password and verifies with the database kept in another server called as RADIUS server. If the RADIUS server says OK, then RAS allows the user to use the Internet.
Step3: Actually, RAS consists of TWO parts. One is modem shelf(bottom one) another one is Router shelf(the top one).The packet which is released by the user PC is converted into analog(voice) signals by the user's modem.(The telephone exchanges will route only the voice not the data, hence this modulation is necessary). This voice signals reaches the RAS Modem Shelf. This RAS modem demodulates the voice signal again into data. This data goes to the upper shelf which is router. Router shelf routes the packet to another Router. This router in turn routes the packet on the Internet.
The Type-A node which will be available only in bigger cities will be have more number of equipments (such as Servers,Routers,Switches). The following is image of a Type-A Node.
-Shivaji From India