Explain the architecture of the Internet.

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24.Explain the architecture of the Internet.

Architecture of the Internet:

Let us assume client calls his or her ISP over a dial-up telephone line, as shown in Fig. 11. The modem is a card within the PC that converts the digital signals the computer produces to analog signals that can pass unhindered over the telephone system. These signals are transferred to the ISP's POP (Point of Presence), where they are removed from the telephone system and injected into the ISP's regional network. From this point on, the system is fully digital and packet switched. If the ISP is the local Telco, the POP will probably be located in the telephone switching office where the telephone wire from the client terminates. If the ISP is not the local Telco, the POP may be a few switching offices down the road.

Overview of the Internet


The ISP's regional network consists of interconnected routers in the various cities the ISP serves. If the packet is destined for a host served directly by the ISP, the packet is delivered to the host. Otherwise, it is handed over to the ISP's backbone operator.

At the top of the food chain are the major backbone operators, companies like AT&T and Sprint. They operate large international backbone networks, with thousands of routers connected by high-bandwidth fiber optics. Large corporations and hosting services that run server farms (machines that can serve thousands of Web pages per second) often connect directly to the backbone. Backbone operators encourage this direct connection by renting space in what are called carrier hotels, basically equipment racks in the same room as the router to allow short, fast connections between server farms and the backbone.

If a packet given to the backbone is destined for an ISP or company served by the backbone, it is sent to the closest router and handed off there. However, many backbones, of varying sizes, exist in the world, so a packet may have to go to a competing backbone. To allow packets to hop between backbones, all the major backbones connect at the NAPs discussed earlier. Basically, a NAP is a room full of routers, at least one per backbone. A LAN in the room connects all the routers, so packets can be forwarded from any backbone to any other backbone.

In addition to being interconnected at NAPs, the larger backbones have numerous direct connections between their routers, a technique known as private peering. One of the many paradoxes of the Internet is that ISPs who publicly compete with one another for customers often privately cooperate to do private peering (Metz, 2001).

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