Explain Store and Forward Packet Switching.

Before delving into the specifics of the network layer, it is essential to understand the context in which network layer protocols operate. This context can be visualized in Figure 5-1. The major components of the network are the Internet Service Provider’s (ISP) equipment, such as routers connected by transmission lines, shown inside the shaded oval, and the customers’ equipment, shown outside the oval.

Host H1 is directly connected to one of the ISP’s routers, A, perhaps as a home computer that is plugged into a DSL modem. In contrast, H2 is on a Local Area Network (LAN), which might be an office Ethernet, with a router, F, owned and operated by the customer. Although router F is shown outside the oval because it does not belong to the ISP, for the purposes of this chapter, routers on customer premises are considered part of the ISP network because they run the same algorithms as the ISP’s routers (and our main concern here is algorithms).

The equipment is used as follows: A host with a packet to send transmits it to the nearest router, either on its own LAN or over a point-to-point link to the ISP. The packet is stored at the router until it has fully arrived, and the link has finished its processing by verifying the checksum. Then, the packet is forwarded to the next router along the path until it reaches the destination host, where it is delivered. This mechanism is known as store-and-forward packet switching, as discussed in previous chapters.

By understanding this context, we can better grasp the role of network layer protocols in facilitating communication between hosts across the ISP’s network and customer premises equipment.

•A host with a packet to send transmits it to the nearest router.
•The packet is received, verified, and stored .
•Then it is forwarded to the next router.
This step can be repeated many times.
•Finally the packet reaches the destination host.

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