tailieunhanh - The Illustrated Network- P48

The Illustrated Network- P48:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 17 MPLS and IP Switching 439 the world over. ATM was part of an all-encompassing vision of networking known as broadband ISDN B-ISDN which would support all types of voice video and data applications though virtual channels and virtual connections . In this model the Internet would yield to a global B-ISDN network and TCP IP to ATM. Does this support plan for converged information sound familiar Of course it does. It s pretty much what the Internet and TCP IP do today without B-ISDN or ATM. But when ATM was first proposed the Internet and TCP IP could do none of the things that ATM was supposed to do with ease. How did ATM handle the problems of mixing support for bulk data transfer with the needs of delay-sensitive voice and bandwidth-hungry and delay-sensitive video ATM was the international standard for what was known as cell relay there were cell relay technologies other than ATM now mostly forgotten . The cell relay name seems to have developed out of an analogy with frame relay. Frame relay relayed switched Layer 2 frames through network nodes instead of independently routing Layer 3 packets. The efficiency of doing it all at a lower layer made the frame relay node faster than a router could have been at the time. Cell relay took it a step further doing everything at Layer 1 the actual bit level . But there was no natural data unit at the physical layer just a stream of bits. So they invented one 53 bytes long and called it the cell apparently in comparison to the cell in the human body which is very small can be generic and everything else is built up from them. Technically in data protocol stacks cells are a shim layer slipped between the bits and the frames because both bits and frames are still needed in hardware and software at source and destination. Cell relay ATM relayed switched cells through network nodes. This could be done entirely in hardware because cells were all exactly the same size. Imagine how fast ATM switches would be compared to