tailieunhanh - The Illustrated Network- P78

The Illustrated Network- P78: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 30 Voice over Internet Protocol 739 interface look like a real telephone. The best that Avaya does is place a small keypad on the screen so that you don t have to type the numbers in. Before you can make a call you have to log in to the server. A simple log-in ID and password is used and then the screen shown in Figure appears. It shows the extension the computer is acting as its IP address this capture is not from wincli2 so the addresses have been changed to the private range the VoIP server s IP address and the gateway VoIP address. The call status is shown also and this screen was captured while the call was in progress. The first thing that becomes obvious when capturing VoIP sessions is the blizzard of packets presented. The actual session from dialing through conversation to hangup lasted less than 30 seconds and the log-in process registration and call setup took only a few seconds of that in this 30-second window some 756 packets passed back and forth from the VoIP client to server. Most of them were small packets using the Real-Time Protocol RTP which carries 20 bytes of voice coded at 8 Kbps the standard . A portion of the FIGURE Avaya log-on screen with a call in progress. 740 PART VII Media FIGURE RTP packets carrying 20 bytes of voice shown highlighted in the bottom pane. conversation between client and gateway is shown in Figure . The gateway address is now accessed from wincli2 and therefore different from that shown in Figure . In addition to the TCP packets which are used to set up the connection to the server and the RTP packets carrying the voice bits and the RTCP packets with status information there are other control packets that serve to remind us that we are not in the data world anymore. The voice world uses a unique language and an often obscure one at that. This VoIP implementation speaks a signaling protocol family for voice. The main signaling protocols seen during the .