Slide 1 Page 1 of 9 Polycom University

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1 Slide 1 Page 1 of 9

2 Slide 2 Welcome to Network Communication part 1, a module in the Polycom Fundamentals series. In this short module we will talk about the OSI model and how it fits in with sending real-time media across a network, and we will begin with a recap of the familiar OSI 7-layer model. It is recommended that if you are not already familiar with the OSI model that you consider Network+ or similar training to become so prior to proceeding on the path to the PCVE certification. This module is approximately 5 minutes long. Page 2 of 9

3 Slide 3 And here it is. If you are working on your computer, the data you enter in your application moves down into each layer, being encapsulated in each as it goes. As the data is encapsulated in each layer, specific information is added which enables the data to be unpacked at the receiving end. Finally, when the data hits the physical layer, the 0s and 1s travel across the network, and the data is unpacked, removing each level of encapsulation and moving up each layer until it appears in the application as the original data which was sent. Page 3 of 9

4 Slide 4 To recap, each layer has a specific function. The Physical layer is responsible for the binary data transmission by defining the relationship between the a device and the transmission medium. The Data Link layer provides structure and reliability for data between connected systems. Data from the Physical layer is put into the frame we mentioned in the last module at this point, and is addressed using a MAC address. The Network layer adds IP addressing to allow the data to be transmitted between networks. The Transport layer adds some reliability measures, including the ability to re-transmit damaged data. The Session layer defines the relationship between the source and destination computer such as handshaking protocols. The Presentation layer defines the format for data, compression and encryption, and, finally, the Application layer defines the type of data are you navigating a web page, or sending an ? Page 4 of 9

5 Slide 5 Moving on from here, the TCP/IP model is a more compact and specific version which directly relates to TCP and IP data transmission and is usually described using a diagram similar to the one seen here. Each of the four TCP/IP model layers more or less correlate to one or more OSI model layers; there are specific IETF protocols which slot into each of the TCP/IP model layers, but due to the mechanisms required for them to do their work they don t always fit neatly into just one layer. This diagram can also sometimes be seen as a five layer model with the Link, or Network Access, layer depicted separately. Page 5 of 9

6 Slide 6 Here are some the protocols and how they fit into the TCP/IP model. With the knowledge you have it should be fairly clear why they sit in the layers they do. The connections between Ethernet and the Link layer, IP and the Internet layer, TCP, UDP and the Transport layer and so on. Page 6 of 9

7 Slide 7 So, how does this all relate to making a video call? Well, rest assured that it does, of course. This is a diagram of the protocol stack for H.323. Here you can see IP inside the Internet layer, then UDP and TCP and RTP inside the Transport layer, each aspect of our video call sits atop either TCP for call control and signaling, and UDP for audio, video, content and far-end camera control. Page 7 of 9

8 Slide 8 And here is the protocol stack for SIP. Although it is similar, you can see here an example of how SIP differs from H.323; it is able to use TCP or UDP, depending on how it is configured. Because SIP covers both control and signaling, it replaces H.225 and H.245, though audio and video are separate like H.323; remember that SIP does not determine which video and audio codecs to use. Page 8 of 9

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