OSI分层模型(改).ppt

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OSI分层模型(改).ppt

Slide 3 of 5: Purpose: This figure orients the students to the network layer. Emphasize: Network layer is where IP occurs. Transition: The following discusses Layer 4, the transport layer, of the OSI reference model. Slide 4 of 5: Purpose: This figure orients the students to the transport layer. Emphasize: The Transport layer of the OSI reference model is implemented by various protocols; one of which is TCP. TCP uses ports and encapsulates the data in ‘segments’. TCP is connection oriented so it offers reliable service. The other major transport layer protocol discussed in this course is UDP. It offers speed but no reliability because it is connectionless. Transition: The following presents the entire OSI stack again. Purpose: This figure illustrates de-encapsulation. Emphasize: At the destination, the headers at each layer are stripped off as the data moves back up the stack. Purpose: This section describes the physical layer in more detail. Note: 802.3 is responsible for LANs based on the carrier sense multiple access collision detect (CSMA/CD) access methodology. Ethernet is an example of a CSMA/CD network. EIA/TIA-232 and V.35 are physical standards that support synchronous serial. Emphasize: Network topology is not necessarily connected to network technology. For example, many Ethernet networks have a backbone bus topology. However, adding a switch or a hub to an Ethernet network changes it to a star topology. IEEE 802.3u defines the standard for a CSMA/CD LAN operating at 100Mbps, Fast Ethernet. In the case of Ethernet, such as 10BaseT, the first part describes the speed of the cable, the second part describes whether it is baseband or broadband cable, the final part describes the media. So, 10BaseT is 10 Mbps baseband twisted-pair cable. Emphasize: All devices attached to a hub are on the same collision and broadcast domain. A hub is a layer one device. Purpose: This figure compares hubs in a CSMA/CD environment to a highway with multiple access points. E

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