31 Days Before Your CCNA Exam

16 31 Days Before Your CCNA Exam■■■■■Internet Protocol (IP): Provides a unique global address to computers for communicatingover the network.Address Resolution Protocol (ARP): Finds a host’s hardware address when only the IPaddress is known.Internet Control Message Protocol (ICMP): Used to send error and control messagesincluding reachability to another host and availability of services.Ethernet: The most popular LAN standard for framing and preparing data for transmissiononto the media.Frame Relay: Also a framing standard; one of the most cost-effective WAN technologiesused to connect LANs.Protocol Data Units and EncapsulationAs application data is passed down the protocol stack on its way to be transmitted across the networkmedia, various protocols add information to it at each level. This is commonly known as theencapsulation process. The data structure at any given layer is called a protocol data unit (PDU).Table 30-3 lists the PDUs at each layer of the OSI model.Table 30-3OSI LayerApplicationPresentationSessionTransportNetworkData linkPhysicalPDUs at Each Layer of the OSI ModelPDUDataDataDataSegmentPacketFrameBitsThe communication process from any source to any destination can be summarized with the followingsteps:1. Creation of data at the application layer of the originating source end device2. Segmentation and encapsulation of data as it passes down the protocol stack in the sourceend device3. Generation of the data onto the media at the network access layer of the stack4. Transportation of the data through the internetwork, which consists of media and any intermediarydevices5. Reception of the data at the network access layer of the destination end device6. Decapsulation and reassembly of the data as it passes up the stack in the destination device7. Passing this data to the destination application at the application layer of the destination enddevice