31 Days Before Your CCNA Exam

Day 29 27The TCP/IP Network Access LayerIP depends on the network access layer to deliver IP packets across a physical network. Therefore,the network access layer defines the protocols and hardware required to deliver data across somephysical network by specifying exactly how to physically connect a networked device to the physicalmedia over which data can be transmitted.The network access layer includes a large number of protocols to deal with the different types ofmedia that data can cross on its way from source device to destination device. For example, datamight need to travel first on an Ethernet link, then cross a Point-to-Point (PPP) link, then a FrameRelay link, then an Asynchronous Transfer Mode (ATM) link, and then finally an Ethernet link tothe destination. At each transition from one media type to another, the network access layer providesthe protocols, cabling standards, headers, and trailers to send data across the physical network.Many times, a local link address is needed to transfer data from one hop to the next. For example, inan Ethernet LAN, Media Access Control (MAC) addresses are used between the sending device andits local gateway router. At the gateway router—depending on the needs of the outbound interface—the Ethernet header might be replaced with a Frame Relay header that will include data-link connectionidentifier (DLCI) addresses. In Frame Relay, DLCI addresses serve the same purpose as MACaddresses in Ethernet—to get the data across the link from one hop to the next so that the data cancontinue its journey to the destination. Some protocols, such as Point-to-Point Protocol (PPP), do notneed a link address because only one other device is on the link that can receive the data.With the network access layer, we can now finalize our web page example. The following greatlysimplifies and summarizes the process of requesting and sending a web page:1. Web client sends an HTTP request.2. TCP encapsulates the HTTP request.3. IP encapsulates the transport segment into a packet, adding source and destination addresses.4. Network access layer encapsulates packet in a frame, addressing it for the local link.5. Network access layer sends the frame out as bits on the media.6. Intermediary devices process the bits at the network access and Internet layers, and then forwardthe data toward the destination.7. Web server receives the bits on the physical interface and sends up through the networkaccess and Internet layers.8. Web server sends a TCP acknowledgement back to the requesting web client.9. Web server sends the HTTP response down to the transport layer.10. TCP encapsulates the HTTP data.11. IP encapsulates the transport segment into a packet, adding source and destination addresses.12. Network access layer encapsulates packet in a frame, addressing it for the local link.13. Network access layer sends the frame out as bits on the media.14. Lower layers process and send the response to the requesting web client.15. Response travels back to the source over multiple data links.