Network Level
The network layer is responsible for transmitting and routing data packets over the network. The Internet uses the Internet Protocol or IP as its network layer. Each node on the network has an address, which of course is called the IP address. Data is sent as IP packets.
A transport layer connection is made up up of a large number of IP packets exchanged by the client and server. The Internet Protocol (IP) is very simple: a packet has a source, a destination and a payload, and it's passed from one node in the network to another until it gets to the destination. The IP does not notice that a packet gets lost. It just never gets to the destination. If a particular node cannot pass the packet to the next node along the normal route, it will do its best to find an alternative path. That's why IP is sometimes called a "best-effort" protocol.
When the client sends its TCP connection request, the network layer puts the request in a number of packets and transmits each of them to the server. Each packet can take a different route, and some of the packets may get lost along the way. If they all make it, the transport layer at the server is able to reconstruct the request, and it will prepare a response confirming that a TCP connection has been set up. This response is sent back again in a number of IP packets that will hopefully make it to the client.
The Network Access layer manages all the services and functions necessary to prepare the data for the physical network. These responsibilities include interfacing with the computer's network adapter. Coordinating the data transmission with the conventions of the appropriate access method. You'll learn more about access methods later in this hour. Formatting the data into a unit called a frame and converting that frame into the stream of electric or analog pulses that passes across the transmission medium. Checking for errors in incoming frames. Adding error-checking information to outgoing frames so that the receiving computer can check the frame for errors. Acknowledging receipt of data frames and resending frames if acknowledgment is not received.
The Network Access layer defines the procedures for interfacing with the network hardware and accessing the transmission medium. Below the surface of TCP/IP's Network Access layer, you'll find an intricate interplay of hardware, software, and transmission-medium specifications. Unfortunately, at least for the purposes of a concise description, there are many different types of physical networks that all have their own conventions, and any one of these physical networks can form the basis for the Network Access layer. You'll learn about these physical network types later in this hour. A few examples include Ethernet, Token ring, FDDI, PPP (Point-to-Point Protocol, through a modem), Wireless networks.
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