Network+

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Physical Layer Defines the mechanism for communicating with the transmission medium and the interface hardware.


Although no commercially available networking protocol follows the OSI model exactly, most perform all the same functions.


In the DoD model, the four layers are as follows:


Process/Application Layer The highest layer; applications such as FTP,


Telnet, and others interact through this layer.


Host-to-Host Layer TCP and other protocols add transport data to the


data packet.


Internet Layer Adds IP information to the packet.


Network Access Layer Defines the mechanism for communicating with


the transmission medium and the interface hardware.


Each layer adds its own header and trailer data to the basic data packet and encapsulates the data from the layer above. On the receiving end, this header information is stripped, one layer at a time, until the data arrives at its final destination.


Now let’s look at how TCP and IP work together.

The Transmission Control Protocol


ransmission Control Protocol (TCP) is the transmission layer of the protocol and serves to ensure a reliable, verifiable data exchange between hosts on a network. TCP breaks data into pieces, first wrapping it with the information needed to route it to its destination and then reassembling the pieces at the receiving end of the communications link. The wrapped and bundled pieces are called datagrams. TCP puts a header on the datagram that provides the information needed to get the data to its destination. The most important information in the header includes the source and destination port numbers, a sequence number for the datagram, and a checksum.

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