Page 34 - Handout Computer Network.

network layer provides a service to the transport layer. The network layer moves the data through
the internetwork by encapsulating the data and attaching a header to create a packet (the Layer 3
PDU). The header contains information required to complete the transfer, such as source and
destination logical addresses. The data link layer provides a service to the network layer. It
encapsulates the network layer information in a frame (the Layer 2 PDU).
The frame header contains the physical addresses required to complete the data link
functions, and the frame trailer contains the frame check sequence (FCS), which is used by the
receiver to detect whether the data is in error. This then becomes the data that is passed down to
the physical layer. The physical layer provides a service to the data link layer. The physical layer
encodes the data link frame into a pattern of 1s and 0s (bits) for transmission on the medium (usually
a wire) at Layer 1. Network devices such as hubs, switches, and routers work at the lowest three
layers. Hubs operate at Layer 1, switches operate at Layer 2, and routers at Layer 3.
The first layer that deals with the end-to-end transport between end users is the transport layer (Layer
4).
DoD (TCP/IP) Model Although the OSI reference model is universally recognized, the historical
and technical open standard of the Internet is Transmission Control Protocol/Internet Protocol
(TCP/IP). The TCP/IP reference model and the TCP/IP protocol suite make data communication
possible between any two computers anywhere in the world at nearly the speed of light.
The TCP/IP model has historical importance, just like the standards that allowed the
telephone, electrical power, railroad, television, and videotape industries to flourish. The U.S. DoD
provided funding for the invention of the TCP/IP reference model because it wanted a network that
could survive any conditions, even a nuclear war.
To illustrate further, imagine a world at war, crisscrossed by different kinds of connections,
including wires, microwaves, optical fibers, and satellite links. Then imagine that information/data (in
the form of packets) must flow, regardless of the condition of any particular node or network on the
internetwork (which, in this case, might have been destroyed by the war). The DoD wants its packets
to get through every time, under any conditions, from any one point to any other point. This very
difficult design problem brought about the creation of the TCP/IP model, which has since become the
standard on which the Internet has grown.
When reading about the TCP/IP model layers, remember the original intent of the Internet; it
helps explain why certain things are as they are. The TCP/IP model, as shown in Table 2-1, has four
layers:
• The application layer

• The transport layer
• The Internet layer

• The network access layer

Table 2-1 shown a difference between OSI and TCP/IP Layer Model

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