Page 37 - M. Abrahim Thesis

Computer Network 2021

Figure 2-12 shown a Peer-to-Peer Communications process

Data packets on a network originate at a source and then travel to a destination. Each
layer depends on the service function of the OSI layer below it. To provide this service, the lower
layer uses encapsulation to put the PDU from the upper layer into its data field. Each layer then
adds whatever headers it needs to perform its function. As the data moves through the layers
of the OSI model, additional headers are added. The grouping of data at the Layer 4 PDU is called
a segment. The 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.

41