Page 140 - Computer_Science_F5

to the data through encapsulation, and it has a
Network models
specific name based on the header information.
The concept of network models When the receiving device receives the data, the
plays a crucial role in solving corresponding layer reads the PDU information. Chapter Three: Advanced Computer Networks
the complexities of network Once read, the control information is removed,
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interactions. While numerous and the data is passed up to the next layer.
network models exist, the OSI
model is widely recognised for Figure 3.5 shows how PDUs attach control
its clear-layered approach. The information at each layer for both the TCP/IP
OSI model offers a structured way and OSI models. These figures demonstrate
to understand data transmission how user data from upper layers is prepared
through standardised protocols. for transmission across the network. The data is
The seven OSI models’ layers first passed down to the Transport layer, which
are Application, Presentation, establishes a virtual circuit to the receiving device
Session, Transport, Network, by sending a synchronisation packet. Next, the
Datalink and Physical layers. Each data stream is divided into smaller segments, and
layer has specific responsibilities a Transport layer header (PDU) is added to each
and works together to ensure segment. These segments are numbered to ensure
seamless communication. proper reassembly at the receiving device. Each
segment is then forwarded to the Network layer,
Data encapsulation in network which handles the addressing and routing of the
communication data through the internetwork.
When data is sent from one
device to another across a Application PDU
network, it goes through a Presentation
process called encapsulation.
This process involves adding Upper layer Session
data
protocol information at each Transport Segment
layer of the TCP/IP model. Each TCP head Upper layer
layer communicates only with data
its corresponding layer on the IP header Data Network Packet
receiving device. To facilitate
this communication, each layer
uses Protocol Data Units (PDUs), LLC header Data FCS Data link Frame
which contain control information
attached to the data. Typically, the MAC header Data FCS
control information is added as a Physical Bits
header at the beginning of the data 0101110101001000010
field, but it can also be added as a
trailer. At each layer of the TCP/ Figure 3.5: PDU attaching control information for
IP model, the PDU is attached OSI and TCP/IP

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Student’s Book Form Five

Computer Science Form 5.indd 131 23/07/2024 12:33