Compare and Contrast OSI Model Layers

Open Systems Interconnection Model

A network is two or more computer systems that are linked by a transmission medium and
share one or more protocols that enable them to exchange data. (Network in terms of nodes
and links)
The main purpose of networking is to all two hosts to share data with one another.
The nodes are devices that send, receive, and forward data and the links are the
communications pathways between them.

Open Systems Interconnection (OSI) reference model was created by the International
Organization for Standardization (ISO).
To create a universal standard of communication between devices
The OSI model was meant to help technology vendors create interoperable network devices
and software in the form of protocols so that different vendor networks could work in
peaceable accord with each other.
OSI model is the primary architectural model for networks. It describes how data and
network information are communicated from an application on one computer through the
network media to an application on another computer.

OSI model addresses all the processes required for effective communication and divides
them into logical groupings called seven layers.

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 The OSI model

Data Encapsulation and Decapsulation

Encapsulation refers to attaching new information in the Application Layer data as it is passed onto
next layers in the TCP/IP model. Decapsulation refers to the removal of all these additional
information and extraction of originally existing data, and this process continues till the last layer
A network protocol is a set of logical rules for exchanging data in a structured format (protocol
serve as a common language for devices to enable communication). A network protocol has two
principal functions:
Addressing: Describing where data messages should go
Encapsulation: Describing how data messages should be packaged for transmission.

sender Receiver

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At each level (except the physical layer), the sending node adds a header to the data payload, (i.e.
the data to be transmitted) forming a “chunk” of data called a protocol data unit (PDU). This is the
process of encapsulation.

Layer 1 PDU is called Bit

Layer 7 Application: This is use by network application.

Network applications refer to computer applications that use the internet, like Google
Chrome, Firefox, Outlook, and Skype.
Application layer protocol include:
o HTTP (Hypertext Transfer Protocol) to do web sulfuring i.e. World Wide Web
o FTP (File Transfer Protocol) is a standard network protocol used for the transfer of
files from one host to another over a TCP-based network, such as the Internet.
o DHCP (Dynamic Host Configuration Protocol) used to assign IP to devices on the
network.
o SNMP (Simple Network Management Protocol) is an internet standard protocol
used to monitor and manage network devices connected over an IP.
Layer 6 Presentation: Receives data from the Application layer. The data are informed of
characters and numbers (ASDFG / 25437)
The basic functions of the presentation layer are:
Translation: Convert the data to machine-understanding binary format (1001).
Data compression: bit reduction. i.e., reduce the amount of space used to store the original
files. This helps to receive the files in less time at the destination
Encryption: Protection of the data
Layer 5 Session: Help in setting up and manage connection. i.e. Enabling sending and receiving
of data follow by termination and ending of a session. Its uses APIs like NETBIOS that allow
application on different computer to communicate with each other. For example, server establish
connection before a session and this is done through what is called authentication

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 Authentication: is the process of verifying who you are. (by using User name and
Password)
Authorization: is the process use by server to determine if you have a permission to assess
a file
Layer 4 Transport: This controls the reliability of communication to
Segmentation. Data are divided into small data unit called segments.
o Segments: This contains a port number and sequence number. Port number help to
direct each segment into correct application while sequence number help to
reassemble segment to form correct message at the receiver
Flow Control: Control amount of data transmitted. (Also help in system performance

Error Control: If some data transmitted fails to reach the destination transport layer use
Automatic Repeat Request to re-transmit the loose data.
Protocol at transportation layer include:
Transmission Control Protocol (TCP)
o Connection-oriented Transmission
User Datagram Protocol (UDP)
o Connectionless Transmission
Also, the transport layer is to identify each type of network application by assigning it a port
number. For example, data requested from an HTTP web application can be identified as port 80,
while data sent to an email server can be identified as port 25
Layer 3 Network: This is responsible for moving data around a network of networks, known as
an internetwork or the Internet.
Moves information around an internetwork by using logical network and host IDs.
The main appliance working at layer 3 is the router.
Data unit in Network layer are called Packets
Perform Logical addressing (IPv4 & IPv6), Routing and Path determination

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Layer 2 Data Link: is responsible for transferring data between nodes on the same logical
segment. The data link layer organizes the stream of bits arriving from the physical layer into
structured units called frames.
At the Data Link layer, a segment is one where all nodes can send traffic to one another
using hardware addresses, regardless of whether they share access to the same media.
The data link layer adds control information to the payload in the form of header fields.
These fields include source and destination hardware addresses, plus a basic error check to
test if the frame was received intact.
Devices that operate at the data link layer include:

Network adapter or network interface card (NICs)

Bridge, Switch, and Wireless access point (AP)

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Layer 1 Physical: is responsible for the transmission and receipt of the signals that represent bits
of data from one node to another node. The transmission media types:
Cabled
Wireless
The Physical layer specifies the following:
Physical topology and segments
Physical interface and transmission of signals
Modulation and encoding
Devices that operate at the Physical layer include:

Transceiver: The part of a network interface that sends and receives signals over the
network media
Repeater: A device that amplifies an electronic signal to extend the maximum allowable
distance for a media type.
Hub: A multiport repeater, deployed as the central point of connection for nodes
Media converter: A device that converts one media signaling type to another

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Modem: A device that performs some type of signal modulation and demodulation, such
as sending digital data over an analog line.

OSI Model Summary

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 Configure SOHO Networks
Small office/home office (SOHO) is a category of LAN with a small number of computing hosts
that typically rely on a single integrated appliance for local and Internet connectivity.
Physical Layer Functions
Starting at layer 1, the SOHO router provides the following physical connections:

A number of RJ-45 ports (typically four) to connect to a local cabled network. These are
typically labeled as the LAN ports.
Radio antennas to transmit and receive wireless signals.

Data Link Layer Functions

At layer 2, the SOHO router implements the following functions to make use of its physical layer
adapters:
Ethernet switch—the RJ-45 jacks are connected internally by an Ethernet switch.
Wireless access point—the radio antennas implement some version of the Wi-Fi standard.
The access point functions as a wireless hub, allowing stations (PCs, tablets, smartphones,
and printers) to form a wireless network. The access point it also wired to the Ethernet
switch via an internal port.

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Network Layer Functions
At layer 3, the network layer, the routing part of the SOHO router makes forwarding decisions
between the local private network and public Internet. These zones are distinguished by internet
protocol (IP) addresses. The local network uses a private. IP address range, such as 192.168.1.0/24.
The router runs a dynamic host configuration protocol (DHCP) server to allocate a unique address
to each host that connects to it over either an Ethernet port or via the wireless access point. T

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