The OSI Model Advantages of the reference model
It divided the network communication process into smaller
and simpler components, thus aiding component development, design and
troubleshooting.
It allows multiple-vendor development through
standardization of network components.
It encourages industry standardization by defining what
functions occur at each layer of the model.
It allows various types of network hardware and software to
communicate.
It prevent changes in one layer from affective others, so it
doesn’t hamper development.
The OSI Reference model
The OSI has seven different layers, divided into two groups.
The top three upper layers define how the applications within the end station
will communicate with each other and with users. The bottom four layers define
how data is transmitted end to end.
The application layer
Application layer provides a user interface, this layer of
the OSI model mark the spot where users actually communicate to the computer.
The application layer is also responsible for identifying
and establishing the availability of the intended communication partner and
determines whether sufficient resources for the intended communication exist.
It’s important to remember to remember that the application layer is acting as
an interface between the actual application programs.
Here are a few good examples of these kinds of events:
File transfers
Email
Enabling remote access
Network management activates
Client/server processes
Information location
Many network applications provide service for communication
over enterprise network.
Presentation layer
Presentation layer gets its name from its purpose: it
presents data to the application layer and in responsible for data compression,
decompression, encryption, decryption and code formatting. It’s an OSI model’s
translator, one very effective way of ensuring a successful data transfer is to
convert the data into a standard format before transmission. Presentation layer
ensure that data transferred from the application layer of one system can be
read by the application layer of another one.
The session layer
The session layer is responsible for setting up, managing,
and dismantling sessions between presentation layer entities and keeping user
data separate. It coordinates communication between systems and serves to
organize their communication via three different modes: simplex,
half-duplex and full-duplex.
Simplex: is simple one-way communication, kind of
like saying something and not getting a reply.
Half-duplex: is actual two-way communication, but is
can take place in only one direction at a time. It’s like walkie-talkie.
Full duplex: is exactly like a real conversation where
device can transmit and receive at the same time. Like a telephone
conversation.
Transport layer
The transport layer segment and reassemble data into a data
stream. They provide end-to-end data transport services and can establish a
logical connection between the sending host and destination host on an
internetwork. Transport layer is responsible for providing mechanism for
multiplexing upper-layer application, establishing session, and tearing down
virtual circuits. The transport layer can either be connectionless or
connection-oriented. Transport layer use two types of protocol TCP and UDP.
Feature of TCP.
flow control
Flowing control prevent a sending host on one side of
the connection from overflowing the
buffer in the receiving host.
The segments delivered are acknowledged back to the sender
upon their reception.
Any segments not acknowledge are retransmitted.
Segments are sequenced back into their proper order
upon arrival at their destination.
A manageable data flow is maintained in order to avoid
congestion, overloading and data loss.
Connection-oriented connection
In reliable transport operation, a device that wants
to transmit sets up a connection-
oriented communication with a remote
device by creating a session is called a setup or three-way handshake.
The first “connection agreement” segment is a request for
synchronization.
The second and third segments acknowledge the request and
establish connection parameters.
The final segment is also an acknowledgment. It notifies the
destination host that the connection agreement has been accepted and the actual
connection has been established.
A service is considered connection-oriented if it has the
following characteristics:
A virtual circuit is set up(e.g., call setup or
three-way handshake).
Its uses sequencing.
Its use acknowledgements.
Its use flow control. The types of flow control are
buffering, windowing and congestion avoidance.
Windowing
The quantity of data segments measured in bytes that the
transmitting machine is allowed to send without receiving an acknowledgement
for them is called a windowing.
Acknowledgment
Reliable data delivery ensures the integrity of the stream
of data sent from one machine to the other through s fully functional data
link. It guarantees that the data won’t be lost or duplicated. This is achieved
through something called positive acknowledgment with Retransmission.
Network layer
The network layer also called layer 3 manages device
addressing, tracking the location of devices on the network, and determines the
best way to move data. Two types of packets are used at the network layer: data
packets and route packets.
Data packets
It’s used to transport user data through the internetwork.
Protocols used to support data traffic are called routed protocols; examples
ipv4 and ipv6.
Routed updates packets
Used to update neighboring routers about the networks
connected to all routers within the internetwork, protocols that send route
update packets are called routing protocols ; (e.g. RIP, RIPv2, ISIS , EIGRP
and OSPF).
The Data link layer
The data link layer provides the physical transmission of
the data and handles error notification; this means that the data link layer
will ensure that message is delivered to the proper device on a LAN using
hardware address. The IEEE Data link layer has two sub layers:
Media access control (MAC) 802.3
Define how packets are placed on the media. Contention media
access is “first come/first served”
Logical link control (LLC) 802.2
Responsible for identifying networks layer protocols and
then encapsulating them.
Physical layer
Physical layer does two things: it send bits and received
bits. The physical layer specifies the electrical, mechanical, procedural and
functional requirement for activating, maintain, deactivating a physical link
between end system. This layer is also where you indentify the interface
between the data terminal equipment (DTE) and the data communication equipment
(DCE)
Data circuit-terminating equipment.
The DCE is usually located at the service provider. While
the DTE is attached device.
Data terminal equipment
The service available to the DTE is most often accessed via
a modern or channel service unit (CSU/DSU)..
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