Transport Layer

Before entering another layer in Lower layer, first of all we must know about transport layer.
What is transport layer? What is the function for this layer?
This layer segment and reassembles data into a data stream.

The example device /fitur for this layer is firewall.

There are 2 protocol in this layer that you must know.
The protocol is TCP and UDP.

TCP = transmission control protocol.
UDP = user datagram protocol

TCP is usually used by FTP. UDP is used by voice.

The Difference between TCP and UDP:

TCP
—-
connection oriented
Flow Control
3 way handshake
Overhead -> The sent data is more bigger than total size because it contain header information
Reliable
Header 20 byte

UDP
—
Connectionless
Header 8 byte
Fast data delivery
Unreliable

The Transport Layer is responsible for providing mechanisms for multiplexing upper-layer applications, establishing sessions, and tearing down virtual circuit.
It also hides detail of any network-dependent information from the higher layers by providing transparent data transfer.

Multiplexing is a technic that sending more than 1 information using 1 channel.
The purDatagrampose of multiplexing is to save the total of physical channel like cable, transceiver, optic cable,etc.

MTU (Maximum Transmission Unit) is a maximum data limitation that can being send by a one frame in Network Interface Layer in TCP/IP. Layer Network Interface is not desain to accept or sending frame that contain more than MTU that we config.a one Datagram IP is a must lower or same size with MTU or it can’t be encapsulation for sending.(Comer, 2001)

Application Layer, Presentation Layer & Session Layer

What is Application Layer?

In OSI communications model, the application layer provides server for an application program to ensure that effective communication with another application program in a network isi possible.
The application layer is not the application but it is a service layer that provide service:
1. To Make sure the other party is identified and can be reached.
2. To make sure either the message sender or receiver or both.
3. To make sure that necessary communication resources exist
(for example, is there any modem in the sender’s computer?)
4. To makes sure aggrement at both ends about error recovery procedures, data integrity and privacy
5. To specify protocol and data syntax rules at the application level

The example protocol that working in application layer :
HTTP,HTTPS,FTP,DHCP,TELNET,DNS,SNMP,SMTP,POP3,IMAP,etc.

Protocol is a rule or standart that set or permit that connection, communication and data move is happen between 2 or more device.

In other word, application layer is interface between application with network functionality.

Presentation Layer

Presentation Layer have a function to data translation for data which want to transmitted from application in format that can been transmitted in network.
Presentation layer has responsibility how data can be converted and formated for data transfer.
the example of conversion data is format text ASCII for document, .gif and JPG for image.
This layer create code convert , data translation, encryption and

The example protocol in this level is RTF,MIDI,JPEG,etc.
RTF (Rich Text Format) is a file format that lets you exchange text files between different word processors, even in different operating systems.

Session Layer

Session Layer is responsible for setting up, managing, and then seperate session between Presentation layer entities.
This layer also provides dialogue control between devices, or nodes.
It coordinate communication between systems, and serves to organize their communication by offering three different modes: simplex,half duplex and full duplex.

The protocol that work in this layer is NFS,SQL,RPC,X Windows, ASP, etc.

Basic Concept OSI Layer

Every people who wants to join in networking world must understand what is OSI layer.
we must overcome this concept, because OSI layer is the basic in networking.

What is OSI?
OSI(Open Systems Interconnection) is model product of OSI effort at the ISO. OSI is a resolution of standard and characteristic of a communications system in terms of abstraction layer.
which mean OSI is a model abstraction to help us know and differentiate the function network device and the system work.

OSI layer is consist of 7 layer:
1. Application Layer
2. Presentation Layer
3. Session Layer
4. Transport Layer
5. Network Layer
6. Data Link Layer
7. Physical layer

Every layer has own specific responsibility for process data communication.
For example, one layer has responsibility to create connection between each other media, and the other layer has responsibility to correct if “error” happen during transfer data.
There are a few theory about this OSI layer. Some people difference this 7 layer in two groups (Upper Layer and Lower layer).
Upper layer is consist of Application Layer, Presentation Layer, Session layer.
Lower layer is consist of Transport layer, Network Layer, Data Link layer, Physical layer.

Upper layer is describe on application that user use and how the file being represented on computer.
For network engineer, the main part is lower layer.
lower layer is the main of data communication.

The main purpose of using model OSI is to help network desainer to understand function of each layer that have connection with communication data flow. Include type of network protocol and method of transmission

I will describe the function of each layer in other subject.
May this post can help u understand the OSI layer.