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Introducing about OSI layer






Introducing About OSI Layer. OSI define as Open System Interconnection.

OSI Layer itself consist of seven layer. Such as :

  1. Application (Layer 7)
  2. Presentation (Layer 6)
  3. Session (Layer 5)
  4. Transport (Layer 4)
  5. Network (Layer 3)
  6. Datalink (Layer 2)
  7. Physical (Layer 1 )

Let us discuss these one by one. We will describe around definition, work, and its protocol respectively.

  1. Application Layer

This layer provide the interface between application and the networking functional. When we sat on the chair then operated computer, we met several application. This application will process and ensure your data in order to transmit through the network by using several protocol which is in the application layer.
here the protocol member which work around application layer :

a. Http :

This protocol is supported for web communication which use for www. there is one kind of http. It called https. Https is used for security transaction. https implement for email transmit.

b. Smtp

Smtp consist of Simple Mail Transfer Protocol. This protocol will allow your email communicate to others email server. We can call smtp will transmit message your email inter-email server.

c. POP3

This protocol is used to catch or retrieve your email from email server. The application which is used for this protocol is Ms. Outlook, Mozilla Thunderbird. And many more.

d. SIP

SIP is consist of Session Initiate Protocol. This protocol is used for signaling on VOIP.

e. H.323

Same as above. Use for signaling. But there is different things. SIP is support for TCP and UDP while UDP is not. Moreover SIP is based on TEXT and H.323 is based on biner.

f. FTP

File Transfer Protocol is used for transfer file. For example : when we try to download the file from other hosting, we will use FTP for manage this transmit.

g. DNS

Domain Name system. Is used to naming the ip network address for website application. Thus, everyone will not hardthink and easy to open the website by type the address directly on the browser.

h. Snmp

Simple Network Management Protokol is used for manage the network by remote. Snmp will allow you to manage the network tools like hub, router, switch, workstation,


2. Presentation Layer

Presentation layer will serve to translate data to be transmitted by the application into a format that can be transmitted over the network.

3. Session Layer

Sesion layer is usually called by semi-communication, so why?. because, inside the session layer the connection can be modified, maintained, and destroyed.
the protocol itself is : Telnet

4. Transport Layer

This layer has provided end-to-end communication for service application. This layer has responsible for successfully transmit each packet over the network. These layer provide convenient service such as connection-oriented, reliability, flow control, and multiplexing. There are two protocol type in transmitting packet method. TCP and UDP.

a. TCP

TCP is transmission control protocol. TCP is largely convenient for a application programming to internet host. It use for connection-oriented transmission. The implementation TCP is for data communication in internet. There is one special for TCP. When the packet was sent to the destination but lost on the way, automatically the source itself would send the same packet which lost on the way.
TCP also is used for congestion avoidance

b. UDP

User Datagram Protocol is also part of the protocol inside Transport layer. The UDP is kind TCP connectionless. It means that, every packet is transmitted from one computer to another one over network without build any connection. So, this is very appropriate for VOIP implementation. UDP provide real time connection. Unfortunately, if there is one or several packet lost on the way, the source will not send back. So, the packet is gone just like that.
but UDP is not longer for work alone. There is one protocol to help UDP working. It is RTP. RTP is Real Time Protocol. With this protocol The service of voice can be transmitted real-time.
the UDP send the packet using datagram method.The datagram will not allow handshaking for its first time connection. Even though this method give real-time connection, UDP still unreliable.

5. Network Layer

This Layer is responsible for routing the packet delivery through intermediate router, whereas the datalink layer is responsible for MAC (Media Acces Control), flow control, and error checking. Here are three function of Network layer

  1. Connection Model : Connectionless Communication
  2. Host Addresing
  3. Massege Forwarding
    here are samples protocol which inside Network layer :

    1. IPv4/IPv6
    2. BGP
    3. IPsec
    4. Datalink Layer

6. datalink layer

This layer, provide transmit data by doing any convertion from frame into bit, in order to be processed in Pyhsical layer.This layer is responsible with any process, such as : making a frame, flow control, error correction and transmit again the frame if there is any frame which is considerable failed.

The main job datalink layer is transmitting several bits to any computer destination.

as you know, there are 2 kind of method transferring data/acces control in order not to collide each packet.
a. CSMA CD (Collision Detect)

CSMA/CD (Carrier Sense Multiple Access/Collision Detect) which deals with transmissions after a collision has occurred.

Carrier Sense Multiple Access/Collision Detect (CSMA/CD) is the protocol for carrier transmission access in Ethernet networks. On Ethernet, any device can try to send a frame at any time. Each device senses whether the line is idle and therefore available to be used. If it is, the device begins to transmit its first frame. If another device has tried to send at the same time, a collision is said to occur and the frames are discarded. Each device then waits a random amount of time and retries until successful in getting its transmission sent.

CSMA/CD is specified in the IEEE 802.3 standard.


b. CSMA CA (Collision Avoid)

this method usually use as protocol carrier for wireless network (802.11). same as CSMA CD method. Nevertheless, CSMA CA is greedier than CSMA CD. Why?

In CSMA/CA, as soon as a node receives a packet that is to be sent, it checks to be sure the channel is clear (no other node is transmitting at the time). If the channel is clear, then the packet is sent. If the channel is not clear, the node waits for a randomly chosen period of time, and then checks again to see if the channel is clear. This period of time is called the backoff factor, and is counted down by a backoff counter. If the channel is clear when the backoff counter reaches zero, the node transmits the packet. If the channel is not clear when the backoff counter reaches zero, the backoff factor is set again, and the process is repeated.

Protocol :










7. Physical Layer

Is the physical things that need for transmit data.

here is the pic that show us to know how data can be transmitted by encapsulate each layer :










Below is one sample image osi layer and its protocol.









TCP/IP layer and its protocol :










A picture below show to us the relationship between OSI layer and TCP/IP layer :



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