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OSI model and TCP/IP stack
Bit, Frame, Packet, Segment, Datagram
Information Encapsulation

osi-model1Both OSI model and TCP/IP stack (they roles partly overlap themselves)  describe standards and framework of network communication. If they didn’t exist communication between network appliances of different vendors wouldn’t be possible. On the other words, 7 Layers of OSI model enable protocols being visible to each other. But what exactly those protocols and layers are? What exactly happens with information during receiving and how it  runs on particular network layers ? How information is being changed from electric impulse to the usable form ? 



OSI MODEL

Each PROTOCOL is a set of rules and guidelines which enable two different devices communicate with each other. On other words protocol is a common language for two devices in order to establish connection between them.

Think about each LAYER as about a gearing in huge clockwork. If one of the gear  get rusting, whole clockwork will stop or some functions won’t be working. Anyway the clockwork will be working faulty or at all. Every layer prepares data properly to sending out to the higher layer and you can’t avoid them. So, if you want to reach Application layer you have to pass through all 7 layers. On other words if you took a picture and would like to send its by email to your friend who wants to see this picture on the computer screen, your picture will come a long 7 layers way.

Let’s have a look how particular layers work in this “clockwork”.

7. APPLICATION – responsible for interactions with operating system or application. This layer provides application services for file transfer, e-mail and other network software services (Telnet, FTP, HTTP, RSTP)

6. PRESENTATION – responsible for delivery and formatting data into the form understanding for Application layer. Presentation layer ensures that data from one computer will be readable on the other one (eg. Windows and Unix)

5. SESSION – responsible for establishing, management and terminating connection between applications. Session layer sets up and coordinates conditions of conversation between applications at each end (L2TP , H.264 -audio/video transmission) 

4. TRANSPORT – provides transparent data transfer between nodes and is responsible for end to end error recovery and flow control. When some data get lost network layer will care to bring them back. (TCP, UDP)

3. NETWORK – the layer which brings routing, addressing, error handling, internetworking technologies. Makes path to reach the target node through the network. (IP, ICMP)

2. DATA LINK – is responsible for error detection and frame synchronization. Data link layer consists of two sub-layers: The Media Access Control (MAC) (addresses layer 3 packets with MAC address, error detection, flow control, access control to the medium)  and the Logical Link Control (LLC) (“frames” layer 3 packets, identifies using network layer protocol, lost frames retransmission, makes connection with upper layers. (ARP, PPP, HDLC, Ethernet, 802.11)

1. PHYSICAL – responsible for sending bit stream (electrical signals, light pulse, radio waves) through the network. It provides physical devices including cables, cards, interfaces. (USB, Bluetooth, Ethernet physical layer, SONET)

TCP/IP stack focuses on Layer 3 and 4 which retain their names , Layer 1 & 2 has been merged into one Network Access Layer likely Layers 5,6 & 7 into Application Layer. TCP/IP stack has been made up only in purpose of communication between nodes and focuses on two protocols TCP and IP:

 


 

ENCAPSULATION

7layersbitfr

What we see above is nothing more than encapsulation stages during communication between 2 network nodes. Assuming that the host wants to send an information to another host, our part of information (let’s call it a portion) must pass through a several encapsulation stages before leave NIC (Network Interface Controller). Higher layers of OSI model are responsible for suitable presentation of data and are strongly linked with applications. Lower Layers are responsible for sending and I am going to focus on this invisible part of activity. 

At every layer during encapsulation is being added HEADER or HEADER and TRAILER ( in case of FRAME- Layer 2)

LAYER 4 – At this layer  every portion of application is  encapsulated into SEGMENTS ( in case of TCP) or DATAGRAMS (in case of UDP ).

LAYER 3 – is responsible for encapsulation DATA DATAGRAMS or SEGMENTS into PACKETS.

LAYER 2 –  PACKETS are  encapsulated into FRAMES. On this layer, except the header, TRAILER is added . 

LAYER 1 – Frames are  changed into BITS ( which are  represented by 0 or 1 states). Every bit represents high level or low level of the signal and translated into electric pulse, light wave or radio wave.

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