OSI Layer Model and TCP/IP Model

OSI Layer (Reference Model): Open Systems Interconnection
(developed by ISO – ‘International Organization of Standardization in 1984) 

Interoperable communication between different vendors

1. Physical Layer : Binary Transmission

• It is the first of the osi model
• It establishes a physical connection between the devices using wires and cables.
• Eg : twisted pair and coaxial cable and optical fiber cable(SONET)
• In which datas are transferred in the form of bits (0s,1s)
• Functions : 

1. Bit synchronization: data circuit-terminating equipment (DCE), data terminal equipment (DTE) using Clock signal.
2.  Bit rate control: transmission rate i.e. the number of bits sent per second 
3.  Physical topologies: network structure i.e. bus,star or mesh topology
4.  Transmission mode (Data flow): Simplex,half-duplex and full-duplex.

• Devices : Hub,repeater,Modem,Ethernet Cables,PCs,etc
• Protocol : ADSL Asymmetric digital subscriber line, ISDN Integrated Services Digital Network, SDH Synchronous Digital Hierarchy, SONET Synchronous Optical Networking.

2. Data-Link Layer : Physical Addressing

• The data link layer is responsible for the node to node error-free delivery over the physical layer.
• Datas are transferred in the form of  frames.
• Functions : 

1. Flow control, Error control and Access control
2. Data Link Layer is divided into two sub layers :

           i. Logical Link Control (LLC) - CRC checks and control error and frame synchronization
           ii. Media Access Control (MAC) - Physical Addressing and data encapsulation.

•  Devices: Switches
•  Protocols: ARP, RARP, GARP, PPP, STP, LLDP, VTP

      3. Network Layer : Logical Addressing

• Logical Addressing using IPV4 and IPV6
• Network layer converts the received segment into packets.
• Functions:

            1.Path determination through ip routing
            2.Handles network traffic i.e network conjunction.
            3.Adds source address, destination address and sequence number to each packet.

• Devices: Routers and L3 Switches
• Protocols: IP, ICMP, IGMP, IPsec.

4. Transport Layer: End-To-End connection reliability

• It is the Heart of OSI layer
• Data is transferred in the form of segments.
• Functions: 

1. Establishes End to End Delivery of data
2. Reliability
3. Error control and Flow control
4. Segmentation
5. MUX 

• Devices: Firewall
• Protocols: TCP and UDP

5. Session Layer: Inter-host Communication

• Functions:

1. It has APIs (Application Programming Interfaces). Eg:- NetBIOS(Net Basic Input Output Systems)
2. Session Management: For generating session ID, establishment, maintenance and termination the sessions between the client and the server
3. Synchronization : checkpoints are added to identify the errors
4. Dialog Controller : communication - simplex, half-duplex or full-duplex.
5. Token management i.e preventing two parties from attempting the same critical operation simultaneously
6. Authentication, Authorization  and Accounting.

• Devices:  Gateways, Firewalls, PC's
• Protocols: PPTP, SCP, SDP, SMPP, RPC, API

6. Presentation Layer: Data representation and encryption

• Functions:

1. Translation : ASCII to EBCDIC.
2. Encryption/ Decryption : Data encryption translates the encrypted data(cipher text) into decrypted data (plain text) vice-versa using a key value.
3. Compression/Decompression : Reduces the number of bits that need to be transmitted on the network vice-versa.

• Protocols: NCP,AFP,LLP,SSL(encryption)

7. Application Layer: Network process to Application

• Layer interacting directly with the user.
• Functions:

1. Network Virtual Terminal
2. FTAM-File transfer access and management
3. Mail Services SMTP
4. Directory Services

• Protocols: HTTP,DNS,FTP,DHCP