The OSI model is the part of the Open Systems Interconnection project at the International Organisation for Standardisation.
The OSI model characterizes and provides a standard set of protocols for communication functions of a telecommunication or network system regardless to its underlying architecture.
It provides interoperability and helps in easy troubleshooting of the network.
The OSI model is not a protocol, it is a network architecture that is flexible, robust and inter-operable.
It is divided into seven different layers:
- Physical Layer
- Data link Layer
- Network Layer
- Transport Layer
- Session Layer
- Presentation Layer
- Application Layer
Lets look into each layer one by one,
Layer 7: Application layer
- The application layer is the layer close to the user, which means both the application layer and the user interact with the software application.
- It acts as a interface between application and user.
- Application-layer functions include identifying communication partners, determining resource availability, and synchronising communication.
- Protocols in this layer are BGP, DHCP, DNS, FTP, HTTP, IMAP, LDAP, MGCP, MQTT, NNTP, NTP, POP, ONC/RPC, RTP, RTSP, RIP, SIP, SMTP, SNMP, SSH, Telnet, TLS/SSL, XMPP.
Layer 6: Presentation layer
- The presentation layer handles the delivery and formatting of information. this layer presents data for the application or the network.
- It converts data into the standard format.
- It performs functions such as encryption, decryption, data compression, data decompression and character encoding.
- Protocols used are eXternal Data Representation (XDR), Network Data Representation (NDR), X.25 Packet Assembler/Dis-assembler Protocol (PAD), Lightweight Presentation Protocol (LPP), Internet key exchange (IKE)
Layer 5: Session layer
- Its main function is to manage communication sessions – continuous exchange of information.
- It provides the mechanism for opening, closing and managing a session between end-user application processes.
- It creates a session ID for each application and keeps the data separate for each.
- In case of a connection loss this protocol will try to recover the connection.
- If a connection is not used for a long duration, then session-layer protocol will close or re-open it.
- It provides either full duplex or half-duplex operation and provides synchronisation points in the messages exchanged.
- Protocols used are ADSP, ASP, H.245, ISO-SP, iSNS, L2F, L2TP, NetBIOS, PAP, PPTP, RPC, RTCP, SMPP, SCP, SOCKS, ZIP, SDP, SIP.
Layer 4: Transport layer
- Transport layer provides functions and procedures for transferring data to the destination.
- Its the bridge between hardware and software layer. The heart of the OSI layer.
- The transport layer controls link through flow control, segmentation/de-segmentation, and error control.
- This layer can keep track of the segments and re-transmit those that fail delivery.
- The transport layer also provides the acknowledgement signal on successful transmission of data and sends the next data if no errors occurred.
- It creates segments of the data dividing them into smaller layer.
- Responsible for process to process delivery.
- Protocols used are TCP and UDP.
Layer 3: Network Layer
- The Network layer is responsible for the delivery of individual packets from the source host to destination.
- It is used to communicate between different network.
- This layer uses logical addressing (adds IP address in the header).
- It decides the path in which the information must flow.
- The main functions are Routing and forwarding data, logical addressing, error detection, sequencing of data and flow control.
- Data is in the form of packets.
- Protocols used are RIP, OSPF, BGP, EIGRP, Apple Talk
- Devices: Router, Firewalls, L3 switch
Layer 2: DataLink Layer
- This layer is responsible for moving frames from one node to another.
- It is responsible for communication with the Local Area Network.
- Data shared is in the form of Frames.
- Responsibility of this layer are framing, physical addressing, flow control, error control, access control.
- Physical addressing – also called as MAC addressing or Layer 2 addressing.
- It is divided into two sublayers : Logical Link Control (LLC) and Media Access Control (MAC).
- MAC address is a 48 bit hexadecimal address.
- Protocols used are ARP, RARP, ICMP, IGMP, HDLC, PPP.
- Devices: NIC, Switch, Bridge.
Layer 1: Physical layer
- The Physical layer is responsible for movement of individual bits from one hop to another.
- It coordinates the function required to carry a bit stream over a physical medium.
- Main functions are modification of data into digital signals, establish and termination of connection, effectively share resources among multiple users.
- Devices: NIC, HUB, Repeaters and Modem.
Remembering the OSI Model 7 layers – 8 mnemonic tricks:
From Application to Physical:
All People Seem To Need Data Processing
All Pros Search Top Notch Donut Places
A Penguin Said That Nobody Drinks Pepsi
A Priest Saw Two Nuns Doing Pushups
From Physical to Application:
Please Do Not Throw Sausage Pizza Away
Pew! Dead Ninja Turtles Smell Particularly Awful
People Don’t Need To See Paula Abdul
Pete Doesn’t Need To Sell Pickles Anymore