What is an OSI Model? Describe the functions of different layers of OSI Model.
THE OSI MODEL
International Standards Organization (ISO) specifications for network architecture, called the Open Systems Interconnect or OSI model. Seven-layered model, higher layers have more complex tasks. Each layer provides services for the next higher layer. Each layer communicates logically with its associated layer on the other computer. Packets are sent from one layer to another in the order of the layers, from top to bottom on the sending computer and then in reverse order on the receiving computer.
Application (user interface)
The top most layer of the osi model is the application layer. It serves as the window for application processes to access network services. This layer represents the services that directly uses applications, such as software for file transfers, for database access, and for email. Application layer serves as a window for applications to access network services. handles general network access, flow control and error recovery.
- Used for applications specifically written to run over the network.
- Allows access to network services that support applications.
- Directly represents the services that directly support user applications.
- Handles network access, flow control and error recovery.
External Data Representation (XDR) sits at the presentation level. It converts local representation of data to its canonical form and vice versa. The canonical uses a standard byte ordering and structure packing convention, independent of the host.
- Translates from application to network format and vice-versa.
- All different formats from all sources are made into a common uniform format that the rest of the OSI model can understand.
- Responsible for protocol conversion, character conversion, data encryption / decryption, expanding graphics commands, data compression.
- Sets standards for different systems to provide seamless communication from multiple protocol stacks
- Not always implemented in a network protocol.
Session (syncs and sessions)
The session layer is charged with establishing, watching and terminating the dialogue between the processes on the end systems. This layer takes care of such functions as login, authorization, synchronization and logout.
- Establishes, maintains and ends sessions across the network.
- Responsible for name recognition (identification) so only the designated parties can participate in the session.
- Provides synchronization services by planning check points in the data stream => if session fails, only data after the most recent checkpoint need be transmitted.
- Manages who can transmit data at a certain time and for how long.
- Examples are interactive login and file transfer connections; the session would connect and re-connect if there was an interruption; recognize names in sessions and register names in history.
Transport (packets; flow control & error-handling)
The transport layer is the end systems’ interface to the data transfer network. It hides physical and logical characteristics of the data transfer network from the end systems and takes care of end-to-end control of the data flow.
- Additional connection below the session layer
- Manages the flow control of data between parties across the network
- Divides streams of data into chunks or packets; the transport layer of the receiving computer reassembles the message from packets
- “Train” is a good analogy => the data is divided into identical units
- Provides error-checking to guarantee error-free data delivery, with on losses or duplications
- Provides acknowledgment of successful transmissions; requests retransmission if some packets don’t arrive error-free
- Provides flow control and error-handling
Network (addressing; routing)
The network layer is responsible for the transport of data through the network. This may be done by line switching, i.e. a direct physical connection between the systems is set up for the duration of the transfer. Normally, it is done by packet switching, which means that the network layer divides the data stream into packets. The packets are either sent together over a virtual connection between the end systems or individually as data grams. The network layer is primarily charged with addressing of the end systems and routing of the data stream through the network.
- Translates logical network address and names to their physical address (e.g. computer name ==> MAC address)
- Responsible for o Addressing, Determining routes for sending,
And Managing network problems such as packet switching, data congestion and routing.
- If router can’t send data frame as large as the source computer sends, the network layer compensates by breaking the data into smaller units. At the receiving end, the network layer reassembles the data.
- Think of this layer stamping the addresses on each train car.
Data Link (data frames to bits)
The link layer governs the physical connection between two systems. The bit stream between the systems is divided into units called frames, which the link layer guarantees error-less transmission of between two nodes in the network. The link layer decides which of the parties should send at any given moment (line access) and takes care of flow control of the connection and sequence control of the frames.
- Turns packets into raw bits 100101 and at the receiving end turns bits into packets.
- Handles data frames between the Network and Physical layers.
- The receiving end packages raw data from the Physical layer into data frames for delivery to the Network layer.
- Responsible for error-free transfer of frames to other computer via the Physical Layer.
- This layer defines the methods used to transmit and receive data on the network. It consists of the wiring, the devices use to connect the NIC to the wiring, the signaling involved to transmit / receive data and the ability to detect signaling errors on the network media.
Physical (hardware; raw bit stream)
The physical connection transmits signals between the systems. This may be a wire connection (twisted pair, coaxial cable, optical fiber) or an ether connection (radio link, satellite connection) or some combination.
The physical layer is charged with sending and receiving bits over the physical connection. At this layer, the electrical, mechanical and procedural aspects for the systems’ physical coupling to the connection are defined. The standards at this layer are mainly concerned with plugs and connectors, as well as the interpretation of voltages and control signals.
- Transmits raw bit stream over physical cable
- Defines cables, cards, and physical aspects
- Defines NIC attachments to hardware, how cable is attached to NIC
- Defines techniques to transfer bit stream to cable