OSI Layers

Posted on March 15, 2017 | by Telecom and Networking

OSI Layers

OSI stands for Open System Interconnection and there are seven OSI layers.

Application Layer

The Application Layer is at top of the OSI model. It is the ultimate goal of networking: service providing. The Application Layer uses special networking protocols to provide file, print, message, application, and database services. It also defines how these services are advertised and used. The Application Layer performs two very important tasks:

Service advertisement
- Active
- Passive
Service availability
- OS Call Interception
- Remote Operation
- Collaborative computing

Presentation Layer

The Presentation Layer is the OSI translator. It performs the upper-layer message into the mutually agreed-upon format. At the point of Presentation Layer, the dialogue is open and data transfer begun, but before the Application Layer can read the message, the Presentation Layer take care of following two things:

Translation
- Bit order
- Byte order
- Character-code
- File syntax
Encryption

Session Layer

The Session Layer gets things started by opening and controlling networking dialogues. This layer often helps network services find each other a dialogues. Here are the important tasks that Session Layer does

Connection establishment
Data Transfer
- Simplex
- Half-duplex
- Full-duplex
Connection Release

Transport Layer

The Transport Layer organizes datagrams into segments and delivers them reliably to upper-layer services. If segments are not delivered to the destination correctly by the Network Layer, transport functions come to the rescue. This layer can initiate retransmissions and inform the upper layers that we are trying again. Transport Layer consists of the following components:

Service addressing
- Connection identifier
- Transaction identifier
Segmentation
Transport Control
- Transport error checking
- Transport flow control

Network Layer

The Network Layer is concerned primarily with moving data from Point A to Point B. Although this seems simple first, it can get quite complex as you begin internetworking over complex WANs.
The Network Layer has following functions:

Logical Addressing
Switching
- Circuit switching
- Message switching
- Packet switching
  - Datagram packet switching
  - Virtual-circuit packet switching
Routing
- Route discovery
  - Distance-vector Route Discovery
  - Link State Route Discovery
- Route Selection
  - Dynamic
  - Static
Network Control
- Network flow control
- Network sequencing
- Network error checking

Data link Layer

Data Link Layer organizes physical bits into logical groups called frames. These are contiguous series of bits grouped together as a unit of data. The Data Link Layer also detects and sometimes corrects, errors. It controls data flow and identifies computers on the network through physical addressing.
Data Link layer is organized into two sublayers.

Media Access Control(MAC)
- Media Access protocols
  - Contention
    - CSMA/CD
    - CSMA/CA
  - Token-passing
  - Polling
- Physical Addressing
Logical Link Control (LLC)
- Frame Synchronization
  - Asynchronous
  - Synchronous
  - Isochronous
- Flow Control
  - Window flow control
  - Guaranteed rate flow control
- Error Checking

Physical Layer

The Physical Layer defines the pathway for electronic communication. The Physical Layer handles low-level rules for transmitting bits.This layer encodes or decodes bits and sends or receives the stream of data. This is the pavement of the information superhighway. The Physical Layer Defines :

Electrical Properties
- Resistance
- Impedance
- Capacitance
- Noise
- Attenuation
- Cross-talk
- Propagation
Transmission Media
- Bound Transmission Media
  - Unshielded twisted-pair (UTP)
  - Shielded twisted-pair (STP)
  - Coaxial
  - Fiber Optics
- Unbound Transmission Media
  - Radio wave
  - Microwave
  - Infrared
Transmission Devices
- Communications
  - Modems
  - Multiplexers
  - CSU/DSU
- Network
  - Media Connectors
  - Network Interface Boards
  - Repeaters
  - Hubs
- Internetwork
  - Bridges
  - Routers
  - Gateways
Physical Topology
- Bus
- Ring
- Star
- Mesh
- Cellular
Digital Signalling
- Current-state
- State transition
Analog Signalling
- Amplitude
- Frequency
- Phase
Data Synchronization
- Asynchronous
- Synchronous
  - Guaranteed State change
  - Separate clock signals
  - Oversampling
Data Bandwidth
- Baseband
- Broadband