OSI Layer | Network Communications Software

Open Systems Interconnection OSI layer is a reference, conceptual or logical model that defines network communication and communication with other systems.

OSI Layers:

Application Layer
Presentation Layer
Session Layer
Transport layer
Network Layer
Data Link Layer
Physical Layer

Short Description of Table:
The Application Layer, Presentation Layer, Session Layer are the Upper Layers where as Transport Layer, Network Layer, Data Link Layer and Physical Layer are the Lower Layers. Sending of data and instruction to Network is done from uppermost part of Upper layer to downwards. Receiving from Network occurs from lowest part of Lower Layer to upward.

The Seven different OSI layers are described below in detail:

Physical (Layer 1)

OSI Model, Layer 1 conveys the bit stream - electrical impulse, light or radio signal ? through the network at the electrical and mechanical level. It provides the hardware means of sending and receiving data on a carrier, including defining cables, cards and physical aspects. Fast Ethernet, RS232, and ATM are protocols with physical layer components. Layer 1 Physical examples include Ethernet, FDDI, B8ZS, V.35, V.24, RJ45.

Data Link (Layer 2)

At OSI Model, Layer 2, data packets are encoded and decoded into bits. It furnishes transmission protocol knowledge and management and handles errors in the physical layer, flow control and frame synchronization. The data link layer is divided into two sub layers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sub layer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking. Layer 2 Data Link examples include PPP, FDDI, ATM, IEEE 802.5/ 802.2, IEEE 802.3/802.2, HDLC, Frame Relay.

Network (Layer 3)

Layer 3 provides switching and routing technologies, creating logical paths, known as virtual circuits, for transmitting data from node to node. Routing and forwarding are functions of this layer, as well as addressing, internetworking, error handling, congestion control and packet sequencing. Layer 3 Network examples include AppleTalk DDP, IP, IPX.

Transport (Layer 4)

OSI Layer, Layer 4, provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer. Layer 4 Transport examples include SPX, TCP, UDP.

Session (Layer 5)

This layer establishes, manages and terminates connections between applications. The session layer sets up, coordinates, and terminates conversations, exchanges, and dialogues between the applications at each end. It deals with session and connection coordination. Layer 5 Session examples include NFS, NetBios names, RPC, SQL.

Presentation (Layer 6)

This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer. Layer 6 Presentation examples include encryption, ASCII, EBCDIC, TIFF, GIF, PICT, JPEG, MPEG, MIDI.

Application (Layer 7)

OSI Model, Layer 7, supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. This layer provides application services for file transfers, e-mail, and other network software services. Telnet and FTP are applications that exist entirely in the application level. Tiered application architectures are part of this layer. Layer 7 Application examples include WWW browsers, NFS, SNMP, Telnet, HTTP, FTP.

Network Communications Software 

Network Operating System:

 A Network Operating System is a system software that controls the hardware devices, software, and communications media and channels across a network. It allows various devices to communicate with each other. For Example: Windows NT, Net ware. 


 The set of rules and procedures governing transmission across a network is called Protocol. The principal functions of protocols in a network are line access and collision avoidance. Line access concerns how the sending device gains access to the network to send a message. Collision avoidance refers to managing message transmission so two message do not collide with each other on the network. 
Ethernet protocol:
 The most common protocol is Ethernet 10BaseT. The 10BaseT means that the network has a speed of 10 Mbps. Fast Ethernet is 100BaseT which means that the network has a speed of 100 Mbps. The most common protocol in large corporation is the Gigabit Ethernet which provides network data transmission speed of one billion bits per second.

 The Transmission Control Protocol/Internet Protocol is a file transfer protocol that can send large files of information across networks with assurance that the data will arrive in uncorrupted form. It allows reasonable and error-free transmission between different systems and is the protocol of internet. TCP/IP is has four layers.

Application Layer Application Layer
Presentation Layer
Session Layer
Transport Layer Transport Layer
Network Layer Internet Layer
Data Link Layer Host To Network
(Network Access Layer)
Physical Layer

 The table above shows the Comparison between OSI and TCP/IP.

Types of Data Transmission:

Packet Switching:
 Packet switching is a digital networking communications method that groups all transmitted data into suitably sized blocks, called packets, which are transmitted via a medium that may be shared by multiple simultaneous communication sessions. 

Circuit Switching
Circuit switching is a methodology of implementing a telecommunications network in which two network nodes establish a dedicated communications channel (circuit) through the network before the nodes may communicate. 

Frame Relay
Frame relay is a faster and less expensive version of packet switching. Frame relay is a shared network services that package data into frames that are similar to packet. It does not perform error correction. it can communicate at transmission speeds of 1.544 Mbps. 

Fiber Distributed Data Interface (FDDI):
 FDDI passes data around a ring with a bandwidth of 100 Mbps. It is based on the high-speed, high-capacity capabilities of fibre optics. Implementing FDDI in a LAN is 10 times more expensive. 

Aysnchronous Transfer Mode (ATM)

Asynchronous transfer mode (ATM) is a switching technique used by telecommunication networks that uses asynchronous time-division multiplexing to encode data into small, fixed-sized cells. This is different from Ethernet or Internet, which use variable packet sizes for data or frames. ATM networks are packet -switched, dividing data into uniform cells, each with 53 groups of 8 bytes, eliminating the need for protocol conversion. It provides support for data, video and voice transmission on one communication line. It requires fibre-optic cable and can transmit data upto 2.5 Gigabits per second but it us more expensive than ISDN and DSL. 

Switched Hub Technologies
Switched Hub Technologies are often used to boost local area networks. A switched hub can turn many small LANs into one big LAN. 

Synchronous Optical Network (SONET) 
Synchronous Optical Network (SONET) is an interface standard for transporting digital signals over fibre optic links that allows the integration of transmission from multiple vendors 

T-Carrie System
It is a digital transmission system that defines circuits that operate at different rates, all of which are multiples of basic 64 Kbps used to transmit single voice call. 

Applications of Telecommunications 

• Electonic Mail 
E-mail or Electronic Mail is the exchange of stored messages in computers through telecommunication. One can also send non-text files, such as graphic images and sounds, as attachments in binary streams. E-mail messages are basically encoded in ASCII text. E-mail was one of the first applicaition of the Internet and It is still the most popular application over the web

• Videoconferencing 
Video conferencing is simply the conduction of a video conference with groups of hardware and software interacting correspondingly, transmitting and receiving video and audio signals from two or more different part of the world or different geographic locations.

• Electronic Data Interchange 
Electronic Data Interchange or EDI is the electronic interchange or exchange of business documents, files and different data in structured data form and without manual intervention.

• Electronic Fund Transfer
Electronic Funds Transfer (EFT) is the transferring of money from one bank to another without any paper money changing hands. For an example: Direct Deposit, in which payroll is deposited straight into an employee's bank account. It is used for both credit transfers, such as payroll payments, and for debit transfers, such as mortgage payments. 

• Fascimiles 

• Telecommuting 

• Distance Learning 

• Telematics

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