Computer Study Notes - The OSI Model

Dear Readers,

The Open System Interconnection (OSI) model defines a networking framework to implement protocols in seven layers. You must at first understand that OSI model is not tangible rather it is conceptual. You can encounter questions related to OSI model in Computer section of upcoming NICL AO and other banking recruitment exams. Although from bank exam's point of view you do not need to dive much deep into the technicality of the topic and networking concepts but a basic knowledge is required as questions can be framed from the OSI Models concept. Keep on reading to know the basic concepts and terminology of OSI Model.

The International Standards Organization (ISO) developed the Open Systems Interconnection (OSI) model. Layers 1-4 are considered the lower layers, and mostly concern themselves with moving data around. Layers 5-7, the upper layers, contain application-level data. Each layer has a protocol data unit which is an open-system interconnection (OSI) term used in telecommunications that refers to a group of information added or removed by a layer of the OSI model. OSI layer may also have specific protocols which are a set of rules that governs the communications between computers on a network.

LAYER 1- PHYSICAL LAYER

The physical layer, the lowest layer of the OSI model, is concerned with the transmission and reception of the unstructured raw bit stream over a physical medium. It provides the hardware means of sending and receiving data on a carrier network.

Networking Device – Hub, Network Interface Card (NIC), repeater, gateway

Protocol Data Unit - Bit

Some Protocols - Ethernet

The physical layer of the network focuses on hardware elements, such as cables, repeaters, and network interface cards. By far the most common protocol used at the physical layer is Ethernet. For example, an Ethernet network (such as 10BaseT or 100BaseTX) specifies the type of cables that can be used, the optimal topology (star vs. bus, etc.), the maximum length of cables, etc.

LAYER 2 – DATA LINK LAYER

When obtaining data from the Physical layer, the Data Link layer checks for physical transmission errors and packages bits into data "frames". The data link layer provides error-free transfer of data frames from one node to another over the physical layer, allowing layers above it to assume virtually error-free transmission over the link.

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.

Networking Device – Bridge, Ethernet Switches and multi layer switches, proxy server, gateway

Protocol Data Unit - Frame

Some Protocols – Ethernet, Point to Point Protocol (PPP)

LAYER 3 – NETWORK LAYER

The network layer controls the operation of deciding which physical path the data should take based on network conditions, priority of service, and other factors. When data arrives at the Network layer, the source and destination addresses contained inside each frame are examined to determine if the data has reached its final destination. If the data has reached the final destination, then network layer formats the data into packets delivered up to the Transport layer. Otherwise, the Network layer updates the destination address and pushes the frame back down to the lower layers.

Networking Device – Router, multi layer switches, gateway, proxy server

Protocol Data Unit - Packets

Some Protocols – Address Resolution Protocol (ARP), IPv4/IPv6, Internet Protocol, Routing Information Protocol (RIP), IPX.

LAYER 4 – TRANSPORT LAYER

The Transport Layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers. The transport layer controls the reliability of communications through flow control, segmentation, and error control. Two great examples of transport protocols are TCP (as in TCP/IP) and UDP.

Networking Device – proxy server, gateway

Protocol Data Unit – Segments for TCP, Datagram for UDP

Some Protocols – SPX, TCP

TCP, paired with IP, is by far the most popular protocol at the transport level. If the IPX protocol is used at the network layer, then it is paired with SPX at the transport layer.

LAYER 5 – SESSION LAYER

The session layer sets up, coordinates and terminates conversations. Services include authentication and re-connection after an interruption. It allows session establishment between processes running on different stations.

Networking Device – gateway, Logical Ports

Protocol Data Unit – Data/Session

Some Protocols – AppleTalk Data Stream Protocol, Remote Procedure Call Protocol (RPC)

LAYER 6 – PRESENTATION LAYER

As the sixth layer of the OSI model, the presentation layer is primarily responsible for managing two networking characteristics: protocol and architecture. Whereas, protocol defines a standard set of guidelines under which the network operates, the network's architecture determines what protocol applies. Encryption is typically done at this level too.

Networking Device – gateway

Protocol Data Unit – Data/ Encoded User Data

Some Protocols – Musical instrument digital interface (MIDI), Moving picture experts group (MPEG)

LAYER 7 – APPLICATION LAYER

The application layer serves as the window for users and application processes to access network services. 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.

Networking Device – gateway

Protocol Data Unit – Data

Some Protocols – DNS, FTP, SMTP, POP3, IMAP, Telnet, HTTP

Takeaways from Study Notes

Layer 7: Application layer - provides access to available network resources
Layer 6: Presentation layer - translates, encrypts, and compresses data
Layer 5: Session layer - establishes, manages, and terminates communicative sessions
Layer 4: Transport layer - provides reliable process-to-process message delivery and error recovery
Layer 3: Network layer - moves packets from source to destination providing inter networking capabilities
Layer 2: Data link layer - organizes bits into frames providing node-to-node delivery
Layer 1: Physical layer - transmits bits over a medium establishing mechanical and electrical specifications