OSI LAYER MODEL (PART 1)

OSI layer model

The seven levels that computer systems utilize to interact over a network are described by the Open System Interconnection (OSI) model. In the early 1980s, all significant computer and telecommunications businesses embraced it as the first universal architecture for network communications.

The OSI model consists of seven (7) layer that start with the physical layer, data link, network, transport, session, presentation and ends with the application layer as shown below

7	Application  Network services to application	data
6	Presentation Data formatting and encryption	data
5	Session Interhost communication	data
4	Transport End-to-end connections and reliability	segments(TCP) Datagrams(UDP)
3	Network Routing and IP (logical addressing)	packets
2	Data link MAC and LLC (physical addressing)	frames
1	Physical  Media, signal and binary transmission	BITS

Why do we use the OSI Layer Model?

• Gives a thorough description of the activities and services that can take place at each stratum.

• Explains how each layer interacts with the levels immediately above and below.

• Preventing changes in technology or capabilities from impacting layers above and below.

The OSI Layer

1. Physical Layer

Describe the functional, mechanical, electrical, and procedural ways that physical connections can be activated, maintained, and deactivated to facilitate bit transfer to and from a network device.

• Enables the data connection layer to accept a full frame.

• It is encoded as a sequence of signals that are broadcast across local media.

Functions 

• Physical components

• Encoding

• Signaling 

Data transfer

• Bandwidth - capacity to a medium to carry data

• Throughput - measure of the transfer of bits across the media

2. Data Link Layer

Describe methods for exchanging data frames between devices over a common media.

• Controlling and providing access to media

• Destination and source physical address

Data Link Sublayers

• LLC communicates with the network layer

• MAC defines the media access processes

Data Link Layer Standards

• IEEE

• ITU

• ISO

• ANSI

The Frame

Consists of header, data, and trailer as shown below.

3. Network Layer

The network layer offers services to allow the exchange of discrete chunks of data between recognised end devices through the network. It explains the protocols used to route and address communications inside an internetwork.

4. Transport Layer

For individual connections between the end devices, the transport layer defines services to split, transmit, and reassemble the data. It outlines common services and operations that implement TCP and UDP protocols and offer systematic and dependable data transport between source and destination sites.

• Establishing a brief communication channel between two programmes in order to transfer data.

• Provides Connection-oriented data stream support, Reliability, Flow control, Multiplexing.

Multiplexing 

A group of methods for allowing numerous signals to go via one connection simultaneously.

Functions

1. Track individual conversations

2. Segment Data and Reassemble Segments

3. By giving each application a port number, you can identify the applications.

5. Session Layer

The presentation layer receives services from the session layer to handle data exchange and structure its conversation.

6. Presentation Layer

Data exchanged across application layer services can be represented consistently by the presentation layer.

7. Application Layer

For communications between processes, protocols are present at the application layer. The protocols outline the control information and format required for typical internet tasks.