Computer Networking - The OSI (Open System Interconnection) Model

ISO or International Standard Organization

- ISO develops the OSI (Open System Interconnection) model.

- It is formed by United Nation (UN) consists of representatives from different industrialized nations.

OSI (Open System Interconnection) model

- OSI is just a “model” (not a protocol!) on how all devices on the network should communicate. 

The 7 Layers of OSI Model

Layer 7 – Application Layer

Layer 6 – Presentation Layer

Layer 5 – Session Layer

Layer 4 – Transport Layer

Layer 3 – Network Layer

Layer 2 – Data Link Layer

Layer 1 – Physical Layer

The Application Layer

- Application layer is a layer closest to the user that provides network services.

- It acts as an interface between the actual application program and the Presentation layer. 

- It is also responsible for identifying and establishing the availability of desired communication partner verifying sufficient resources exist for communication.

- Application layer synchronizes agreement on procedures for error recovery and data integrity.

- The protocols that function on this layer are SMTP, FTP, TFTP, HTTP, SNMP, etc.

The Presentation Layer

- Presentation layer accepts the data from Application layer. 

- Its main task is to format the data into a common language like ASCII and EBCDIC.

- It is also responsible for compressing the amount of data to transfer it more quickly. It has the ability to hide information by means of encryption to ensure protection as the data travels through the network.

- Services include on this layer are PICT, TIFF, JPEG, MIDI, MPEG, Quick Time, etc.

The Session Layer

- Session layer accepts the data from Presentation layer.

- It establishes, manages, and terminates sessions between two communicating hosts. 

- It also synchronizes dialogue between the two hosts and manages their data exchange.

- Protocols which function on this layer are NFS, SQL, RPC, X-Window, ASP, DNA SCP,etc.

Note: Application, Presentation, and Session Layers are responsible for communicating from user interface to an application program. They are sometimes referred as the Upper-Layers and they know nothing about networking or internetworking!

The Transport Layer

- Transport layer accepts the data from Session layer.

- It breaks up data into segments for an ease of transmission.

- It establishes, maintains, and properly terminates virtual circuits.

- It also ensures that the data are sent and received intact!

- There are two ways to transmit data packets: connectionless and connection-oriented.

- Protocols that function on this layer are UDP, TCP, SPX, NetBEUI, etc.

Note: Devices functioning from Application Layer to Transport Layer include hosts and gateways.

The Network Layer

- Network layer accepts the data segments from Transport layer.

- It manages software and hardware addressing (I mean logical addressing not the physical! I will explain them later.), packet routings between hosts and networks and determines the best way or path to move or transfer data.

- Protocols that function on this layer are ICMP, ARP, RARP, RIP, IGRP, EIGRP, EIGRP, OSPF, EGP, IGMP, etc.

- Devices functioning up to this layer include Routers, Brouters, and Layer-3 Switches.  

The Data Link Layer 

- Data Link layer is concerned with physical addressing, network topology, network access, error notification and flow control.

- It assures that messages are delivered to proper device on a LAN using hardware address (MAC address) and translates message from the Network layer into bits for the Physical layer to transmit.

- It formats the message into frame and adds header containing the hardware address and source address.

- Protocols that function on this layer are HDLC and LLC.

- Device functioning up to this point are Network Interface Card (NIC), bridge, and switch.

The 2 Sub-layers of Data Link Layer

1. MAC Sublayer – It defines how packets are placed on the media.

2. Logical Link Control (LLC) Sublayer – It identifies Network layer protocols and then encapsulating them.

Note: frame = packet + MAC address

The Physical Layer

- Physical layer specifies voltage, wire speed, and pin-out of cables.

- It is responsible for moving bits of data through physical media.

- It also defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between end systems.

- Protocols that function on this layer are the cabling, signaling and connection standards that I’m going to discuss later on this lecture note.

- Devices functioning up to this point are repeaters, hubs, media access units (MAU), and transceivers.