The OSI Model

Techie     July 2022

Definition

The Open System Interconnection (OSI) model is a conceptual framework consisting of seven layers that describe the universal standard of communication functions of a telecommunication system or computing system.

Even though the modern internet uses the OSI model to help visualize how networks function, the simpler TCP/IP model is used for practical application. The updated TCP/IP model is similar to the OSI model, the only difference is that it has only 5 layers since it collapses layers 5, 6 and 7 onto just the Application layer.



1. Physical Layer (Layer 1)

This layer deals with physical transmission media. It essentially puts the data on the physical media as bits and receives it in the same way. Connectors, pinouts, cables, electrical currents are defined within this layer.

Functions

  • Encoding and signaling.
  • Physical data transmission.
  • Hardware specifications.
  • Topology and design.

Protocols

  • Bluetooth
  • Ethernet
  • DSL
  • ISDN
  • 802.11
  • Wi-Fi
  • IEEE 802.3
  • IEEE 802.15
  • IEEE 802.16
  • IEEE 1394
  • USB

Protocol Data Unit(PDU)

  • Bits

Devices

  • Network hubs
  • Cabling
  • Repeaters
  • Network adapters or modems


This is the second layer of the model. It handles data transfer between nodes on a network segment across the physical layer.

Functions

  • Logical Link Control(LLC).
  • Media Access Control (MAC).
  • Data framing.
  • Addressing.
  • Error detection and handling.
  • Defining requirements of physical layer.

Protocols

  • ARP
  • ATM
  • CDP
  • FDDI
  • Frame Relay
  • HDLC
  • MPLS
  • PPP
  • STP
  • Token Ring
  • SDLC
  • IEEE 802.3
  • Frame Relay
  • ITU-T G.hn DLL
  • X.25

Protocol Data Unit(PDU)

  • Frames

Devices

  • Bridge
  • Switch
  • Access point


3. Network Layer (Layer 3)

The network layer receives frames from the data link layer, and delivers them to their intended destinations among based on the addresses contained inside the frame. It is responsible for packet forwarding including routing through intermediate routers.

Functions

  • Routing
  • Logical addressing
  • Datagram encapsulation
  • Fragmentation and reassembly
  • Error handling and diagnostics

Protocols

  • ICMP
  • IGMP
  • IPsec
  • IPv4
  • IPv6
  • IPX
  • RIP

Protocol Data Unit(PDU)

  • Packets

Devices

  • Routers
     


4. Transport Layer (Layer 4)

The transport layer layer provides end-to-end communication services for applications by collecting message segments from applications, and transmitting them into the network.

Functions

  • Segmentation and reassembly
  • Proccess-level addresing
  • Connections
  • Multiplexing/demultiplexing

Protocols

  • TCP
  • UDP
  • SCTP
  • DCCP

Protocol Data Unit(PDU)

  • Segment

Devices

  • Gateways
  • Firewalls


5. Session Layer (Layer 5)

The session layer manages connections (sessions) between end-user application processes. Essentially, it tracks the dialogs between systems, which are also called sessions.

Functions

  • Data synchronization
  • Management and termination
  • Session establishment

Protocols

  • NetBIOS
  • NFS
  • PAP
  • SCP
  • SQL
  • ZIP
  • SAP
  • SIP
  • L2TP
  • PPTP

Protocol Data Unit(PDU)

  • Data

Devices

  • Gateways
  • Firewalls
  • PC’s


6. Presentation Layer (Layer 6)

The presentation layer presents the data in a readable format to the application layer.

Functions

  • Data format and translation
  • Compression and decompression
  • Encryption and decryption

Protocols

  • JPEG
  • MIDI
  • MPEG
  • PICT
  • TIFF
  • MIME
  • XDR
  • TLS
  • SSL

Protocol Data Unit(PDU)

  • Data

Devices

  • Gateways
  • Firewalls
  • PC’s


7. Application Layer (Layer 7)

The application layer is used by end-user software such as web browsers and email clients. It provides an interface between the software application on a system and the network, hence does not include the application itself, but provides services that an application requires.

Functions

  • Manages user application services

Protocols

  • DNS
  • DHCP
  • FTP
  • HTTPS
  • IMAP
  • LDAP
  • NTP
  • PO3
  • RTP
  • RTSP
  • SSH
  • SIP
  • SMTP
  • SNMP
  • SMPP
  • Telnet
  • TFTP
  • NFS
  • Gopher

Protocol Data Unit(PDU)

  • Data

Devices

  • Gateways
  • Firewalls
  • PC’s
  • Phones
  • Servers

Thanks for reading, see you in the next one!