What is the Open Systems Interconnection model (OSI model)?

Map Of Internet 1973


This page is about the Open Systems Interconnection (OSI) model that describes layers for data transmission.


The elements of each layer are:


The level 5 (session) and 6 (presentation) are nowadays implemented in the level 7 (application) and above

Level Name Data unit name Data Unit example Description Protocol
7 Application layer Message Message Higher level (Resource Sharing, Remote file access, …) HTTP, SMTP, FTP, DHCP, DNS, Telnet, SNMP, …
6 Presentation layer Message Message Data Translation (Character encoding, data compression and encryption/decryption) MIME, XDR, ASN.1, ASCII, PGP
5 Session layer Message Message Session functionality Named pipe, NetBIOS, SAP, PPTP, RTP, SOCKS, SPDY, X.225
4 Transport layer Packet Segment Packet
Datagram Packet
Data transmission between network points TCP, UDP, SCTP, DCCP, SPX
3 Network layer Packet IP Packet Addressing, routing and traffic control IP Protocol, ICMP, IPsec, IGMP, IPX, IS-IS, Apple Talk, X.25, PLP
2 Data link layer Packet Frame Frames transmission between two physical nodes connected ATM, ARP, …
1 Physical layer Bit Bit, Symbol Raw bit transmission over a physical medium IEEE 802.3 (Ethernet cable), IEEE 802.11 (Wifi), USB, Bluetooth, Rs232 (modem, printer, …)

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