Table of Contents Link to heading
Layered Model Link to heading
OSI Reference Model Link to heading
It was developed by the International Organisation for Standardisation (ISO) to provide a road map for non-proprietary protocol development.
Many of the OSI protocols are no longer in use, but knowledge of the model as a reference is a basic expectation for networking professionals.
Many professionals refer to the layers by number rather than name, so it is important to know both.
OSI Protocol Stack Link to heading
APSTNDP - All People Seem To Need Dominos Pizza
PDNTSPA - Please Do Not Tell Sales People Anything
- Layers 6 to 5: not commonly referred to in most instances.
- Layers 7 to 3: primary concern is communications between applications.
- Layers 2 to 1: primary concern is moving raw data cross the network.
| # | Layer | OSI Protocol Suite | Functional Description |
|---|---|---|---|
| 7 | Application | HTTP, SMTP, FTP, Telnet, DNS, SSH, IRC, Modbus, SEP2, DNP3, IEC 61850, DNP3, ICCP, BACnet, OpenADR, GOOSE | Performs services for the applications used by end users |
| 6 | Presentation | SSL, SSH, IMAP, FTP | Provides for common representation of the data transferred between
application layer services E.g. tells the application layer whether there is an encryption or whether it is a .png image |
| 5 | Session | API, Socket, NFS, SQL, SMB, RPC, P2P, SCP, SDP, SIP, H.323, tunneling | Provides services to the presentation layer to organise its dialogue
to manage data exchange and sessions between users E.g. synchronises multiple web sessions and voice and video data in web conferences |
| 4 | Transport | TCP, UDP, ECN, SCTP, DCCP | Defines services to segment, transfer, and reassemble the data for individual communications between the end devices |
| 3 | Network | IPv4, IPv6, ICMPv4, ICMPv6, ARP, IGMP | Provides services to exchange the individual pieces of data over the
network between identified end devices E.g. creates and addresses packets for end-to-end delivery through intermediary devices in other networks |
| 2 | Data Link | Ethernet, SLLIP, PPP, FDDI | Describes methods for exchanging data frames between devices over a
common media and provides error control E.g. creates and addresses frames for host-to-host delivery on the local LANs and between WAN devices |
| 1 | Physical | Coax, Fibre, Wireless, DSL, RS 232, UTP cables (CAT 5, 6) | Describes the mechanical, electrical, functional, and procedural means
to activate, maintain, and deactivate physical connections for bit
transmission to and from a network device E.g. encodes the binary digits that represent data link layer frames into signals and to transmit and receive these signals across the physical media that connect network devices. |
Model Comparision Link to heading
The protocols that make up the TCP/IP protocol suite can also be described in terms of the OSI reference model. In the OSI model, the network access layer and the application layer of the TCP/IP model are further divided to describe discrete functions that must occur at these layers.
At the network access layer, the TCP/IP protocol suite does not specify which protocols to use when transmitting over a physical medium; it only describes the handoff from the Internet layer to the physical network protocols. OSI Layers 1 and 2 discuss the necessary procedures to access the media and the physical means to send data over a network.
OSI Layer 3, the network layer, maps directly to the TCP/IP Internet layer. This layer is used to describe protocols that address and route messages through an internetwork.
OSI Layer 4, the transport layer, maps directly to the TCP/IP Transport layer. This layer describes general services and functions that provide ordered and reliable delivery of data between source and destination hosts.
The TCP/IP application layer includes a number of protocols that provide specific functionality to a variety of end user applications. The OSI model Layers 5, 6, and 7 are used as references for application software developers and vendors to produce products that operate on networks.
Both the TCP/IP and OSI models are commonly used when referring to protocols at various layers. Because the OSI model separates the data link layer from the physical layer, it is commonly used when referring to these lower layers.