The OSI Model

The Open Systems Interconnection Basic Reference Model (OSI Reference Model or OSI Model for short) is a layered, abstract description for communications and computer network protocol design, developed as part of the Open Systems Interconnection initiative. It is also called the OSI seven layer model. It simply defines, in universal language, the interconnection and transfer of information between two intelligent devices.

OSI Model
7 Application layer
6 Presentation layer
5 Session layer
4 Transport layer
3 Network layer
2 Data link layer
1 Physical layer

The OSI model describes the flow of data in a network, from the lowest layer (the physical connections) up to the layer containing the user’s applications. Data going to and from the network is passed layer to layer. Each layer is able to communicate with the layer immediately above it and the layer immediately below it. This way, each layer is written as an efficient, streamlined software component. When a layer receives a packet of information, it checks the destination address, and if its own address is not there, it passes the packet to the next layer.

When two computers communicate on a network, the software at each layer on one computer assumes it is communicating with the same layer on the other computer. For example, the Transport layer of one computer communicates with the Transport layer on the other computer. The Transport layer on the first computer has no regard for how the communication actually passes through the lower layers of the first computer, across the physical media, and then up through the lower layers of the second computer.

The Layers

The Application layer represents the level at which applications access network services. This layer represents the services that directly support applications such as software for file transfers, database access, and electronic mail.

The Presentation layer translates data from the Application layer into an intermediary format. This layer also manages security issues by providing services such as data encryption, and compresses data so that fewer bits need to be transferred on the network.

The Session layer allows two applications on different computers to establish, use, and end a session. This layer establishes dialog control between the two computers in a session, regulating which side transmits, plus when and how long it transmits.

The Transport layer handles error recognition and recovery. It also repackages long messages when necessary into small packets for transmission and, at the receiving end, rebuilds packets into the original message. The receiving Transport layer also sends receipt acknowledgments.

The Network layer addresses messages and translates logical addresses and names into physical addresses. It also determines the route from the source to the destination computer and manages traffic problems, such as switching, routing, and controlling the congestion of data packets.

The Data Link layer packages raw bits from the Physical layer into frames (logical, structured packets for data). This layer is responsible for transferring frames from one computer to another, without errors. After sending a frame, it waits for an acknowledgment from the receiving computer.

The Physical layer transmits bits from one computer to another and regulates the transmission of a stream of bits over a physical medium. This layer defines how the cable is attached to the network adapter and what transmission technique is used to send data over the cable.

Examples at Each Layer

7. Application layer:
HL7, Modbus, SIP, SSI, HTTP, RIP, SMTP, SMPP, SNMP, FTP, Telnet, NFS, NTP, RTP, ISUP, INAP, MAP, TUP, TCAP, AFP, FTAM, X.400, X.500, DAP, APPC

6. Presentation layer:
TDI, ASCII, EBCDIC, MIDI, MPEG, MIME, XDR, SSL, TLS (Not a separate layer), AFP, ISO 8823, X.226

5. Session layer:
Named pipes, NetBIOS, SAP, SDP; port numbering in TCP and UDP, session establishment in TCP, telnet part of text based application protocols, ASP, ADSP, ZIP, PAP, ISO 8327, X.225, NWLink.

4. Transport layer:
NetBEUI, TCP, UDP, SCTP, ATP, NBP, AEP, RTMP, TP0-TP4, SPX.

3. Network layer:
NetBEUI, Q.931, IP, ICMP, IPsec, ARP, BGP, OSPF, RIP, MTP-3, SCCP, DDP, X.25, (PLP), CLNP, IPX (CCI office network), RRC (Radio Resource Control)

2. Data link layer:
Ethernet (including 802.11 WiFi Data Link layer), token ring, FDDI, PPP, HDLC, Q.921, Frame Relay, ATM, Fibre Channel, PPP, SLIP, MTP-2, LocalTalk, TokenTalk, EtherTalk, AppleTalk, Remote Access, PPP, X.25 (LAPB), Token Bus, IEEE 802.3 framing, Ethernet II framing, SDLC, MAC (Media Access Control), ATM, Bluetooth (PAN-Profile), FDDI • Frame Relay, GPRS, Modems.

1. Physical layer:
EIA-232, V.35, V.34, I.430, I.431, T1, E1, 10BASE-T, 100BASE-TX, POTS, SONET, DSL, 802.11b, 802.11g, MTP-1, EIA-422, EIA-485, STP, PhoneNet, Bluetooth RF, Ethernet physical layer, ISDN.

Complex protocols, such as BACnet, span multiple layers depending on the application.