Open Systems Interconnection (OSI) Reference Model

At one time, most vendors agreed to support OSI in one form or another, but the OSI was too loosely defined and proprietary standards were too entrenched. Except for the OSI-compliant X.400 and X.500 e-mail and directory standards, which are still widely used, what was once thought to become the universal communications standard now serves as the teaching model for all other protocols.

Most of the functionality in the OSI model exists in all communications systems, although two or three OSI layers may be incorporated into one.

Example of how the OSI Layers work using an e-mail sent from the computer on the left.

Data travels from the sending computer down through all the layers to the physical layer where the data is put onto the network cabling, and then sent to the physical layer of the receiving computer where the process reverses and the data travels up through the layers to the application layer of the receiving computer.

E-mail sent from John: Meet me at Carl's 1:30 John		E-mail received from John: Meet me at Carl's 1:30 John
Identify sender and intended receiver; is there an e-mail application available?	APPLICATION layer 7	Identified sender and intended receiver; found e-mail application.
Encode data with X coding key; use ASCII characters.	PRESENTATION layer 6	Decoded data with X decoding key; used ASCII characters.
Initiate and terminate the session according to X protocol.	SESSION layer 5	Initiated and terminated the session according to X protocol.
Make sure all data is sent intact.	TRANSPORT layer 4	Make sure all data has arrived intact.
Keep track of how many hops; open shortest path First; Go to IP address 255.65.0.123	NETWORK layer 3	Keep track of how many hops; opened the shortest path First; Went to IP address 255.65.0.123
Is the initial connection set up? Put data into frames according to X standard.	DATA LINK layer 2	The initial connection set up. Decoded data in frame according to X standard.
Send as electrical signal over the network cable at X voltage, and X Mbps.	PHYSICAL layer 1	Receive electrical signal over the network cable at X voltage, and X Mbps.

A look at each of the OSI layers , and the role it plays.

APPLICATION layer 7	Gives user applications access to network. This layer represents the services, that directly support the user applications such as software for file transfers, database access, and E-mail
PRESENTATION layer 6	The presentation layer, usually part of an operating system, converts incoming and outgoing data from one presentation format to another. Presentation layer services include data encryption and text compression.
SESSION layer 5	Opens manages, and closes conversations between two computers. It performs name recognition and the functions such as security, needed to allow two applications to communicate over the network, also provides error handling.
TRANSPORT layer 4	This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer. Sequences data packets, and requests retransmission of missing packets. It also repackages messages for more efficient transmission over the network.
NETWORK layer 3	Establishes, maintains and terminates network connections. Routes data packets across network segments. Translates logical addresses and names into physical addresses.
DATA LINK layer 2	Transmits frames of data from computer to computer on the same network segment. Ensures the reliability of the physical link established at layer 1. Standards define how data frames are recognized and provide the necessary flow control and error handling at the frame set. The data link layer is divided into two sublayers: The Media Access Control (MAC) layer and the Logical Link Control (LLC) layer. The MAC sublayer controls how a computer on the network gains access to the data and permission to transmit it. The LLC layer controls frame synchronization, flow control and error checking.
PHYSICAL layer 1	The Physical layer defines all the electrical and physical specifications for devices. This includes the layout of pins, voltages, and cable specifications. Hubs, repeaters and network adapters are physical-layer devices. Defines cabling and connections. Transmits data over the physical media.

Some common network devices and protocols and where they are implemented in the OSI model.

OSI LAYER	DEVICES	PROTOCOLS
APPLICATION layer 7		SNMP, SMTP, FTP, TELNET, HTTP, NCP, SMB, AppleTalk
PRESENTATION layer 6		NCP, AFP, TDI
SESSION layer 5		NetBIOS
TRANSPORT layer 4		NetBEUI, TCP, SPX, NWlink
NETWORK layer 3	Routers, layer 3 (or IP) switches.	IP, IPX, NWlink, NetBEUI
DATA LINK layer 2	Bridges and switches, Ethernet incorporates both this layer and the Physical layer.	-
PHYSICAL layer 1	Hubs, repeaters, network adapters, Parallel SCSI buses. Various physical-layer Ethernet incorporates both this layer and the data-link layer. Token ring, FDDI, and IEEE 802.11.	-

The unofficial other OSI Layer 2.5

While not a part of the official OSI model, the term "Layer 2.5" has been used to categorize some protocols that operate between the data link layer 2 and the network layer 3. For example, Multiprotocol Label Switching (MPLS) operates on packets (layer 2) while working with IP addresses (layer 3) and uses labels to route packets differently.

Interfaces

In addition to standards for individual protocols in transmission, there are also interface standards for different layers to talk to the ones above or below, usually operating system specific. For example, Microsoft Winsock, and Unix's sockets and System V Transport Layer Interface, are interfaces between applications (layers 5 and above) and the transport (layer 4). NDIS and ODI are interfaces between the media (layer 2) and the network protocol (layer 3).

Layer	Examples	TCP/IP	AppleTalk	OSI	Novell IPX
Application	HL7 Modbus SIP	HTTP SMTP SMPP SNMP FTP Telnet NFS NTP	AFP PAP	FTAM X.400 X.500 DAP
Presentation	TDI ASCII EBCDIC MIDI MPEG	XDR SSL TLS	AFP PAP	ISO 8823 X.226
Session	Named Pipes NetBIOS SAP SDP	Session establishment for TCP	ASP ADSP ZIP	ISO 8327 X.225	NWLink
Transport	NetBEUI	TCP UDP RTP SCTP	ATP NBP AEP RTMP	TP0 TP1 TP2 TP3 TP4 OSPF	SPX RIP
Network	NetBEUI Q.931	IP ICMP IPsec ARP RIP BGP	DDP	X.25 (PLP) CLNP	IPX
Data Link	Ethernet Token Ring FDDI PPP HDLC Q.921 Frame Relay ATM Fibre Channel		LocalTalk TokenTalk EtherTalk Apple Remote Access PPP	X.25 (LAPB) Token Bus	IEEE 802.3 framing Ethernet II framing
Physical	RS-232 V.35 V.34 Q.911 T1 E1 100BASE-TX ISDN SONET DSL		Localtalk on shielded, Localtalk on unshielded (PhoneNet)	X.25 (X.21bis) EIA/TIA-232 EIA/TIA-449 EIA-530 G.703

notes:

X.400 An ISO and ITU standard for addressing and transporting e-mail messages. It conforms to layer 7 of the OSI model and supports several types of transport mechanisms, including Ethernet, X.25, TCP/IP, and dial-up lines.

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X.500 An ISO and ITU standard that defines how global directories should be structured. X.500 directories are hierarchical with different levels for each category of information, such as country, state, and city. X.500 supports X.400 systems.

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Media Access Control Layer is one of two sublayers that make up the Data Link Layer of the OSI model. The MAC layer is responsible for moving data packets to and from one Network Interface Card (NIC) to another across a shared channel.

The MAC sublayer uses MAC protocols to ensure that signals sent from different stations across the same channel don't collide.

Different protocols are used for different shared networks, such as Ethernets, Token Rings, Token Buses, and WANs.