Open Systems Interconnection (OSI) Reference Model

The Open Systems Interconnection (OSI) Reference Model is a modular framework for developing standards that are based on a division of network operations into seven, sets of network services.

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.


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.


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.