What is the Open System Interconnect (OSI) model?
The Open Systems Interconnection (OSI) reference model was an architectural model for open networking systems that was developed by the International Organization for Standardization (ISO) in Europe in 1974. The OSI reference model was intended as a basis for developing universally accepted networking protocols. So, the OSI reference model is an idealized model of the logical connections that must occur in order for network communication to take place. Most protocol suites, such as TCP/IP, DECnet, and Systems Network Architecture (SNA), map loosely to the OSI reference model. The OSI model is not a protocol but it is good for understanding how various protocols within a protocol suite function and interact.
The OSI reference model has seven logical layers, as shown in the following table.
The OSI Reference Model
7 Application layer
Interfaces user applications with network functionality, controls
how applications access the network, and generates error
messages. Protocols at this level include HTTP, FTP, SMTP, and NFS.
6 Presentation layer
Translates data to be transmitted by applications into a format
suitable for transport over the network. Redirector software, such as the
Workstation service for Microsoft Windows NT, is located at this
level. Network shells are also defined at this layer.
5 Session layer
Defines how connections can be established, maintained, and
terminated. Also performs name resolution functions. This layer enables
applications running at two workstations to coordinate their
communications into a single session.
4 Transport layer
Sequences packets so that they can be reassembled at the
destination in the proper order. Generates acknowledgments and retransmits
packets. Assembles packets after they are received. If a duplicate packet
arrives, the transport layer recognizes it as a duplicate and discards
it. TCP and SPX are transport-layer protocols.
3 Network layer
Defines logical host addresses, creates packet headers, and routes
packets across an internetwork using routers and Layer 3 switches. Strips
the headers from the packets at the receiving end. This layer is
responsible for the entire route of a packet, from source to
destination. IP and IPX are examples of network-layer protocols.
2 Data-link layer
Specifies how data bits are grouped into frames, and specifies
frame formats. Responsible for error correction, flow control, hardware
addressing (such as MAC addresses), and how devices such as hubs, bridges,
repeaters, and Layer 2 switches operate. The Project 802 specifications
divide this layer into two sublayers, the logical link control (LLC) layer
and the media access control (MAC) layer. The MAC layer deals with network
access and network control. The LLC layer, operating just above the MAC
layer, is concerned with sending and receiving the user massages.
1 Physical layer
Defines network transmission media, signaling methods, bit
synchronization, architecture (such as Ethernet or Token Ring), and
cabling topologies. Defines how network interface cards (NICs) interact
with the media (cabling). You can think this layer as the hardware layer.