7 Layers of the Osi Reference Model

The Application Layer (Layer 7) refers toincluded. Examples include: TCP, UDP, and SPX.
communication services to applications and is theNetwork Layer (Layer 3) defines end-to-end
interface between the network and thedelivery of data packets and defines logical
application. Most application layer protocols offeraddressing to accomplish this. It also defines how
services that programs use to access therouting works and how routes are learned. The
network, such as the Simple Mail TransferOSI Reference Model fragments a packet into
Protocol (SMTP).smaller packets to accommodate media with
Application layer protocols often include thesmaller maximum transmission unit sizes. The
session and presentation layer functions. A typicalInternet Protocol (IP) is the cornerstone of the
protocol stack consists of four separateTransmission Control Protocol/Internet Protocol
protocols, which run at the application, transport,(TCP/IP) suite and the most frequently used
network, and data-link layers.Examples include:network layer protocol. Other examples include:
Telnet, HTTP, FTP, Internet browsers, NFS, SMTPIPX, AppleTalk DDP, NetBIOS, NetBEUI and ICMP.
gateways, SNMP, X.400 mail, and FTAM.Data Link Layer (Layer 2) is the channel between
The Presentation Layer (Layer 6) defining datathe computer's networking hardware and
formats, such as ASCII text, EBCDIC text,networking software. This layer is concerned with
binary, BCD, and JPEG and is responsible forgetting data across one particular link or medium.
formatting data exchange..The data link protocols define delivery across an
Encryption also is defined as a presentation layerindividual link. These protocols are necessarily
service. Examples include: JPEG, ASCII, EBCDIC,concerned with the type of media in use. To
TIFF, GIF, PICT, encryption, MPEG, and MIDI.implement a data-link layer protocol, the following
The Session Layer (Layer 5) defines how twohardware and software is needed: Network
computers establish, synchronize, maintain, andinterface cards (NICs), which are also called
end communication sessions. This includes theNetwork adapters; Network adapter drivers;
control and management of multiple bidirectionalNetwork cables and ancillary connecting hardware;
messages so that the application can be notified ifand Network hubs. NICs and hubs are designed
only some of a series of messages arefor specific data-link layer protocols, and it cannot
completed. This allows the presentation layer tomix with other products or different kinds of
have a seamless view of an incoming stream ofprotocols. Some network cables are
data.protocol-specific, while others can be used with
The presentation layer can be presented withdifferent protocols. The most popular data-link
data if all flows occur in some cases. There arelayer protocol is Ethernet while Token Ring is also
no separate session layer protocols as there arecommon. Data-link layer protocol specifications
at the lower layers. Session layer functions areinclude the three basic elements: A format for the
integrated into other protocols that includeframe, A mechanism for controlling access to the
presentation and application layer functions. Thenetwork medium, One or more physical layer
transport, network, data-link, and physical layersspecifications for use with the protocol Examples
are concerned with the proper transmission ofinclude: IEEE 802.3/802.2, HDLC, Frame Relay, PPP,
data across the network. The session layer andFDDI, ATM, and IEEE 802.5/802.2.
above are not involved in that part of thePhysical Layer (Layer 1) deals with the physical
communications process. The session layercharacteristics of the transmission medium.
provides 22 services. The most important ofConnectors, pins, use of pins, electrical currents,
these services are called dialog control and dialogencoding, and light modulation are all part of
separation. Examples include: RPC, SQL, NFS,different physical layer specifications. It defines
NetBIOS names, AppleTalk ASP, and DECnet SCP.the nature of the network's hardware elements.
Transport Layer (Layer 4) defines severalIt also defines what kind of network interface
functions, including the choice of protocols. Theadapter should be used and the hub that should
most important Layer 4 functions are errorbe used. Different kinds of copper or fiber optic
recovery and flow control. The transport layercable can be used, and a wide variety of wireless
may provide for retransmission, i.e., errorsolutions. Usually when a LAN is set up, the
recovery, and may use flow control to preventphysical layer specifications are related to the
unnecessary congestion by attempting to senddata-link layer. The data-link layer protocol,
data at a rate that the network canEthernet, helps with the different kinds of options
accommodate, or it might not, depending on thein the physical layer. Examples includes:
choice of protocols. Multiplexing of incoming dataEIA/TIA- 232, V.35, EIA/TIA-449, V.24, RJ-45,
for different flows to applications on the sameEthernet, 802.3, 802.5, FDDI, NRZI, NRZ, and
host is also performed. Reordering of the incomingB8ZS.
data stream when packets arrive out of order is