OSI model
The Open System Interconnection is an ISO standard for worldwide
communications that defines a framework for implementing protocols in seven layers. Control
is passed from one layer to the next, down and back up the hierarchy from
station to station. There are seven layers as defined in the OSI model:
Application - Layer 7: The top layer defines the language and syntax that programs use to communicate
with other programs. For example, a client program uses commands to request
data from a server program. Common functions at this layer are opening, closing,
reading files, writing files, transferring files, e-mail messages, executing
remote jobs, and obtaining directory information about network resources.
Presentation - Layer 6: When data is transmitted between different types of computer systems, this
layer negotiates and manages how data is represented and encoded. For example, it
provides a common reference between ASCII and EBCDIC machines, as well as
between different floating point and binary formats. SunÆs XDR and OSIÆs ASN.1 are two protocols used for this purpose. This layer is also used for
encryption and decryption.
Session - Layer 5: Provides communication coordination. Determines one-way or two-way
communications and manages the dialogue; for example, verifying that the previous request
has been fulfilled before sending the next. It can also provide fast recovery
in the event of a connection failure. This layer is often not used, or its
services are sometimes incorporated into the transport layer.
Transport - Layer 4: Responsible for end to end validity and integrity of the transmission. If a
packet gets lost in a router somewhere in the enterprise internet, the transport
layer will detect the error. It ensures that if an 8 MB file is sent, the full
8 MB is received. ôOSI transport servicesö include layers 1 through 4, and are collectively responsible for delivering a
error-free and complete message or file from sending to receiving station.
Network - Layer 3: Establishes the route between the sending and receiving stations. The node to
node function of the data link layer (layer 2) is extended across the entire
internetwork, because a routable protocol contains a network address in addition
to a station addresses. This layer is the switching function of the dial-up
telephone system, as well as the functions performed by routable protocols such as
IP, IPX, SNA, and AppleTalk. If all stations are contained within a single network
segment (that is, they are not separated by a router), then the routing
capability in this layer is not required.
Data Link - Layer 2: Responsible for node to node validity and integrity of the transmission. The
transmitted bits are divided into frames; for example, an Ethernet, Token Ring, or FDDI frame in local area networks (LANs). Layers 1 and 2 are required for
every type of communications.
Physical - Layer 1: Responsible for passing bits onto and receiving them from the connecting
medium. This layer has no understanding of the meaning of the bits, but deals with
the electrical and mechanical characteristics of the signals and signaling
methods. For example, it includes the RTS and CTS signals in an RS-232 environment,
as well as TDM and FDM techniques for multiplexing data on a line.