Definition: The OSI model defines internetworking in terms of a vertical stack of seven layers. The upper layers of the OSI model represent software that
implements network services like encryption and connection management. The lower layers of the OSI model implement more primitive, hardware-oriented functions
like routing, addressing, and flow control.
The OSI model was introduced in 1984. Although it was designed to be an abstract model, the OSI model remains a practical framework for today's key network
technologies like Ethernet and protocols like IP.
The OSI model should be used as a guide for how data is transmitted over the network. It is an abstract representation of the data pathway and should be
treated as such.
The OSI model was specifically made for connecting open systems. These systems are designed to be open for communication with almost any other system. The model
was made to break down each functional layer so that overall design complexity could be lessened. The model was constructed with seven layers for the flow of
information. These are:
Data link layer
1.1 Application layer
Provides a means for the user to access information on the network through an application. This layer is the main interface for the user to interact with the application and therefore the network.
The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the
software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope
of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing
communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an
application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the
requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application
Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol (SMTP).
1.2 Presentation layer
Manages the presentation of the information in an ordered and meaningful manner. This layer's primary function is the syntax and semantics of the data transmission. It converts local host computer data representations into a standard network
format for transmission on the network. On the receiving side, it changes the network format into the appropriate host computer's format so that data can be
utilized independent of the host computer. ASCII and EBCDIC conversions, cryptography, and the like are handled here.
The presentation layer provides a variety of coding and conversion functions that are applied to application layer data. These functions ensure that
information sent from the application layer of one system would be readable by the application layer of another system. Some examples of presentation layer
coding and conversion schemes include common data representation formats, conversion of character representation formats, common data compression schemes,
and common data encryption schemes.
Common data representation formats, or the use of standard image, sound, and video formats, enable the interchange of application data between different types
of computer systems. Using different text and data representations, such as EBCDIC and ASCII, uses conversion schemes to exchange information with systems.
Standard data compression schemes enable data that is compressed. or encrypted at the source device to be properly decompressed, or deciphered at the destination.
Presentation layer implementations are not typically associated with a particular protocol stack. Some well-known standards for video include QuickTime and
Motion Picture Experts Group (MPEG). QuickTime is an Apple Computer specification for video and audio, and MPEG is a standard for video compression and coding.
Among the well-known graphic image formats are Graphics Interchange Format (GIF), Joint Photographic Experts Group (JPEG), and Tagged Image File Format (TIFF).
GIF is a standard for compressing and coding graphic images. JPEG is another compression and coding standard for graphic images, and TIFF is a standard coding
format for graphic images.
1.3 Session layer
Coordinates dialogue/session/connection between devices over the network. This layer manages
communications between connected sessions. Examples of this layer are token management (the session layer manages who has the token) and network time
The session layer establishes, manages, and terminates communication sessions. Communication sessions consist of service requests and service responses that
occur between applications located in different network devices. These requests and responses are coordinated by protocols implemented at the session layer.
Some examples of session-layer implementations include Zone Information Protocol (ZIP), the AppleTalk protocol that coordinates the name binding process; and
Session Control Protocol (SCP), the Decent Phase IV session layer protocol.
1.4 Transport layer
Responsible for reliable transmission of data and service specification between hosts. The
major responsibility of this layer is data integrity--that data transmitted between hosts is reliable and timely. Upper layer data grams are broken down into
network-sized data grams if needed and then implemented using appropriate transmission control. The transport layer creates one or more than one network
connection, depending on conditions. This layer also handles what type of connection will be created. Two major transport protocols are the TCP
(Transmission Control Protocol) and the UDP (User Data gram Protocol).
Important features of Transport layer:
Transport layer ensures reliable service.
Breaks the message (from sessions layer) into smaller packets, assigns sequence number and sends them.
Reliable transport connections are built on top of X.25 or IP.
In case IP, lost packets arriving out of order must be reordered.
Important features of TCP/UDP:
TCP/IP Widely used for network/transport layer (UNIX).
TCP (Transport Control Protocol): This is a connection oriented protocol.
UDP (Universal Data gram Protocol): This is a connectionless transport layer protocol.
Application programs that do not need connection-oriented protocol generally use UDP.
1.5 Network layer
Responsible for the routing of data (packets) through the network; handles the addressing and
delivery of data. This layer provides for congestion control, accounting information for the network, routing, addressing, and several other functions. IP
(Internet Protocol) is a good example of a network layer protocol. Network layer does not deal with lost messages.
Important features of Network layer protocols:
Concerned with the transmission of packets.
Choose the best path to send a packet (routing).
The routing may be complex in a large network (e.g. Internet).
Routing packets through a network may be accomplished by using simple static routes or by using complex dynamic routing algorithms.
1.6 Data link layer
Provides for the reliable delivery of data across a physical network. This layer deals
with issues such as flow regulation, error detection and control, and frames. This layer has the important task of creating and managing what frames are sent
out on the network. The network data frame, or packet, is made up of checksum, source address, destination address, and the data itself. The largest packet size
that can be sent defines the maximum transmission Unit (MTU).
Important features of Data link layer:
Handles errors in the physical layer.
Groups bits into frames and ensures their correct delivery.
Adds some bits at the beginning and end of each frame plus the checksum.
Receiver verifies the checksum.
If the checksum is not correct, it asks for retransmission. (Send a control message).
Consists of two sub layers:
Logical Link Control (LLC) defines how data is transferred over the cable and provides data link service to the higher layers.
Medium Access Control (MAC) defines who can use the network when multiple computers are trying to access it simultaneously (i.e. Token passing, Ethernet
The data link layer provides reliable transit of data across a physical network link. Different data link layer specifications define different network and
protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control. Physical addressing
(as opposed to network addressing) defines how devices are addressed at the data link layer. Network topology consists of the data link layer specifications that
often define how devices are to be physically connected, such as in a bus or a ring topology. Error notification alerts upper-layer protocols that a transmission
error has occurred, and the sequencing of data frames reorders frames that are transmitted out of sequence. Finally, flow control moderates the transmission of
data so that the receiving device is not overwhelmed with more traffic than it can handle at one time.
The protocols used in Data link layer are SLIP, PPP, MTU, and CSLP.
1.7 Physical layer
Handles the bit-level electrical/light communication across the network channel. The physical
layer defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between
communicating network systems. Physical layer specifications define characteristics such as media, voltage levels, timing of voltage changes, physical data rates,
maximum transmission distances, and physical connectors.
Basically, this layer ensures that a bit sent on one side of the Network is received correctly on the other side.
Data travels from the application layer of the sender, down through the levels, across the nodes of the network service, and up through the levels of the receiver
To keep track of the transmission, each layer "wraps" the preceding layer's data and header with its own header. A small chunk of data will be
transmitted with multiple layer headers attached to it. On the receiving end, each layer strips off the header that corresponds to its respective level.
Physical layer is concerned with the following:
Physical interface characteristics like electrical, and mechanical specifications,
Number of bits of second to be transmitted,
Transmission type like duplex or half-duplex etc.
Frequently used Physical layer protocols:
Some of the important standards that deal with physical layer specifications are:
RS-232(for serial communication lines), X.21, EIA 232, and G730.
Physical layer and Data link layer implementations can be categorized as either LAN or WAN specifications.