OSI Network

OSI network model in full stands for Open System Interconnection Reference Model. This is a theoretical depiction of the computer network protocols and layered communications. OSI network models divide network architecture to seven layers namely the presentation, application, transport, physical layers, data-link, session and network layer; thus the name OSI Seven Layer Model.

The OSI speciﬁcations were conceived and implemented by two international standards organizations: the International Organization for Standardization (ISO) and the International Telecommunication Union-Telecommunication Standardization Sector (ITU-T). This chapter provides a summary of the OSI protocol suite and illustrates itsmapping to the general OSI reference model.

Open Systems Interconnection Reference Model:


Layer No.	Layer Name	Description
7	Application	Consists of standard communication services and applications that everyone can use.
6	Presentation	Ensures that information is delivered to the receiving system in a form that the system can understand.
5	Session	Manages the connections and terminations between cooperating systems.
4	Transport	Manages the transfer of data. Also assures that the received data are identical to the transmitted data.
3	Network	Manages data addressing and delivery between networks.
2	Data link	Handles the transfer of data across the network media.
1	Physical	Defines the characteristics of the network hardware.

Layer one which is also called the physical layer, describes physical and electrical device designs. It particularly describes the association between physical mediums and devices. All these will consist of layouts of cable specifications, pins, repeaters, voltages, hubs, host bus adapters and network adapters. The data link layer, also known as layer II, will offer both technical and serviceable means of transferring data connecting network entities. It will similarly identify and probably correct mistakes that might take place in layer one.

The third network layer offers the technical and serviceable ways through which changeable data length sequences could be moved from sources to destinations using a single or numerous networks. While still doing that, it will maintain quality services demanded by layer IV. Layer III carries out network routing roles and may additionally carry out reassembly, fragmentation and report errors associated with delivery. This layer also supports operations of routers and sends data all the way through the extended networks, improving internet speed.

Layer 4 is the transport layer that provides transparent data transfers amongst end users, offering dependable services in data transfer to the top layers. Layer IV has assigned link reliability using error control, desegmentation/segmentation and flow control. A number of procedures are connection and state oriented. This implies that layer IV can manage to track all the segments and then transmit again the ones that fail. The fifth layer that is known as Session layer manages dialogues between computers. It sets up, controls and gives an end to the connections that exist between remote and local applications. The layer offers for simplex, half-duplex or full-duplex. It also institutes rebooting procedures, check-pointing and termination.

The sixth layer identifies the presentation layer that institutes context among later application entities where higher layer entities could use dissimilar semantics and syntax. However, provided presentation services should be able to understand all of them and their mappings as well. Units for the presentation service data will then be summed up to units in form of session protocol data then moved to the stack. The last, seventh application layer is the Open System Interconnection Reference Model that is the closest to end users. This means that all OSI application layers and users will directly interrelate with these software applications. The application layer works together with the software applications, which in turn implement a communicating constituent. These types of programs for application fall in the outer region of the OSI model scope.

OSI Protocols Application Processes

An application process is the element of an application that provides the interface between the application itself and the OSI application layer. Some of the standard OSI application processes include the following:

Common Management-Information Protocol (CMIP)—Performs network management functions, allowing the exchange of management information between ESs and management stations. CMIP is speciﬁed in the ITU-T X.700 recommendation and is functionally similar to
the Simple Network-Management Protocol (SNMP) and NetView.
Directory Services (DS)—Serves as a distributed directory that is used for node identiﬁcation and addressing in OSI internetworks. DS is speciﬁed in the ITU-T X.500 recommendation.
File Transfer, Access, and Management (FTAM)—Provides ﬁle-transfer service and distributed ﬁle-access facilities.
Message Handling System (MHS)—Provides a transport mechanism for electronic messaging applications and other applications by using store-and-forward services.
Virtual Terminal Protocol (VTP)—Provides terminal emulation that allows a computer system to appear to a remote ES as if it were a directly attached terminal.