Osi Model

UNDERSTANDING THE OSI MODEL AND THE RELATIONSHIP WITH TCP/IP

Table Of Contents
Letter of Transmittal
Abstract
Table of Contents
Written Presentation
References
Abstract
The Open Systems Interconnection (OSI) model is a reference tool for understanding data communications between any two networked systems. It divides the communications processes into seven layers. Each layer both performs specific functions to support the layers above it and offers services to the layers below it. The three lowest layers focus on passing traffic through the network to an end system. The top four layers come into play in the end system to complete the process.
This presentation will provide you with an understanding of each of the seven layers, including their functions and their relationships to each other. This will provide you with an overview of the network process, which can then act as a framework for understanding the details of computer networking. Also this paper will explain how the 802 specifications expanded the OSI reference model by dividing the data link layer into two layers.
Finally, this paper will draw comparisons between the theoretical OSI model and the functional
TCP/IP model. Although TCP/IP has been used for network communications before the adoption of the
OSI model, it supports the same functions and features in a differently layered arrangement.
The history of the development of the OSI model is, for some reason, a little-known story. Much of the work on the design of OSI was actually done by a group at Honeywell Information Systems, headed by Mike Canepa, with Charlie Bachman as the principal technical member. This group was chartered, within Honeywell, with advanced product planning and with the design and development of prototype systems. In the early and middle '70s, the interest of Canepa 's group was primarily on database design and then on distributed database design. By the mid-70s, it become clear that to support database machines, distributed access, and the like, a structured distributed communications architecture would be required. The group studied some of the existing solutions, including IBM 's system network architecture (SNA), the work on protocols being done for ARPANET, and some of the concepts of presentation services being developed for standardized database systems. The result of this effort was the development by 1977 of a seven-layer architecture known internally as the distributed systems architecture (DSA).
Meanwhile, in 1977 the British Standards Institute proposed to the International Organization for
Standardization (ISO) that a standard architecture was needed to define the communications infrastructure for distributed processing. As a result of this proposal, ISO formed a subcommittee on
Open Systems Interconnection (Technical Committee 97, Subcommittee 16). The American National
Standards Institute (ANSI) was charged to develop proposals in advance of the first formal meeting of the subcommittee.
Bachman and Canepa participated in these early ANSI meetings and presented their seven-layer model. This model was chosen as the only proposal to be submitted to the ISO subcommittee. When the ISO group met in Washington, DC in March of 1978, the Honeywell team presented their solution.
A consensus was reached at that meeting that this layered architecture would satisfy most requirements of Open Systems Interconnection, and had the capacity of being expanded later to meet new requirements. A provisional version of the model was published in March of 1978. The next version, with some minor refinements, was published in June of 1979 and eventually standardized in 1984. The resulting OSI model is essentially the same as the DSA model developed in 1977.
OSI Model Physical Layer
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 voltage levels, timing of voltage changes, physical data rates, maximum transmission distances, and physical connectors. Physical layer implementations can be categorized as either LAN or WAN specifications.
OSI Model Data Link Layer
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 Institute of Electrical and Electronics Engineers (IEEE) has subdivided the data link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC). The Logical Link
Control sublayer of the data link layer manages communications between devices over a single link of a network. LLC is defined in the IEEE 802.2 specification and supports both connectionless and connection-oriented services used by higher-layer protocols. IEEE 802.2 defines a number of fields in data link layer frames that enable multiple higher-layer protocols to share a single physical data link.
The Media Access Control sublayer of the data link layer manages protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses, which enable multiple devices to uniquely identify one another at the data link layer.
OSI Model Network Layer
The network layer defines the network address, which differs from the MAC address. Some network layer implementations, such as the Internet Protocol (IP), define network addresses in a way that route selection can be determined systematically by comparing the source network address with the destination network address and applying the subnet mask. Because this layer defines the logical network layout, routers can use this layer to determine how to forward packets. Because of this, much of the design and configuration work for internetworks happens at Layer 3, the network layer.
OSI Model Transport Layer
The transport layer accepts data from the session layer and segments the data for transport across the network. Generally, the transport layer is responsible for making sure that the data is delivered errorfree and in the proper sequence. Flow control generally occurs at the transport layer. Flow control manages data transmission between devices so that the transmitting device does not send more data than the receiving device can process. Multiplexing enables data from several applications to be transmitted onto a single physical link. Virtual circuits are established, maintained, and terminated by the transport layer. Error checking involves creating various mechanisms for detecting transmission errors, while error recovery involves acting, such as requesting that data be retransmitted, to resolve any errors that occur. The transport protocols used on the Internet are TCP and UDP.
OSI Model Session Layer
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 DECnet Phase IV session layer protocol.
OSI Model Presentation Layer
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. Conversion schemes are used to exchange information with systems by using different text and data representations, such as EBCDIC and ASCII. Standard data compression schemes enable data that is compressed at the source device to be properly decompressed at the destination. Standard data encryption schemes enable data encrypted at the source device to be properly 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.
OSI Model Application Layer
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 layer. Some examples of application layer implementations include Telnet, File Transfer Protocol (FTP), and Simple Mail Transfer Protocol
(SMTP).
The TPC/IP Model
The TCP/IP model does not exactly match the OSI model. There is no universal agreement regarding how to describe TCP/IP with a layered model but it generally agreed that there are fewer levels that the seven layers of the OSI model. Most descriptions present from three to five layers. In this technical reference document the layers of the TCP/IP model are defined as follows:
Application Layer
This layer is broadly equivalent to the application, presentation and session layers of the OSI model.
It gives an application access to the communication environment. Examples of protocols found at this layer are Telnet, FTP (File Tranfer Protocol), SNMP (Simple Network Management Protocol), HTTP
(Hyper Text Transfer Protocol) and SMTP (Simple Mail Transfer Protocol).
Transport Layer
The transport layer is similar to the OSI transport model, but with elements of the OSI session layer functionality. This layer provides an application layer delivery service. The two protocols found at the transport layer are TCP (Transmission Control Protocol) and UDP (User Datagram Protocol). Either of these two protocols are used by the application layer process, the choice depends on the application 's transmission reliability requirements.
Internet Layer
This layer is responsible for the routing and delivery of data across networks. It allows communication across networks of the same and different types and carries out translations to deal with dissimilar data addressing schemes. IP (Internet Protocol) and ARP (Address Resolution Protocol) are both to be found at the Internet layer.
Network Access
The combination of datalink and physical layers deals with pure hardware (wires, satellite links, etc.) and access methods such as CSMA/CD (carrier sensed multiple access with collision detection).
Ethernet exists at the network access layer - its hardware operates at the physical layer and its medium access control method (CSMA/CD) operates at the datalink layer.
In conclusion, the understanding of the OSI model and the relationship it has with TCP/IP is an important task to learn if a person wants to get a full understanding of how computer systems communicate with one another in the world wide web or in a corporate setting.
Personally, I have found that the OSI model relates to just about everything that I have done as an
IT consultant. During computer migrations and configuring desktops to be networked on the corporate land, enables the workstations to communicate via the OSI model and the TCP/IP model. Having to map network drives enables users to have extra disk space other than just their hard drive to store data.
When a user retrieves data from a network drive, the total process is through the seven layers of the
OSI model. Configuring email accounts enables users to communicate via email transactions, a process that uses the OSI model and the TCP/IP model.
References
1. Network Plus Guide to Networks (2002)
2. Ethernet Tutorial (2001)
3. Microsoft 's guide to the OSI model (2004)

References: 1. Network Plus Guide to Networks (2002) 2. Ethernet Tutorial (2001) 3. Microsoft 's guide to the OSI model (2004)