N+ Osi Supplemental Osi Model

OSI REFERENCE MODEL Open Systems Interconnection A seven layer architecture that standardizes levels of service and types of interaction for computers exchanging information through a network. It is used to describe the flow of data between the physical connection to the network and the end-user application. This model initial development in 1977, by the ISO (International Standards Organization), redesigned and released for general use in 1984, is the best known and most widely used model for describing networking environments. It is largely based on the SNA, Systems Network Architecture, model developed by IBM in 1974, but could not be used because of prioritization by IBM.

General Overview Layer Number

Layer Name

Data Unit Name

7

Application

6

Presentation

5

Session

4

Transport

Segments

3

Network

Packets or Datagrams

Most Gateways, Routers and Brouters

Transport routes, message handling and transfer.

2

Data Link

Frames

Bridges PPP

Coding, addressing, and transmitting information.

1

Physical

Signals or Bits

Repeaters SLIP & PPP

Hardware connections.

Sub Layer

Messages

Services and Hardware Devices Protocol Support

Focus

Some Gateways

Program to Program transfer of information.

Redirector Service

Text formatting and display code conversion. Establishing, maintaining, and coordinating communication.

Media Access Control MAC Logical Link Control LLC

Accurate delivery, service quality.

OSI REFERENCE MODEL

Layer Descriptions Application: The top (seventh) layer of the OSI model. This layer serves as the window which application processes use to access the network services. This layer represents the services that directly support the user applications such as software for file transfers, database access, and e-mail. Basically it connects the user to the network. Some of the more popular application protocols that provide application-to-application interaction and data exchange include, FTAM, FTP, SMTP, Telnet and in the case of NetWare NCP, (NetWare Core Protocol), used to exchange information between NetWare servers and clients.

Presentation: The sixth layer of the OSI model. This layer determines the form used to exchange data between networked computers. At the sending computer, this layer translates data from a format sent down from the Application layer into a commonly recognized, intermediary format. At the receiving end, this layer translates the intermediary format into a format useful to that computers Application layer. Also manages network security issues by providing services such as data encryption/de-encryption. Provides rules for data transfer and provides data compression/de-compression to reduce the number of bits that need to be transmitted.

Session: The fifth layer of the OSI model. This layer allows two applications on different computers to establish, use or maintain, and end or terminate a connection called a session. This layer performs name recognition and the functions, such as security, needed to allow two applications to communicate over the network. This layer also implements dialog control between to nodes regulating which side transmits, when, for how long, and so on. Also this layer attempts to handle any errors generated by the upper layers.

Transport: The fourth layer of the OSI model. It provides the upper layers with a communication channel. This layer ensures that messages are delivered errorfree, in sequence, and with no losses or duplications, through the use of checksums. This layer repackages messages into segments, for their efficient transmission over the network. At the receiving end, the layer unpacks the segments, reassembles the original messages, and sends an acknowledgment, ACK, of receipt.

OSI REFERENCE MODEL Network: The third layer of the OSI model. This layer is responsible for addressing messages and translating logical addresses and names into physical addresses. Assembles/disassembles packets from/to segments for network transmissions. Determines the route from the source to the destination computer. It determines which path the data should take based on network conditions, priority of service, and other factors. Also manages traffic problems such as switching, routing, and controlling the congestion of data packets on the network.

Data Link: The second layer in the OSI model. This layer packages raw bits from the physical layer into data frames. Controls access to the communication channel, the flow of data, identifies specific computers on the network and detects and corrects transmission errors that occurred in the physical layer. This layer contains two Sub-Layers as defined by the IEEE 802 specification, (see IEEE Project 802). The Media Access Control and Logical Link Control sub-layers. The 802 specification was developed to specify the cable and topology characteristics of all wiring systems. MAC, the Media Access Control, sub-layer communicates directly with the network adapter card and is responsible for delivering error-free data between two computers on the network. AKA the MAC controls the means by which multiple devices share the media channel and provides addressing information for communications between network devices. LLC, the Logical Link Control, sub-layer is the upper sub-layer that manages data link communications and defines the use of logical interface points called Service Access Points, (Sap’s), that are used by computers to transfer information from the LLC sub-layer to the upper OSI layers. So it could be said that the LLC establishes and maintains logical links between communicating devices.

Physical: The first (bottom most) layer of the OSI model. This layer addresses the transmission of unstructured raw data bit streams over the physical medium, the networking cable. The physical relates the electrical/optical, mechanical, and functional interfaces to the cable. The physical layer also carries the signals that transmit data generated by all of the higher OSI layers.

IEEE Project 802 Institute of Electrical and Electronics Engineers, Inc. A networking model developed by the IEEE. Project 802, named for the year and month it began (February 1980), defines LAN standards for the Physical and Data Link layers of the OSI model. The 802 project divides the Data Link layer into two sub-layers: Media Access Control (MAC) and Logical Link Control (LLC). Project 802 specifically defines the functionality of ETHERNET NETWORKS, but today is used by non ethernet networks, for the Logical Link Control sub-layer in Standard 802.2 and defined functionality for the Media Access Control sub-layer and the Physical layer in Standards 802.3, 802.4, and 802.5. 802.3, defines standards for logical bus, (straight line), networks, such as Ethernet, that use a mechanism called carrier-sense multiple access with collision detection (CSMA/CD). This protocol regulates network traffic by allowing a broadcast only when the wire is clear and no other computer is broadcasting. 802.4, defines standards for a token-passing bus network, not the IBM Token Ring topology. This is a bus layout that uses a broadcast. Every computer receives all the data but only the ones addressed respond to the broadcast. A token that travels the wire determines which computer is able to broadcast. An example of a token passing network using this standard is Digital’s Arc-Net. 802.5, defines standards for token-passing ring networks. This is a logical ring network that transmits at either 4 Mpbs or 16 Mbps. Even though this is called a ring, it uses a hub and is configured as a star. A token traveling around the physical ring inside the hub determines which computer may send data.