Transcript Document
CIT 513: Introduction to Computer Communications Mohammed A. Saleh http://ifm.ac.tz/staff/msaleh/CIT513.html 1 The OSI Reference Models What is the OSI reference model? In the 1970 the International Standards Organization (ISO) developed the Open Systems Interconnection (OSI) reference model to define the basic standards for network communication It sets up the rules that all computers have to use no matter what protocol they are using The OSI model is made up seven (7) layers. 2 Cont … •Application •Presentation •Session •Transport •Network •Data Link •Physical •All •Away •People •Pizza •Seem •Sausage •To •Throw •Need •Not •Data •Do •Processing •Please Easy way to remember the 7 layers of the OSI model 3 Cheat Sheet 4 Cont … This is what I would like to call the cheat sheet of the OSI model. The sheet gives out what the OSI model is comprised of, not everything but covers what you need to know at this level The sheet clearly shows that there are two OSI models, one on the sending computer and the other on the receiving computer From the sending computer the arrows are pointing downwards whereas on the receiving side the arrows are pointing upwards 5 Cont … The reason for the upward and downward direction is to show how data travels through the OSI model This is how data travels through logically. Even though data travels down travels in the upward and downward directions, the dashed lines represents the virtual communication that takes place. Virtual communication means once data travels across the network, the receiving physical layer only acknowledges what the sending physical layer did to the data and then passes it up to the data link layer. 6 Cont … As data travels down the layers, each layers adds its own piece of information to the data Each layer adds a slice to one side of the data, except data link layer that also adds information to the opposite on the other side. Whatever is added to the data by the layers can also be said that the layers add header information to the data (a) (b) (a) Original data from the application layer moving down the other layers 7 (b) Data after having added header information Cont … the additional data added by the data link layer is known as trailer information The reason is to make sure the data sent is not damaged by the air Once the data arrives on the receiving side, it is stripped off as it moves up the layers, leaving only the original sent data Video on the OSI Layers 8 About the OSI Model The OSI model divides network architecture into seven layers. The term layer refer to a collection of similar functions that provide service to the layer above it and requests service from the layer below it. Each layer is responsible for a very specific task. The function of one layer is not known by other layers, which makes the OSI model a suitable model to be used by network communications. 9 Functions of the OSI Layers Physical Layer It identifies the physical characteristics of the network, including the following specifications: Hardware: The type of media used on the network such as type of cable, type of connector, and pin out format for cables. Topology: The physical layer identifies the topology to be used in the network. Common topologies include ring, mesh, star, and bus. Data-Link Layer Is responsible for getting data to the physical layer so that it can be transmitted over the network Also responsible for error detection, error correction, and hardware addressing 10 Cont … The term frame is used to describe the logical grouping of data at the data-link layer. The data-link layer has two distinct sub-layers the Media Access Control (MAC) sub-layer and the Logical Link Control (LLC) sub-layer. MAC layer: The MAC address is defined at this layer. The MAC address is the physical or hardware address burned into each NIC. LLC layer The LLC layer is responsible for the error and flowcontrol mechanisms of the data-link layer 11 Cont … Network Layer The primary responsibility of the network layer is routing providing mechanisms by which data can be passed from one network system to another It does not specify how the data is passed, but rather provides the mechanisms to do so. Functionality at the network layer is provided through protocols Protocols at the network layer are also responsible for route selection, which refers to determining the best path for the data to take throughout the network 12 Cont … In contrast to the data-link layer, which uses MAC addresses to communicate on the LAN, network protocols use software configured addresses and special routing protocols to communicate on the network. The main protocol operating at this layer is the Internet Protocol (IP) The term packet is used to describe the logical grouping of data at the network layer. Transport Layer It provides mechanisms to transport data between network devices Primarily it does this in three ways: 13 Cont … Error checking Protocols at the transport layer ensure that data is sent or received correctly. Service addressing Protocols such as TCP support many network services. The transport layer makes sure that data is passed to the right service at the upper layers of the OSI model. Segmentation: To traverse the network, blocks of data need to be broken down into packets that are of a manageable size for the lower layers to handle Session Layer Is responsible for managing and controlling the synchronization of data between applications on two devices It does this by establishing, maintaining, and breaking 14 sessions. Cont … Presentation Layer Its basic function is to convert the data intended for or received from the application layer into another format Transforms data into the form that only the application layer can accept Some common data formats handled by the presentation layer include the following: Graphics files JPEG, TIFF, GIF. Text and data: translate data into different formats such as American Standard Code for Information Interchange (ASCII) and the Extended Binary Coded Decimal Interchange Code (EBCDIC). Sound/video MPEGs, QuickTime video, and MIDI files 15 Cont … A very important function of the presentation layer is encryption, which is the scrambling of data so that it can't be read by anyone other than the intended recipient. Application Layer The function of the application layer is to take requests and data from the users and pass them to the lower layers of the OSI model Incoming information is passed to the application layer, which then displays the information to the users The most common misconception about the application layer is that it represents applications that are used on a system such as a Web browser, word processor, or a 16 spreadsheet Cont … Instead, the application layer defines the processes that enable applications to use network services, like web, email, file transfer, terminal emulation programs Each of the services has is represented by a corresponding protocol; Service Protocol Web Hyper Text Transfer Protocol (HTTP) Email Simple Mail Transfer Protocol (SMTP) File Transfer File Transfer Protocol (FTP) T E programs telnet 17 Questions