1. 1 Lecture 31 The OSI Reference Model & Transmission Media Overview  Lecture Objectives.  A Brief Look into the OSI Seven-Layer Architecture.  Transmission Media & their Characteristics.  Preview: Data Communications (cont’d).

2. 2 Lecture 31 Lecture Objectives  To understand the basic functions of the different layers of the OSI reference model  To know the different data transmission media and the main characteristics

3. 3 Lecture 31 The OSI Reference Model

4. 4 Lecture 31 The OSI Reference Model We now highlight what happens in the layers OSI model, which allows dissimilar computers to transfer data between themselves: 7.Application Layer : Represents the level at which applications access network services. This layer represents the services that directly support applications such as software for file transfers, database access, and electronic mail. 6.Presentation Layer : Translates data from the Application layer into an intermediary format. This layer also manages security issues by providing services such as data encryption, and compresses data so that fewer bits need to be transferred on the network. 5.Session Layer :Allows two applications to establish, use and disconnect a connection between them called a session. This layer establishes dialog control between the two computers in a session, regulating which side transmits, plus when and how long it transmits. 4.Transport Layer : Ensures that data is delivered error free, in sequence and with no loss, duplications or corruption. This layer also repackages data by assembling long messages into lots of smaller messages for sending, and repackaging the smaller messages into the original larger message at the receiving end. 5.Network Layer: This is responsible for addressing messages and data so they are sent to the correct destination, and for translating logical addresses and names (like a machine name BAREED) into physical addresses. This layer is also responsible for finding a path through the network to the destination computer and manages traffic problems, such as switching, routing, and controlling the congestion of data packets. The only layer concerned with routing between networks.

5. 5 Lecture 31 The OSI Reference Model 2.Data-Link Layer This layer takes the data frames or messages from the Network Layer and provides for their actual transmission. At the receiving computer, this layer receives the incoming data and sends it to the network layer for handling. The Data-Link Layer also provides error-free delivery of data between the two computers by using the physical layer. It does this by packaging the data from the Network Layer into a frame that includes error detection information. At the receiving computer, the Data-Link Layer reads the incoming frame, and generates its own error detection information based on the received frame data. After receiving all of the frame, it then compares its error detection value with that of the incoming frames, and if they match, the frame has been received correctly. After sending a frame, the sender waits for an acknowledgment from the receiving computer. A frame looks like, 1. Physical Layer : Transmits bits from one computer to another and regulates the transmission of a stream of bits over a physical medium. This layer defines how the cable is attached to the network adapter and what transmission technique is used to send data over the cable. Always implemented in hardware.

6. 6 Lecture 31 Sending Data Via the OSI Model  In reality, data is passed from one layer down to the next lower layer at the sending computer, till its finally transmitted onto the network cable by the Physical Layer. As the data it passed down to a lower layer, it is encapsulated into a larger unit (in effect, each layer adds its own layer information to that which it receives from a higher layer). At the receiving end, the message is passed upwards to the desired layer, and as it passes upwards through each layer, the encapsulation information is stripped off.

7. 7 Lecture 31 James Bond Meets The 7 Layer OSI Model James Bond meets Number One on the 7th floor of the spy headquarters building. Number One gives Bond a secret message that must get through to the US Embassy across town. Bond proceeds to the 6th floor where the message is translated into an intermediary language, encrypted and miniaturized. Bond takes the elevator to the 5 th floor where Security checks the message to be sure it is all there and puts some checkpoints in the message so his counterpart at the US end can be sure he’s got the whole message. On the 4 th floor the message is analyzed to see if it can be combined with some other small messages that need to go to the US end. Also if the message was very large it might be broken into several small packages so other spies can take it and have it reassembled on the other end. The 3 rd floor personnel check the address on the message and determine who the addressee is and advising Bond of the fastest route to the Embassy. On the 2 nd floor the message is put into a special courier pouch(packet). It contains the message, the sender and destination ID. It also warns the recipient if other pieces are still coming. Bond proceeds to the 1 st floor where Q has prepared the Aston Martin for the trip to the Embassy. Bond departs for the US Embassy with the secret packet in hand. On the other end the process is reversed. Bond proceeds from floor to floor where the message is decoded. The US Ambassador is very grateful the message got through safely. "Bond, please tell Number One I’ll be glad to meet him for dinner tonight". Taken from http://www.pe.net/~rlewis/Resources/james.html

8. 8 Lecture 31 Transmission Media : Introduction  Encode data as energy and transmit energy  Decode energy at destination back into data  Energy can be electrical, light, radio, sound,...  Each form of energy has different properties and requirements for transmission  Transmitted energy is carried through some sort of medium  Transmitter encodes data as energy and transmits energy through medium  Media can be copper, glass, air,...

9. 9 Lecture 31 Transmission Media: Introduction (cont’d)  A transmission medium (or link) is the physical wiring part of a data communications system on which the data is transmitted.  There are different kinds of transmission media. Important points to consider when choosing a transmission media are:  Transmission Rate and Bandwidth  Distance and Attenuation  Security: Interference Characteristics  Cost and Ease of Installation  The popular Transmission media are:  1. Twisted pair wire  2. Coaxial cable  3. Fiber optic  4. Satellite microwave  5. Terrestrial microwave  6. Radio

10. 10 Lecture 31 Transmission Media and their Characteristics  1. Twisted pair wire: Twisted pair is made of two copper wires which are covered by plastics or materials that are similar. The two insulated wires are twisted together to form the communication line. The purpose of twisting the wires is to avoid electrical interference. Twisted pair should be familiar to us, since it is widely used in our telephone communication either in the home or the office. Twisted pair can be combined into a cable that may consist of two, four, fifty, or hundreds of pairs. Twisted pair is the basis of transmission media. It supports most of the local telephone network.

11. 11 Lecture 31 Transmission Media and their Characteristics (cont’d)  2. Coaxial cable: Coaxial cable contains two conductors, which are insulated from one another for improved performance. The inner conductor is covered by the dielectric. The outer conductor is covered by a shield. Widely used in cable TV to connect the home viewers to the cable companies. Local network, short-run computer system links, and long- distance telephone transmission are also the important applications of coaxial cable.

12. 12 Lecture 31 Transmission Media and their Characteristics (cont’d)  3. Fiber Optic: The optical fiber contains three parts. The shape of optical fiber is like a cylinder. The inner most part is the core, which contains one or more thin fibers that are made of plastics or glasses. Each of the fibers in the core is covered by the cladding. The cladding is a glass or plastic coating which has different optical properties from the core. The outer most part of the optical fiber is made of plastic or other materials, which is called jacket. The purpose of the jacket is to prevent the destruction from the environment. There are a light-emitting diode ( light source ) in one side of the fiber and a photodiode ( light- sensitive receptor ) in the other side of the fiber. The fiber optics equipment transmits the light pulses from one side to the other through the plastic or glass. Because of the better performance and the greater data rate of optical fiber, the application of optical fiber has increased dramatically in long-distance communication and military application

13. 13 Lecture 31 Transmission Media and their Characteristics (cont’d)  4. Terrestrial microwave: A pair of microwave antennas which are equipped on the top of towers will transmit the signals through the air. The shape of a microwave antenna is similar to a dish and is about ten feet wide. The microwave antennas are usually equipped in higher place in order to achieve better transmission effect. Terrestrial microwave is used as an alternative to coaxial cable and optical fiber, since terrestrial microwave transmission needs less repeaters than that of coaxial cable. Voice transmission, television transmission, and the short point-to-point links between building are the major application of microwave transmission.

14. 14 Lecture 31 Transmission Media and their Characteristics (cont’d)  5. Satellite Microwave: A satellite consists of several transponders (frequency bands) which receive signals from one frequency band. This action is called uplink. After this action, the satellite will amplify or repeat the signals to the other frequency band. This action is called downlink. A satellite link is like a microwave link. Satellite will link two or more ground stations, like point-to-point link or broadcast link. The most important applications for satellite communication will be long-distance telephone transmission, private business networks, and television distribution.

15. 15 Lecture 31 Transmission Media and their Characteristics (cont’d)  6. Broadcast Radio: The major difference between microwave and broadcast radio transmission is that the broadcast radio can have many directions, and microwave is only point to point transmission. In other words, the broadcast radio can transmit data to many destinations at the same time. The broadcast radio covers the range of frequency from 3 to 300 Hz. From 30MHz to 1GHz, this range is most effective for broadcast radio to transmit data.  Some important points about transmission media:  Twisted cable is inexpensive, well understood, easy to install BUT is sensitive to noise, covers short distances, has limited capacity and can easily be tapped.  Coaxial cable has high bandwidth, covers long distances, is immune to noise BUT like twisted cable, can be easily tapped  Coaxial cable has higher data rate and bandwidth than that of twisted pair.  The repeater spacing of twisted pair and coaxial cable is similar.  Fiber optics has superior performance compared with twisted pair and coaxial cable.  Fiber optic is secure, has no electrical interference, covers long distances and has large capacity BUT it is costly, difficult to install and repair