-.pdf

Data Communications and Networking Networking-C403 ng C403 ng ng- Dr....

Data Communications and Networking Networking-C403 ng C403 ng ng- Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi Chapter One: Introduction Data communications are the exchange of data between two devices via some form of transmission medium such as a wire cable. For data communications to occur, the communicating devices must be part of a communication system made up of a combination of hardware (physical equipment) and software (programs). The effectiveness of a data communications system depends on four fundamental characteristics: · Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user. · Accuracy: The system must deliver the data accurately. Data that have been altered in transmission and left uncorrected are unusable. · Timeliness: The system must deliver data in a timely manner. Data delivered late are useless. In the case of video and audio. · Jitter: Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. A data communications system has five components (see Figure (1.1)). Fig. (1.1): Five components of data communication system. 1. Message. The message is the information (data) to be communicated. Popular forms of information include text, numbers, pictures, audio, and video. 2. Sender. The sender is the device that sends the data message. It can be a computer, workstation, telephone handset, video camera, and so on. 3. Receiver. The receiver is the device that receives the message. It can be a computer, workstation, telephone handset, television, and so on. 4. Transmission medium. The transmission medium is the physical path by which a message travels from sender to receiver. Some examples of transmission media include twisted-pair wire, coaxial cable, fiber-optic cable, and radio waves. 1 Data Communications and Networking Networking-C403 ng-C403 ng Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi 5. Protocol. A protocol is a set of rules that govern data communications. Data Flow Communication between two devices can be simplex, half-duplex, or full-duplex as shown in Figure (1.2). Fig. (1.2): Data flow (simplex, half-duplex and full-duplex). Simplex In simplex mode, the communication is unidirectional, as on a one-way street. Only one of the two devices on a link can transmit; the other can only receive. Half-Duplex In half-duplex mode, each station can both transmit and receive, but not at the same time. Full-Duplex In full-duplex (called duplex), both stations can transmit and receive simultaneously. 2 Data Communications and Networking- Networking-C403 ng C403 Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi NETWORKS A network is a set of devices (often referred to as nodes) connected by communication links. A node can be a computer, printer, or any other device capable of sending and/or receiving data generated by other nodes on the network. Network Criteria: · Performance o Depends on Network Elements. o Measured in terms of Delay and Throughput. · Reliability o Failure rate of network components. o Measured in terms of availability/robustness. · Security o Data protection against corruption/loss of data due to: · Errors. · Malicious users. Networks Advantages: · File Sharing: The major advantage of a computer network is that is allows file sharing and remote file access. · Resource Sharing: Files, modems, printers… · Increased Storage Capacity: As there is more than one computer on a network which can easily share files, the issue of storage capacity gets resolved to a great extent. · Increased Cost Efficiency: There are many software available in the market which are costly and take time for installation. Computer networks resolve this issue as the software can be stored or installed on a system or a server and can be used by the different workstations. Networks Disadvantages: · Security Issues: Physical access, computer hacker can get unauthorized access by using different tools. · Rapid Spread of Computer Viruses: If any computer system in a network gets affected by computer virus, there is a possible threat of other systems getting affected too. 3 Data Communications and Networking- Networking-C403 ng C403 Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi · Expensive Set Up: The initial set up cost of a computer network can be high depending on the number of computers to be connected. Costly devices like routers, switches, hubs, etc., can add up to the bills of a person trying to install a computer network. He will also have to buy NICs (Network Interface Cards) for each of the workstations, in case they are not inbuilt. · Dependency on the Main File Server: In case the main File Server of a computer network breaks down, the system becomes useless. In case of big networks, the File Server should be a powerful computer, which often makes it expensive. Physical Structures · Type of Connection o Point-to-Point A point-to-point connection provides a dedicated link between two devices. o Multipoint A multipoint (also called multidrop) connection is one in which more than two specific devices share a single link (see Figure (1.3)). In a multipoint environment, the capacity of the channel is shared, either spatially or temporally. If several devices can use the link simultaneously, it is a spatially shared connection. If users must take turns, it is a timeshared connection. Fig. (1.3): Types of connections: (Point-to-Point and Multipoint). 4 Data Communications and Networking- Networking-C403 ng C403 Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi Physical topology Fig. (1.4): Categories of topology. Mesh topology Every device has a dedicated point-to-point link to every other device. The term dedicated means that the link carries traffic only between the two devices it connects. To find the number of physical links in a fully connected mesh network with n nodes, we first consider that each node must be connected to every other node. Node 1 must be connected to n - I nodes, node 2 must be connected to n – 1 nodes, and finally node n must be connected to n - 1 nodes. We need n(n - 1) physical links. However, if each physical link allows communication in both directions (duplex mode), we can divide the number of links by 2. In other words, we can say that in a mesh topology, we need n(n -1) /2. Duplex-mode links: To accommodate that many links, every device on the network must have n – 1 input/output (VO) ports (see Figure (1.5)) to be connected to the other n - 1 stations. Fig. (1.5): A fully connected mesh topology (five devices) 5 Data Communications and Networking- Networking-C403 ng C403 Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi The advantages of mesh topology: 1. Use of dedicated links guarantees that each connection can carry its own data load, thus eliminating the traffic problems that can occur when links must be shared by multiple devices. 2. a mesh topology is robust 3. the advantage of privacy or security 4. Point-to-point links make fault identification and fault isolation easy. The disadvantages of this topology related to the amount of cabling and the number of I/O ports required: 1. Every device must be connected to every other device, installation and reconnection are difficult. 2. The sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can accommodate. 3. The hardware required to connect each link (I/O ports and cable) can be prohibitively expensive. One practical example of a mesh topology is the connection of telephone regional offices in which each regional office needs to be connected to every other regional office Star Topology In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub. The devices are not directly linked to one another. If one device wants to send data to another, it sends the data to the controller, which then relays the data to the other connected device (see Figure (1.6)). Fig. (1.6): A star topology connecting four stations 6  Data Communications and Networking- Networking-C403 ng C403 Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi The advantages of this topology: 1. A star topology is less expensive than a mesh topology 2. It easy to install and reconfigure 3. Include robustness. If one link fails, only that link is affected. All other links remain active. 4. Easy fault identification and fault isolation. The disadvantages of this topology 1. One big disadvantage of a star topology is the dependency of the whole topology on one single point, the hub. If the hub goes down, the whole system is dead. 2. Each node must be linked to a central hub. For this reason, often more cabling is required in a star than in some other topologies (such as ring or bus). The star topology is used in local-area networks (LANs). Bus Topology The preceding examples all describe point-to-point connections. A bus topology, on the other hand, is multipoint. One long cable acts as a backbone to link all the devices in a network (see Figure (1.7)). Nodes are connected to the bus cable by drop lines and taps. Fig. (1.7): A bus topology connecting three stations. The advantages of this topology: 1. Ease of installation. Backbone cable can be laid along the most efficient path, then connected to the nodes by drop lines of various lengths. 2. In this way, a bus uses less cabling than mesh or star topologies. The disadvantages of this topology: 1. Include difficult reconnection and fault isolation. 2. Direction of origin, creating noise in both directions. 3. Signal reflection at the taps can cause degradation in quality. This degradation can be controlled by limiting the number and spacing of devices connected to a given length of cable 7  Data Communications and Networking- Networking-C403 ng C403 Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi 4. Fault or break in the bus cable stops all transmission, even between devices on the same side of the problem. The damaged area reflects signals back in the direction of origin, creating noise in both directions Bus topology was the one of the first topologies used in the design of early local area networks. Ring Topology In a ring topology, each device has a dedicated point-to-point connection with only the two devices on either side of it. A signal is passed along the ring in one direction, from device to device, until it reaches its destination. Each device in the ring incorporates a repeater. When a device receives a signal intended for another device, its repeater regenerates the bits and passes them along (see Figure (1.8)).. Fig. (1.8): A ring topology connecting six stations. The advantages of this topology: 1. Easy to install and reconfigure. 2. To add or delete a device requires changing only two connections. 3. Fault isolation is simplified. The disadvantages of this topology 1. Unidirectional traffic can be a disadvantage in a simple ring, a break in the ring (such as a disabled station) can disable the entire network. Hybrid Topology A network can be hybrid. For example, we can have a main star topology with each branch connecting several stations in a bus topology as shown in Figure (1.9). 8 Data Communications and Networking- Networking-C403 ng C403 Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi Fig. (1.9): A hybrid topology: a star backbone with three bus networks. Network classification 1-Local Area Networks (LANs) A local area network (LAN) is usually privately owned and links the devices in a single office, building, or campus (see Figure (1.10)). Depending on the needs of an organization and the type of technology used, a LAN can be as simple as two PCs and a printer in someone's home office; or it can extend throughout a company and include audio and video peripherals. Currently, LAN size is limited to a few kilometers. In addition to size, LANs are distinguished from other types of networks by their transmission media and topology. In general, a given LAN will use only one type of transmission medium. The most common LAN topologies are bus, ring, and star. Early LANs had data rates in the 4 to 16 megabits per second (Mbps) range. Today, however, speeds are normally 100 or 1000 Mbps. Fig. (1.10): An isolated LAN connecting 12 computers to a hub in a closet 9 Data Communications and Networking- Networking-C403 ng C403 Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi 2-Wide Area Networks (WANs) A wide area network (WAN) provides long-distance transmission of data, image, audio, and video information over large geographic areas that may comprise a country, a continent, or even the whole world. A WAN can be as complex as the backbones that connect the Internet or as simple as a dial-up line that connects a home computer to the Internet. We normally refer to the first as a switched WAN and to the second as a point-to-point WAN (Figure (1.11)).The switched WAN connects the end systems, which usually comprise a router (internetworking connecting device) that connects to another LAN or WAN. The point-to-point WAN is normally a line leased from a telephone or cable TV provider that connects a home computer or a small LAN to an Internet service provider (lSP). This type of WAN is often used to provide Internet access. Fig. (1.11): WANs: a switched WAN and a point-to-point WAN An early example of a switched WAN is X.25, a network designed to provide connectivity between end users. A good example of a switched WAN is the asynchronous transfer mode (ATM) network, which is a network with fixed-size data unit packets called cells. 3-Metropolitan Area Networks (MANs) A metropolitan area network (MAN) is a network with a size between a LAN and a WAN. It normally covers the area inside a town or a city. It is designed for customers who need a high- speed connectivity, normally to the Internet, and have endpoints spread over a city or part of city. A good example of a MAN is the part of the telephone company network that can provide a high-speed DSL line to the customer. Another example is the cable TV network that originally 10 Data Communications and Networking Networking-C403 ng-C403 ng Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi was designed for cable TV, but today can also be used for high-speed data connection to the Internet. Interconnection of Networks: Internetwork Today, it is very rare to see a LAN, a MAN, or a LAN in isolation; they are connected to one another. When two or more networks are connected, they become an internetwork, or internet as in Figure (1.12). Fig. (1.12): A heterogeneous network made of four WANs and two LANs. 11 Data Communications and Networking- Networking-C403 ng C403 Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi THE INTERNET The Internet has revolutionized many aspects of our daily lives. It has affected the way we do business as well as the way we spend our leisure time. The Internet is a communication system that has brought a wealth of information to our fingertips and organized it for our use. Fig. (1.13): Hierarchical organization of the Internet. There are international service providers, national service providers, regional service providers, and local service providers. The Internet today is run by private companies, not the government. PROTOCOLS AND STANDARDS A protocol is a set of rules that govern data communications. A protocol defines what is communicated, how it is communicated, and when it is communicated. The key elements of a protocol are: · Syntax o Structure or format of the data o Indicates how to read the bits - field delineation · Semantics o Interprets the meaning of the bits 12 Data Communications and Networking Networking-C403 ng-C403 ng Dr. Dr.Haider HaiderM. M.Al-Mashhadi Al- Al-Mashhadi o Knows which fields define what action · Timing o When data should be sent and what o Speed at which data should be sent or speed at which it is being received. Communication Standards The rules that are established to ensure compatibility among similar communications products and services. Communication standards specify how a particular communication product, service, or interface will operate. · The way that a standard is developed is through consensus of the members of the standards organization. Standards Organization Main telecommunication focus Internet web address International: International Telephone And Data http://www.itu.ch Telecommunication Union- Communication Telecommunication http://www.iso.ch Standardization Section (ITU-T) Communications Standards Of All International Organization for Types (Coordinate With The ITU-T) Standardization (ISO) http://ietf.org Sets Standards For How The Internet Engineering Task Force Internet will operate (IETF) United States: American National Standards data communication in general http://www.ansi.org Institute (ANSI) Interfaces, connections, media. Electronic Industries Association http://eia.org facsimile (EIA) http://www.ieee.org 802 LAN standards Institute of Electrical and http://nist.gov Electronics Engineers (IEEE) Standards of all types National Institute of Standards and Technology (NIST) http://neca.org North American WAN standards National Exchange Carriers Association (NECA) 13  (Network l) Chapter Two OSI Model Third stage  THE OSI MODEL Established in 1947, the International Standards Organization (ISO) is a multinational body dedicated to worldwide agreement on international standards. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection (OSI) model. It was first introduced in the late 1970s. ISO is the organization. OSI is the model. 1  Figure 1 Seven layers of the OSI model 2  Figure 2 The interaction between layers in the OSI model 3  Figure 3 An exchange using the OSI model 4  7 Application OSI REFERENCE MODEL 6 Presentation 1. Physical Layer a) Convert the logical 1’s and 0’s coming from layer 2 into electrical signals. Data encoding 5 Session (bits to waves b) Transmission of the electrical signals over a 4 Transport communication channel. Electrical properties Cabling 3 Network C) Interconnect methods (topology / devices) Main topics: 2 Data Link Transmission mediums Encoding Modulation 1 Physical RS232 and RS422 standards Repeaters Hubs (multi-port repeater) 5  Layer 1 : Physical layer The physical layer is responsible for movements of individual bits from one hop (node) to the next. Protocol : ARP , Fiber , USB Devices: HUB , Repeater Data form : Bits 6 7 Application OSI REFERENCE MODEL 6 2. Data Link Layer Presentation a) Error control to compensate for the imperfections of the physical layer. 5 Session b) Flow control to keep a fast sender from swamping a slow receiver. 4 Transport Main topics: 3 Network Framing methods Error detection and correction methods Flow control 2 Data Link Frame format IEEE LAN standards 1 Physical Bridges Switches (multi-port bridges) 7 Layer 2 : Data link layer The data link layer is responsible for moving frames from one hop (node) to the next. Protocol : ISDN , ATM ,Ethernet , PPP Devices: Switch , Bridge Data form : Frames 8  Figure 4 Hop-to-hop delivery 9  7 Application OSI REFERENCE MODEL 6 3. Network Layer Presentation a) Controls the operation of the subnet. 5 Session b) Routing packets from source to destination. c) Logical addressing. 4 Transport Main topics: Internetworking 3 Network Routing algorithms Internet Protocol (IP) addressing 2 Data Link Routers 1 Physical 10  Layer 3 : Network layer The network layer is responsible for the delivery of individual packets from the source host to the destination host. Protocol : IP , ARP , ICMP Devices: Router Data form : Packet 11  Figure 5 Source-to-destination delivery 12  7 Application OSI REFERENCE MODEL 6 Presentation 4. Transport Layer a) Provides additional Quality of Service. 5 Session b) Heart of the OSI model. Main topics: 4 Transport Connection-oriented and connectionless services 3 Network Transmission Control Protocol (TCP) User Datagram Protocol (UDP) 2 Data Link ACK 1 Physical 13  Layer 4 :Transport layer The transport layer is responsible for the delivery of a message from one process to another. Protocol : TCP , UDP Data form : Segment 14  Protocols at the transport layer Transmission control protocol (TCP), Connection oriented Connection established before sending data Reliable in-order delivery congestion control user datagram protocol (UDP) Connectionless Sending data without establishing connection Fast but unreliable unordered delivery delay guarantees 15  7 Application OSI REFERENCE MODEL 6 5. Session Layer Presentation a) Allows users on different machines to establish sessions (dialogue) between them. 5 Session b) One of the services is managing dialogue control. 4 Transport c) Token management. 3 Network d) Synchronization. 2 Data Link 1 Physical 16 Layer 5 : Session layer The session layer is responsible for dialog control and synchronization Protocol : SQL , RPC , NFS Data form : Data 17  7 Application OSI REFERENCE MODEL 6 6. Presentation Layer Presentation a) Concerned with the syntax and semantics of the information. 5 Session b) Preserves the meaning of the information. 4 Transport c) Data compression. d) Data encryption. 3 Network Protocols: SSL : WEB applications, with HTTP 2 Data Link SSH : Telnet and FTP ( email ) 1 Physical 18  Layer 6 : Presentation layer The presentation layer is responsible for translation, compression, and encryption. Data form : Data 19  7 Application OSI REFERENCE MODEL 6 7. Application Layer :allows user to interface with the Presentation network! a) Provides protocols that are commonly 5 Session needed. 4 Transport Main topics: File Transfer Protocol (FTP) 3 Network HyperText Transfer Protocol (HTTP),(HTTPS) Simple Mail Transfer Protocol (SMTP) Simple Network Management Protocol (SNMP) 2 Data Link Network File System (NFS) Telnet 1 Physical 20  Layer 7 : Application layer The application layer is responsible for providing services to the user. Protocol : HTTP , FTP , SMTP , DNS , TELNET Data form : Data 21  Most common Protocols DNS – (Domain name service) Matches domain names IP addresses HTTP –(Hypertext Transfer Protocol) Used to transfer d between clients/servers using a web browser SMTP & POP3 –( simple mail transmission Prot.) used send email messages from clients to servers over the internet FTP – ( file Transmission Prot.) allows the download/upl of files between a client/server Telnet – allows users to login to a host from a remote location and take control as if they were sitting at the machine (virtual connection) DHCP –(Dynamic Host Configuration Protocol.) assigns addresses, subnet masks, default gateways, DNS serve 22 etc. To users as they login the network  Summary of layers 23  OSI Layers 24  TRANSMISSION MEDIA 1. Guided Data is sent via a wire or optical cable. Twisted Pair Two copper wires are twisted together to reduce the effect of crosstalk noise. (e.g. Cat5, UTP, STP) Unshielded Twisted-Pair (UTP) cables Shielded Twisted-Pair (STP) cables Baseband Coaxial Cable A 50-ohm cable used for digital transmission. Used in 10Base2 and 10Base5. Broadband Coaxial Cable A 75-ohm cable used for analog transmission such as Cable TV. 25  TRANSMISSION MEDIA Fiber Optic Cables is a network cable that contains strands of glass fibers inside an insulated casing. They're designed for long- distance, high-performance data networking, and telecommunications. Compared to wired cables, fiber optic cables provide higher bandwidth and transmit data over longer distances. Fiber optic cables support much of the world's internet, cable television, and telephone systems 26  TRANSMISSION MEDIA 2. Unguided Data is sent through the air. Line-of-sight Transmitter and receiver must “see” each other, such as microwave system. Communication Satellites A big microwave repeater in the sky. Data is broadcasted, and can be “pirated.” Radio Term used to include all frequency bands, such as FM, UHF, and VHF television. 27  Computer Network I Chapter Three: Data and Signals  Data and Signals One of the major functions of the physical layer is to move data in the form of electromagnetic signals across a transmission medium. Whether you are collecting numerical statistics from another computer, sending animated pictures from a design workstation connections. To be transmitted, data must be transformed to electromagnetic signals.  ANALOG AND DIGITAL Data can be analog or digital. The term analog data refers to information that is continuous; digital data refers to information that has discrete states. Analog data take on continuous values. Digital data take on discrete values.  Analog and Digital Signals Signals can be analog or digital; Analog signals can have an infinite number of values in a range. Digital signals can have only a limited number of values. Sine Wave The sine wave is the most fundamental form of a periodic analog signal. When we visualize it as a simple oscillating curve, its change over the course of a cycle is smooth and consistent, a continuous, rolling flow - A sine wave can be represented by three parameters: the peak amplitude, the frequency, and the phase. These three parameters fully describe a sine wave. - Period and Frequency Period refers to the amount of time, in seconds, a signal needs to complete 1 cycle. Frequency refers to the number of periods in 1 s. Frequency and period are the inverse of each other. - - Example (3.2): The period of a signal is 100 ms. what is its frequency in kilohertz? Solution: First we change 100 ms to seconds, and then we calculate the frequency from the period (1 Hz = 10−3 kHz). - Bandwidth and Signal Frequency The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. Figure (3.11) shows the concept of bandwidth. The figure depicts two composite signals, one periodic and the other nonperiodic. The bandwidth of the periodic signal contains all integer frequencies between 1000 and 5000 (1000, 1001, 1002,...). The bandwidth of the nonperiodic signals has the same range, but the frequencies are continuous. - Example (3.6): If a periodic signal is decomposed into five sine waves with frequencies of 100, 300, 500, 700, and 900 Hz, what is its bandwidth? Draw the spectrum, assuming all components have a maximum amplitude of 10 V Solution Let fh be the highest frequency, fl the lowest frequency, and B the bandwidth. Then - Example (3.7): A periodic signal has a bandwidth of 20 Hz. The highest frequency is 60 Hz. What is the lowest frequency? Draw the spectrum if the signal contains all frequencies of the same amplitude. Solution: Let fh be the highest frequency, fl the lowest frequency, and B the bandwidth. Then - Fig. (3.14): The bandwidth for Example (3.7). DIGITAL SIGNALS In addition to being represented by an analog signal, information can also be represented by a digital signal. For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage. A digital signal can have more than two levels. In this case, we can send more than 1 bit for each level. - Example (3.8) A digital signal has eight levels. How many bits are needed per level? We calculate the number of bits from the formula: - TRANSMISSION IMPAIRMENT Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise - A-Attenuation Attenuation means a loss of energy. When a signal, simple or composite, travels through a medium, it loses some of its energy in overcoming the resistance of the medium. To compensate for this loss, amplifiers are used to amplify the signal. Figure (3.20) shows the effect of attenuation and amplification. - To show the loss or gain of energy the unit “decibel” is used. dB = 10log10 P2/ P1 ; where P1 - input signal, P2 - output signal Example (3.14): Suppose a signal travels through a transmission medium and its power is reduced to one-half. This means that P2 is (1/2)P1. In this case, the attenuation (loss of power) can be calculated as: - Example (3.15): A signal travels through an amplifier, and its power is increased 10 times. This means that P2 = 10P1. In this case, the amplification (gain of power) can be calculated as:  B-Noise There are different types of noise * Thermal - random noise of electrons in the wire creates a extra signal - * Induced - from motors and appliances, devices act are transmitter antenna and medium as receiving antenna. Crosstalk - same as above but between two wire * Impulse - Spikes that result from power lines, lightning, etc - Signal to Noise Ratio (SNR) To measure the quality of a system the SNR is often used. It indicates the strength of the signal with the noise power in the system. n It is the ratio between two powers: SNR=Average Signal Power/Average Noise Power) n It is usually given in dB and referred to as SNRdB. Example (3.16): The power of a signal is 10 mW and the power of the noise is 1 μW; what are the values of SNR and SNRdB ? Solution The values of SNR and can be calculated as follows: SNR = 10,000µW / 1µW=10,000  DATA RATE LIMITS A very important consideration in data communications is how fast we can send data, in bits per second, over a channel. Data rate depends on three factors: 1. The bandwidth available. 2. The level of the signals we use. 3. The quality of the channel (the level of noise). Capacity of a System * The bit rate of a system increases with an increase in the number of signal levels we use to denote a symbol. * A symbol can consist of a single bit or “n” bits. *The number of signal levels = 2^ n. * As the number of levels goes up, the spacing between level decreases ->increasing the probability of an error occurring in the presence of transmission impairments. Noiseless Channel: Nyquist Bit Rate For a noiseless channel, the Nyquist bit rate formula defines the theoretical maximum bit rate Example (3.19): Consider a noiseless channel with a bandwidth of 3000 Hz transmitting a signal with two signal levels. The maximum bit rate can be calculated as: Example (3.20): Consider the same noiseless channel with a bandwidth of 3000 Hz transmitting a signal with four signal levels (for each level, we send 2 bits). The maximum bit rate can be calculated as: Example (3.21): We need to send 265 kbps over a noiseless channel with a bandwidth of 20 kHz. How many signal levels do we need? Solution We can use the Nyquist formula as shown: Noisy Channel: Shannon Capacity Shannon’s theorem gives the capacity of a system in the presence of noise. Example (3.22): Consider an extremely noisy channel in which the value of the signal-to-noise ratio is almost zero. In other words, the noise is so strong that the signal is faint. For this channel the capacity C is calculated as: C = B log2(1 + SNR)  Example (3.23): We can calculate the theoretical highest bit rate of a regular telephone line. A telephone line normally has a bandwidth of 3000. The signal-to-noise ratio is usually 3162. For this capacity is calculated aschannel the Propagation Time Propagation time measures the time required for a bit to travel from the source to the destination. Propagation Delay = Distance/Propagation speed Example (3.28): What is the propagation time if the distance between the two points is 12,000 km? Assume the propagation speed to be 2.4 x 10^8 m/s in cable. Transmission Time Transmission time: The time at which all the bits in a message arrive at the destination. (Difference in arrival time of first and last bit) Example (3.29): What are the propagation time and the transmission time for a 2.5-kbyte message (an e-mail) if the bandwidth of the network is 1 Gbps? Assume that the distance between the sender and the receiver is 12,000 km and that light travels at 2.4 x 10^8 m/s. Example (3.30): What are the propagation time and the transmission time for a 5-Mbyte message (an image) if the bandwidth of the network is 1 Mbps? Assume that the distance between the sender and the receiver is 12,000 km and that light travels at 2.4 x 10^8 m/s.  Network I Chapter 4 Digital Transmission 4.1  4-1 DIGITAL-TO-DIGITAL CONVERSION In this section, we see how we can represent digital data by using digital signals. The conversion involves three techniques: line coding, block coding, and scrambling. Line coding is always needed; block coding and scrambling may or may not be needed. Topics discussed in this section: Line Coding Line Coding Schemes 4.2  Line Coding Converting a string of 1‟s and 0‟s (digital into a sequence of signals that denote the and 0‟s (digital signal). For example a high voltage level (+V) cou represent a “1” and a low voltage level (0 V) could represent a “0”. 4.3  Characteristic of these line coding techniques 1-There should be self-synchronizing i.e., both receiver and sender clock should be synchronized. 2-There should have some error-detecting capability. 3-There should be immunity to noise and interference. 4-There should be less complexity. 5-There should be no low frequency component (DC- component) as long distance transfer is not feasible for low frequency component signal.. 4.4  Figure 4.1 Line coding and decoding 4.5  Mapping Data symbols onto Signal levels A data symbol (or element) can consist of a number of data bits: 1 , 0 or 11, 10, 01, …… A data symbol can be coded into a single signal element or multiple signal elements 1 -> +V, 0 -> -V 1 -> +V and -V, 0 -> -V and +V The ratio „r‟ is the number of data elements carried by a signal element. 4.6  Relationship between data rate and signal rate The data rate defines the number of bits sent per sec - bps. It is often referred to the bit rate. The signal rate is the number of signal elements sent in a second and is measured in bauds. It is also referred to as the modulation rate. Goal is to increase the data rate whilst reducing the baud rate. 4.7  Figure 4.2 Signal element versus data element 4.8  Data rate and Baud (signal)rat The baud or signal rate can be expressed as: S = c x N x 1/r bauds S is signal rate N is data rate or bit rate c is the case factor (worst, best & avg.) r is the ratio between data element / signal element 4.9  Example 4.1 A signal is carrying data in which one data element is encoded as one signal element ( r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if c is between 0 and 1? Solution We assume that the average value of c is 1/2. The baud rate is then 4.10  Line encoding C/Cs Self synchronization - the clocks at the sender and the receiver must have the same bit interval. If the receiver clock is faster or slower it will misinterpret the incoming bit stream. 4.11  Figure 4.4 Line coding schemes 4.12  Unipolar All signal levels are on one side of the time axis - either above or below NRZ - Non Return to Zero scheme is an example of this code. The signal level does not return to zero during a symbol transmission. 1=+ 0=0 4.13  Figure 4.5 Unipolar NRZ scheme 1 = Positive 0 =Zero 4.14  The advantages of unipolar NRZ are: 1-Simple to implement. 2-Requires relatively low bandwidth. The disadvantages of unipolar NRZ are: 1-There is a significant DC component, which means that power is wasted due to heating of the wires in the transmission line. It also means that channel links must be DC-coupled, because AC-coupled links will reject the signal's DC component. 2-There is no mechanism for embedding a clock signal into the line code. Long sequences of ones or zeros can cause loss of synchronisation at the receiver due to the absence of voltage transitions. 4.15  Polar It uses two voltage levels, one positive and one negative. of the many existing variations of polar encodin we examine four of the most popular: non retu to zero (NRZ), return to zero (RZ), Manchester, and differential Manchester. 4.16   Polar : NRZ The value of the signal is always either positive or nega Polar NRZ scheme can be implemented with two voltag E.g. +V for 1 and -V for 0. There are two versions: non return to zero (NRZ) NRZ-Level (NRZ-L): A positive voltage usually means the bit is a (0), while a negative voltage means the bit is a (1). NRZ-Inversion (NRZ-I): An inversion of the voltage level represents a 1 bit. It is transition between a positive and a negative voltag A (0) bit is represented by no change. The signal is inverted if a (1) is encountered. 4.17  Figure 4.6 Polar NRZ-L and NRZ-I schemes NRZ-Level (NRZ-L): A positive voltage usually means the bit is a (0), while a negative voltage means the bit is a (1). NRZ-Inversion (NRZ-I): A (0) bit is represented by no change. The signal is inverted if a (1) is encountered 4.18   Polar : RZ The Return to Zero (RZ) scheme uses three voltage values: (positive, negative, and zero). A positive voltage means (1) and a negative voltage means (0). A halfway through each bit interval, the signal returns to zero.(half wave) A (1) bit is actually represented by positive-to-zero. A (0) bit is represented by negative-to-zero. 4.19  Figure 4.7 Polar RZ scheme A (0) bit is represented by negative-to-zero. A (1) bit is actually represented by positive-to-zero. 4.20  Polar : RZ The advantages of Polar RZ are − *It is simple. *No low-frequency components are present. The disadvantages of Polar RZ are − 1-No error correction. 2-No clock is present. 3-Occupies twice the bandwidth of Polar NRZ. 4-The signal droop is caused at places where the signal is non-zero at 0 Hz. 4.21   Polar: Manchester Manchester coding consists of combining the idea NRZ-L and the idea of RZ (transition at the middle the bit). In this scheme, the duration of the bit is divided i two levels. The voltage remains at one level during the first h and moves to the other level in the second half. A negative-to-positive transition represents binar A positive-to-negative transition represents binar 4.22   Polar: Manchester A positive-to-negative transition represents binary (0). A negative-to-positive transition represents binary (1). 4.23   Polar: Differential Manchester Differential Manchester coding consists of combining the NRZ-I and RZ schemes. There is always a transition at the middle of the but the bit values are determined at the beginn of the bit. If the next bit is 0, there is a transition; if the ne bit is 1, there is none. 4.24   Polar : Differential Manchester A (0) bit is represented by no change. The signal is inverted if a (1) is encountered 4.25  POLAR : DIFFERENTIAL MANCHESTER A (0) bit is represented by no change. 4.26 The signal is inverted if a (1) is encountered  Figure 4.8 Polar biphase: Manchester and differential Manchester schemes A (0) bit is represented by no change. The signal is inverted if a (1) is encountered 4.27   Bipolar : AMI (alternates Mark Inversio Code uses 3 voltage levels: ( Positive , Zero, Negative ) to represent the symbols (note no transitions to zero as in RZ). Voltage level for one symbol is at “0” and the other alternates between (Positive & Negative). Bipolar Alternate Mark Inversion (AMI) :the “0” symbol is represented by zero voltage and the “1” symbol alternates between +V and -V. Pseudo-ternary is the reverse of AMI. 4.28  The advantages of bipolar AMI are: 1-Easy to implement. 2-Same signalling rate as other NRZ schemes. 3-Uses less power than polar NRZ line coding schemes. 4-The signal has no DC-component. 5-Baseline wandering is not an issue. 6-Avoidance of polar ambiguity. The disadvantages of bipolar AMI are: *No embedded clock signal. Long sequences of zeros can cause loss of synchronisation at the receiver due to the absence of voltage transition 4.29   Bipolar : Pseudo-ternary Also, code uses 3 voltage levels: ( Positive , Zero, Negative ) to represent the symbols (no not transitions to zero as in RZ). But, Pseudo-ternary :is the reverse of AMI - the “1” symbol is represented by zero voltage and the “0” symbol alternates between +V an -V. AMI 0 = zero , 1= Positive than Negative Pseudo-ternary 1 = zero , 0= Positive than Negative 4.30  Figure 4.9 Bipolar schemes: AMI and pseudoternary AMI 0 = zero , 1= Positive than 1= Negative Pseudo-ternary 1 = zero , 0= Positive than Negative 4.31`  Advantage , Disadvantages of Bipolar Advantages 1-It is simple. 2-No low-frequency components are present. 3-Occupies low bandwidth than unipolar and polar NRZ schemes. 4-This technique is suitable for transmission over AC coupled lines, as signal drooping doesn’t occur here. 5-A single error detection capability is present in this. Disadvantages 1-No clock is present. 2-Long strings of data causes loss of synchronization. 4.32  Thank You 4.33