GROUP MEMBER NAME LONG SANGHENG NAME PHAL PHEARUN
GROUP MEMBER: NAME: LONG SANGHENG NAME: PHAL PHEARUN 1
1 Introduction to Data Communications 2 Network Model 3 Data and Signals 2
Introduction to Data Communications 3
1. Data Communications 1. 1. Communication Data communications are the transfer of data from one device to another via some form of transmission medium. Data Communications are the exchange or sharing of data or information in form of binary code (1 and 0) between two communicating devices via any form of transmission medium. Example: Local Area Network The remote communication takes place over distance and it is called telecommunication. Tele is Greek for "far"). Example: Internet 4
The five components that make up a data communications system are the message, sender, receiver, medium, and protocol. 5
2. Direction of Data Flow Transmission mode is referred to direction of data that flow between two devices. Data flow between two devices can occur in one of three ways: § simplex § half-duplex § or full-duplex 6
. Simplex Mode Unidirectional Communication Only one of the two devices on a link can transmit and the other can only receive Example: Keyboard, Radio, TV… Simplex mode is like one way street Practical example is radio broadcasting 7
. Half-duplex Mode Bidirectional Communication Each station can both transmit and receive, but not at the same time When one device is sending, the other can only receive, and vice versa §The half-duplex mode is like a one-lane road (small road) with traffic allowed in both directions. When cars are traveling in one direction, cars going the other way must wait. 8
. Full-duplex Mode (Duplex) § Bidirectional Communication § Both stations can transmit and receive simultaneously § In full-duplex mode, signals going in one direction share the capacity of the link with signals going in the other direction. § This sharing can occur in two ways: A. The link must contain two physically separate transmission paths, one for sending and the other for receiving. B. The capacity of the channel is divided between signals traveling in both directions The full-duplex mode is like a two-way street with traffic flowing in both directions at the same time 9
3. Type of Connection Type of connection is referred to the way of two or more devices connected through a link. A link is a communications pathway that transfers data from one device to another. There are two possible types of connections: Point-to-Point Connection Multipoint Connection 10
4. Physical Topology refers to the physical or logical arrangement of a network. Devices may be arranged in a mesh, star, bus, or ring topology. There are four basic topologies possible: mesh, star, bus, and ring. 11
Mesh Topology In a 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. 12
A network can be categorized as a local area network, Metropolitan area network and Wide area network. A LAN is a data communication system within a home or building, or campus. A WAN is a data communication system spanning states, countries, or the whole world. An internet is a network of networks. It is a collection of many separate networks. 13
Star Topology In a star topology, each device has a dedicated point-to-point link only to a central controller, usually called a hub or switch. The devices are not directly linked to one another. 14
Bus Topology A bus topology is multipoint connection. One long cable acts as a backbone to link all the devices in a network. 15
Ring Topology In a ring topology, packets are sent in one direction around the circle from computer to computer, until it reaches its destination. Each computer looks at each packet to decide whether the packet was intended for it. If not, the packet is passed on to the next computer in the ring. 16
5. Network Categories Local Area Network Metropolitan Area Network Wide Area Network 17
Local Area Network A LAN is a data communication system within a home or building, or campus. Early LANs had data rates in the 4 to 16 megabits per second (Mbps) range. Today, however, speeds are normally 100 or 1000 Mbps or 10000 Mbps. 18
Metropolitan Area Network 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. A good example of MAN, a bank with multiple branches might uses a MAN 19
Wide Area Network A wide area network (WAN) provides long-distance transmission of data such as image, audio, and video… over large geographic areas that may comprise a country, a continent, or even the whole world. 20
Internetwork or Internet Today, it is very rare to see a LAN, a MAN, or a WAN in isolation; they are connected to one another. When two or more networks are connected, they become an internetwork, or internet. As an example, assume that an organization has two offices, one on the east coast and the other on the west coast. 21
Network Model 22
6. Network Model The International Standards Organization created a model called the Open Systems Interconnection, which allows diverse systems to communicate. A network is a combination of hardware and software that sends data from one location to another. § The hardware consists of the physical equipment that carries signals from one point of the network to another. § The software is a collection of instructions (segment of codes) that enable a user to interact with the computer or have the computer perform specific tasks for them. Without software, the computer would be useless. 23
6. 1 Layered Tasks We use the concept of layers in our daily life. As an example, let us consider two friends who communicate through postal mail. The process of sending a letter to a friend would be complex if there were no services available from the post office. The following figure shows the steps in this task. Sender Receiver Carrier 24
6. 2 The OSI Model An ISO standard that covers all aspects of network communications is the Open Systems Interconnection model. It was first introduced in the late 1970 s. § An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture. § The purpose of the OSI model is to show to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software. § It is a model for designing a network architecture that is flexible, robust, and interoperable. 25
The OSI Model 7. Application layer 6. Presentation layer 5. Session layer 4. Transport Layer 3. Network layer 2. Data Link (MAC) layer 1. Physical layer 26
1. Physical Layer The physical layer coordinates the functions required to transmit a bit stream over a physical medium. § The physical layer coordinates the functions required to carry a bit stream over a physical medium. § It deals with the mechanical and electrical specifications of the interface and transmission medium. Note: The physical layer is responsible for movements of individual bits from one node (device) to the next. 27
2. Data Link Layer The data link layer is responsible for delivering frames from one station to the next without errors. § The data link layer transforms the physical layer to a reliable link. § It makes the physical layer appear error-free to the upper layer (network layer). Note: The data link layer is responsible for moving frames from one node to the next. 28
3. Network Layer The network layer is responsible for the source-to-destination delivery of a packet across multiple network links. § If two devices are connected to the same link, usually there is no need for a network layer (Router). However, if the two devices are attached to different networks (links) with connecting devices between the networks, there is often a need for the network layer to accomplish source-to-destination delivery. Note: The network layer is responsible for the delivery of individual packets from the source host to the destination host. 29
4. Transport Layer The transport layer is responsible for the process-to-process delivery of the entire message. § The transport layer is responsible for process-to-process delivery of the entire message. § A process is an application program running on a host. Provide logical communication between application processes running on different hosts 30
Note: The transport layer is responsible for the delivery of a message (segments) from one process to another 31
5. Session Layer The session layer establishes, maintains, and synchronizes the interactions between communicating devices. This layer allows applications on connecting systems to communicate using a session. It opens, uses, and closes this communication link. 32
6. Presentation Layer The presentation layer ensures interoperability between communicating devices through transformation of data into a mutually agreed upon format. Translation: The presentation layer at the sender changes the information from its sender-dependent format into a common format. The presentation layer at the receiving machine changes the common format into its receiver-dependent format. Note: The presentation layer is responsible for translation, compression, and encryption. 33
7. Application Layer The application layer enables the users to access the network. § The application layer enables a user, whether human or software, to access the network. It provides support for services such as electronic mail, remote file access and transfer, browsing the World Wide Web, and so on. Note: The application layer is responsible for providing services to the user. 34
6. 3 Internet Model or TCP/IP Model In The layers in the TCP/IP model or protocol suite do not exactly match those in the OSI model. The original TCP/IP Model was defined as having four layers: host-to-network, internet, transport, and application. However, when TCP/IP is compared to OSI, we can say that the TCP/IP model is made of five layers: physical, data link, network, transport, and application. TCP/IP is a five-layer hierarchical protocol suite developed before the OSI model. 35
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Data and Signals 37
What is Data? Data is information that has been translated into a form that is more convenient (easy) to move or process. Relative to today's computers and transmission media, data is information converted into binary digital form. A signal is an electric current or electromagnetic field used to convey data from one place to another. 38
1. Analog and Digital 1. 1. 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. For example, an analog clock, that has hour, minute, and second hands gives information in a continuous form; the movements of the hands are continuous. On the other hand, a digital clock that reports the hours and the minutes will change suddenly from 8: 05 to 8: 06. 39
§ Analog data, such as the sounds made by a human voice. When someone speaks, an analog wave is created in the air. This can be captured by a microphone and converted to an analog signal and converted to a digital signal. Data can be analog or digital. Analog data are continuous and take continuous values. Digital data have discrete states and take discrete values. § Digital data is data stored in computer memory in the form of 0 s and 1 s. They can be converted to a digital signal or modulated into an analog signal for transmission across a medium. 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. Data can be analog or digital. Analog data are continuous and take continuous values. Digital data have discrete states and take discrete values. 40
1. 2. Analog and Digital Signals § An analog signal has many levels of intensity over a period of time. As the wave moves from value A to value B, it passes through many number of values along its path. § A digital signal, on the other hand, can have only a limited number of defined values. Although each value can be any number, it is often as simple as 1 and 0. 41
2. 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. 42
2. 1 Bit Rate and Bit Interval § Bit Rate Most digital signals are non-periodic, and thus period and frequency are not appropriate characteristics. Another term-bit rate (instead of frequency) is used to describe digital signals. The bit rate is the number of bits sent in 1 second, expressed in bits per second (bps). 43
§ Bit Interval is a duration (expressed in second) of one bit. It is the inverse of bit rate. Bit Interval = 1/Bit rate 1 s = 8 bit intervals Bit Interval 44
2. 2 Transmission of Digital Signals A computer network is designed to send information from one point to another. This information needs to be converted to either a digital signal or an analog signal for transmission. In digital transmission, the required bandwidth is proportional (~) to the bit rate; if we need to send bits faster, we need more bandwidth. 45
2. 3 Digital-To-Digital Conversion 2. 3. 1. Line Coding Line coding is the process of converting digital data to digital signals. We assume that data, in the form of text, numbers, graphical images, audio, or video, are stored in computer memory as sequences of bits. Line coding converts a sequence of bits to a digital signal. At the sender, digital data are encoded into a digital signal; at the receiver, the digital data are recreated by decoding the digital signal 46
Line Coding There are some characteristics of line coding such as: == Signal level (element) vs. data level (element) == Pulse (signal) rate vs. bit rate == DC components == Self-synchronization == Signal level (element) vs. data level In data communications, our goal is to send data elements. A data element is the smallest entity that can represent a piece of information: this is the bit In digital data communications, a signal element carries data elements. A signal element is the shortest unit (short period) of a digital signal. 47
. Line Coding == Signal level (element) vs. data level 48
. Line Coding == Signal level (element) vs. data level 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 signal elements § 1 -> +V, 0 -> -V § 1 -> +V and -V § 0 -> -V and +V 49
2. 3. 2. Unipolar § Unipolar encoding is very simple. ü 1 is represented by a high positive level (+A volts) ü 0 is represented by a zero level (0 volts). 50
2. 3. 3. Polar § Polar encoding uses two voltage levels, one positive and one negative. § There are four most popular types of polar encoding: 51
Non-Return to Zero (NRZ) 1). NRZ-L (level) version – voltage level determines the bit value: 1 negative voltage 0 positive voltage 2). NRZ-I (invert) version – voltage change or no-change determines the bit value (no change = 0, change = 1) 52
Non-Return to Zero (NRZ) 53
Non-Return to Zero (NRZ) 54
Return to Zero (RZ) In return to zero encoding three values are used: positive, negative, and zero. ü 0 = (-V), ü 1 = (+V), ü AND signal must return to zero halfway through each bit interval 55
Manchester encoding uses an inversion at middle of each bit interval for both synchronization and bit representation. ü 0 = pos-to-neg transition in middle of interval (H-to-L=0); ü 1 = neg-to-pos transition in middle of interval (L-to-H=1) 56
Different Manchester § Inversion at middle of each bit interval for synchronization. § 0=transition at beginning of interval § 1=no transition at beginning of interval 57
Different Manchester Another example 58
2. 3. 4. Bipolar § Bipolar encoding, like RZ, uses three voltage levels: (+, -, and 0). Unlike RZ (0 -V), however, ü The zero level in bipolar encoding is used to represent binary 0. ü The 1 s are represented by alternating positive and negative voltages. A common bipolar encoding is called alternate mark inversion (AMI). A zero voltage represents binary 0. Binary 1 s are represented by alternating positive and negative voltages. 59
3. Data Transmission modes 3. 1 Parallel Transmission Multiple bits are sent with each clock tick. Faster transmission, expensive due to N wires, used for short distances only. 60
3. 2 Serial Transmission 1 bit is sent with each clock tick. Slower but cheaper. Since the communication within a device is parallel a converter is required. 61
Applications Serial transmission is between two computers or from a computer to an external device located some distance away. Parallel transmission either takes place within a computer system (on a computer bus) or to an external device located a close distance away. Examples of parallel mode transmission include connections between a computer and a printer (parallel printer port and cable) Examples of serial mode transmission include connections between a computer and a modem. Example of Synchronous Communication: Real time, f 2 f conversation, telephone calls, chat rooms, . . Example of Asynchronous: Some delay between initial and reply, letters, email, facebook, … 62
4. Analog Transmission § What is Analog Transmission? Is a transmission method of conveying voice, data, image, or video information using a continuous signal which varies in amplitude, phase, or frequency. 4. 1 Digital to Analog Modulation Converts a digital signal to an analog signal 63
Digital to Analog Modulation § When you transmit data from one PC to another across public access phone line, the original data are digital, but because telephone wires carry analog signals, the digital data must be converted to analog. Modems enable remote sites to communicate through the plain old telephone system (POTS) 64
Digital to Analog Modulation Types of digital-to-analog modulation: Mechanisms for modulating digital data into an analog signal: Amplitude shift keying (ASK), Frequency shift keying (FSK), Phase shift keying (PSK), and Quadrature amplitude modulation–QAM (combine: ASK+PSK). QAM is the most efficient of these options and is the mechanism commonly used in all modern modems today. 65
Digital to Analog Modulation v Amplitude Shift Keying-ASK: - The two binary values are represented by two different amplitudes of the carrier signal. (Commonly one level is 0 and another one is 1). - Frequency and Phase are remaining constant while Amplitude changes 66
Digital to Analog Modulation v Amplitude Shift Keying-ASK: Relation bit rate and baud rate : 67
Digital to Analog Modulation v Amplitude Shift Keying-ASK: Bandwidth of ASK - The bandwidth of a signal is the total range of frequencies occupied by that signal. - When we decompose an ASK –modulated signal, we get a spectrum of many simple frequencies. However, the most important ones are those between: (fc-Nbaud/2) and (fc+Nbaud/2 )with the carrier frequency fc at the middle. 68
Digital to Analog Modulation v Amplitude Shift Keying-ASK: - Advantage: simplicity and low implementation costs - Disadvantage: ASK is very susceptible to noise interference. Noise caused by various phenomena such as heat or electromagnetic induction created by other sources. - Application: ASK is used to transmit digital data over optical fiber communication because the noise is less. 69
Digital to Analog Modulation v Frequency Shift Keying: § In FSK, frequency of the carrier signal is varied to represent binary 1 or 0. § Both peak amplitude and phase remain constant. the 70
v Frequency Shift Keying: Bandwidth for FSK The bandwidth required for FSK transmission is equal to the baud rate of the signal plus the frequency shift ( difference between the two carrier frequencies). BW = fc 1 – fc 0 + N baud Advantage: - Less effected by noise Disadvantage - Require large bandwidth Application: - Normally used in high frequency radio transmission, LAN or coaxial cable. 71
Phase Shift Keying: 72
v. Phase Shift Keying: - In PSK, the phase of the carrier is varied to represent binary 1 or 0. Peak amplitude and frequency remain constant as the phase changes. For example we start with a phase of 0º to represent binary 0, then we can change with a phase of 180º to represent binary 1. - The phase of the signal during each bit duration is constant, and its value depends on the bit (0 or 1). Ø The 2 -PSK Method The above method is often called 2 -PSK, because two different phases (0 and 180 degree) are used. 73
v Phase Shift Keying: - The diagram represents of this method is called constellation or phase-diagram (2 -PSK). 74
v Phase Shift Keying: Relation bit rate and baud rate in 2 -PSK 75
v Phase Shift Keying: Ø The 4 -PSK Method: Instead of utilizing only two variations signal, each phase shift represent 1 bit, we can use 4 variations signal and each phase shift represent 2 bit . 76
v Phase Shift Keying: Ø The 4 -PSK Method: - A phase of 00 now represents 00; 900 represents 01; 1800 represents 10; and 2700 represents 11. This technique is called 4 -PSK or Q-PSK. The pair of bits represented by each phase is called a dibit. We can transmit data twice as efficiently using 4 -PSK as we can using 2 -PSK 77
v Phase Ø Shift Keying: The 8 -PSK characteristics : - We can extend 4 -PSK to 8 -PSK. Instead of 90 degree, we now vary the signal by shifts of 45 degree. With 8 different phases, each shift can represent 3 bits (1 Tribit) at a time. When we have 4 possible phase, we can send 2 bits at a time-(2 x 2=4). When we have 8 possible phases, we can send 3 bits at a time-(2 x 2 x 2=8) 78
v Phase Shift Keying: Advantage: - PSK is less effected by noise than ASK and requires same bandwidth as ASK and less than FSK. Disadvantage: - Difficult to detect phase shift in case of phase difference when it is too small. Application: - Normally used in MODEM 79
4. 2 Modulation of Analog Signals § Analog-to-analog conversion is the representation of analog information by an analog signal. 80
4. 2 Modulation of Analog Signals § Why we need Analog-to-Analog Modulation? There are two principal reasons for combining an analog signal with a carrier at frequency fc. 1. Higher frequency may be needed for effective transmission data over long distance. 2. Modulation permits FDM (frequency-division multiplexing). FDM is a technique that allows a single transmission medium such as a cable or optical fiber to be shared by many signals which have different frequencies. An example of a system using FDM is cable television, in which many television channels are carried simultaneously on a single cable. 81
Modulation of Analog Signals Type of analog to analog modulation 82
Modulation of Analog Signals Amplitude Modulation-AM: - Amplitude of carrier signal varies with changing amplitude of the modulating signal - Frequency and phase remain unchanged 83
Modulation of Analog Signals AM Bandwidth: - Bandwidth of an AM signal = 2 x bandwidth of modulating Signal. 84
Modulation of Analog Signals AM Bandwidth: v AM radio - The bandwidth of an audio signal (speech + music) is usually 5 k. Hz => each AM radio station needs a min bandwidth of 10 k. Hz - AM radio station needs a minimum BW of 10 k. Hz (By FCC-Federal Communications Commission) - AM stations are allowed carrier frequencies anywhere between 530 - 1700 k. Hz; each station’s carrier frequency must be separated from those on either side by at least 10 k. Hz, to avoid interference. Ex: If one station uses a fc of 1100 k. Hz, the next station’s fc cant be lower than 1110 k. Hz. 85
Modulation of Analog Signals AM Bandwidth: v AM radio Modulation of Analog Signals AM Bandwidth: Ex 15: We have an audio signal with a bandwidth of 4 KHz. What is the bandwidth needed if we modulate the signal using AM? Solution: An AM signal requires twice the bandwidth of the original signal: 86 BWt=2*BWm=2*4=8 k. Hz
Modulation of Analog Signals Frequency Modulation-FM: - Frequency of carrier signal is changed - Amplitude and phase remain unchanged 87
Modulation of Analog Signals Note: Ø AM radio has wider coverage than FM radio Ø FM radio has better sound quality than AM radio because the higher frequency of FM waves means they can carry more information. Ø FM signals can carry two channels of sound to enable a stereo broadcast, whereas AM radio waves are mono (1 audio channel or speaker). Ø AM signals are also less prone(susceptible) to ambient (around) electrical interference, whereas FM signals are not. 88
THE END 89
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