Computer Network Fundamentals
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This slides will be very useful for engineering students studying in India.Based on Anna university syllabus Chennai
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Computer Network Fundamentals
- 1. N.Jagadish kumar Assistant professor-IT 1 FUNDAMENTALS & LINK LAYER
- 2. 2 Overview
- 3. 3 Building a network A network is a set of devices connected by a media link Devices are often called as nodes They are the serial lines connected by dumb terminals to mainframe computer Difference=>Carry many different types of data and support a wide range of applications
- 4. Data Communications 4 • Data – facts, concepts, and instructions • Data – Information presented in whatever form is agreed upon by the parties creating and using the data • Represented by binary information units (in the form of 0s and 1s) • Data Communications – Exchange of data between two devices via some form of transmission medium such as a wire cable
- 5. Effectiveness of Data Communications 5 Effectiveness of a data communications system depends on three 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. Delivering the data in the same order that they are produced and without significant delay (real time transmission)
- 6. 6 Five components of datacommunication Message: Information to be communicated. Consist of text, numbers, pictures, sound or video - combination
- 7. 7 Five components of datacommunication Sender: Device that sends the data message- May be computer, workstation. telephone handset, video camera and etc.
- 8. 8 Five components of data communication Receiver: The device that receives the message. Can be computer, work station, telephone handset, television and so on.
- 9. 9 Five components of data communication Medium: Physical path by which a message travels from sender to receiver. Eg :Twisted pair wire, coaxial cable, fibre optic cable or radio waves
- 10. 10 Five components of data communication Protocol: Set of rules that governs data communication. It represents an agreement between the communicating devices. Without a protocol, two devices may be connected but not communicating
- 11. Data Representation 11 Information Today comes in different forms such as text, numbers, images, audio and video Text: Represented as a bit pattern, a sequence of bits 0s or 1s. Different codes are used ASCII (7 bits per symbol) Extended ASCII (8 bits per symbol) Unicode (16 bits – supports different languages) ISO (32 bits)
- 12. Data Representation 12 Numbers: Converted to a binary number – to simplify the mathematical operations Images: Represented by bit patterns Each pixel is assigned a bit pattern Black and White Image – 1 bit per pixel Gray scale images – depends on number of levels in gray scale Colour images: Each pixel has 3 bit patterns (RGB) • Audio / Video: – Converted in to Analog/Digital
- 13. Direction of data flow 13 Simplex Simplex – unidirectional; one transmits, other receives
- 14. 14 Half-duplex Half-duplex – each can transmit/receive; communication must alternate
- 15. 15 Full-duplex Full-duplex – both can transmit/receive simultaneously
- 16. Networks 16 Set of devices (nodes) connected by media.
- 17. 17 Requirements The requirements to build a network are, Perspectives Scalable Connectivity Cost effective resource sharing Support for common services Interoperability Managebility Performance Connectivity=> Connections among a set of computers Links and nodes Types of links=> point to point link, multipoint link
- 18. Applications • Peer-Peer model: – No fixed clients or servers – Each host can act as both client & server • Examples: Napster, Gnutella, KaZaA 18
- 19. Applications 19 WWW • Instant Messaging (Internet chat, text messaging on cellular phones) • Peer-to-Peer • Internet Phone • Video-on-demand • Distributed Games • Remote Login (Telnet) • File Transfer
- 20. Network Criteria 20 Performance Can be measured by transit time and response time. Affected by number of users, type of medium, connected HW/SW Reliability Measured by frequency of failure, recovery time, robustness in a catastrophe Security – protection from unauthorized access, viruses / worms
- 21. Line Configuration 21 The way two or more communication devices attach to a link Link – Physical communication pathway that transfer data from one device to another Two possible line configurations Point-to-point Multipoint
- 24. Topology 24 Physical or logical arrangement Topology of a network is the geometric representation of the relationship of all the links and linking devices to one another Basic types: Mesh, star, bus, ring,hybrid
- 26. Mesh Topology Dedicated point-to-point links to every other device n(n-1)/2 links an each device will have n-1 I/O ports Advantages: Dedicated links – no traffic problems Robust Privacy/Security Easy fault identification and isolation Disadvantages: More amount of cabling and I/O ports requirement Installation and reconnection is difficult Expensive 26
- 27. Star Topology Dedicated point-to-point links to central controller (hub) Controller acts as exchange Advantages less expensive robustness Disadvantages More cabling requirement than ring and bus topologies 27
- 28. Bus Topology Multipoint configuration One cable acts as a backbone to link all devices Advantages : Ease of installation, less cabling Disadvantages : Difficult reconnection and fault isolation, a fault/break in the bus cable stops all transmission 28
- 29. Ring Topology Dedicated point-to-point configuration to neighbours Signal is passed from device to device until it reaches destination Each device functions as a repeater Advantages : easy to install and reconfigure Disadvantages :limited ring length and no: of devices; break in a ring can disable entire network 29
- 30. Categories of Networks 30 Based on size, ownership, distance covered, and physical architecture Local Area Network (LAN) – smaller geographical area Metropolitan Area Network (MAN) – network extended over an entire city Wide Area Network (WAN) – large geographical area
- 32. 32 LAN Privately owned and links the devices in a single office, building or campus
- 33. 33 LAN
- 34. 34 MAN Designed to extend over an entire city – May be a single network such as cable television network – may be a means of connecting a number of LANS into a larger network
- 35. Metropolitan Area Network (MAN) 35 home cable headend cable distribution network (simplified) Typically 500 to 5,000 homes A Cable TV Network is an example of a MAN
- 36. 36 WAN Provides long distance transmission of data, voice, image and video information over large geographic areas – a country/continent/world
- 37. Protocols and Standards 37 Protocols: Set of rules that governs data communications Defines what is communicated, how it is communicated, and when it is communicated Key elements Syntax : Structure/ format of data –order in which it is presented Semantics : meaning of each section of bits- how pattern to be interpreted – What action to be taken Timing: When data to be sent and how fast they can be sent
- 38. Protocols and Standards 38 Standards: Essential in creating and maintaining an open and competitive market for equipment manufacturers and in guaranteeing national and international interoperability of data and telecommunications technology and processes – De facto: Standards that have not been approved by an organized body but have been adopted as standards through widespread use – De jure: legislated by an officially recognized body
- 39. Protocols and Standards 39 Standards Organizations • International organization for Standardization (ISO) • International Telecommunication Union – Telecommunication Standards Sector (ITU-T) • American National Standards Institute (ANSI) Institute of Electrical and Electronics Engineers (IEEE) • Electronic Industries Assoctiation (EIA) Regulatory Agencies Federal Communication Commission (FCC) Internet Standards Internet Draft Request for Comment (RFC)
- 40. ISO/OSI MODEL ISO/OSI MODEL The Open Systems Interconnection (OSI) architecture has been developed by the International Organization for Standardization (ISO) to describe the operation and design of layered protocol architectures. This forms a valuable reference model and defines much of the language used in data communications.
- 41. ISO/OSI MODEL
- 42. 42 FUNCTIONS OF LAYERS Describes a seven-layer abstract reference model for a network Architecture. Purpose of the reference model was to provide a framework for the development of protocols
- 43. 43 PHYSICAL LAYER 43 FUNCTIONS OF LAYERS Physical characteristics of interfaces and media Representation of bits Data rate Synchronization of bits: DATALINK LAYER Framing Physical Addressing Flow Control Error control& Access control
- 44. 44 FUNCTIONS OF LAYERS NETWORK LAYER Logical Addressing Routing TRANSPORT LAYER Service-point (port) addressing Segmentation and reassembly Connection control Flow control Error Control
- 45. 45 FUNCTIONS OF LAYERS SESSION LAYER Dialog Control Synchronization PRESENTATION LAYER Translation Encryption Compression APPLICATION LAYER Network Virtual terminal File transfer, access and management Mail services & Directory services
- 46. 46 Switched Network Long distance transmission Don’t bother about the content of data Transfers the data until it reaches the destination Types=> packet switched and circuit switched
- 47. 47 Packet switching Used when the users are need of intermittent channel usage Messages are broken into individual small packets Switch to switch packet transmission Handles traffic well Approaches=> datagram and virtual circuit
- 48. 48 Datagram=>connection-less Disadvantages=> Suffers from lot of overhead The packets are not reassembled at the receiver Virtual circuit=> connection oriented Consist of 3 phases=> Establishment phase=> set up connection and ensures QoS Data transfer phase=> performs error control, flow control services Connection release phase=>Connection termination
- 51. 51 Comparison S.no Virtual Circuit Datagram 1 Circuit setup is required Circuit setup is not required 2 Each packet contains a short VC number as address Each packet contains full source and destination address 3 Route chosen when VC is setup and all packets follow this route Each packet is routed independently 4 All packets are lost incase of router failure Only crashed packets are lost 5 Congestion control is easy using buffers Difficult congestion control
- 52. 52 Circuit Switching=> End to end path setup beginning of a session Dedicated to an application And then released at the end of the session Example=> Telephone system Message Switching=> Data stored at the switching office and then forwarded later Uses store and forward network Example=> Telegraph system
- 53. 53 Addressing It uses four levels of addresses, Physical(link) addresses=> defined by its LAN or WAN Logical(IP)addresses=>universal communications Port addresses=>label assigned to the communication process through telnet and FTP Specific addresses=>designed by users for some application
- 54. 54 TCP/IP layers and its addresses Specific Addresses Port Address Logical Address Physical Address
- 55. 55 Network software Socket Application Programming Interface Socket=>interface between the application layer and transport layer Also referred as API between application and network Associated with host address and a port address Host address=>IP address of a host where client and server program is loaded Port address=>communication port used by the client or server program
- 56. 56 Syntaxes for Socket Connection Socket creation=> int socket(int family, int type, int protocol); Binding=>int bind(int sd,struct sockaddr *name, int namelen); Listening=> int listen(int sd, int backlog); Accepting=> int accept(int sd, struct *addr, int *addrlen); Closing=>int close(int sd);
- 57. 57 Protocol Implementation OS uses process or thread=>communication Implementation=> Process per protocol model Process per message model Message Buffers=>
- 58. 58 Performance Measured in various parameters Bandwidth=> measured in hertz and bps Throughput=>Measures the speed of data transmission Latency=>Time required for sending a message to reach destination Bandwidth delay product=>number of bits that can fill the channel Jitter=>Data packets reaching at receiver at different times
- 59. Transmission Media Transmission media can be divided into two broad categories Guided Unguided Guided media It includes twisted-pair cable, coaxial cable, and fiber-optic cable. Guided media, which are those that provide a conduit from one device to another. Unguided media It is usually air.
- 60. Transmission Media Coaxial cable Coaxial cable carries signals of higher frequency ranges than twisted pair cable. It has a central core conductor of solid or stranded wire enclosed in an insulating sheath. This in turn encased in an outer conductor of metal foil ,braid or a combination of the two.
- 61. 61 Transmission Media Fiber Optic Cable. A fiber optic cable is made of glass or plastic and transmit signals in the form of light. There are two modes for propagating light along optical channels. Multimode Single mode.
- 62. LINE CODING&SCHEMES Line Coding is the process of converting binary data , a sequence of bits , to a digital signal
- 63. Unipolar encoding uses only one voltage level. LINE CODING&SCHEMES Polar encoding uses two voltage levels (positive and negative).Polar encoding uses two voltage levels (positive and negative).
- 64. POLAR ENCODING&SCHEMES NRZ-LNRZ-L :The level of the signal is dependent upon the:The level of the signal is dependent upon the state of thestate of the bitbit NRZ-I:NRZ-I: The signal isThe signal is inverted if a 1 is encounteredinverted if a 1 is encountered..
- 65. RZ encoding: POLAR ENCODING&SCHEMES Manchester encodingManchester encoding: the: the transition at the middle of the bittransition at the middle of the bit is used for both synchronization and bit representation.is used for both synchronization and bit representation.
- 66. 66 Differential Manchester encoding:Differential Manchester encoding: TheThe transition at the middletransition at the middle of the bit is used only forof the bit is used only for synchronization.synchronization. The bit representation is defined by theThe bit representation is defined by the inversion or non-inversion or non- inversioninversion at theat the beginning of the bitbeginning of the bit.. POLAR ENCODING&SCHEMES
- 67. BIPOLAR ENCODING Bipolar AMI encoding:Bipolar AMI encoding: Use three levels: positive, zero, andUse three levels: positive, zero, and negative.negative.
- 68. 68 Modem Standards The term modem is a composite word that refers to the two functional entities that make up the device; a signal modulator and a signal demodulator. A modulator creates a bandpass analog signal from binary data. A demodulator recovers the binary data from the modulated signal.
- 69. TELEPHONE MODEMS Traditional telephone lines can carry frequencies between 300 and 3300 HZ, giving them BW of 3000 Hz. The effective BW of a telephone line being used for data transmission is 2400 Hz,covering the range from 600 to 3000 Hz.
- 70. Modem standards V-series standards published by the ITU-T. V.32: The V.32 calls for 32-QAM with a baud rate of 2400. Because only 4 bits of each pentabit represents data, the resulting speed is 4*2400=9600 V.32 bits: The V.32 bits modem support 14,400-bps transmission. The V.32 bits uses 128-QAM transmission. V.34 bits: The V.34 bits modem support 28,800-bps transmission with a 960-point constellation to a bit rate of 33,600 with a 1664-point constellation.
- 71. Traditional modems In traditional modems data exchange is between two computers, A and B,through digital telephone network. After modulation by the modem, an analog signal reaches the telephone company Switching station. It is sampled and digitized to be passed through the digital network. The quantization noise introduced in the signal at the sampling point limits the data rate according to the capacity. This limit is 33.6 Kbps.
- 73. 56K Modems Communication is via the Internet. In Uploading, the analog signal must still be sampled at the switching station, which means the data rate in the uploading is limited to33.6 Kbps. There is no sampling in downloading. Data rate in downloading is 56Kbps
- 74. 56K Modems
- 75. RS 232 INTERFACE RS 232 is a standard interface by EIA and RS232C is the latest version of this interface. RS232 standard follows –ve logic. Logic1 is represented by negative voltage. Logic0 is represented by +ve voltage. Level 1 varies from -3 to -15v and level 0 varies from 3 to15v
- 76. 76 Data Link Layer design issues Services provided to the network layer=> Transfers the data from the network layer on the source to network layer on the destination The important services are, unacknowledged connectionless services Acknowledged connection service Acknowledged connection-oriented service
- 77. 77 Framing=> separates a message from one source to a destination or from other messages to other destinations Fixed size framing Variable size framing o Character oriented protocol=> byte stuffing o Bit oriented protocol=>data as sequence of bits and interpreted by the upper layer as text, graphics, audio, etc.
- 78. 78 Error Control Ensures proper sequencing and safe delivery of frames at destination An acknowledgement is send by destination node Positive acknowledgement=>frame is successfully delivered Negative acknowledgement=>frame not sent and the frame is to be retransmitted
- 79. 79 Flow control =>the data flow is regulated Includes a feedback mechanism requesting transmitter to retransmit the data Damaged frames Lost frame Lost acknowledgement frames Most common retransmission technique=> ARQ(Automatic repeat request)
- 80. 80 Error Detection & Correction Introduction Data can be corrupted during transmission For reliable communication, errors must be detected and corrected Error Bits flow from one point to another They subject to unpredictable changes because of interference is rightly called as an error
- 81. 81 Error Detection Types of Errors Single-Bit Errors only one bit in the data unit has changed
- 82. 82 Error Detection Burst Errors of length ‘n’: 2 or more bits in the data unit have changed
- 83. 83 Error Detection Error Detection Sender transmits every data unit twice Receiver performs bit-by-bit comparison between that two versions of data Any mismatch would indicate an error, which needs error correction. Redundancy Shorter group of bits may be appended to the end of each unit
- 84. 84 Error Detection Types of Redundancy Checks Parity Check Simple Parity Check Two Dimensional Parity Check / Longitudinal Redundancy Check (LRC) Cyclic Redundancy Check (CRC) Check Sum
- 85. 85 Parity check Parity A redundant bit called Parity Bit is added to every data unit Even Parity: total number of 1‘s in the data unit becomes even Odd Parity: total number of 1‘s in the data unit becomes odd
- 86. 86 LONGTITUDINAL REDUNDANCY CHECK LRC - Performance Detects all burst errors up to length n (number of columns) If two bits in one data unit are damaged and two bits in exactly same positions in another data unit are also damaged. Now the checker will not detect an error
- 88. 88 Cyclic redundancy check Powerful error detection scheme Rather than addition, binary division is used A sequence of redundant bits, called CRC or CRCremainder is appended to the data unit At the receiver side, the incoming data unit is divided by the same predetermined number. If there is no remainder, the data unit is accepted If there is a remainder, the receiver indicates that the data unit has been damaged during transmission
- 90. 90 Error Correction Error Correction-Hamming Code Hamming Code uses the relationship between data and redundancy bits. For example: a 7-bit ASCII code requires 4 redundancy bits, which are placed in positions 1, 2, 4 and 8 i.e. x0,x1,x2,x3 and so on.
- 91. 91 Error Correction –Hamming Code In the Hamming Code, each bit for one combination of data bits as below: r1: bits 1, 3, 5, 7, 9, 11 r2: bits 2, 3, 6, 7, 10, 11 r3: bits 4, 5, 6, 7 r4: bits 8, 9, 10, 11
- 92. 92 Error Correction –Hamming Code
- 93. 93 Data Link Control Flow Control Error Control Flow Control: How much data sender can transmit before receiving the acknowledgement Error Control: Error Detection + Error Correction ARQ o Any time, an error is discovered in an exchange, specified frames are retransmitted
- 94. 94 Flow and Error Control Mechanisms Flow and Error Control Mechanisms Stop and Wait ARQ Go-Back ARQ Selective Repeat ARQ Stop and Wait ARQ Simplest flow and error control mechanism The sending device keeps a copy of the last frame transmitted until it receives an acknowledgement Frames - alternately numbered as 0 and 1
- 95. 95 Stop-and-Wait Sender keeps a copy of the last frame until it receives an acknowledgement. For identification, both data frames and acknowledgements (ACK) frames are numbered alternatively 0 and 1. Sender has a control variable (S) that holds the number of the recently sent frame. (0 or 1) Receiver has a control variable ® that holds the number of the next frame expected (0 or 1). Sender starts a timer when it sends a frame. If an ACK is not received within a allocated time period, the sender assumes that the frame was lost or damaged and resends it Receiver send only positive ACK if the frame is intact. ACK number always defines the number of the next expected frame
- 96. 96 Stop-and-Wait ARQ, lost ACK frame When a receiver receives a damaged frame, it discards it and keeps its value of R. After the timer at the sender expires, another copy of frame 1 is sent.
- 97. 97 Stop-and-Wait, lost ACK frame • If the sender receives a damaged ACK, it discards it. • When the timer of the sender expires, the sender retransmits frame 1. • Receiver has already received frame 1 and expecting to receive frame 0 (R=0). Therefore it discards the second copy of frame 1.
- 98. 98 Stop-and-Wait, delayed ACK frame • The ACK can be delayed at the receiver or due to some problem • It is received after the timer for frame 0 has expired. • Sender retransmitted a copy of frame 0. However, R =1 means receiver expects to see frame 1. Receiver discards the duplicate frame 0. • Sender receives 2 ACKs, it discards the second ACK.
- 99. 99 Piggybacking • A method to combine a data frame with ACK. • Station A and B both have data to send. • Instead of sending separately, station A sends a data frame that includes an ACK. • Station B does the same thing. • Piggybacking saves bandwidth.
- 100. 100 Disadvantage of Stop-and-Wait • In stop-and-wait, at any point in time, there is only one frame that is sent and waiting to be acknowledged. • This is not a good use of transmission medium. • To improve efficiency, multiple frames should be in transition while waiting for ACK. • Two protocol use the above concept, – Go-Back-N ARQ – Selective Repeat ARQ
- 101. 101 Go-Back-N ARQ We can send up to W frames before worrying about ACKs. We keep a copy of these frames until the ACKs arrive. This procedure requires additional features to be added to Stop- and-Wait ARQ.
- 102. 102 Sequence Numbers • Frames from a sender are numbered sequentially. • We need to set a limit since we need to include the sequence number of each frame in the header. • If the header of the frame allows m bits for sequence number, the sequence numbers range from 0 to 2 m – 1. for m = 3, sequence numbers are: 1, 2, 3, 4, 5, 6, 7. • We can repeat the sequence number. • Sequence numbers are: 0, 1, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3, 4, 5, 6, 7, 0, 1, …
- 103. 103 Sender Sliding Window • At the sending site, to hold the outstanding frames until they are acknowledged, we use the concept of a window. • The size of the window is at most 2m -1 where m is the number of bits for the sequence number. • Size of the window can be variable, e.g. TCP. • The window slides to include new unsent frames when the correct ACKs are received
- 104. 104 Receiver Sliding Window • Size of the window at the receiving site is always 1 in this protocol. • Receiver is always looking for a specific frame to arrive in a specific order. • Any frame arriving out of order is discarded and needs to be resent. • Receiver window slides as shown in fig. Receiver is waiting for frame 0 in part a.
- 105. 105 Control Variables • Sender has 3 variables: S, SF, and SL • S holds the sequence number of recently sent frame • SF holds the sequence number of the first frame • SL holds the sequence number of the last frame • Receiver only has the one variable, R, that holds the sequence number of the frame it expects to receive. If the seq. no. is the same as the value of R, the frame is accepted, otherwise rejected.
- 106. 106 Go-Back-N ARQ, normal operation • The sender keeps track of the outstanding frames and updates the variables and windows as the ACKs arrive.
- 107. 107 Go-Back-N ARQ, lost frame • Frame 2 is lost • When the receiver receives frame 3, it discards frame 3 as it is expecting frame 2 (according to window). • After the timer for frame 2 expires at the sender site, the sender sends frame 2 and 3. (go back to 2)
- 108. 108 Go-Back-N ARQ, damaged/lost/delayed ACK • If an ACK is damaged/lost, we can have two situations: • If the next ACK arrives before the expiration of any timer, there is no need for retransmission of frames because ACKs are cumulative in this protocol. • If ACK1, ACK2, and ACk3 are lost, ACK4 covers them if it arrives before the timer expires. • If ACK4 arrives after time-out, the last frame and all the frames after that are resent. • Receiver never resends an ACK. • A delayed ACK also triggers the resending of frames
- 109. 109 Go-Back-N ARQ, sender window size • Size of the sender window must be less than 2 m . Size of the receiver is always 1. If m = 2, window size = 2 m – 1 = 3. • Fig compares a window size of 3 and 4. Accepts as the 1st frame in the next cycle-an error
- 110. 110 Selective Repeat ARQ, sender and receiver windows• Go-Back-N ARQ simplifies the process at the receiver site. Receiver only keeps track of only one variable, and there is no need to buffer out-of-order frames, they are simply discarded. • However, Go-Back-N ARQ protocol is inefficient for noisy link. It bandwidth inefficient and slows down the transmission. • In Selective Repeat ARQ, only the damaged frame is resent. More bandwidth efficient but more complex processing at receiver. • It defines a negative ACK (NAK) to report the sequence number of a damaged frame before the timer expires.
- 111. 111 Selective Repeat ARQ, lost frame • Frames 0 and 1 are accepted when received because they are in the range specified by the receiver window. Same for frame 3. • Receiver sends a NAK2 to show that frame 2 has not been received and then sender resends only frame 2 and it is accepted as it is in the range of the window.
- 112. 112 Selective Repeat ARQ, sender window size • Size of the sender and receiver windows must be at most one-half of 2 m . If m = 2, window size should be 2 m /2 = 2. Fig compares a window size of 2 with a window size of 3. Window size is 3 and all ACKs are lost, sender sends duplicate of frame 0, window of the receiver expect to receive frame 0 (part of the window), so accepts frame 0, as the 1st frame of the next cycle – an error.