PSD2A Computer Networks. Unit : I-V

Transcription

1 PSD2A Computer Networks Unit : I-V 1

2 UNIT 1 Introduction Network Hardware Software Reference Models OSI and TCP/IP models Example networks: Internet, ATM, Ethernet and Wireless LANs Physical layer Theoretical basis for data communication - guided transmission media PSD2A Computer Networks 2

3 Definition A computer network is a set of computers connected together for the purpose of sharing resources. The most common resource shared today is connection to the Internet. PSD2A Computer Networks 3

4 Network Architecture TM Network architecture is the design of a communication network. It is a framework for the specification of a network's physical components and their functional organization and configuration, its operational principles and procedures, as well as data formats use.4 PSA2D PSD2A Computer Networks 4

5 Business Applications of Networks A network with two clients and one server. PSA2D Computer Networks 5

6 Types of Network TM PSA2D Computer Networks 6

7 Design Issues for the Layers Addressing Error Control Flow Control Multiplexing Routing PSD2A Computer Networks 7

8 Network Hardware Local Area Networks Metropolitan Area Networks Wide Area Networks Wireless Networks Home Networks Internetworks PSD2A Computer Networks 8

9 Network Software Protocol Hierarchies Design Issues for the Layers Connection-Oriented and Connectionless Services Service Primitives The Relationship of Services to Protocols PSD2A Computer Networks 9

10 Connection-Oriented and Connectionless Services PSD2A Computer Networks 10

11 The OSI Reference Model TM PSD2A Computer Networks All People Seem To Need Data Processing 11

12 Reference Models PSD2A Computer Networks 12

13 The ARPANET PSD2A Computer Networks 13

14 UNIT 2 Wireless transmission Communication Satellites Telephones structure Local loop, trunks and multiplexing, switching. Data link layer Design issues Error detection and correction. PSD2A Computer Networks 14

15 Transmission Media Transmission medium -The physical path between transmitter and receiver. Repeaters or amplifiers may be used to extend the length of the medium. Communication of electromagnetic waves is guided or unguided. Guided media :: waves are guided along a physical path (e.g, twisted pair, coaxial cable and optical fiber). Unguided media:: means for transmitting but not guiding electromagnetic waves (e.g., the atmosphere and outer space). PSD2A Computer Networks 15

16 Transmission Media Choices Twisted pair Coaxial cable Optical fiber Wireless communications PSD2A Computer Networks 16

17 Twisted Pair Cable Two insulated wires arranged in a spiral pattern. Copper or steel coated with copper. The signal is transmitted through one wire and a ground reference is transmitted in the other wire. Typically twisted pair is installed in building telephone wiring. Local loop connection to central telephone exchange is twisted pair. PSD2A Computer Networks 17

18 Coaxial Cable Discussion divided into two basic categories for coax used in LANs: 50-ohm cable [baseband] 75-ohm cable [broadband or single channel baseband] In general, coaxial cable has better noise immunity for higher frequencies than twisted pair. Coaxial cable provides much higher bandwidth than twisted pair. PSD2A Computer Networks 18

19 Optical Fiber Optical fiber : A thin flexible medium capable of conducting optical rays. Optical fiber consists of a very fine cylinder of glass (core) surrounded by concentric layers of glass (cladding). A signal-encoded beam of light (a fluctuating beam) is transmitted by total internal reflection. Attenuation in the fiber can be kept low by controlling the impurities in the glass. PSD2A Computer Networks 19

20 Major Components of the Telephone System Local loops Analog twisted pairs going to houses and businesses Trunks Digital fiber optics connecting the switching offices Switching offices Where calls are moved from one trunk to another PSD2A Computer Networks 20

21 PSTN PSD2A Computer Networks 21

22 Modem Modulator-demodulator. Modulates digital signal at the source and demodulates received signal at the destination. How to transmit faster? Nyquist says that capacity is achieved at 2*H*log 2 V. So there is no point sampling faster than 2*H. But, can try to send more bits per sample. PSD2A Computer Networks 22

23 Multiplexing PSD2A Computer Networks 23

24 Frequency Division Multiplexing TM FDM Useful bandwidth of medium exceeds required bandwidth of channel Each signal is modulated to a different carrier frequency Carrier frequencies separated so signals do not overlap (guard bands) e.g. broadcast radio Channel allocated even if no data. PSD2A Computer Networks 24

25 Wavelength Division Multiplexing TM Multiple beams of light at different frequency Carried by optical fiber A form of FDM Each color of light (wavelength) carries separate data channel 1997 Bell Labs 100 beams Each at 10 Gbps Giving 1 terabit per second (Tbps) Commercial systems of 160 channels of 10 Gbps now available Lab systems (Alcatel) 256 channels at 39.8 Gbps each 10.1 Tbps Over 100km PSD2A Computer Networks 25

26 Time Division Multiplexing TM Data rate of medium exceeds data rate of digital signal to be transmitted. Multiple digital signals interleaved in time May be at bit level of blocks Time slots preassigned to sources and fixed Time slots allocated even if no data Time slots do not have to be evenly distributed amongst sources. PSD2A Computer Networks 26

27 Switching Techniques In large networks there might be multiple paths linking sender and receiver. Information may be switched as it travels through various communication channels. There are three typical switching techniques available for digital traffic. Circuit Switching Message Switching Packet Switching PSD2A Computer Networks 27

28 Circuit Switching TM Circuit switching is a technique that directly connects the sender and the receiver in an unbroken path. Telephone switching equipment, for example, establishes a path that connects the caller's telephone to the receiver's telephone by making a physical connection. With this type of switching technique, once a connection is established, a dedicated path exists between both ends until the connection is terminated. Routing decisions must be made when the circuit is first established, but there are no decisions made after that time. PSD2A Computer Networks 28

29 Message Switching TM With message switching there is no need to establish a dedicated path between two stations. When a station sends a message, the destination address is appended to the message. The message is then transmitted through the network, in its entirety, from node to node. Each node receives the entire message, stores it in its entirety on disk, and then transmits the message to the next node. This type of network is called a store-and-forward network. PSD2A Computer Networks 29

30 Packet Switching TM Packet switching can be seen as a solution that tries to combine the advantages of message and circuit switching and to minimize the disadvantages of both. There are two methods of packet switching: Datagram and virtual circuit. PSD2A Computer Networks 30

31 Data Link Layer Design Issues Services Provided to the Network Layer Framing Error Control Flow Control Functions of the Data Link Layer Provide service interface to the network layer Dealing with transmission errors Regulating data flow Slow receivers not swamped by fast senders PSD2A Computer Networks 31

32 Framing A character stream. (a) Without errors. (b) With one error. PSD2A Computer Networks 32

33 Error Detection and Correction Error-Correcting Codes Error-Detecting Codes Key idea: redundant (additional) bits (use only 2 m out of 2 n code words) Key metrics: how many errors can be corrected/detected; what burst length PSD2A Computer Networks 33

34 Hamming Distance Some codes words are valid; others are invalid Hamming distance between two code words is number of bits that must be flipped to change from one to the other If Hamming distance is d then d single bit errors needed to change one word to the other Hamming distance of a code is the minimum Hamming distance between two valid code words PSD2A Computer Networks 34

35 CRC Schemes CRC Cyclic Redundancy Check or polynomial code Consider bits of a message to be coefficients of a polynomial M(x) x 3 + 0x 2 + 1x 1 + 1x 0 Of course real messages will be much longer and hence of higher degree Agree on a small-degree generator polynomial G(x) of degree r Divide x r M(x) by G(x) using modulo 2 division (no carries or borrows) getting the remainder polynomial R(x) Transmit T(x) = x r M(x) - R(x); note that this has remainder 0 when divided by G(x) Receiver rejects frame if the remainder it computers is not 0 PSD2A Computer Networks 35

36 Error-Detecting Codes TM Calculation of the polynomial code checksum. PSD2A Computer Networks 36

37 UNIT-3 Elementary data link protocols Sliding window protocols Data Link Layer in the Internet Medium Access Layer Channel Allocation Problem Multiple Access Protocols. PSD2A Computer Networks 37

38 Elementary Data Link Protocols An Unrestricted Simplex Protocol A Simplex Stop-and-Wait Protocol Simplex Protocol for a Noisy Channel Sliding Window Protocols A One-Bit Sliding Window Protocol A Protocol Using Go Back N A Protocol Using Selective Repeat PSD2A Computer Networks 38 Slide number / Total slides

39 Sliding Window Protocols A sliding window of size 1, with a 3-bit sequence number. (a) Initially. (b) After the first frame has been sent. (c) After the first frame has been received. (d) After the first acknowledgement has been received. PSD2A Computer Networks 39

40 The Data Link Layer in the Internet A home personal computer acting as an internet host. PSD2A Computer Networks 40

41 Channel Allocation Problem Static channel allocation Assumptions for dynamic Assumptions for Dynamic Channel Allocation 1. Independent traffic 2. Single channel 3. Observable Collisions 4. Continuous or slotted time 5. Carrier sense or no carrier sense PSD2A Computer Networks 41

42 Multiple Access Protocols TM ALOHA Carrier Sense Multiple Access Collision-free protocols Limited-contention protocols Wireless LAN protocols PSD2A Computer Networks 42

43 Multiple Access Protocols Random Access Protocols Aloha Slotted Aloha Carrier sense multiple access (CSMA): Ethernet Group random access Controlled Access Protocols Predetermined allocation (TDMA) Reservation protocols Token passing protocols PSD2A Computer Networks 43

44 Aloha If you have a packet, just send it. If multiple people try it and so there is collision, then try resending it later! Theoretical analysis (based on Poisson distribution) shows a throughput of only 18%. Slotted Aloha Synchronous, that is time is divided into slots Slot size is equal to the transmission time of a packet When you are ready, transmit at the start of the time slot. Doubles the efficiency of Aloha (38% throughput) But requires synchronization! PSD2A Computer Networks 44

45 Carrier Sense Multiple Access CSMA Listen to channel. If busy then wait for a random time and then listen again. If not busy then transmit Collision may still happen p-persistent CSMA Quite like CSMA. But, when the channel is idle then you transmit with probability p. Otherwise, even when it is idle, you wait for a random time before you listen to the channel. PSD2A Computer Networks 45

46 CSMA/CD CSMA with collision detection(cd): Listen while you are sending packets Stop sending when collision happens Wait random time before you attempt to resend. IEEE standard Used in coaxial cable. You do exponetial backoff. PSD2A Computer Networks 46

47 UNIT-4 Network layer Design issues Routing algorithms Congestion control algorithms IP protocol IP Address Internet Control Protocol. PSD2A Computer Networks 47

48 Network Layer Design Issues TM Store-and-forward packet switching Services provided to transport layer Implementation of connectionless service Implementation of connection-oriented service Comparison of virtual-circuit and datagram networks PSD2A Computer Networks 48

49 Store-and-Forward Packet Switching ISP s equipment The environment of the network layer protocols. PSD2A Computer Networks 49

50 Services Provided to the Transport Layer 1. Services independent of router technology. 2. Transport layer shielded from number, type, topology of routers. 3. Network addresses available to transport layer use uniform numbering plan - even across LANs and WANs PSD2A Computer Networks 50

51 Implementation of Connectionless Service A s table (initially) A s table (later) C s Table E s Table Routing within a datagram network PSD2A Computer Networks 51

52 Implementation of TM Connection-Oriented Service ISP s equipment A s table C s Table E s Table Routing within a virtual-circuit network PSD2A Computer Networks 52

53 Routing Algorithms Optimality principle Shortest path algorithm Flooding Distance vector routing Link state routing Routing in ad hoc networks PSD2A Computer Networks 53

54 The Optimality Principle (a) A network. (b) A sink tree for router B. PSD2A Computer Networks 54

55 Shortest Path Algorithm TM The first five steps used in computing the shortest path from A to D. The arrows indicate the working node 55 PSD2A Computer Networks

56 Distance Vector Routing (a) A network. (b) Input from A, I, H, K, and the new routing table for J. PSD2A Computer Networks 56

57 Link State Routing 1. Discover neighbors, learn network addresses. 2. Set distance/cost metric to each neighbor. 3. Construct packet telling all learned. 4. Send packet to, receive packets from other routers. 5. Compute shortest path to every other router. PSD2A Computer Networks 57

58 Broadcast Routing TM Reverse path forwarding. (a) A network. (b) A sink tree. (c) The tree built by reverse path forwarding. PSD2A Computer Networks 58

59 Multicast Routing (a) A network. (b) A spanning tree for the leftmost router. (c) A multicast tree for group 1. (d) A multicast tree for group 2. PSD2A Computer Networks 59

60 Congestion Control Algorithms Approaches to congestion control Traffic-aware routing Admission control Traffic throttling Load shedding PSD2A Computer Networks 60

61 Congestion Control Algorithms When too much traffic is offered, congestion sets in and performance degrades sharply. PSD2A Computer Networks 61

62 The Network Layer in the Internet The Internet is an interconnected collection of many networks. PSD2A Computer Networks 62

63 Unit 5 Transport layer design issues Connection management Addressing, Establishing & Releasing a connection Simple Transport Protocol Internet Transport Protocol (TCP) Network Security: Cryptography. PSD2A Computer Networks 63

64 The Transport Service Services Provided to the Upper Layers Transport Service Primitives Berkeley Sockets An Example of Socket Programming: An Internet File Server PSD2A Computer Networks 64

65 Elements of Transport Protocols Addressing Connection Establishment Connection Release Flow Control and Buffering Multiplexing Crash Recovery PSD2A Computer Networks 65

66 Transport Protocol TM (a) Environment of the data link layer. (b) Environment of the transport layer. PSD2A Computer Networks 66

67 Connection Release The two-army problem. PSD2A Computer Networks 67

68 Multiplexing (a) Upward multiplexing. (b) Downward multiplexing. PSD2A Computer Networks 68

69 A Simple Transport Protocol The Example Service Primitives The Example Transport Entity The Example as a Finite State Machine PSD2A Computer Networks 69

70 The Internet Transport Protocols: UDP Introduction to UDP Remote Procedure Call The Real-Time Transport Protocol PSD2A Computer Networks 70

71 The TCP Service Model Port Protocol Use 21 FTP File transfer 23 Telnet Remote login 25 SMTP 69 TFTP Trivial File Transfer Protocol 79 Finger Lookup info about a user 80 HTTP World Wide Web 110 POP-3 Remote access 119 NNTP USENET news Some assigned ports. PSD2A Computer Networks 71

72 Cryptography Introduction to Cryptography Substitution Ciphers Transposition Ciphers One-Time Pads Two Fundamental Cryptographic Principles PSD2A Computer Networks 72

73 Transposition Ciphers A transposition cipher. PSD2A Computer Networks 73

74 Communication Security TM IPsec Firewalls Virtual Private Networks Wireless Security Security PGP Pretty Good Privacy PEM Privacy Enhanced Mail S/MIME PSD2A Computer Networks 74