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Midterm Exam Review

Midterm Exam Review Communication Networks A communication network provides a general solution to the problem of connecting many devices: Connect each device to a network node (router) Network nodes exchange information and carry the information from a source device to a destination device

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Midterm Exam Review

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  1. Midterm ExamReview COMP361 by M. Hamdi

  2. Communication Networks • A communication network provides a general solution to the problem of connecting many devices: • Connect each device to a network node (router) • Network nodes exchange information and carry the information from a source device to a destination device • Note: Network nodes do not generate information • Connect devices to a single shared medium (LAN) COMP361 by M. Hamdi

  3. Communication Networks • A generic communication network: Other names for Device: station, host, terminal Other names for Node: switch, router, gateway COMP361 by M. Hamdi

  4. Classification of Communications • Communication networks can be classified based on the way in which the nodes exchange information: • Communication Network • Switched Communication Network • Circuit-Switched Communication Network • Packet-Switched Communication Network • Datagram Network • Virtual Circuit Network • Broadcast Communication Network COMP361 by M. Hamdi

  5. Broadcast Network Examples Packet Radio Network Satellite Network Bus Local Network COMP361 by M. Hamdi

  6. Circuit Switching • A node in a circuit-switching network: COMP361 by M. Hamdi

  7. Circuit Switching COMP361 by M. Hamdi

  8. Packet Switching COMP361 by M. Hamdi

  9. Datagram Packet Switching COMP361 by M. Hamdi

  10. Virtual-Circuit Packet Switching COMP361 by M. Hamdi

  11. Network Technologies • Telephone Networks • IP Networks • ATM Networks COMP361 by M. Hamdi

  12. Three Network Technologies • Telephone Network • The largest worldwide computer network, specialized for voice • Switching technique: Circuit-switching • Internet • A newer global and public information infrastructure • Switching technique: Datagram packet switching • ATM • Was intended to replace telephone networks and data networks, but lost momentum due the success of the Internet • Switching technique: VC packet switching COMP361 by M. Hamdi

  13. Telephone Networks Starting in 1876, the public switched telephone network (PSTN) has become a global infrastructure for voice communications COMP361 by M. Hamdi

  14. Addressing and Routing • Each subscriber has an address (telephone number) • Addresses are hierarchical • The information contained in a telephone address is exploited when establishing a route from caller to callee Country code Number of local exchange Subscriber number Area code 852 2358 6984 My office number COMP361 by M. Hamdi

  15. The Internet - A Network of Networks • The Internet is a loose collection of networks • Networks are organized in a (loose) multi-layer hierarchy COMP361 by M. Hamdi

  16. What defines the Internet • Use of a globally unique address space (Internet Addresses) • Support of the Transmission Control Protocol/Internet Protocol (TCP/IP) suite for communications • The physical networks widely differ (cable, optical, wireless, radio, etc.) - IP on top of ANYTHING. COMP361 by M. Hamdi

  17. Internet Addresses • Each network interface on the Internet has a unique global address, called the IP address. • An IP address: • is 32 bits long • encodes a network number and a host number • IP addresses are written in a dotted decimal notation. means: • 10000000 in 1 st Byte • 10001110 in 2 nd Byte • 10001000 in 3 rd Byte • 10010011 in 4 th Byte COMP361 by M. Hamdi

  18. Domain Names and IP Addresses • Users and applications on the Internet normally do not use IP addresses directly. No one says: • Rather users and applications use domain names: http://www.cs.ust.hk • A service on the Internet, called the Domain Name System (DNS) performs the translation between domain names and IP addresses COMP361 by M. Hamdi

  19. Traditional Network Infrastructure COMP361 by M. Hamdi

  20. B-ISDN COMP361 by M. Hamdi

  21. Protocol Architecture • Layered Protocol Architectures • OSI Reference Model • TCP/IP Protocol Stack COMP361 by M. Hamdi

  22. Need for Protocols • The task of exchanging information between devices • requires a high degree of cooperation between the involved parties • can be quite complex • Protocols are a set of rules and conventions. By enforcing that communicating parties adhere to a common protocol, communication is made possible. • The complexity of the communication task is reduced by dividing it into subtasks: • Each subtask is implemented independently. • Each subtask provides a service to another subtask. COMP361 by M. Hamdi

  23. OSI Reference Model • In 1977 the International Standardization Organization (ISO) developed a model for a layered network architecture • This effort was completed in 1983 and is known as the Open Systems Interconnection (OSI) Reference Model • The OSI model defines seven layers: • Layer 7: Application Layer • Layer 6: Presentation Layer • Layer 5: Session Layer • Layer 4: Transport Layer • Layer 3: Network Layer • Layer 2: Data Link Layer • Layer 1: Physical Layer • (Layer 0: Interconnection Media) COMP361 by M. Hamdi

  24. OSI Layers COMP361 by M. Hamdi

  25. OSI Layers and Encapsulation COMP361 by M. Hamdi

  26. OSI Model in a Switched Communication Network COMP361 by M. Hamdi

  27. TCP/IP Protocol Suite • The TCP/IP protocol suite was first defined in 1974 • The TCP/IP protocol suite is the protocol architecture of the Internet • The TCP/IP suite has four layers: Application, Transport, Internet, and Network Interface Layer COMP361 by M. Hamdi

  28. Encapsulation in the TCP/IP Suite • As data is moving down the protocol stack, each protocol is adding layer-specific control information. COMP361 by M. Hamdi

  29. Comparison of OSI Model and TCP/IP Suite COMP361 by M. Hamdi

  30. Physical Layer • Fundamentals • Transmissions factors • Transmission Media COMP361 by M. Hamdi

  31. Physical Layer • The physical layer deals with transporting bits between two machines. • The goal is to understand what happens to a signal as it travels across some physical media. COMP361 by M. Hamdi

  32. Theoretical Basis for Data Communication • Fourier AnalysisFourier showed that a periodic function g(t) can be represented mathematically as an infinite series of sines and cosines: • fis the function's fundamentalfrequency • T=1/f is the function's period • an  and bn are the amplitudes of the nth harmonics COMP361 by M. Hamdi

  33. Theoretical Basis for Data Communication • The series representation of g(t) is called its Fourierseries expansion. • In communications, we can always represent a data signal using a Fourier series by imagining that the signal repeats the same pattern forever. COMP361 by M. Hamdi

  34. Theoretical Basis for Data Communication • We can compute the coefficients  an and bn • Suppose we use voltages (on/off) to represent ``1''s and ``0''s, and we transmit the bit string ``011000010'. The signal would look as follows: COMP361 by M. Hamdi

  35. Theoretical Basis for Data Communication COMP361 by M. Hamdi

  36. Theoretical Basis for Data Communication • Points to note about the Fourier expansion • The more terms in the expansion, the more exact our representation becomes. • The expression represents the amplitude or energy of the signal (e.g., the harmonics contribution to the wave). COMP361 by M. Hamdi

  37. Theoretical Basis for Data Communication • Conclusion: it's essentially impossible to receive the exact signal that was sent. The key is to receive enough of the signal so that the receiver can figure out what the original signal was. • Note: ``bandwidth'' is an overloaded term. Engineers tend to use bandwidth to refer to the spectrum of signals a channel carries. In contrast, the term ``bandwidth'' is often also used to refer to the data rate of the channel, in bps. COMP361 by M. Hamdi

  38. Nyquist Theorem • Noise-free channel • Limiting factor on transmission is channel BW • If bandwidth is B, highest signal rate is 2B • Multi-level signaling: C = 2B log2 M; where: C is the data rate B is the bandwidth M is the number of levels • For example, a noiseless 3-kHz channel cannot transmit binary signals at a rate exceeding 6000 bps. COMP361 by M. Hamdi

  39. Shannon’s Theorem • If random noise is present, the situation deteriorates rapidly. The amount of noise present is measured by the ratio of the signal power to the noise power, called the signal-to-noise ratio (S/N). • Usually, the ratio itself is not quoted; instead, the quantity 10 log10S/N is given. These units are called decibels (dB). • Maximum number of bits/sec=Hlog2(1+S/N) • For telephone line: 3000log2(1+30dB)30000bps. COMP361 by M. Hamdi

  40. Transmission Media • The purpose of the physical layer is to transport a raw bit stream from one machine to another. • Various physical media can be used for the actual transmission. • Each one has its own niche in terms of bandwidth, delay, cost, and ease of installation and maintanence. • Media are roughly grouped into guided media, such as copper wire and fiber optics, and unguided media such as radio and lasers through air. COMP361 by M. Hamdi

  41. Transmission Media • Twisted Pair • Coaxial Cable • Fiber Optic COMP361 by M. Hamdi

  42. Transmission Media:Wireless Transmission • Radio : omnidirectional, AM, FM Radio, TV, ALOHA data network • Microwave : directional • Terrestrial Microwave, long-haul common carrier, government communications. • Satellite Microwave • A communication satellite is a microwave relay station. COMP361 by M. Hamdi

  43. Data Link Layer • Framing • Error Detection • Flow Control • Error Control (via Retransmission) COMP361 by M. Hamdi

  44. Introduction Main Task of the data link layer: • Provide error-free transmission over a physical link COMP361 by M. Hamdi

  45. Introduction • The PDU at the Data Link Layer (DL-PDU) is typically called a Frame. A Frame has a header, a data field, and a trailer • Example COMP361 by M. Hamdi

  46. Framing • Problem: Identify the beginning and the end of a frame in a bit stream • Solution (bit-oriented Framing): A special bit pattern (flag) signals the beginning and the end of a frame (e.g., "01111110") – use bit stuffing • Problem: The sequence “01111110” must not appear in the data of the frame COMP361 by M. Hamdi

  47. Error Control • Two basic approaches to handle bit errors: • Error-correcting codes • Too many additional bits are needed for correction (used only in simplex communication (e.g., satellite)) • Error-detecting codes plus retransmission • Used if retransmission of corrupted data is feasible • Receiver detects error and requests retransmission of a frame. COMP361 by M. Hamdi

  48. Cyclic-Redundancy Codes (CRC) General Method: • The transmitter generates an n-bit check sequence number (known as Frame Checksum Sequence (FCS)) from a given k-bit frame such that the resulting (k+n)-bit frame is divisible by some number • The receiver divides the incoming frame by the same number • If the result of the division does not leave a remainder, the receiver assumes that there was no error COMP361 by M. Hamdi

  49. Step 2: CRC Encoding Method Define: • M(x): Data block is a polynomial (= Message, Frame) • P(x): "Generator Polynomial" which is known to both sender and receiver (degree of P(x) is n) COMP361 by M. Hamdi

  50. Step 2: CRC Encoding Method • (I) Append n zeros to M(x), i.e., M(x)*x^n • (II) Divide M(x)*x^n by P(x) and obtain: • M(x)*x^n = Q(x)P(x) + R(x) • (III) Set T(x) = M(x)*x^n + R(x). T(x) is the encoded message Note: T(x) is divisible by P(x). Therefore, if the received message does not contain an error then it can be divided by P(x). COMP361 by M. Hamdi

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