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Wireless Networks

Wireless Networks

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Wireless Networks

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  1. Wireless Networks Instructor: Fatima Naseem Lecture # 02 Computer Engineering Department, University of Engineering and Technology, Taxila

  2. Protocols and the TCP/IP Suite Chapter 4

  3. Key Features of a Protocol • Syntax • Concerns the format of the data blocks • Semantics • Includes control information for coordination and error handling • Timing • Includes speed matching and sequencing

  4. Agents Involved in Communication • Applications • Exchange data between computers (e.g., electronic mail) • Computers • Connected to networks • Networks • Transfers data from one computer to another

  5. Principles to Arrive at Layers • Layer should be created where different abstraction needed • Each layer performs well defined functions • Layer boundaries should be chosen to minimize flow across the interfaces • No of layers should be • Large enough so that distinct functions are not thrown in same layer • Small enough to avoid repetition

  6. TCP/IP Layers • Physical layer • Network access layer • Internet layer • Host-to-host, or transport layer • Application layer

  7. TCP/IP Physical Layer • Covers the physical interface between a data transmission device and atransmission medium or network • Physical layer specifies: • Characteristics of the transmission medium • The nature of the signals • The data rate • Other related matters

  8. TCP/IP Network Access Layer • Concerned with the exchange of data between an end system and the network to which it's attached • Software used depends on type of network • Circuit switching • Packet switching (e.g., X.25) • LANs (e.g., Ethernet) • Others

  9. T:TCP/IP Internet Layer • Uses internet protocol (IP) • Provides routing functions to allow data to traverse multiple interconnected networks • Implemented in end systems and routers

  10. TCP/IP Host-to-Host, or Transport Layer • Commonly uses transmission control protocol (tcp) • Provides reliability during data exchange • Completeness • Order

  11. TCP/IP Application Layer • Logic supports user applications • Uses separate modules that are peculiar to each different type of application

  12. Protocol Data Units (PDUs)

  13. Common TCP/IP Applications • Simple mail transfer protocol (SMTP) • Provides a basic electronic mail facility • File Transfer Protocol (FTP) • Allows files to be sent from one system to another • TELNET • Provides a remote logon capability

  14. Layers of the OSI Model • Application • Presentation • Session • Transport • Network • Data link • Physical

  15. OSI Application Layer • Provides access to the OSI environment for users • Provides distributed information services

  16. OSI Presentation Layer • Provides independence to the application processes from differences in data representation (syntax)

  17. OSI Session Layer • Provides the control structure for communication between applications • Establishes, manages, and terminates connections (sessions) between cooperating applications

  18. OSI Transport Layer • Provides reliable, transparent transfer of data between end points • Provides end-to-end error recovery and flow control

  19. OSI Network Layer • Provides upper layers with independence from the data transmission and switching technologies used to connect systems • Responsible for establishing, maintaining, and terminating connections

  20. OSI Data link Layer • Provides for the reliable transfer of information across the physical link • Sends blocks (frames) with the necessary synchronization, error control, and flow control

  21. OSI Physical Layer • Concerned with transmission of unstructured bit stream over physical medium • Deals with accessing the physical medium • Mechanical characteristics • Electrical characteristics • Functional characteristics • Procedural characteristics

  22. Comparison of OSI and TCP/IP

  23. TCP/IP Architecture Dominance • TCP/IP protocols matured quicker than similar OSI protocols • When the need for interoperability across networks was recognized, only TCP/IP was available and ready to go • OSI model is unnecessarily complex • Accomplishes in seven layers what TCP/IP does with fewer layers

  24. Elements of Standardization within OSI Framework • Protocol Specification • Format of protocol data units (PDUs) exchanged • Semantics of all fields • Allowable sequence of PDUs • Service Definition • Functional description that defines what services are provided, but not how the services are to be provided • Addressing • Entities are referenced by means of a service access point (SAP)

  25. Internetworking Terms • Communication network – facility that provides a data transfer service among devices attached to the network • Internet – collection of communication networks, interconnected by bridges/routers • Intranet – internet used by an organization for internal purposes • Provides key Internet applications • Can exist as an isolated, self-contained internet

  26. Internetworking Terms • End System (ES) – device used to support end-user applications or services • Intermediate System (IS) – device used to connect two networks • Bridge – an IS used to connect two LANs that use similar LAN protocols • Router - an IS used to connect two networks that may or may not be similar

  27. Functions of a Router • Provide a link between networks • Provide for the routing and delivery of data between processes on end systems attached to different networks • Provide these functions in such a way as not to require modifications of the networking architecture of any of the attached subnetworks

  28. Network Differences Routers Must Accommodate • Addressing schemes • Different schemes for assigning addresses • Maximum packet sizes • Different maximum packet sizes requires segmentation • Interfaces • Differing hardware and software interfaces • Reliability • Network may provide unreliable service

  29. Antennas and Propagation Chapter 5

  30. Introduction • An antenna is an electrical conductor or system of conductors • Transmission - radiates electromagnetic energy into space • Reception - collects electromagnetic energy from space • In two-way communication, the same antenna can be used for transmission and reception

  31. Radiation Patterns • Radiation pattern • An antenna radiates in all directions but does not perform well in all directions • One way of performance characterization is radiation pattern • Graphical representation of radiation properties of an antenna • Depicted as two-dimensional cross section • The distance from the antenna to each point on the radiation pattern is proportional to the power radiated in that direction

  32. Radiation Patterns • Beam width (or half-power beam width) • Measure of directivity of antenna • Reception pattern • Receiving antenna’s equivalent to radiation pattern

  33. Types of Antennas • Isotropic antenna (idealized) • Radiates power equally in all directions • Dipole antennas • Half-wave dipole antenna (or Hertz antenna) • Quarter-wave vertical antenna (or Marconi antenna) • Parabolic Reflective Antenna

  34. Antenna Gain • Antenna gain • Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) • Effective area • Related to physical size and shape of antenna

  35. Antenna Gain • Relationship between antenna gain and effective area • G = antenna gain • Ae= effective area • f = carrier frequency • c = speed of light (» 3 ´ 108 m/s) •  = carrier wavelength

  36. Propagation Modes • Ground-wave propagation • Sky-wave propagation • Line-of-sight propagation

  37. Ground Wave Propagation

  38. Ground Wave Propagation • Follows contour of the earth • Can Propagate considerable distances • Frequencies up to 2 MHz • Example • AM radio

  39. Sky Wave Propagation

  40. Sky Wave Propagation • Signal reflected from ionized layer of atmosphere back down to earth • Signal can travel a number of hops, back and forth between ionosphere and earth’s surface • Reflection effect caused by refraction • Examples • Amateur radio • CB radio

  41. Line-of-Sight Propagation

  42. Line-of-Sight Propagation • Transmitting and receiving antennas must be within line of sight • Satellite communication – signal above 30 MHz not reflected by ionosphere • Ground communication – antennas within effective line of site due to refraction • Refraction – bending of microwaves by the atmosphere • Velocity of electromagnetic wave is a function of the density of the medium • When wave changes medium, speed changes • Wave bends at the boundary between mediums

  43. Line-of-Sight Equations • Optical line of sight • Effective, or radio, line of sight • d = distance between antenna and horizon (km) • h = antenna height (m) • K = adjustment factor to account for refraction, rule of thumb K = 4/3

  44. Line-of-Sight Equations • Maximum distance between two antennas for LOS propagation: • h1 = height of antenna one • h2 = height of antenna two

  45. LOS Wireless Transmission Impairments • Attenuation and attenuation distortion • Free space loss • Noise • Atmospheric absorption • Multipath • Refraction • Thermal noise

  46. Attenuation • Strength of signal falls off with distance over transmission medium • Attenuation factors for unguided media: • Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal • Signal must maintain a level sufficiently higher than noise to be received without error • Attenuation is greater at higher frequencies, causing distortion

  47. Free Space Loss • Free space loss, ideal isotropic antenna • Pt = signal power at transmitting antenna • Pr = signal power at receiving antenna •  = carrier wavelength • d = propagation distance between antennas • c = speed of light (» 3 ´ 10 8 m/s) where d and  are in the same units (e.g., meters)

  48. Free Space Loss • Free space loss equation can be recast: