Introduction

1. Introduction • Basic concepts • Terminology

2. Ubiquitous Computing • Computers everywhere. • Also means ubiquitous communication • Users connected anywhere/anytime. • PC (laptop, palmtop) equivalent to cell phone. • Networking computers together is critical!

3. Computer Network • Provide access to local and remote resources. • Collection of interconnected end systems: • Computing devices (mainframes, workstations, PCs, palm tops) • Peripherals (printers, scanners, terminals). • Applications: location transparency.

4. Computer Networks (cont’d) • Components: • End systems (or hosts), • Routers/switches/bridges, and • Links (twisted pair, coaxial cable, fiber, radio, etc.).

5. Communication Model Network Source Destination

6. Example PTN Modem Modem Source Destination Source System Destination System PTN: Public Telephone Network

7. Connecting End Systems Dedicated link Multiple access / shared medium

8. Connecting End Systems (cont’d) Router Switched network Router: switching element; a.k.a., IMPs (Interface Message Processors) in ARPAnet’s terminology.

9. Shared Communication Infrastructure • Shared medium: • Examples: ethernet, radio. • How to acquire channel: medium access control protocols. • Switched networks: • Shared infrastructure consisting of point-to-point links. • Circuit- versus packet-switching.

10. Circuit Switching • Establish dedicated path (circuit) between source and destination. • Example: telephone network. • +’s: dedicated resources(stream-oriented). • -’s: lower resource utilization (e.g.,bursts).

11. Packet Switching S1 D1 D2 S2 • Data split into transmission units, or packets. • Routers: store packets briefly store packets and forward them: store-and-forward. • Efficient resource use: statistical multiplexing. • Ability to accommodate bursts.

12. (Switched) Network Topologies Ring Tree Star Irregular

13. Protocol • Set of rules that allow peering entities to communicate. • Example: 2 friends talking on the phone. • Peering entities or peers: user application programs, file transfer services, e-mail services, etc.

14. Network Architecture • Protocol layers: reduce design complexity. • Main idea: each layer uses the services from lower layer and provide services to upper layer. • Higher layer shielded from the implementation details of lower layers. • Interface between layers must be clearly defined: services provided to upper layer.

15. Example 1: ISO OSI Model • ISO: International Standards Organization • OSI: Open Systems Interconnection. Application Presentation Session Transport Network Data link Physical

16. OSI ISO 7-Layer Model • Physical layer: transmission of bits. • Data link layer: reliable transmission over physical medium; synchronization, error control, flow control; media access in shared medium. • Network layer: routing and forwarding; congestion control; internetworking.

17. OSI ISO 7-Layer Model (cont’d) • Transport layer: error, flow, and congestion control end-to-end. • Session layer: manages connections (sessions) between end points. • Presentation layer: data representation. • Application layer: provides users with access to the underlying communication infrastructure.

18. Example 2: TCP/IP Model • Model employed by the Internet. ISO OSI TCP/IP Application Application Presentation Session Transport Transport Internet Network Network Access Data link Physical Physical

19. TCP/IP Protocol Suite: • Physical layer: same as OSI ISO model. • Network access layer: medium access and routing over single network. • Internet layer: routing across multiple networks, or, an internet. • Transport layer: end-to-end error, congestion, flow control functions. • Application layer: same as OSI ISO model.

20. The Internet: Some History • Late 1970’s/ early 1980’s: the ARPANET (funded by ARPA). • Connecting university, research labs and some government agencies. • Main applications: e-mail and file transfer. • Features: • Decentralized, non-regulated system. • No centralized authority. • No structure. • Network of networks.

21. The Internet (cont’d) • Early 1990’s, the Web caused the Internet revolution: the Internet’s killer app! • Today: • Almost 60 million hosts as of 01.99. • Doubles every year.

22. Topics for Further Reading • Some Internet governing entities: • IAB • IETF • IRTF • The Internet’s standardization process. • Other network standardization bodies. • Other networks (Bitnet, SNA, etc).

23. Physical Layer • Sending raw bits across “the wire”. • Issues: • What’s being transmitted. • Transmission medium.

24. Basic Concepts • Signal: electro-magnetic wave carrying information. • Time domain: signal as a function of time. • Analog signal: signal’s amplitude varies continuously over time, ie, no discontinuities. • Digital signal: data represented by sequence of 0’s and 1’s (e.g., square wave).

25. Time Domain • Periodic signals: • Same signal pattern repeats over time. • Example: sine wave • Amplitude (A) • Period (or frequency) (T = 1/f) • Phase(f)

26. Frequency Domain • Signal consists of components of different frequencies. • Spectrum of signal: range of frequencies signal contains. • Absolute bandwidth: width of signal’s spectrum.

27. Example: S(f) • Spectrum of S(f) extends from f1 to 3f1. • Bandwidth is 2f1. f 1 3 2

28. Bandwidth and Data Rate • Data rate: rate at which data is transmitted; unit is bits/sec or bps (applies to digital signal). • Example: 2Mbits/sec, or 2Mbps. • Digital signal has infinite frequency components, thus infinite bandwidth. • If data rate of signal is W bps, good representation achieved with 2W Hz bandwidth.

29. Baud versus Data Rate • Baud rate: number of times per second signal changes its value (voltage). • Each value might “carry” more than 1 bit. • Example: 8 values of voltage (0..7); each value conveys 3 bits, ie, number of bits = log2V. • Thus, bit rate = log2V * baud rate. • For 2 levels, bit rate = baud rate.

30. Data Transmission 1 • Analog and digital transmission. • Example of analog data: voice and video. • Example of digital data: character strings • Use of codes to represent characters as sequence of bits (e.g., ASCII). • Historically, communication infrastructure for analog transmission. • Digital data needed to be converted: modems (modulator-demodulator).

31. Digital Transmission • Current trend: digital transmission. • Cost efficient: advances in digital circuitry (VLSI). • Advantages: • Data integrity: better noise immunity. • Security: easier to integrate encryption algorithms. • Channel utilization: higher degree of multiplexing (time-division mux’ing).

32. Transmission Impairments • Cause received signal to differ from original, transmitted signal. • Analog data: quality degradation • Digital data: bit errors. • Types of impairments: • Attenuation. • Delay distortion. • Noise.

33. Attenuation 1 • Weakening of the signal’s power as it propagates through medium. • Function of medium type • Guided medium: logarithmic with distance. • Unguided medium: more complex (function of distance and atmospheric conditions).

34. Attenuation 2 • Problems and solutions: • Insufficient signal strength for receiver to interpret it: use amplifiers/repeaters to boost/regenerate signal. • Error due to noise interference (level is not high enough to be distinguished from noise): use amplifiers/repeaters. • Attenuation increases with frequency: special amplifiers to amplify high-frequencies.

35. Delay Distortion • Speed of propagation in guided media varies with frequency. • Different frequency components arrive at receiver at different times. • Solution: equalization techniques to equalize distortion for different frequencies.

36. Noise • Noise: undesired signals inserted anywhere in the source/destination path. • Different categories: thermal (white), crosstalk, impulse, etc.

37. Decibel and Signal-to-Noise Ratio • Decibel (dB): measures relative strength of 2 signals. • Example: S1 and S2 with powers P1 and P2. NdB = 10 log10 (P1/P2) • Signal-to-noise ratio (S/N): • Measures signal quality. • S/NdB = 10 log10 (signal power/noise power)

38. Channel Capacity 1 • Rate at which data can be transmitted over communication channel. • Noise-free channel: Nyquist Theorem • Limitation of data rate is signal’s bandwidth. • Given channel bandwidth W, highest signal rate (or baud rate) is 2W. • From receiver’s point of view: sampling at rate 2W can reconstruct signal.

39. Channel Capacity 2 • Using data rate, • C = 2W log2V, where V is number voltage levels. • Same bandwidth, increasing number of signal levels, increases data rate, but more complex signal recognition at receiver and more noise-prone. • This is a theoretical upper bound, since channels are noisy.

40. Channel Capacity 3 • Noisy channel: Shannon’s Theorem • Given channel with W (Hz) bandwidth and S/N (dB) signal-to-noise ratio, C (bps) is • C = W log2 (1+S/N) • Theoretical upper bound since assumes only thermal noise (no impulse noise, etc).

41. Transmission Media • Physically connect transmitter and receiver carrying signals in the form electromagnetic waves. • Types of media: • Guided: waves guided along solid medium such as copper twisted pair, coaxial cable, optical fiber. • Unguided: “wireless” transmission (atmosphere, outer space).

42. Guided Media: Examples 1 • Twisted Pair: • 2 insulated copper wires arranged in regular spiral. Typically, several of these pairs are bundled into a cable. • Cheapest and most widely used; limited in distance, bandwidth, and data rate. • Applications: telephone system (home-local exchange connection). • Unshielded and shielded twisted pair.

43. Examples 2 • Coaxial Cable • Hollow outer cylinder conductor surrounding inner wire conductor; dielectric (non-conducting) material in the middle. • Applications: cable TV, long-distance telephone system, LANs. • +’s: Higher data rates and frequencies, better interference and crosstalk immunity. • -’s: Attenuation and thermal noise.

44. Examples 3 • Optical Fiber • Thin, flexible cable that conducts optical waves. • Applications: long-distance telecommunications, LANs. • +’s: greater capacity, smaller and lighter, lower attenuation, better isolation,

45. Unguided, Wireless Media • Microwave: directional, LOS transmission. • Satellite: directional, LOS, large delay, high bandwidth. • Radio: omnidirectional (broadcast), single hop (cellular), multi-hop (ad hoc net’s). • Infrared: directional, LOS transmission, cannot penetrate obstacles and used outdoors.

46. Data Encoding • Transforming original signal just before transmission. • Both analog and digital data can be encoded into either analog or digital signals. Chapter 4 EE/CS 450 Fall 99

47. Digital/Analog Encoding Encoding: g(t) g(t) (D/A) Encoder Digital Medium Decoder Source Destination Source System Destination System Modulation: g(t) g(t) (D/A) Modulator Analog Medium Demodulator Source Destination Source System Destination System

48. Encoding Considerations • Digital signaling can use modern digital transmission infrastructure. • Some media like fiber and unguided media only carry analog signals. • Analog-to-analog conversion used to shift signal to use another portion of spectrum for better channel utilization (frequency division mux’ing).

49. Digital Transmission Terminology • Data element: bit. • Signaling element: encoding of data element for transmission. • Unipolar signaling: signaling elements have same polarization (all + or all -). • Polar signaling: different polarization for different elements.

50. More Terminology • Data rate: rate in bps at which data is transmitted; for data rate of R, bit duration (time to emit 1 bit) is 1/R sec. • Modulation rate = baud rate (rate at which signal levels change).