1. DNT 1013 DATA COMMUNICATIONS ------------------------------------------ CHAPTER 2: PHYSICAL LAYER AND MEDIA Prepared By: Mdm Noor Suhana Bt Sulaiman FKMT-NT, TATiUC

2. DATA AND SIGNALS

3. 3 Introduction Data are entities that convey meaning (computer files, music on CD, results from a blood gas analysis machine) Signals are the electric or electromagnetic encoding of data (telephone conversation, Web page download) Computer networks and data/voice communication systems transmit signals Data and signals can be analog or digital

4. 4 To be transmitted, data must be transformed to electromagnetic signals. Physical Layer

5. Analog and Digital Data can be analog or digital Data can be analog or digital Analog data refers to information that is continuous Analog data take on continuous values Analog signals can have an infinite number of values in a range Digital data refers to information that has discrete states Digital data take on discrete values Digital signals can have only a limited number of values In data communications, we commonly use periodic analog signals and nonperiodic digital signals.

6. 6 Analog and Digital Table 2-1 Four combinations of data and signals

7. 7 Data and Signals Data are entities that convey meaning within a computer or computer system Signals are the electric or electromagnetic impulses used to encode and transmit data

8. Comparison of Analog and Digital Signals

9. 9 Fundamentals of Signals All signals have three components: Amplitude Frequency Phase Amplitude The height of the wave above or below a given reference point

10. 10 Fundamentals of Signals..cont..

11. 11 Fundamentals of Signals Frequency The number of times a signal makes a complete cycle within a given time frame; frequency is measured in Hertz (Hz), or cycles per second Spectrum – range of frequencies that a signal spans from minimum to maximum Bandwidth – absolute value of the difference between the lowest and highest frequencies of a signal For example, consider an average voice The average voice has a frequency range of roughly 300 Hz to 3100 Hz The spectrum would be 300 – 3100 Hz The bandwidth would be 2800 Hz

12. 12 Fundamentals of Signals (continued)

13. 13 Fundamentals of Signals Phase The position of the waveform relative to a given moment of time or relative to time zero A change in phase can be any number of angles between 0 and 360 degrees Phase changes often occur on common angles, such as 45, 90, 135, etc.

14. 14 Fundamentals of Signals (continued)

15. Frequency Change in a short span of time means high frequency. Change over a long span of time means low frequency. If a signal does not change at all, its frequency is zero If a signal changes instantaneously, its frequency is infinite. Frequency is the rate of change with respect to time.

16. Frequency and Period Frequency and period are the inverse of each other. Units of period and frequency

17. Two Signal with the Same Amplitude but Different Frequency

18. Examples The power we use at home has a frequency of 60 Hz. What is the period of this sine wave ? The period of a signal is 100 ms. What is its frequency in kilohertz?

19. Bandwidth The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal

20. DIGITAL TRANSMISSION

21. Digital Signal In addition to being represented by an analog signal, information can also be represented by a digital signal. For example, a 1 can be encoded as a positive voltage and a 0 as zero voltage. A digital signal can have more than two levels. In this case, we can send more than 1 bit for each level.

22. Two Digital Signal One with two signal levels and the other with four signal levels

23. Example A digital signal has 8 levels. How many bits are needed per level? We calculate the number of bits from the formula Each signal level is represented by 3 bits. A digital signal has 9 levels. How many bits are needed per level? Each signal level is represented by 3.17 bits. The number of bits sent per level needs to be an integer as well as a power of 2. Hence, 4 bits can represent one level.

24. Example Assume we need to download files at a rate of 100 pages per minute. A page is an average of 24 lines with 80 characters in each line where one character requires 8 bits. What is the required bit rate of the channel? A digitized voice channel is made by digitizing a 4-kHz bandwidth analog voice signal. We need to sample the signal at twice the highest frequency (two samples per hertz). Assume that each sample requires 8 bits. What is the required bit rate?

25. Example HDTV uses digital signals to broadcast high quality video signals. There are 1920 by 1080 pixels per screen, and the screen is renewed 30 times per second. Also, 24 bits represents one color pixel. What is the bit rate for high-definition TV (HDTV)? The TV stations reduce this rate to 20 to 40 Mbps through compression.

26. The Time and Frequency Domain

27. Baseband Transmission A digital signal is a composite analog signal with an infinite bandwidth.

28. 28 Transmitting Digital Data with Analog Signals Three basic techniques: Amplitude shift keying Frequency shift keying Phase shift keying

29. 29 Amplitude Shift Keying One amplitude encodes a 0 while another amplitude encodes a 1 (a form of amplitude modulation)

30. 30 Amplitude Shift Keying

31. 31 Amplitude Shift Keying

32. 32 Frequency Shift Keying One frequency encodes a 0 while another frequency encodes a 1 (a form of frequency modulation)

33. 33 Frequency Shift Keying

34. 34 Phase Shift Keying One phase change encodes a 0 while another phase change encodes a 1 (a form of phase modulation)

35. 35 Phase Shift Keying (continued)

36. 36 Phase Shift Keying Quadrature phase shift keying Four different phase angles used 45 degrees 135 degrees 225 degrees 315 degrees

37. 37 Phase Shift Keying (continued)

38. 38 Phase Shift Keying Quadrature amplitude modulation As an example of QAM, 12 different phases are combined with two different amplitudes Since only four phase angles have two different amplitudes, there are a total of 16 combinations With 16 signal combinations, each baud equals 4 bits of information (2 ^ 4 = 16)

39. 39 Phase Shift Keying (continued)

40. ANALOG TRANSMISSION

41. Analog to Digital Transmission A digital signal is superior to an analog signal. The tendency today is to change an analog signal to digital data. In this section we describe two techniques, pulse code modulation and delta modulation.

42. Component of PCM Encoder

43. 43 Pulse Code Modulation The analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values

44. 44 Pulse Code Modulation (continued)

45. 45 Pulse Code Modulation The analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values

46. 46 Pulse Code Modulation (continued)

47. 47 Pulse Code Modulation The more snapshots taken in the same amount of time, or the more quantization levels, the better the resolution

48. 48 Pulse Code Modulation

49. 49 Pulse Code Modulation Since telephone systems digitize human voice, and since the human voice has a fairly narrow bandwidth, telephone systems can digitize voice into either 128 or 256 levels These are called quantization levels If 128 levels, then each sample is 7 bits (2 ^ 7 = 128) If 256 levels, then each sample is 8 bits (2 ^ 8 = 256)

50. 50 Pulse Code Modulation How fast do you have to sample an input source to get a fairly accurate representation? Nyquist says two times the highest frequency Thus, if you want to digitize voice (4000 Hz), you need to sample at 8000 samples per second

51. 3 Different Sampling Method for PCM

52. Recovery of Sampled Sine Wave for Different Sampling Rate Sampling at the Nyquist rate can create a good approximation of the original sine wave. Oversampling can also create the same approximation, but is redundant and unnecessary. Sampling below the Nyquist rate does not produce a signal that looks like the original sine wave.

53. Example We want to digitize the human voice. What is the bit rate, assuming 8 bits per sample? Solution The human voice normally contains frequencies from 0 to 4000 Hz. So the sampling rate and bit rate are calculated as follows:

54. Component of PCM Decoder

55. 55 Delta Modulation

57. Delta Modulation Component

58. 58 The Relationship Between Frequency and Bits Per Second Higher data transfer rates How do you send data faster? Use a higher frequency signal (make sure the medium can handle the higher frequency) Use a higher number of signal levels In both cases, noise can be a problem

59. BANDWIDTH UTILIZATION: MULTIPLEXING AND SPREADING

60. 60 Introduction Under simplest conditions, medium can carry only one signal at any moment in time For multiple signals to share a medium, medium must somehow be divided, giving each signal a portion of the total bandwidth Current techniques include: Frequency division multiplexing Time division multiplexing Code division multiplexing

61. 61 Frequency Division Multiplexing Assignment of nonoverlapping frequency ranges to each “user” or signal on a medium Thus, all signals are transmitted at the same time, each using different frequencies A multiplexor accepts inputs and assigns frequencies to each device

62. 62 Frequency Division Multiplexing The multiplexor is attached to a high-speed communications line A corresponding multiplexor, or demultiplexor, is on the end of the high-speed line and separates the multiplexed signals

63. 63 Frequency Division Multiplexing (continued)

64. 64 Frequency Division Multiplexing (continued) Analog signaling is used to transmit the data Broadcast radio and television, cable television, and cellular telephone systems use frequency division multiplexing This technique is the oldest multiplexing technique Since it involves analog signaling, it is more susceptible to noise

65. Time Division Multiplexing Time-division multiplexing (TDM) is a type of digital or (rarely) analog multiplexing. Two or more signals or bit streams are transferred apparently simultaneously as sub-channels in one communication channel, but are physically taking turns on the channel. The time domain is divided into several recurrent timeslots of fixed length, one for each sub-channel. A sample byte or data block of sub-channel 1 is transmitted during timeslot 1, sub-channel 2 during timeslot 2, etc. One TDM frame consists of one timeslot per sub-channel. After the last sub-channel the cycle starts all over again with a new frame, starting with the second sample, byte or data block from sub-channel 1, etc

66. Code division multiplexing Method for transmitting multiple digital signals simultaneously over the same carrier frequency (the same channel). Although used in various radio communications systems, the most widely known application of CDMA is for cellphones. As of 2009, there were more than 460 million CDMA cellular users worldwide, with more than half in Asia. Verizon and Sprint are CDMA carriers in the U.S., while TELUS uses CDMA in Canada. QUALCOMM designs the chips for the CDMA air interface. CDMA provides up to 10 times the calling capacity of earlier analog networks (AMPS) and up to five times the capacity of GSM systems. CDMA is also the basis for the WCDMA and HSPA 3G technologies used by GSM carriers

67. Code division multiplexing Code division multiple access (CDMA) is a channel access method utilized by various radio communication technologies. One of the basic concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a single communication channel. This allows several users to share a bandwidth of different frequencies. This concept is called multiplexing.

68. Code division multiplexing CDMA employs spread-spectrum technology and a special coding scheme (where each transmitter is assigned a code) to allow multiple users to be multiplexed over the same physical channel. By contrast, time division multiple access (TDMA) divides access by time, while frequency-division multiple access (FDMA) divides it by frequency. CDMA is a form of "spread-spectrum" signaling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.

69. TRANSMISSION MEDIA

70. 70 Data Codes The set of all textual characters or symbols and their corresponding binary patterns is called a data code There are three common data code sets: EBCDIC ASCII Unicode

71. 71 EBCDIC

72. 72 ASCII

73. 73 Unicode Each character is 16 bits A large number of languages / character sets For example: T equals 0000 0000 0101 0100 r equals 0000 0000 0111 0010 a equals 0000 0000 0110 0001

74. 74 Data and Signal Conversions In Action: Let us transmit the message “Sam, what time is the meeting with accounting? Hannah.” This message leaves Hannah’s workstation and travels across a local area network

75. 75 Data and Signal Conversions In Action:

76. 76 Data and Signal Conversions In Action:

77. 77 Data and Signal Conversions In Action:

78. Transmission Mode The transmission of binary data across a link can be accomplished in either parallel or serial mode. In parallel mode, multiple bits are sent with each clock tick. In serial mode, 1 bit is sent with each clock tick. While there is only one way to send parallel data, there are three subclasses of serial transmission: asynchronous, synchronous, and isochronous.

79. Data Transmission and Modes

80. Parallel Transmission

81. Serial Transmission

82. Asynchronous Transmission We send 1 start bit (0) at the beginning and 1 or more stop bits (1s) at the end of each byte. There may be a gap between each byte. It is “asynchronous at the byte level,” bits are still synchronized; their durations are the same.

83. Synchronous Transmission We send bits one after another without start or stop bits or gaps. It is the responsibility of the receiver to group the bits.

84. Transmission medium Transmission medium is 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). Repeaters or amplifiers may be used to extend the length of the medium.

85. Transmission media choice Twisted pair Coaxial cable Optical fiber Wireless communications

86. Digital Transmission Media Bit Rates

87. Twisted Pair 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.

88. Twisted Pair..cont.. Limited in distance, bandwidth and data rate due to problems with attenuation, interference and noise Issue: cross-talk due to interference from other signals “shielding” wire (shielded twisted pair (STP)) with metallic braid or sheathing reduces interference. “twisting” reduces low-frequency interference and crosstalk.

89. Category 3 corresponds to ordinary voice-grade twisted pair found in abundance in most office buildings. Category 5 (used for Fast Ethernet) is much more tightly twisted. UTP (Unshielded Twisted Pair)

90. Digital Subscriber Line (DSL) Telphone companies originally transmitted within the 0 to 4kHZ range to reduce crosstalk. Loading coils were added within the subscriber loop to provide a flatter transfer function to further improve voice transmission within the 3kHZ band while increasing attenuation at the higher frequencies.

91. DSL..cont.. the network transmits downstream at speeds ranging from 1.536 Mbps to 6.144Mbps asymmetric bidirectional digital transmisssions users transmit upstream at speeds ranging [higher frequencies] from 64 kbps to 640 kbps 0 to 4kHZ used for conventional analog telephone signals

92. DSL..cont.. ITU-T G992.1 ADSL standard uses Discrete Multitone (DMT) that divides the bandwidth into a large number of small subchannels. A splitter is required to separate voice signals from the data signal. The binary information is distributed among the subchannels. Each subchannel uses QAM. DMT adapts to line conditions by avoiding subchannels with poor SNR.

93. Asymmetric Digital Subscriber Line (ADSL) Uses existing twisted pair lines to provide higher bit rates that are possible with unloaded twisted pairs (i.e., no loading coils on subscriber loop.)

94. 10 Base-T 10 Mbps baseband transmission over twisted pair. Two Cat 3 cables, Manchester encoding, Maximum distance - 100 meters Ethernet hub

95. Coaxial Cable Center conductor Dielectric material Braided outer conductor Outer cover

96. Coaxial Cable…cont.. 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, coax has better noise immunity for higher frequencies than twisted pair. Coaxial cable provides much higher bandwidth than twisted pair. However, cable is ‘bulky’.

97. Base Band Coax 50-ohm cable is used exclusively for digital transmissions Uses Manchester encoding, geographical limit is a few kilometers. 10Base5 Thick Ethernet :: thick (10 mm) coax 10 Mbps, 500 m. max segment length, 100 devices/segment, awkward to handle and install. 10Base2 Thin Ethernet :: thin (5 mm) coax 10 Mbps, 185 m. max segment length, 30 devices/segment, easier to handle, uses T-shaped connectors.

98. Broad Band Coax 75-ohm cable (CATV system standard) Used for both analog and digital signaling. Analog signaling – frequencies up to 500 MHZ are possible. When FDM used, referred to as broadband. For long-distance transmission of analog signals, amplifiers are needed every few kilometers.

99. Optical Fibre 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. Total internal reflection occurs in the core because it has a higher optical density (index of refraction) than the cladding. Attenuation in the fiber can be kept low by controlling the impurities in the glass.

100. Optical Fibre..cont.. core cladding jacket light cc (a) Geometry of optical fiber (b) Reflection in optical fiber

101. Optical Fibre..cont.. Lowest signal losses are for ultrapure fused silica – but this is hard to manufacture. Optical fiber acts as a wavelength guide for frequencies in the range 10 **14 to 10 **15 HZ which covers the visible and part of the infrared spectrum. Three standard wavelengths : 850 nanometers (nm.), 1300 nm, 1500 nm. First-generation optical fiber :: 850 nm, 10’s Mbps using LED (light-emitting diode) sources. Second and third generation optical fiber :: 1300 and 1500 nm using ILD (injection laser diode) sources, gigabits/sec.

102. Optical Fibre..cont.. Attenuation loss is lower at higher wavelengths. There are two types of detectors used at the receiving end to convert light into electrical energy (photo diodes): PIN detectors – less expensive, less sensitive APD detectors ASK is commonly used to transmit digital data over optical fiber {referred to as intensity modulation}.

103. Optical Fibre..cont.. Three techniques: Multimode step-index Multimode graded-index Single-mode step-index Presence of multiple paths  differences in delay  optical rays interfere with each other. A narrow core can create a single direct path which yields higher speeds. WDM (Wavelength Division Multiplexing) yields more available capacity.

104. Optical Fibre..cont.. (a) Multimode fiber: multiple rays follow different paths (b) Single mode: only direct path propagates in fiber direct path reflected path

105. SWITCHING

106. Core networks Graph of interconnected routers One fundamental question: how is data passed through the net? Circuit switching Packet switching Circuit switching Dedicated line through the network Packet switching Discrete data units are sent through the network

107. Core networks: Circuit Switching End-to-end resource reservation for a ”session” Link bandwidth, router capacity Dedicated resources (no sharing) Guaranteed throughput Setup phase is required

108. Core networks: Circuit Switching Historical: Analog telephone networks Network consists of resources Cables Switches with relays Establish a physical connection Relays switch to connect cables physically Create a circuit Guaranteed resources No difference between talking and silence Modern: Networks consist of resources Cables Routers or switches Network resources can be shared Establish a connection Switches reserve part of available resource Division of link bandwidth into parts Frequency division Time division

109. Core networks: Packet Switching Each end-to-end data stream is divided into packets Data streams share network resources Each packets uses the entire bandwidth of a link Resources are used as needed Competition for resources: Combined resource need can exceed the available resources Congestion: packets are queued in front of “thin” links Store and forward: packets move one link at a time Send over a link Wait for your turn at the next link Division of bandwidth Dedicated allocation Resource reservation

110. Core networks: Packet switching A B C 10 Mbps Ethernet 1.5 Mbps 45 Mbps D E statistical multiplexing Queue of packets that wait for link access

111. Packet switching versus circuit switching 1 Mbps link Each user 100Kbps when “active” Active 10% of the time, at random times Circuit switching max 10 users Loss probability: 0% Waste: ~90% capacity Packet switching >10 may be active concurrently! Loss probability >0% Waste: < 90% capacity Packet switching allows more users in the net! N users 1 Mbps link

112. Packet switching versus circuit switching Good for data with “bursty” behavior Resource sharing No ”setup phase” required In a congested network: delay and packet loss Protocols required for reliable traffic and congestion control How to achieve a behavior like that of circuit switching? Bandwidth guarantees are required for audio/video applications Unsolved problem so far! Is packet switching always the best approach?

113. Delay in packet switching networks Packet experience delay on the way from sender to receiver four sources of delay in each hop. Node processing: Checking for bit errors Determining the output link Queuing Waiting for access to the output link Depends on the congestion level of the router A B node processing queueing

114. USING TELEPHONE AND CABLE NETWORKS FOR DATA

115. Physical medium Physical link: a sent bit propagates through the link Closed media: Signals propagate in cable media (copper, fiber) Open media: Signals propagate freely, e.g. radio. Twisted Pair (TP) Two isolated copper cables Category 3: traditional telephone cables, 10 Mbps Ethernet Category 5 TP: 100Mbps Ethernet Category 6 TP: 1Gpbs Ethernet

116. Physical medium: coax, fiber Coaxial cable Wire (signal carrier) in a wire (shielding) baseband: a single channel on a cable broadband: multiple channels on a cable Bi-directional Typically used for 10Mbs Ethernet. Fiber optic cable Optical fiber that carries light impulses High-speed transfer: High-speed point-to-point transmission Low error rate Longer distances 100Mbps, 1Gbps Ethernet

117. Physical media: radio Radio Signal in electromagnetic spectrum No physical ”cable” Bi-directional Effects of environment on the distribution: Reflection Obstruction by blocking objects Interferences Types of radio links microwaves E.g. up to 45 Mbps LAN 2Mbps, 11Mbps, 54Mbps wide-area GSM, 9,8 Kbps satellite Up to 50Mbps per channel (or several thinner channels) 270 Msec end-to-end delay (limited by speed of light).

118. 118 Summary Data and signals are two basic building blocks of computer networks All data transmitted is either digital or analog Data is transmitted with a signal that can be either digital or analog All signals consist of three basic components: amplitude, frequency, and phase Two important factors affecting the transfer of a signal over a medium are noise and attenuation Four basic combinations of data and signals are possible: analog data converted to an analog signal, digital data converted to a digital signal, digital data converted to an analog signal, and analog data converted to a digital signal

119. 119 Summary (continued) To transmit analog data over an analog signal, the analog waveform of the data is combined with another analog waveform in a process known as modulation Digital data carried by digital signals is represented by digital encoding formats For digital data to be transmitted using analog signals, digital data must first undergo a process called shift keying or modulation Three basic techniques of shift keying are amplitude shift keying, frequency shift keying, and phase shift keying

120. 120 Summary (continued) Two common techniques for converting analog data so that it may be carried over digital signals are pulse code modulation and delta modulation Data codes are necessary to transmit the letters, numbers, symbols, and control characters found in text data Three important data codes are ASCII, EBCDIC, and Unicode

121. And one truly last word… ThAnKs