Digital Communication 4


Published on

Digital Communication - Multiplexing

Published in: Education, Business, Technology

Digital Communication 4

  1. 1. DIgItal CommunICatIon ECE 422l IV. Bandwidth Utilization 2013
  2. 2. Bandwidth utilization •Bandwidth is the precious commodity in communication •Bandwidth utilization is the wise use of available bandwidth to achieve specific goals. 2
  3. 3. Bandwidth utilization Efficiency can be achieved by multiplexing; privacy and anti-jamming can be achieved by spreading. 3
  4. 4. MULTIPLEXING • Whenever the bandwidth of a medium linking two devices is greater than the bandwidth needs of the devices, the link can be shared. • Is the set of techniques that allows the simultaneous transmission of multiple signals across a single data link. • As data and telecommunications use increases, so does traffic. 4
  5. 5. Dividing a link into channels 5
  6. 6. Categories of Multiplexing 6
  7. 7. Frequency-division multiplexing 7
  8. 8. Frequency-division multiplexing FDM is an analog multiplexing technique that combines narrow band analog signals. 8
  9. 9. FDM process 9
  10. 10. FDM demultiplexing example 10
  11. 11. Frequency-division multiplexing Applications: • Broadcasting • Telephone and data communication system • Cable television • Data distribution networks 11
  12. 12. Example 1 Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration, using the frequency domain. Assume there are no guard bands. 12
  13. 13. Example 1 Solution We shift (modulate) each of the three voice channels to a different bandwidth, as shown in next slide. We use the 20- to 24-kHz bandwidth for the first channel, the 24- to 28-kHz bandwidth for the second channel, and the 28- to 32-kHz bandwidth for the third one. Then we combine them 13
  14. 14. Example 1 14
  15. 15. Example 2 Five channels, each with a 100-kHz bandwidth, are to be multiplexed together. What is the minimum bandwidth of the link if there is a need for a guard band of 10 kHz between the channels to prevent interference? 15
  16. 16. Example 2 Solution For five channels, we need at least four guard bands. This means that the required bandwidth is at least 5 × 100 + 4 × 10 = 540 kHz, as shown in next slide 16
  17. 17. Example 2 17
  18. 18. Example 3 Four data channels (digital), each transmitting at 1 Mbps, use a satellite channel of 1 MHz. Design an appropriate configuration, using FDM. 18
  19. 19. Example 3 Solution The satellite channel limited to 1MHz. We divide this among the four data channel that is each channel is allocated with 250 kHz bandwidth. Since the system is using FDM (analog) and the input signal is in digital, we are going to used digital to analog modulation. One solution is 16QAM modulation. 19
  20. 20. Example 3 20
  21. 21. Analog hierarchy (AT&T) Message Channel 21
  22. 22. Example 4 The Advanced Mobile Phone System (AMPS) uses two bands. The first band of 824 to 849 MHz is used for sending, and 869 to 894 MHz is used for receiving. Each user has a bandwidth of 30 kHz in each direction. How many people can use their cellular phones simultaneously? 22
  23. 23. Example 4 Solution Each band is 25 MHz. If we divide 25 MHz by 30 kHz, we get 833.33. In reality, the band is divided into 832 channels. Of these, 42 channels are used for control, which means only 790 channels are available for cellular phone users. 23
  24. 24. Wavelength Division Multiplexing • The first commercial use of WDM was to use different frequencies of light for transmitters on each end of one fiber. • The velocity of propagation is equal to the product of the wavelength and the frequency – vp = λ * f – In a vacuum, the speed of light, C, is 3x108 m/s 24
  25. 25. Wavelength Division Multiplexing • Also called Dense wavelength division multiplexing • The development of the erbium-doped fiber amplifier (EDFA) made DWDM possible. • Easy to do with fiber optics and optical sources • Give each message a different wavelength (frequency) 25
  26. 26. Wavelength Division Multiplexing • multiplexes multiple data streams onto a single fiber optic line.(4, 8, 16, 32, and 180 lasers in the transmitter) • C band EDFAs operate from 1530 nm to 1560 nm. • The bandwidth of a C Band system is 4 trillion Hz. • Frequencies used chosen from the ITU Grid. 26
  27. 27. ITU Grid 27
  28. 28. Wavelength Division Multiplexing • Each signal carried on the fiber can be transmitted at a different rate from the other signals. • Different wavelength in a light pulse travels through an optical fiber at different speeds – Blue light is slower than red light • Each wavelength takes a different transmission path 28
  29. 29. Wavelength Division Multiplexing 29 29
  30. 30. Wavelength Division Multiplexing 30
  31. 31. Wavelength Division Multiplexing Merits Enhanced capacity - >100 gbps Full dup;ex transmission Easier to configure reliable Demerits close wavelength may cause interference(0.8nm) physical limitation of the fiber limited to two point circuit or a combination of many two point circuit requires mod/demoulator as many as the propagating wavelenght 31
  32. 32. Time Division Multiplexing is a digital multiplexing technique for combining several low-rate channels into one high-rate one. 32
  33. 33. Time Division Multiplexing 33
  34. 34. Time Division Multiplexing Sharing of the signal is accomplished by dividing available transmission time on a medium among users. Digital signaling is used exclusively. Time division multiplexing comes in two basic forms: 1. Synchronous time division multiplexing, and 2. Statistical, or asynchronous time division multiplexing. 34
  35. 35. Synchronous TDM The original time division multiplexing. The multiplexor accepts input from attached devices in a round-robin fashion and transmit the data in a never ending pattern. T-1 and ISDN telephone lines are common examples of synchronous time division multiplexing. 35
  36. 36. Synchronous TDM 36
  37. 37. Synchronous TDM In synchronous TDM, the data rate of the link is n times faster, and the unit duration is n times shorter. 37
  38. 38. Synchronous TDM • If one device generates data at a faster rate than other devices, then the multiplexor must either sample the incoming data stream from that device more often than it samples the other devices, or buffer the faster incoming stream. • If a device has nothing to transmit, the multiplexor must still insert a piece of data from that device into the multiplexed stream. 38
  39. 39. Synchronous TDM 39
  40. 40. Synchronous TDM 40
  41. 41. Synchronous TDM So that the receiver may stay synchronized with the incoming data stream, the transmitting multiplexor can insert alternating 1s and 0s into the data stream. 41
  42. 42. Synchronous TDM • One eight-bit PCM code from each channel is called a TDM frame and the time it takes to transmit one TDM frame is called frame time and it is equal to reciprocal of sample rate. 42
  43. 43. Synchronous TDM Three types popular today: •T-1 multiplexing (the classic) •ISDN multiplexing •SONET (Synchronous Optical NETwork) 43
  44. 44. T1 Digital Carrier System • North American Telephone standards • Recognized by ITU-T - Recommendation G.733 • The T1 (1.54 Mbps) multiplexor stream is a continuous series of frames of both digitized data and voice channels. • Each channel contains an 8 bit PCM code and is sampled at 8000 times per second 44
  45. 45. T1 Digital Carrier System A digital carrier system is a communications system that uses digital pulse rather than analog signals to encode information. 45
  46. 46. 46
  47. 47. T-1 line for multiplexing telephone lines 47
  48. 48. T-1 frame structure 48
  49. 49. T1 Digital Carrier System • The multiplexer has 24 independent inputs and one time-division multiplexed output. The 24 PCM output signals are sequentially selected and connected through the multiplexer to the transmission line. • A transmitting portion of a Channel Bank digitally encodes the 24 analog channels, adds signaling information into each channel, and multiplexes the digital stream onto the transmission medium. 49
  50. 50. T1 Digital Carrier System The line calculated as: speed is Each of the 24 channels contains an eight-bit PCM code and is sampled 8000 times a second. Each channel is sampled at the same rate, but may not be at the same time. 50
  51. 51. T1 Digital Carrier System • Later, an additional bit called the framing bit is added to each frame. The framing bit occurs once per frame and is recovered at the receiver and its main purpose is to maintain frame and sample synchronization between TDM transmitter and receiver. 51
  52. 52. T1 Digital Carrier System • As a result of this extra bit, each frame now contains 193 bits and the line speed for a T1 digital carrier system is 1.544 Mbps. { 193 bits × 8000 frames = 1.544 Mbps} . 52
  53. 53. Superframe TDM Format • The 8-kbps signaling rate used with the early digital channel banks was excessive for signaling on standard telephone voice circuits. • Therefore, with modern channel banks, a signaling bit is substituted only into the least-significant bit (LSB) of every sixth frame. 53
  54. 54. 54
  55. 55. Superframe TDM Format • Within each super- frame are 12 consecutively numbered frames (1 to12). The signaling bits are substituted in frames 6 and 12, the MSB into frame 6, and the LSB into frame 12. Frames 1 to 6 are called the A highway, with frame 6 designated the A channel signaling frame. Frames 7 to 12 are called the B high way, with frame 12 designated the B channel signaling frame. 55
  56. 56. Superframe TDM Format • To identify frames 6 and 12, a different framing bit sequence is used for the odd- and even-numbered frames. The odd frames (frames 1, 3, 5, 7, 9, and 11) have an alternating 1/0 pattern, and the even frames (frames 2, 4, 6, 8, 10, and 12) have a 00 1110repetitive pattern. • As a result, the combined framing bit pattern is 1000 11011100 56
  57. 57. Superframe TDM Format • D4 channel banks time-division multiplex 48 voice-band telephone channels and operate at a transmission rate of 3.152 Mbps, which is slightly more than twice the line speed for 24channel D1, D2, or D3 channel banks because with D4 channel banks, rather than transmitting a single framing bit with each frame, a 10-bit frame synchronization pattern is used. 57
  58. 58. Superframe TDM Format • Line speed is calculated as: total no of bits is 8 bits/channel × 48 channels = 384 bits/frame An additional 10 bits are added for frame: so 394 bits/frame. Therefore, line speed of DS-1C system is 394×8000 = 3.152 Mbps. 58
  59. 59. Superframe TDM Format 59
  60. 60. Extended Superframe 60
  61. 61. Fractional T Carrier Service 61
  62. 62. Fractional T Carrier Service • Fractional T1 systems distribute the channels (i.e., bits) in a standard T1 system among more than one user, allowing several subscribers to share one T1line. • The above figure shows four subscribers combining their transmissions in a special unit called a data service unit/channel service unit (DSU/CSU). A DSU/CSU is a digital interface that provides the physical connection to a digital carrier network. User 1 is allocated 128 kbps, user 2 - 256 kbps, user 3 - 384 kbps, and user 4 - 768 kbps for a total of 1.536 kbps (8 kbps is reserved for the framing bit). 62
  63. 63. North American Digital Multiplexing Hierarchy 63
  64. 64. North American DS System • The basic building block for digital transmission standards begins with the DS0 signal level. – One voice equivalent – 8 bits/sample x 8,000 samples/second • The DS1 signal has 24 voice equivalents – 193 bits per frame • 24 x 8 bits per channel • 1 framing bit 64
  65. 65. D-Type Channel Banks • D type Channel Bank refers to the terms used in T1 technology. • Channel Bank defines the type of formatting that is required for transmission on T1 trunk. • The purpose of a Channel Bank in the telephone company is to form the foundation of multiplexing and de multiplexing the 24 voice channels (DS0). • D type Channel Bank is one of the types of Channel Bank which is used for digital signals. • There are five kinds of Channel Banks that are used in the System: D1, D2, D3, D4, and DCT (Digital Carrier Trunk). 65
  66. 66. North American Digital Multiplexing hierarchy 66
  67. 67. North American Digital Multiplexing hierarchy • A special device called muldem (multiplexers/ demultiplexer) is used to upgrade from one level in the hierarchy to the next-higher level. • They handle bit-rate conversions in both directions and are designated as M12, M23 etc. which identifies the respective input and output digital signals. • As shown, an M12 muldem interfaces DS-1 and DS-2 digital signals. 67
  68. 68. North American Digital Multiplexing hierarchy • As shown, an M12 muldem interfaces DS-1 and DS-2 digital signals. • Digital signals are routed at central locations called digital cross-connects (DSX), which are convenient for making patchable interconnections and routine maintenance and troubleshooting. Each digital signal (i.e. DS-1, DS-2, etc) has its own digital switch (DSX-1, DSX-2/). 68
  69. 69. One Frame of a DS1 Signal 69
  70. 70. DS and T line rates 70
  71. 71. T1 Carrier System • T1 carrier systems were designed to combine PCM and TDM techniques for the transmission of 24 64-kbps channels with each channel capable of carrying digitally encoded voice band telephone signals or data. The transmission bit rate (line speed) for a T1 carrier is 1.544 Mbps. • All 24 DS-0 channels combined has a data rate of 1.544 Mbps; this digital signal level is called DS1. Therefore T1 lines are sometimes referred to as DS-1 lines. 71
  72. 72. T2 Carrier System • T2 carriers time-division multiplex 96 64-kbps voice or data channels into a single 6.312 Mbps data signal for transmission over twisted-pair copper wire upto 500 miles over a special LOCAP (low capacitance) metallic cable. • Higher transmission rates make clock synchronization even more critical. 72
  73. 73. T3 Carrier System • T3 carriers time-division multiplex 672 64kbps voice or data channels for transmission over a single 3A-RDS coaxial cable. The transmission bit rate is 44.736 Mbps and coding technique used with T3 carriers is binary three zero substitution (B3ZS). 73
  74. 74. T4M Carrier System • T4M carriers time division multiplex 4032 64-kbps voice or data channels for transmitting over a single T4M coaxial cable upto 500 miles. The transmission rate is very high (274.16kbps) making substituting patterns impractical. They transmit scrambled unipolar NRZ digital signals. 74
  75. 75. T5 Carrier System • T5 carriers time-division multiplex 8064 64-kbps voice or data channels and transmits them at 560.16 Mbps over a single coaxial cable. 75
  76. 76. European TDM 30 + 2 System • In Europe, a different version of T carrier lines is used called E lines. • With the basic E1 system, a 125µs frame is divided into 32 equal time slots. 76
  77. 77. European TDM 30 + 2 System • The European TDM system multiplexes 32 DS0 channels together. – Channel 0 is used for synchronizing (framing) and signaling. – Channels 1-15 and 17-31 are used for voice. – Channel 16 is reserved for future use as a signaling channel. – Time slot 17 is used for a common signaling channel (CSC). 77
  78. 78. European TDM 30 + 2 System • The total signal rate is 2.048 Mbps (64 kbps * 32 channels) • The signalling for all 32 voice-band channels is accomplished on the common signalling channel. Consequently, 32 voiceband channels are time-division multiplexed into each E1 frame. And the line speed can be given as 256 bits/frame × 8000 frames/second = 2.408 Mbps 78
  79. 79. European TDM 30 + 2 System 79
  80. 80. Integrated Service Digital Network •Is a telephone system which provide digital telephone and data services. •designed to provide access to voice and data services simultaneously •there is no digital to analog conversion. •Developed by CCITT (Comate Consultative International Telephonique Telegraphs) to limitation of POTS (Plane old Telephone system). 80
  81. 81. ISDN frame The ISDN multiplexor stream is also a continuous stream of frames. Each frame contains various control and sync info. 81
  82. 82. Channels of ISDN 1. B Channel • Carries voice, data, video etc. • functions at a constant 64 kbps. • can be used for packet and circuit switching applications. 82
  83. 83. Channels of ISDN 2. D Channel (Denial) •is used to convey user signaling massages. •used out of band signaling . This means that network related signals are carried on a separate channel than used data. 83
  84. 84. Channels of ISDN 3. H Channel •Have a considerably higher transfer rate than B channels. •sustains rates of approximately 1920 mbps. effectively meet the needs of real time video conferencing, digital quality audio and other services requiring a much higher bandwidth. 84
  85. 85. Use of ISDN • • • • • • Electronic library Inter connection. Electronic resources accessing. Images, sound and video retrieval. Video conferencing. Call center. Internet Access. 85
  86. 86. Synchronous Optical Network SONET was developed by ANSI; SDH was developed by ITU-T. 86
  87. 87. SONET Standards • Fiber optics use synchronous optical network (SONET) standards. • The initial SONET standard is OC-1, this level is known as synchronous transport level 1 (STS-1). – It has a synchronous frame structure at a speed of 51.840 Mbps. – OC-1 is an envelope containing a DS3 signal (28 DS1 signals or 672 channels). 87
  88. 88. SONET • Used in massive data rates 88
  89. 89. SONET/SDH rates 89
  90. 90. SONET SONET defines four layers: path, line, section, and photonic. 90
  91. 91. SONET 91
  92. 92. STS Circuits 92
  93. 93. SONET FRAMES Each synchronous transfer signal STS-n is composed of 8000 frames. Each frame is a two-dimensional matrix of bytes with 9 rows by 90 × n columns. 93
  94. 94. An STS-1 and an STS-n frame 94
  95. 95. SONET A SONET STS-n signal is transmitted at 8000 frames per second. Each byte in a SONET frame can carry a digitized voice channel. 95
  96. 96. TDM Comparisons 96
  97. 97. Statistical TDM • improves the efficiency of a TDM system. – Channel units do not have reserved time slots. – Time slots are dynamically assigned. • Also called stat muxs, intelligent multiplexers, and asynchronous multiplexers. 97
  98. 98. Statistical TDM • A statistical multiplexor transmits only the data from active workstations (or why work when you don’t have to). • If a workstation is not active, no space is wasted on the multiplexed stream. • A statistical multiplexor accepts the incoming data streams and creates a frame containing only the data to be transmitted. 98
  99. 99. Statistical TDM 99
  100. 100. Statistical TDM To identify each piece of data, an address is included. 100
  101. 101. Statistical TDM If the data is of variable size, a length is also included. 101
  102. 102. Statistical TDM More precisely, the transmitted frame contains a collection of data groups. 102
  103. 103. Statistical TDM A statistical multiplexor does not require a line over as high a speed line as synchronous time division multiplexing since STDM does not assume all sources will transmit all of the time! Good for low bandwidth lines (used for LANs) Much more efficient use of bandwidth! 103
  104. 104. TDM slot comparison 104
  105. 105. Example 5 • The data rate for each one of the 3 input connection is 1 kbps. If 1 bit at a time is multiplexed (a unit is 1 bit), What is the duration of: (a) each input slot, (b) each output slot, (c) each frame? 105
  106. 106. Example 5 106
  107. 107. Example 5 Solution a. The data rate of each input connection is 1 kbps. This means that the bit duration is 1/1000 s or 1 ms. The duration of the input time slot is 1se 1 ms (same as bit duration). c/ 1 000 (3signals * 1 kbps)/ 3 channels = 1 kbps 107 bits
  108. 108. Example 5 (continued) b. The duration of each output time slot is onethird of the input time slot. This means that the duration of the output time slot is 1/3 ms. 1/3 ms per channel 1 ms 108
  109. 109. Example 5 (continued) c. Each frame carries three output time slots. So the duration of a frame is 3 × 1/3 ms, or 1 ms. “The duration of a frame is the same as the duration of an input unit.“ 1 ms 109
  110. 110. Example 6 Consider a synchronous TDM with a data stream for each input and one data stream for the output. The unit of data is 1 bit. Find : (a) the input bit duration, (b) the output bit duration, (c) the output bit rate, (d) the output frame rate. 110
  111. 111. Example 6 111
  112. 112. Example 6 Solution a. The input bit duration is the inverse of the bit rate: 1/1 Mbps = 1 μs. b. The output bit duration ( is one-fourth of the input bit duration, or ¼ μs. 112
  113. 113. Example 6 (continued) c. The output bit rate is the inverse of the output bit duration or 1/(4μs) or 4 Mbps. This can also be deduced from the fact that the output rate is 4 times as fast as any input rate; so the output rate = 4 × 1 Mbps = 4 Mbps. d. The frame rate is always the same as any input rate. So the frame rate is 1,000,000 frames per second. 113
  114. 114. Example 7 Four 1-kbps connections are multiplexed together. A unit is 1 bit. Find: (a) the duration of 1 bit before multiplexing, (b) the transmission rate of the link, (c) the duration of a time slot (d) the duration of a frame. 114
  115. 115. Example 7 Solution We can answer the questions as follows: a. The duration of 1 bit before multiplexing is: 1 / 1 kbps, or 0.001 s (1 ms). b. The rate of the link is 4 times the rate of a connection, or 4 kbps. 115
  116. 116. Example 7 (continued) c. The duration of each time slot is one-fourth of the duration of each bit before multiplexing, or 1/4 ms or 250 μs. Note that we can also calculate this from the data rate of the link, 4 kbps. The bit duration is the inverse of the data rate, or 1/4 kbps or 250 μs. 116
  117. 117. Example 7 (continued) d. The duration of a frame is always the same as the duration of a unit before multiplexing, or 1 ms. another way: Each frame in this case has four time slots. So the duration of a frame is 4 times 250 μs, or 1 ms. 117
  118. 118. Interleaving 118
  119. 119. Example 8 Four channels are multiplexed using TDM. If each channel sends 100 bytes /s and we multiplex 1 byte per channel, •show the frame traveling on the link, •the size of the frame, •the duration of a frame, •the frame rate, •the bit rate for the link. 119
  120. 120. Example 8 Solution Each frame carries 1 byte from each channel; •the size of each frame = 4 bytes, or 32 bits. •each channel = 100 bytes/sec and a frame = 1 byte •frame rate must be 100 frames / sec. •The bit rate is 100 × 32, or 3200 bps. 120
  121. 121. Example 8 121
  122. 122. Example 9 A multiplexer combines four 100-kbps channels using a time slot of 2 bits. •What is the frame rate? •What is the frame duration? •What is the bit rate? •What is the bit duration? 122
  123. 123. Example 9 Solution •The frame rate is : (4 channels * 100 kbps)/ (2 bits/channel* 4 channel) = 50,000 frames per second. •The frame duration is: 1sec/50,000 frames = 20 μs. • the bit rate is: 50,000 frames/s × 8bit/frame = 400,000 bits/s. •The bit duration is: 1sec /400,000 bits = 2.5 μs. 123
  124. 124. Example 9 124
  125. 125. Empty slots 125
  126. 126. Multilevel multiplexing 126
  127. 127. Multiple-slot multiplexing 127
  128. 128. Pulse stuffing 128
  129. 129. Synchronization • To ensure that the receiver correctly reads the incoming bits, i.e., knows the incoming bit boundaries to interpret a “1” and a “0”, a known bit pattern is used between the frames. • The receiver looks for the anticipated bit and starts counting bits till the end of the frame. 129
  130. 130. Synchronization • Then it starts over again with the reception of another known bit. • These bits (or bit patterns) are called synchronization bit(s). • They are part of the overhead of transmission. 130
  131. 131. Framing bits 131
  132. 132. Example 10 We have four sources, each creating 250 8-bit characters per second. If the interleaved unit is a character and 1 synchronizing bit is added to each frame, find (a) the data rate of each source, (b) the duration of each character in each source, (c) the frame rate, (d) the duration of each frame, (e) the number of bits in each frame, and (f) the data rate of the link. 132
  133. 133. Example 10 Solution a. The data rate of each source is 250 × 8 = 2000 bps = 2 kbps. b. Each source sends 250 characters per second; therefore, the duration of a character is 1/250 s, or 4 ms. 133
  134. 134. Example 10 (continued) c. Each frame has one character from each source, which means the link needs to send 250 frames per second to keep the transmission rate of each source. d. The duration of each frame is 1/250 s, or 4 ms. Note that the duration of each frame is the same as the duration of each character coming from each source. e. Each frame carries 4 characters and 1 extra synchronizing bit. This means that each frame is 4 × 8 + 1 = 33 bits. 134
  135. 135. Example 11 Two channels, one with a bit rate of 100 kbps and another with a bit rate of 200 kbps, are to be multiplexed. How this can be achieved? What is the frame rate? What is the frame duration? What is the bit rate of the link? 135
  136. 136. Example 11 Solution We can allocate one slot to the first channel and two slots to the second channel. Each frame carries 3 bits. The frame rate is 100,000 frames per second because it carries 1 bit from the first channel. The bit rate is 100,000 frames/s × 3 bits per frame, or 300 kbps. 136
  137. 137. Code Division Multiplexing • Old but now new method • Also known as code division multiple access (CDMA) • An advanced technique that allows multiple devices to transmit on the same frequencies at the same time using different codes • Used for mobile communications 137
  138. 138. Code Division Multiplexing • An advanced technique that allows multiple devices to transmit on the same frequencies at the same time. • Each mobile device is assigned a unique 64-bit code (chip spreading code) • To send a binary 1, mobile device transmits the unique code • To send a binary 0, mobile device transmits the inverse of code 138
  139. 139. Code Division Multiplexing • Receiver gets summed signal, multiplies it by receiver code, adds up the resulting values • Interprets as a binary 1 if sum is near +64 • Interprets as a binary 0 if sum is near –64 139
  140. 140. 140 140
  141. 141. Summary • Multiplexing • Types of multiplexing – TDM • Synchronous TDM (T-1, ISDN, optical fiber) • Statistical TDM (LANs) – FDM (cable, cell phones, broadband) – WDM (optical fiber) – CDM (cell phones) 141
  142. 142. SPREAD SPECTRUM In spread spectrum (SS), we combine signals from different sources to fit into a larger bandwidth, but our goals are to prevent eavesdropping and jamming. To achieve these goals, spread spectrum techniques add redundancy. 142
  143. 143. History of Spread Spectrum Spread Spectrum was actually invented by 1940s Hollywood actress Hedy Lamarr(19132000). An Austrian refugee, in 1940 at the age of 26, she devised together with music composer George Antheil a system to stop enemy detection and jamming of radio controlled torpedoes by hopping around a set of frequencies in a random fashion. 143
  144. 144. History of Spread Spectrum She was granted a patent in 1942 (US pat. 2292387) but considered it her contribution to the war effort and never profited. Techniques known since 1940s and used in military communication systems since 1950s. 144
  145. 145. Introduction to Spread Spectrum “Spread” radio signal over a wide frequency range Several magnitudes higher than minimum requirement Gained popularity by the needs of military communication Proved resistant against hostile jammers Ratio of information bandwidth and spreading bandwidth is identified as spreading gain or processing gain Processing gain does not combat white Noise 145
  146. 146. SPREAD SPECTRUM applications: • able to deal with multi-path • multiple access due to different spreading sequences • spreading sequence design is very important for performance • low probability of interception • privacy • anti-jam capabilities 146
  147. 147. Spread Spectrum Applications Interference ̶ Prevents interference at specific frequencies ̶ E.g. other radio users, electrical systems Military ̶ Prevents signal jamming ̶ Scrambling of ‘secret’ messages ̶ gps 147
  148. 148. Spread Spectrum Applications Wireless LAN security ̶ Prevents ‘eavesdropping’ of wireless links ̶ Prevents ‘hacking’ into wireless LANs CDMA (Code Division Multiple Access) ̶ Multiple separate channels in same medium using different spreading codes 148
  149. 149. Spread Spectrum Criteria A communication system is considered a spread spectrum system if it satisfies the following two criteria: Bandwidth of the spread spectrum signal has to be greater than the information bandwidth. (This is also true for frequency and pulse code modulation!) The spreading sequence has to be independent from the information. Thus, no possibility to calculate the information if the sequence is known and vice versa. 149
  150. 150. Spread Spectrum Classification 150
  151. 151. Spread Spectrum Classification 151
  152. 152. Direct Sequence Spread Spectrum • • • Information signal is directly modulated (multiplicated) by a spreading sequences Commonly used with digital modulation schemes The idea is to modulate the transmitter with a bit stream consisting of pseudo-random noise (PN) that has a much higher rate than the actual data to be communicated Near/far effect Require continuous bandwidth 152
  153. 153. Direct Sequence Spread Spectrum • The use of the high-speed PN sequence results in an increase in the bandwidth of the signal, regardless of what modulation scheme is used to encode the bits into the signal 153
  154. 154. DSSS example 154
  155. 155. Frequency Hopping Spread Spectrum The information signal is transmitted on different frequencies Time is divided in slots Each slot the frequency is changed 155
  156. 156. Frequency Hopping Spread Spectrum The change of the frequency is referred to as slow if more than one bit is transmitted on one frequency, and as fast if one bit is transmitted over multiple frequencies The frequencies are chosen based on the spreading sequences 156
  157. 157. FHSS 157
  158. 158. Frequency selection in FHSS 158
  159. 159. FHSS cycles 159
  160. 160. Bandwidth sharing 160
  161. 161. Time Hopping Spread Spectrum Time divided into frames; each TF long Each frame is divided in slots Each wireless terminal send in exactly one of these slots per frame regarding the spreading sequence No near far effect 161
  162. 162. Time Hopping Spread Spectrum 162
  163. 163. Comparison of different Spread Spectrum Techniques SS Technique Direct Sequence Frequency Hopper advantage • best behavior in multi path rejection • no synchronization • simple implementation • difficult to detect • no need for coherent • bandwidth • less affected by the near far effect 163 disadvantage • near far effect • coherent bandwidth • complex hardware • error correction • needed
  164. 164. Comparison of different Spread Spectrum Techniques SS Technique Time Hopper advantage • high bandwidth efficiency • less complex hardware • less affected by the near far effect 164 disadvantage • error correction needed
  165. 165. Code Division Multiple Access • analog communication systems used frequency division multiplexing • digital systems have employed time division multiplexing • to combine many information signals into a single transmission channel from different sources, these two methods become frequency division multiple access (FDMA) and time-division multiple access (TDMA), respectively. 165
  166. 166. Code Division Multiple Access • Spread-spectrum communication allows a third method for multiplexing signals from different sources: code-division multiple access • allow multiple users to use the same frequency using separate PN codes and a spread-spectrum modulation scheme 166
  167. 167. Reference • Digital Communication – by Sanjay Sharma • Advance Electronic Communication – by Robert Tomasi • World Wide Web • 167
  168. 168. 168