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Data Encoding (Chap. 5)

Data Encoding (Chap. 5). Transforming original signal just before transmission. Both analog and digital data can be encoded into either analog or digital signals. Digital Transmission Terminology. Data element: bit. Signaling element: encoding of data element for transmission.

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Data Encoding (Chap. 5)

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  1. Data Encoding (Chap. 5) • Transforming original signal just before transmission. • Both analog and digital data can be encoded into either analog or digital signals. University of Delaware CPEG 419

  2. 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. University of Delaware CPEG 419

  3. 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). University of Delaware CPEG 419

  4. Switch when a 1 occurs Approach 1: NRZ • But how do you know when to sample? • Phase-locked-loop (PLL) – measures the difference when transitions occur on the wire and when they occur on a local adjustable oscillator, and then make adjustments accordingly. • YOU MUST HAVE TRANSISTIONS TO LOCK ON TO. University of Delaware CPEG 419

  5. Multilevel Binary opposite direction Pros: No DC component. Can be used to force transitions (to help PLL). Cons: We are using 3 levels and could send ?? bits instead of 1 University of Delaware CPEG 419

  6. Scrambling – to help the PLL • If there are not enough transitions, the PLL may have problems. • So we force extra transitions when there are not enough. • Approach 1 – Use special coding so that long strings of zeros (or ones) don’t occur. University of Delaware CPEG 419

  7. Scrambling – to help the PLL • Approach 2 – Use multilevel binary and set illegal transitions to long strings of zeros. • Here, if an octet of zeros occurs, send a special illegal sequence. • The receiver must be able to interpret this special sequence. used in long-distance transmission University of Delaware CPEG 419

  8. Biphase – Differential Manchester(Self-Clocking) A transition always occurs in the middle of the period. A zero is represented by a transition occurring at the beginning of the period. A one is represented by no transition at the beginning of the period. 0 0 1 1 always a transition in the middle Used in CD players and Ethernet University of Delaware CPEG 419

  9. Methods to Encode Digital Signals • NRZ • Multilevel binary • Manchester • Issues: • DC? • Self Clocking? • How big is the spectrum? University of Delaware CPEG 419

  10. Sending Digital Signals over Analog (e.g. Modem) • Amplitude shift keying (ASK) (Amplitude Modulation) • Frequency shift keying (FSK) (Frequency modulation) • Phase shift keying (PK) (Phase Modulation) • Modems use phase and amplitude. University of Delaware CPEG 419

  11. Modulation Techniques ASK FSK PSK University of Delaware CPEG 419

  12. Fixed mistakes on last slide? University of Delaware CPEG 419

  13. Phase-shift Keying • Quadrature phase-shift keying (QPSK) - send 2 bits. 90 0 180 270 University of Delaware CPEG 419

  14. QAM - Quadrature Amplitude Modulation constellation diagrams 90 90 0 180 0 180 270 270 QAM-16 (16 levels, how many bits) QAM - 64 University of Delaware CPEG 419

  15. V32 128 bits: 6 data and 1 parity (error correction) University of Delaware CPEG 419

  16. Use 2400 sample each way - duplex Definition: a duplex connection means that we can send data in both directions at the same time. A simplex or half-duplex connection only sends data in one direction at a time. How fast is V32? The phone system transmits over the 300 to 3400 Hz band. So what data rate can we use? How fast can we send symbols? So 2400 * 6 = 14400 bps What is the baud rate? V.34 2400 baud - with 12 data bits/symbol V.34 bis 2400 baud – with 14 data bits/symbol That’s the fastest there is! To get 56K you send at 4000 baud (if the phone system can handle it) University of Delaware CPEG 419

  17. Digital Subscriber Lines (DSL) • ADSL – A for asymmetric, faster down load speed than up. • The 56kbps or 33kbps modem speed is due to a filter installed at the end office. • If this filter is removed, then the full spectrum of the twisted pair is available (which is?) • But, if you are far from the office, then you can’t get a very high data rate because…? • The DSL standard goes up to 8 Mbps down and 1 Mbps up. University of Delaware CPEG 419

  18. DSL A total of 256 4kHz channels Upstream downstream empty 25kHz (channel 6) channel 6+32 Voice POTS (stands for ?) (channel 0) 250 parallel channels: Each data channel uses QAM 16 (with 1 parity bit). The quality of each channel is monitored and adjusted. So channels may transmit at different speeds What is the maximum data rate? University of Delaware CPEG 419

  19. ADSL configuration voice switch telephone line splitter splitter ADSL Modem DSLAM digital subscriber line access multiplexer TO ISP University of Delaware CPEG 419

  20. Cable - History • Starting in the late 1940s, mountain people put a big antenna on a hill top and ran a cable to them and their friends. • 1970, 1000s of independent cable systems existed. • 1974 Time Inc started HBO University of Delaware CPEG 419

  21. Early CableCommunity Antenna Television Head end University of Delaware CPEG 419

  22. very high bandwidth (fiber) Head end coaxial fiber Internet over Cable A group of homes share the cable. (As oppose to DSL.) The number of homes per group is around 500-2000. University of Delaware CPEG 419

  23. Cable Spectrum 108MHz Upstream 5 – 42 MHZ 750MHz 550MHz TV shopping HBO Downstream data TV FM 54 MHz 88MHz Each TV channel gets its own frequency. This is called frequency multiplexing University of Delaware CPEG 419

  24. Multiplexing It cost the same to install a high bandwidth connection as it does to install a low bandwidth one. So the approach is to install a few high bandwidth connections and make it seem like many low bandwidth connections. One high bandwidth link is shared by many. But how? University of Delaware CPEG 419

  25. Frequency Division Multiplexing • FDM • Useful bandwidth of medium exceeds required bandwidth of channel • Each signal is modulated to a different carrier frequency • Carrier frequencies separated so signals do not overlap (guard bands) • e.g. broadcast radio • Channel allocated even if no data • ADSL uses FDM University of Delaware CPEG 419

  26. FDM Channel 1 Channel 2 Channel n University of Delaware CPEG 419

  27. Analog Carrier Systems • AT&T (USA) • Hierarchy of FDM schemes • Group • 12 voice channels (4kHz each) = 48kHz • Range 60kHz to 108kHz • Supergroup • 60 channel • FDM of 5 group signals on carriers between 420kHz and 612 kHz • Mastergroup • 10 supergroups University of Delaware CPEG 419

  28. Back to Cable • Why put downstream data at highend of spectrum? • When they made the system they put in amplifiers that could work at these high frequencies (why?) • So the downstream was all set to go. But upstream amplifiers had to be installed. • Downstream data also uses FDM: 6MHz or 8MHz channel width with QAM-64 (what is the data rate?) • But with overhead you only get 27Mbps. • The upstream also uses FDM, but with QPSK (2 bits per symbol). University of Delaware CPEG 419

  29. Cable modems • On modem boot-up, the headend tells the modem which channel to use for upstream and downstream transmission. • Many users may share the same channel. • To share the same channel, cable uses time division multiplexing (TDM). University of Delaware CPEG 419

  30. Time Division Multiplexing (TDM) channel 1 channel 2 channel n University of Delaware CPEG 419

  31. TDM • Data rate of medium exceeds data rate of digital signal to be transmitted • Multiple digital signals interleaved in time • May be at bit level of blocks • Time slots preassigned to sources and fixed • Time slots allocated even if no data • Time slots do not have to be evenly distributed amongst sources University of Delaware CPEG 419

  32. TDM vs FDM FDM TDM University of Delaware CPEG 419

  33. TDM • Must provide synchronizing mechanism • Added digit framing • One control bit added to each TDM frame • Looks like another channel - “control channel” • Identifiable bit pattern used on control channel • e.g. alternating 01010101…unlikely on a data channel • Can compare incoming bit patterns on each channel with sync pattern University of Delaware CPEG 419

  34. Back to Cable • Downstream is easy. The headend just transmits packets as it wants. Each packet has a label, so the modem can detect that the packet belongs to it. • Upstream is hard. • The upstream channel is shared. • TDM is used, but still each TDM slot is shared by many users. • What happens if two users try to send upstream data on the same channel? University of Delaware CPEG 419

  35. Cable: Upstream Media Access (link layer) • Time is divided into minislot. It is possible to transmit 8 bytes in one minislot. • The modem asks the headend if it can transmit a packet. • The headend responds with an acknowledgment and tells the modem which minislots it can use. • Problem: How can the modem sent the request without permission to use minislots? • Solution: on boot-up, the headend tells the modem which minislots it can use for requesting minislots and the headend never allocates these minislots for upstream data. • Problem: These special control minislots are shared by many users (why?), so what happens if two users make a request at the same time? • Solution: If two users transmit at the same time, the signal cannot be understood by the headend and is ignored. Thus, no acknowledgement is made. The competing modems then wait a random amount of time and try again. It if fails again, then they wait a random amount of time again, but the maximum time they might wait is doubled. University of Delaware CPEG 419

  36. Cable MAC (media access control ) headend modem minislot } modem sends a request for upstream minislots Headend gets request. Thinks about it. And sends acknowledgement with which minislots to use. modem sends data data does not overlap with request minislot upstream request for bandwidth minislots time University of Delaware CPEG 419

  37. Cable MAC: Contention Two modems send request for bandwidth at the same time. The headend can figure out what was transmitted and ignores it The modems wait for the acknowledgement that will never arrive. The amount of time they wait is random. This time the red modem gets through. The blue modem tries again, but the green modem also sends a request. Now the blue doubles the maximum random amount of time it waits before sending another request. Green gets through University of Delaware CPEG 419

  38. Cable vs. DSL • Cable could give higher bandwidth, but it might give less. It depends on the number of users. • If there are too many users in a group, the cable operator has to put in a fiber and headend. That cost money, so they try not to do it. • DSL can promise 1Mbps down and 256kbps up, and you will likely get it. • Note that sharing bandwidth is much more efficient than assigning each user a fixed chunk. • The telephone system is very reliable. When was the last time you picked up the phone and there was no dial tone (major earthquake or huge storm). Cable will go down when the power goes out. University of Delaware CPEG 419

  39. A F D E E B G G C C A F F D E Mobile Phone University of Delaware CPEG 419

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