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Data and Computer Communications

Data and Computer Communications. Chapter 5 – Signal Encoding Techniques. Eighth Edition by William Stallings Lecture slides by Lawrie Brown. Signal Encoding Techniques. Even the natives have difficulty mastering this peculiar vocabulary —The Golden Bough , Sir James George Frazer.

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Data and Computer Communications

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  1. Data and Computer Communications Chapter 5 – Signal Encoding Techniques Eighth Edition by William Stallings Lecture slides by Lawrie Brown

  2. Signal Encoding Techniques Even the natives have difficulty mastering this peculiar vocabulary —The Golden Bough, Sir James George Frazer

  3. PENGODEAN DATA • Karakter data yang akandikirimdarisuatutitikketitik yang lain tidakdapatdikirimsecaralangsung. • Karakter-karakter data tersebutharusdikodekanterlebihdahuludengankode yang dikenalolehsetiap terminal yang ada • Ttujuandaripengodeanadalahmenjadikansetiapkarakterdalamsebuahinformasi digital kedalambentukbiner agar dapatditransmisikan.

  4. KODE-KODE • Kode yang digunakanpadakomunikasi data : 1.BCD (Binary Coded Decimal) 2.SBCDIC (Standard Binary Coded Decimal Interchange Code) 3. EBCDIC (Extended Binary Coded Decimal Interchange Code) 4. BOUDDOT 5.ASCII (American Satndard Code for Information Interchange)

  5. KODE BCD : Binary Coded Decimal Kode biner yang mewakili 0 – 9, yang merupakan kombinasi 4 bit, Kombinasi kode maksimal 16, tapi yang digunakan hanya 0-9 dan tidak bisa untuk huruf.

  6. KODE SBCDIC: Standard Binary Coded Decimal Interchange Code Kombinasi 6 bit , kombinasi maksimal 64 kode, 10 kode untuk angka, 26 kode untuk huruf, sisanya untuk kode yang lain.

  7. KODE EBCDIC :Extended Binary Coded Decimal Interchange Code Kode 8 bit, maksimal karakter 256 Pada EBCDIC, hight –order bits atau 4 bit pertama disebut zone bit dan low-order bits atau 4 bit kedua disebut dengan numeric bit. Merupakan pengembangan dari kode 6 bit yang dipakai untuk kartu berlubang. Dipakai pada komputer generasi ketiga, seperti pada IBM

  8. KODE BOUDOT Terdiri dari 5 bit yang digunakan pada terminal teletype dan teleprinter Terdiri dari 25 sampai 32 kombinasi dengan kode huruf dan gambar yang berbeda.

  9. KODE ASCII:American Standard Code for Information Interchange ASCII 7 Bit standar alfabet no. 5 berdasarkan ISO Kode ini menggunakan 7 bit dan bit yang kedelapan digunakan untuk posisi pengecekan bit secara even atau odd parity. Memiliki 128 kombinasi yang selalu digunakan. 32 kode untuk fungsi kendali, yang lain untuk alphanumeric dan karakter khusus. KODE ASCII

  10. Dari 128 kode ASCII, 34 untuk keperluan khusus seperti :

  11. Kode ASCII 8 bit, mempunyai kombinasi bit yang lebih banyak, sehingga karakter-karakter yang tidak dapat diwakili 7 bit, dapat dibuat dengan kode ASCII 8 bit UNICODE Digunakan untuk menuliskan kata-kata sesuai dengan bahasa ibu . Dengan kata lain, negara-negara yang berbeda menggunakan karakter berbeda.

  12. Signal Encoding Techniques

  13. Macam-macamTeknikPengkodean 1. Digital data, digital signals: simplest form of digital encoding of digital data (Data digital, sinyal digital) 2. Digital data, analog signal: A modem converts digital data to an analog signal so that it can be transmitted over an analog (Data digital, sinyal analog) 3. Analog data, digital signals: Analog data, such as voice and video, are often digitized to be able to use digital transmission facilities (Data analog, sinyal digital) 4. Analog data, analog signals: Analog data are modulated by a carrier frequency to produce an analog signal in a different frequency band, which can be utilized on an analog transmission system (Data analog,sinyal analog)

  14. Macam-macamTeknikPengkodean • Data digital, sinyal digital: Pengkodean digital untuk data digital adalah menugaskan satu tingkat tegangan ke biner 1 atau sedangkan lainnya ke biner 0.Sedangkan untuk yang lebih kompleks memerlukan sustu sinkronisasi data. • Data digital,sinyal analog:Sebuah modem mengubah data digital menjadi sinyal analog sehingga dapat ditransmisikan melalaui jalur analog. Teknik dasar yang digunakan adalah amplitude shift keying(ASK), frequency shift keying (FSK), dan phase shift keying (PSK). • Data analog,sinyal digital:Seperti suara dan video didigtalkan supaya dapat ditransmisikan pada fasilitas transmisi digital.Salah satu teknik yang digunalan adalah pulse code modulation, yang mencakup sampling data analog secara periodeik dan menguantifikasikan sampel-sampel tersebut. • Data analog, sinyal analog: Data analog dimodulasi oleh sebuah frekuensi pembawa untuk menghasilkan sinyal analog dengan band frekuensi yang berbeda yang dapat digunakan pada sistem transmisi analog. Teknik yang dipergunakan adalah AM, FM, PM (phase modulation).

  15. Digital Data, Digital Signal • Digital signal • discrete, discontinuous voltage pulses • each pulse is a signal element • binary data encoded into signal elements

  16. Data Digital, Sinyal Digital • Data digital merupakan data yang memiliki deretan nilai yang berbeda dan memiliki ciri-ciri tersendiri.Salah satu contoh data digital adalah teks, bilangan bulat dan karakter yang lain. • permasalahannya adalah data tersebut

  17. Some Terms • unipolar • polar • data rate • duration or length of a bit • modulation rate • mark and space

  18. Istilah-istilah yang berhubungandengan data digital-sinyal digital: • Elemensinyaladalahtiappulsadarisinyal digital, data binerditransmisikandenganmeng-encode-kantiap bit data menjadielemen-elemensinyal. • Sinyalunipolaradalahsemuaelemensinyal yang mempunyaitanda yang sama, yaitupositifsemuaataunegatifsemua. • Sinyal polar adalahelemen-elemensinyaldimanasalahsatukondisilogikdiwakilioleh level teganganpositifdan yang lainnyaoleh level tegangannegatif. • Durasiataulebarsuatu bit adalahwaktu yang diperlukanolehpemancaruntukmemancarkan bit tersebut. • Modulation rate adalahkecepatandimana level sinyalberubah, dimanadinyatakandalam bauds atauelemensinyalperdetik • Mark dan space menyatakan digit biner 1 dan 0

  19. Tugaspesawatpenerimadalammengartikansinyal digital : • RX harusmengetahui timing tiap bit • RX harusmenentukanapakah level sinyaldalamposisi bit high(1) atau Low(0) • Prosesdiatasdilakuakdenganproses Sampling • Faktor yang menetukankesuksesan RX : • Data rate (kecepatan data) : peningkatankecepatan data akanmeningkatkan BIT ERROR RATE /BER ( kecepatankesalahan bit) • S/N : peningkatan S/N akanmenurunkankecepatan bit • Bandwidth : peningkatan bandwidth dapatmeningkatkankecepatan data

  20. Interpreting Signals • need to know • timing of bits - when they start and end • signal levels • factors affecting signal interpretation • signal to noise ratio • data rate • bandwidth • encoding scheme

  21. Comparison of Encoding Schemes • signal spectrum • clocking • error detection • signal interference and noise immunity • cost and complexity

  22. Encoding Schemes

  23. Nonreturn to Zero-Level(NRZ-L) • two different voltages for 0 and 1 bits • voltage constant during bit interval • no transition I.e. no return to zero voltage • such as absence of voltage for zero, constant positive voltage for one • more often, negative voltage for one value and positive for the other

  24. Nonreturn to Zero Inverted • nonreturn to zero inverted on ones • constant voltage pulse for duration of bit • data encoded as presence or absence of signal transition at beginning of bit time • transition (low to high or high to low) denotes binary 1 • no transition denotes binary 0 • example of differential encoding since have • data represented by changes rather than levels • more reliable detection of transition rather than level • easy to lose sense of polarity

  25. NRZ Pros & Cons • Pros • easy to engineer • make good use of bandwidth • Cons • dc component • lack of synchronization capability • used for magnetic recording • not often used for signal transmission

  26. Multilevel BinaryBipolar-AMI • Use more than two levels • Bipolar-AMI • zero represented by no line signal • one represented by positive or negative pulse • one pulses alternate in polarity • no loss of sync if a long string of ones • long runs of zeros still a problem • no net dc component • lower bandwidth • easy error detection

  27. Multilevel BinaryPseudoternary • one represented by absence of line signal • zero represented by alternating positive and negative • no advantage or disadvantage over bipolar-AMI • each used in some applications

  28. Multilevel Binary Issues • synchronization with long runs of 0’s or 1’s • can insert additional bits, cf ISDN • scramble data (later) • not as efficient as NRZ • each signal element only represents one bit • receiver distinguishes between three levels: +A, -A, 0 • a 3 level system could represent log23 = 1.58 bits • requires approx. 3dB more signal power for same probability of bit error

  29. Manchester Encoding • has transition in middle of each bit period • transition serves as clock and data • low to high represents one • high to low represents zero • used by IEEE 802.

  30. Differential Manchester Encoding • midbit transition is clocking only • transition at start of bit period representing 0 • no transition at start of bit period representing 1 • this is a differential encoding scheme • used by IEEE 802.5

  31. Biphase Pros and Cons • Con • at least one transition per bit time and possibly two • maximum modulation rate is twice NRZ • requires more bandwidth • Pros • synchronization on mid bit transition (self clocking) • has no dc component • has error detection

  32. Modulation Rate

  33. Scrambling • use scrambling to replace sequences that would produce constant voltage • these filling sequences must • produce enough transitions to sync • be recognized by receiver & replaced with original • be same length as original • design goals • have no dc component • have no long sequences of zero level line signal • have no reduction in data rate • give error detection capability

  34. B8ZS and HDB3

  35. Digital Data, Analog Signal • main use is public telephone system • has freq range of 300Hz to 3400Hz • use modem (modulator-demodulator) • encoding techniques • Amplitude shift keying (ASK) • Frequency shift keying (FSK) • Phase shift keying (PK)

  36. Data digital, SinyalAnalog • Contohumumtransmisi data digital denganmenggunakansinyal analog adalah Public Telephone Network. Perangkat yang dipakaiadalah MODEM (modulator-demodulator) yang mengubah data digital kesinyal analog (modulator) dansebaliknyamengubahsinyal analog menjadisinyal digital (demodulator). • Teknik-teknikPenyandian: • Tigateknikpenyandianuntukmengubah data digital menjadisinyal analog : • Amplitudo Shift Keying (ASK) • Frequency Shift Keying (FSK) • Phase Shift Keying (PSK)

  37. Modulation Techniques

  38. Amplitude Shift Keying • encode 0/1 by different carrier amplitudes • usually have one amplitude zero • susceptible to sudden gain changes • inefficient • used for • up to 1200bps on voice grade lines • very high speeds over optical fiber

  39. Amplitudo Shift Keying (ASK) Dua biner diwakilkan dengan dua amplitudo frekuensi pembawa yang berbeda. Salah satu dari amplitudo adalah nol; satu digit biner yang ditunjukkan mellaui keberadaan sinyal paad amplitudo yang konstan dari suatu sinyal pembawa. Dapat dinyatakan sebagai berikut : A cos (2Π f1t + Θc ) biner 1 S(t) = 0 biner 0

  40. Binary Frequency Shift Keying • most common is binary FSK (BFSK) • two binary values represented by two different frequencies (near carrier) • less susceptible to error than ASK • used for • up to 1200bps on voice grade lines • high frequency radio • even higher frequency on LANs using co-ax

  41. Frequency Shift Keying (FSK) Dua biner diwakili oleh dua frekuensi berbeda yang dekat dengan pembawa frekuensi atau dinyatakan sebagai A cos (2Π f1t + Θc ) biner 1 S(t) = A cos (2Π f2t + Θc ) biner 0 Kecepatan data dapat mencapai 1200 bps pada voice grade line, dipakai untuk transmisi radio frekuensi tinggi dan juga jaringan lokal dengan frekuensi tinggi yang memakai kabel koaksial.

  42. Multiple FSK • each signalling element represents more than one bit • more than two frequencies used • more bandwidth efficient • more prone to error

  43. Phase Shift Keying • phase of carrier signal is shifted to represent data • binary PSK • two phases represent two binary digits • differential PSK • phase shifted relative to previous transmission rather than some reference signal

  44. Phase Shift Keying (PSK) Biner 0 diwakilkan dengan mengirim suatu sinyal dengan fase yang sama terhadap fase sebelumnya dan biner 1 diwakilkan dengan mengirim suatu sinyal dengan fase berlawanan dengan sinyal yang dikirim sebelumnya, atau dinyatakan sebagai berikut : A cos (2Π f1t + Π ) biner 1 S(t) = A cos (2Π f2t ) biner 0

  45. Quadrature PSK • get more efficient use if each signal element represents more than one bit • eg. shifts of /2 (90o) • each element represents two bits • split input data stream in two & modulate onto carrier & phase shifted carrier • can use 8 phase angles & more than one amplitude • 9600bps modem uses 12 angles, four of which have two amplitudes

  46. QPSK and OQPSK Modulators

  47. Performance of Digital to Analog Modulation Schemes • bandwidth • ASK/PSK bandwidth directly relates to bit rate • multilevel PSK gives significant improvements • in presence of noise: • bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK • for MFSK & MPSK have tradeoff between bandwidth efficiency and error performance

  48. Quadrature Amplitude Modulation • QAM used on asymmetric digital subscriber line (ADSL) and some wireless • combination of ASK and PSK • logical extension of QPSK • send two different signals simultaneously on same carrier frequency • use two copies of carrier, one shifted 90° • each carrier is ASK modulated • two independent signals over same medium • demodulate and combine for original binary output

  49. QAM Modulator

  50. QAM Variants • two level ASK • each of two streams in one of two states • four state system • essentially QPSK • four level ASK • combined stream in one of 16 states • have 64 and 256 state systems • improved data rate for given bandwidth • but increased potential error rate

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