Data and Computer Communications

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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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### 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

PENGODEAN DATA
• Karakter data yang akandikirimdarisuatutitikketitik yang lain tidakdapatdikirimsecaralangsung.
• Karakter-karakter data tersebutharusdikodekanterlebihdahuludengankode yang dikenalolehsetiap terminal yang ada
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)

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.

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.

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.

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.

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

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.

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)

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

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.
Some Terms
• unipolar
• polar
• data rate
• duration or length of a bit
• modulation rate
• mark and space
• Sinyalunipolaradalahsemuaelemensinyal yang mempunyaitanda yang sama, yaitupositifsemuaataunegatifsemua.
• Sinyal polar adalahelemen-elemensinyaldimanasalahsatukondisilogikdiwakilioleh level teganganpositifdan yang lainnyaoleh level tegangannegatif.
• Durasiataulebarsuatu bit adalahwaktu yang diperlukanolehpemancaruntukmemancarkan bit tersebut.
• Mark dan space menyatakan digit biner 1 dan 0

• 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
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
Comparison of Encoding Schemes
• signal spectrum
• clocking
• error detection
• signal interference and noise immunity
• cost and complexity
• two different voltages for 0 and 1 bits
• voltage constant during bit interval
• such as absence of voltage for zero, constant positive voltage for one
• more often, negative voltage for one value and positive for the other
• 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
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
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
Multilevel BinaryPseudoternary
• one represented by absence of line signal
• zero represented by alternating positive and negative
• each used in some applications
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
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.
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
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
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
Digital Data, Analog Signal
• main use is public telephone system
• has freq range of 300Hz to 3400Hz
• use modem (modulator-demodulator)
• encoding techniques
• Frequency shift keying (FSK)
• Phase shift keying (PK)

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 :
• Frequency Shift Keying (FSK)
• Phase Shift Keying (PSK)
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

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

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
• even higher frequency on LANs using co-ax

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.

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

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

• 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
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
• 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
QAM Variants
• each of two streams in one of two states
• four state system
• essentially QPSK
• combined stream in one of 16 states
• have 64 and 256 state systems
• improved data rate for given bandwidth
• but increased potential error rate
Analog Data, Digital Signal
• digitization is conversion of analog data into digital data which can then:
• be transmitted using NRZ-L
• be transmitted using code other than NRZ-L
• be converted to analog signal
• analog to digital conversion done using a codec
• pulse code modulation
• delta modulation
Pulse Code Modulation (PCM)
• sampling theorem:
• “If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all information in original signal”
• eg. 4000Hz voice data, requires 8000 sample per sec
• strictly have analog samples
• Pulse Amplitude Modulation (PAM)
• so assign each a digital value
Delta Modulation
• can move up or down one level () at each sample interval
• has binary behavior
• since function only moves up or down at each sample interval
• hence can encode each sample as single bit
• 1 for up or 0 for down
PCM verses Delta Modulation
• DM has simplicity compared to PCM
• but has worse SNR
• issue of bandwidth used
• eg. for good voice reproduction with PCM
• want 128 levels (7 bit) & voice bandwidth 4khz
• need 8000 x 7 = 56kbps
• data compression can improve on this
• still growing demand for digital signals
• use of repeaters, TDM, efficient switching
• PCM preferred to DM for analog signals
Analog Data, Analog Signals
• modulate carrier frequency with analog data
• why modulate analog signals?
• higher frequency can give more efficient transmission
• permits frequency division multiplexing (chapter 8)
• types of modulation
• Amplitude
• Frequency
• Phase
Analog ModulationTechniques
• Amplitude Modulation
• Frequency Modulation
• Phase Modulation
Summary
• looked at signal encoding techniques
• digital data, digital signal
• analog data, digital signal
• digital data, analog signal
• analog data, analog signal