Encoding and Transmission of Data CSIS 625 Week 2 Encoding and Transmission of Data Copyright 2001, 2002 - Dan Oelke Portions Copyright 2000 - Dmitry Gringauz For use by students of CSIS 625 for purposes of this class only. CSIS 625

Overview Analog and Digital Signals Encoding and Modulation Vocabulary Analog Signals Digital Signals Encoding and Modulation Digital to Digital Conversion Analog to Digital Conversion Digital To Analog Conversion Analog to Analog Conversion CSIS 625

Analog and Digital Signals Signal - an electromagnetic wave that transfers information Analog Signal - Continuous set of data Real Numbers Digital Signals - Discrete set of data Integer Numbers Often binary (1 or 0 only) Digital Signal Analog Signal CSIS 625

Periodic vs Aperiodic Signals A signal that completes a pattern in a measureable time frame Aperiodic Signal A signal that does not exhibit a pattern All aperiodic signals can be shown to be a combination of periodic signals APeriodic Signal Periodic Signal CSIS 625

Signal definitions Amplitude - The “height” of a signal. Measured in Volts, Amps, Watts, etc. Period - The amount of time to complete one cycle Frequency - The number of periods per second. Measured in Hertz (Hz) Amplitude Period CSIS 625

Phase The position of a sine wave relative to time zero. Measured in degrees. 0 Degrees 90 Degrees 1/4 Cycle 180 Degrees 1/2 Cycle 270 Degrees 3/4 Cycle CSIS 625

Bandwidth Bandwidth - A range of frequencies Analog - measured in Hz Bandwidth = High-Freq – Low-Freq Spectrum - synonym - used only in analog measurements. Bandwidth in digital realm - often used to refer to bits-per-second CSIS 625

Bit Rate Most digital signals are aperiodic Period and frequency are not appropriate to describe digital signals Bit Interval - time to send one bit Bit rate - number of bits send in a second. Measured in bits per second bps - Bits Per Second Do NOT use Hz when you mean bps or vice-versa CSIS 625

Decomposing a digital signal A digital signal can be decomposed into an infinite number of simple sine waves It is not practical or necessary to send all of these components Significant Bandwidth - Those frequencies necessary to recreate a digital bit pattern Significant Bandwidth is related to bit rate Greater bit rate = Greater significant bandwidth CSIS 625

Medium Bandwidth and Significant Bandwidth All transmission mediums have limited bandwidth The significant bandwidth of a digital bit rate must fit within the limited bandwidth of the medium that carries it. CSIS 625

Encoding Information must often be encoded before being sent over a medium Four basic types of encoding Digital to Digital Analog to Digital Digital to Analog Analog to Analog Encoding schemes may be stacked Voice to digital data to radio waves CSIS 625

Digital to Digital Encoding Using a digital signal to represent digital data Binary data is translated to different voltage, current, or light pulses that can be transported over the medium. Types Unipolar - uses 1 signal level Polar - uses 2 signal levels Bipolar - uses 2 signal levels and 0 CSIS 625

Digital signal encoding formats 0 1 0 0 1 1 0 0 0 1 Unipolar NRZL NRZI RZ Manchester Differential Manchester Bipolar-AMI Pseudoternary CSIS 625

Unipolar Encoding Simplest scheme Uses two signal levels 1’s are encoded with signal present 0’s are encoded by absence of a signal (Sometimes inverse of the above) Long run of 0s or 1s can’t be handled by some mediums CSIS 625

Unipolar encoding - synchronization When a signal isn’t varying, receiver can’t determine beginning and ending of each bit Solutions: A separate line with a clock signal Asynchronzous Serial lines wrap each byte with start and stop bit Scrambling of data to ensure enough transitions Use of additional coding schemes like 8b10b CSIS 625

Polar Encoding Uses a positive and a negative signal but not a zero level Several types of Polar encoding NRZ - Non-Return to Zero RZ - Return to Zero Biphase CSIS 625

Non-Return to Zero - Level NRZL - Non-Return to Zero - Level Simple - exactly like Polar, except 1’s are encoded with positive signal 0’s are encoded with negative signal (Sometimes inverse of the above) Same synchronization problems and solutions CSIS 625

Non-Return to Zero - Invert on Ones NRZI - Non-Return to Zero - Invert on Ones A change in voltage level indicates a 1 No change in voltage level indicates a 0 Synchronization less of a problem Every 1 bit causes a signal change A string of 0’s still causes problems Same synchronization solutions CSIS 625

Return to Zero RZ - Return to Zero Not strictly polar - uses 0 in addition to positive and negative Works like NRZL, except it goes to zero between each bit. Transition to/from zero provides for synchronization Because there are more transisitions (2 per bit time) it has a higher significant bandwidth than NRZ CSIS 625

Manchester Coding A biphase mechanism Inversion of signal in middle of each bit low to high transition is 1 high to low transition is 0 Mid-bit inversion provides for both data and synchronization information May have transition between bits so that right transition can be made in middle of a bit CSIS 625

Differential Manchester A biphase mechanism Always has a mid-bit inversion to provide timing information Inversion at beginning of bit time provides data Presence of inversion means 0 No inversion means 1 CSIS 625

Bipolar AMI Bipolar Alternate Mark Inversion Mark comes from old telegraphy - means 1 Encoding 0 = lack of signal (0) 1 = positive or negative values alternating for successive ones CSIS 625

Pseudoternary Same as Bipolar AMI, but inverts 1s and 0s Encoding 0 = positive or negative values alternating for successive zeros 1 = lack of signal (0) CSIS 625

B8ZS Bipolar 8-Zero Substitution A modification of Bipolar AMI to solve the synchronization problem that occurs when a long string of 0s occurs Substitutes 8 consecutive 0s with fixed pattern that contains 2 AMI violations 1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0 Binary-AMI B8ZS V = Bipolar AMI Violation V CSIS 625

HDB3 High Density Bipolar - 3 Zeros Similar to B8ZS Substitutes 4 zeros with a pattern that contains 1 AMI violation 1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 1 0 Binary-AMI HDB3 V = Bipolar AMI Violation V CSIS 625

Analog to Digital Encoding Digitizing - analog to digital conversion Approximate analog information with a digital signal Reduces infinite number of analog values to a finite number of digital values. Codec - Coder-Decoder Analog to digital converter CSIS 625

Pulse Amplitude Modulation (PAM) First step to analog to digital encoding Sample analog amplitude information at equal intervals PAM alone not useful as measurements are still analog values CSIS 625

Pulse Code Modulation (PCM) Modifies PAM output to create completely digital signal PCM quantizes Take the samples from PAM and assigns digital values to each measurement. Nyquist theorem - To ensure accurate reproduction of a signal, the sample rate must be twice the highest frequency of the original signal CSIS 625

PCM & Telephony Telephony system uses 8 bits (256 levels) when quantizing A non-linear set of quantizing levels is used so that quiet sounds are accurately reproduced 300-3300Hz is voice range. 8kHz sample rate is used to cover this range 8kHz * 8 bits/sample = 64,000 bps CSIS 625

DM - Delta Modulation Analog data is approximated using a staircase function that moves up or down by one level each sampling time. Digital data is a stream of 1s and 0s that specify the up and down steps. Can be implemented using simple components. Not as good quality as PCM Quantizing noise when slope changes slowly Slope overload noise when slope changes fast CSIS 625

Digital to Analog Conversion ASK - Amplitude Shift Keying FSK - Frequency Shift Keying PSK - Phase Shift Keying QAM - Quadrature Amplitude Modulation combination of ASK & PSK CSIS 625

Bit rate vs. Baud Rate & Carrier Signal Bit rate is Bits per Second Baud Rate is number of signal units per second Baud rate is less than or equal bit rate Don’t mix them up! Carrier Signal high frequency signal that is modified to carry digital signal CSIS 625

ASK - Amplitude Shift Keying Amplitude of signal varied for 1 or 0 Frequency and phase remain constant Very susceptible to noise On-Off-Keying - signal and no-signal Example: 1 BIT 1 BIT 1 1 BIT 1 BIT 1 CSIS 625

FSK- Frequency Shift Keying Frequency of the carrier signal is varied to represent a 1or 0. Avoids many of the noise problems of Amplitude Shift keying Example: 1 BIT 1 BIT 1 1 BIT 1 BIT 1 CSIS 625

PSK - Phase Shift Keying The phase of the carrier signal is varied to represent a 1 or 0. Avoids noise problems of ASK Uses less bandwidth than FSK Example: 1 BIT 1 BIT 1 1 BIT 1 BIT 1 CSIS 625

QPSK - Quadrature PSK A type of PSK that uses 90° shifts instead of 180° shifts. Allows for 2 bits per baud to be encoded. CSIS 625

DPSK - Differential PSK The bit pattern defines the phase change, instead of the current phase V.22bis standard at 1200 bps uses: 00  90 Degree phase change 01  0 Degree phase change 10  180 Degree phase change 11  270 Degree phase change CSIS 625

Quadrature Amplitude Modulation The phase and amplitude of the carrier signal is varied to give several bits per baud Number of different phases is greater than number of amplitudes Example: 2 amplitudes & 4 phases 3 BITS 000 3 BITS 010 3 BITS 001 3 BITS 111 CSIS 625

Trellis Coded Modulation Uses QAM, but includes extra data Trellis coding is a specific type of convolutional encoding Viterbi Decoder - a specific algorithm for decoding convolutionally encoded data. Convolutional codes add redundancy to the data, which makes it more resistant to noise. Resistance to noise is more important as data rates get higher. CSIS 625

Constellation diagrams Constellation diagram shows relationship between amplitude and phase of different signal levels polar diagram, amplitude shown as distance from center phase shown as degrees around circle 011 010 1 1 101 100 000 001 110 ASK PSK 111 8-QAM 16-QAM CSIS 625

Bandwidth required Amplitude Shift Keying Frequency Shift Keying bandwidth = baud rate * (1 + noise factor) noise factor is 0 in ideal world Frequency Shift Keying bandwidth = (fc1 - fc0) + baud rate Phase Shift Keying & QAM but bit rate is higher because more than one bit per baud CSIS 625

Analog to Analog Encoding AM - Amplitude Modulation The amplitude of the carrier is modified Bandwidth = 2x Bandwidth of modulating signal FM- Frequency Modulation The frequency of the carrier is modified Bandwidth = 10x Bandwidth of modulation signal CSIS 625

Analog to Analog Encoding Phase Modulation The phase of the carrier is modified Phase Modulation and FM are a special case of Angle modulation Observing the signal, it is impossible to tell apart FM and phase modulation CSIS 625

Parallel/Serial Transmission of Data Transmission of Digital Data Serial & Parallel transmission Serial interfaces - DTE & DCE - Modems CSIS 625

Parallel Transmission of Data Send several bits of data at the same time, each one over a separate media link. Typically 8 bits of data sent over 8 wires Examples: Printer cables, SCSI, PCI bus Allows faster transmission of data, but at the cost of multiple wires, multiple transmitters, and multiple receivers Must keep all bits in sync Typically uses a separate clock line CSIS 625

Serial Transmission of Data Sends all bits from node to node over a single media link. Bits are sent one after another - or “serially” May or may not have additional media links for clock, frame, or flow control. Need some method of keeping track of when a byte starts and ends. Asynchronous or Synchronous CSIS 625

Serial - Asynchronous transmission Bits are grouped together into characters Start and stop bits frame the data bits A start bit is sent first Followed by the data bits Followed by a stop bit or bits Variable number of idle bits between characters CSIS 625

Serial - Asynchronous transmission At best - 80% efficient 8 data bits 1 start bit 1 stop bit Allows for about a lot of timing error Example: Start Data Stop CSIS 625

Serial - Synchronous transmission Each byte of data is sent with no extra gaps between bytes. Data is grouped into frames Frame contains Between frames, special idle patterns used Much less overhead that asynchronous Can achieve faster bit rates than asynchronous Requires synchronization method CSIS 625

Data transparency on serial links Data transparency - the ability of a link to send any data pattern Bit stuffing - insertion of extra bits to change a flag pattern so that data transparency is achieved Byte stuffing - insertion of extra bytes to change a flag pattern so that data transparency is achieved Flag character - special bit pattern to show start or end of a frame CSIS 625

Serial - Synchronous transmission Bit-oriented synchronous transmission Uses a special bit pattern at the start and end of the frame (flag character) Data may be any number of bits Uses bit stuffing to replace flag pattern in data Bit stuffing is slightly more efficient than byte stuffing Easier to implement in hardware CSIS 625

Serial - Synchronous transmission Character oriented synchronous transmission Uses a special byte at the start and end of the frame Data must be an even number of 8-bit bytes Uses byte stuffing to replace flag byte in data Byte stuffing makes this slightly less efficient Easier to implement in software CSIS 625

DTE-DCE interface DTE - Data Terminal Equipment A device that is a source or destination for binary digital data DCE - Data Circuit-terminating Equipment A device that interfaces between a DTE and a network Modem is classic DCE example Lots of standards specify DTE to DCE interface More standards for DCE to DCE interface CSIS 625

RS-232 Interface Specifies the mechanical, electrical & functional characteristics of DTE-DCE interface EIA-232 is now the official name Tailored to Computer to modem interface Limited to about 20 Kbps Mechanical less than 50 feet long cable DB-25 connector original standard DB-9 connector now standardized CSIS 625

RS-232 Interface Electrical - Uses NRZL 0 = +3 to +15 volts 1 = -3 to -15 volts 3 pins are all that are necessary Receive Data Transmit Data Ground Other pins are often ignored Null modem - a device that flips receive and transmit lines CSIS 625

Other serial interfaces RS-449 - uses 37 pin connector RS-423 - uses 2-6 volt levels 40 feet - 100 Kbps 4000 feet - 1 Kbps RS-422 - 2-6 Volt balanced transmission 40 feet - 10 Mbps CSIS 625

Balanced transmission Uses two wires with a positive or negative voltage put on the line Compared to unbalanced which using two wires, one as ground and the other as signal. Better noise resistance than unbalanced CSIS 625