thapar university4-1 digital transmission 1.digital-to-digital conversion 2.analog-to-digital...

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Digital Transmission

1. Digital-to-Digital Conversion

2. Analog-to-Digital Conversion

3. Transmission Mode

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Digital-to-Digital Conversion• Involves three techniques:

– Line coding (always needed), block coding, and scrambling

• Line coding: the process of converting digital data to digital signals

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Signal Element and Data Element• Data elements (smallest entity, bit) are what we need to send; signal elements are

what we can send. Data elements are being carried and signal elements are carriers. Ratio r which is the no. of data elements carried by each signal element.

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Data Rate Versus Signal Rate

• Data rate defines the number of data elements (bits) sent in 1s: bps

• Signal rate is the number of signal elements sent in 1s: baud

• Data rate = bit rate, signal rate = pulse rate, modulation rate, baud rate

• S = c x N x 1/r, where N is the date rate; c is the case factor, S is the number of signal elements; r is the number of data elements carried by each signal element

• Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite

• The bandwidth is proportional to the signal rate (baud rate)

• The minimum bandwidth: Bmin = c x N x 1/r

• The maximum data rate: Nmax = 1/c x B x r

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Design Consideration for Line Coding Scheme• In decoding a digital signal, the receiver calculates a running average

of the received signal power. The average is called the baseline.• Baseline wandering

– Long string of 0s and 1s can cause a drift in the baseline (good line coding scheme needs to prevent baseline wondering

• DC components– DC or low frequencies cannot pass a transformer or telephone line

(below 200 Hz)• Self-synchronization: to correctly interpret the signals received from

the sender, the receiver’s bit intervals must corresponds exactly to the sender’s bit intervals (Timings (Clock)).

• Built-in error detection• Complexity (cost)

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Lack of Synchronization

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Line Coding Schemes

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Unipolar Scheme

• One polarity: one level of signal voltage either above or below.• Unipolar NRZ (Non-Return-to-Zero) is simple, because signal does

not return to 0, but – DC component : Cannot travel through microwave or transformer– Synchronization : Consecutive 0’s and 1’s are hard to be synchronized

Separate line for a clock pulse– Normalized power is double that for polar NRZ (not used these days)

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1 for positive 0 for bit 0

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Polar Scheme

• Two polarity: Voltage at the both side of time axis• Problem of DC component is alleviated (NRZ,RZ)

or eliminated (Biphaze)

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Polar NRZ• Voltage are on the both side of time axis. 0 can be positive or 1 can be negative• NRZ-L (Non Return to Zero-Level)

– Level of the voltage determines the value of the bit• NRZ-I (Non Return to Zero-Invert)

– Inversion or the lack of inversion determines the value of the bit. If there is a change the bit is 1 if there is no change the bit is 0.

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Polar NRZ: NRZ-L and NRZ-I

• Baseline wandering problem– Both, but NRZ-L is twice severe

• Synchronization Problem– Both, but NRZ-L is more serious

• NRZ-L and NRZ-I both have an average signal rate of N/2 Bd

• Both have a DC component problem

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RZ (Return to Zero)• Main problem with NRZ encoding occurs when the sender and receiver clocks are not

synchronized. The receiver does not know when one bit has ended and when one bit is starting.

• Provides synchronization for consecutive 0s/1s

• Signal changes not between the bit but during each bit, signal goes to 0 in middle of each bit. It remains there until the beginning of next bit.

• Three values (+, -, 0) are used

– Bit 1: positive-to-zero transition, bit 0: negative-to-zero transition

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Disadvantages: 2 signals to encode a bit so bandwidth is greater

Sudden change of polarity 1’s can be treated as 0 and 0 can be treated as 1’s

Complexity three level of voltage (by seeing above.. this scheme is not used these days)

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Biphase • Combination of RZ and NRZ-L ideas

• Signal transition at the middle of the bit is used for synchronization

• Manchester– Used for Ethernet LAN

– Bit 1: negative-to-positive transition

– Bit 0: positive-to-negative transition

• Differential Manchester– Used for Token-ring LAN

– Bit 1: no transition at the beginning of a bit

– Bit 0: transition at the beginning of a bit

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Polar Biphase• Minimum bandwidth is 2 times that of NRZ

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Duration of bit is divided into 2 halves

Voltage remains at one level during first half and moves to other level in the second half. Transition at the middle of the bit provides synchronization

Combines the idea of RZ and NRZ-I. transition at the middle of the bit.

—Transition occurs at the middle of each bit period—Transition serves as clock and data—Low to high represents binary 1—High to low represents binary 0—Used by IEEE 802.3 (Ethernet)

—Midbit transition occurs always and is used for clocking only—Transition at start of a bit period represents binary 0—No transition at start of a bit period represents binary 1—Note: this is a differential encoding scheme—Used by IEEE 802.5 (token ring LAN)

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)

– No dc component

– Error detection

• Absence of expected transition

Modulation Rate

Multilevel Binary or Bipolar Schemes

• 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 (zeros still a problem)

– No net dc component

– Lower bandwidth

– Easy error detection

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Bipolar Scheme• Three levels of voltage, called “multilevel binary” (+,- and 0)• Bit 0: zero voltage, bit 1: alternating +1/-1

– (Note) In RZ, zero voltage has no meaning• AMI (Alternate Mark Inversion) and pseudoternary (variation of AMI 1 is encoded as 0 voltage and 0 is

encoded as alternating + or -) zero represented by no line signal and one represented by positive or negative pulse, Word marks come from telegraphy and means 1.

• In Psuedoternary One represented by absence of line signal• Zero represented by alternating positive and negative• No advantage or disadvantage over bipolar-AMI

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Benefits with respect to NRZ

• —No loss of sync if a long string of ones (zeros still a problem)

• —No net DC component• —Lower bandwidth• —Easy error detection

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Scrambling• Deliberate distortion or encoding of audio/video signals, or a

data stream, through an electronic device (scrambler) to prevent unauthorized reception in 'plain' or 'readable' form.

• Biphase : not suitable for long distance communication due to its wide bandwidth requirement

• Combination of block coding and NRZ: not suitable for long distance encoding due to its DC component problem

• Bipolar AMI: synchronization problem Scrambling

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B8ZS

• Commonly used in North America• Updated version of AMI with synchronization

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HDB3• High-density bipolar 3-zero• Commonly used outside of North America• HDB3 substitutes four consecutive zeros with 000V or B00V depending on the

number of nonzero pulses after the last substitution– If no. of nonzero pulses after the last substitution is odd the substitution pattern will

be 000V, which makes total no. of nonzero pulses even.– If no. of nonzero pulses after the last substitution is even the substitution pattern will

be B00V, which makes total no. of nonzero pulses even.

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Sampling: Analog-to-Digital Conversion

• Analog information (e.g., voice) digital signal (e.g., 10001011…)

• Codec(Coder/Decoder): A/D converter

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PCM• Pulse Code Modulation (technique to change an analog signal to

digital signal)• Three processes (components)

– The analog signal is sampled– The sampled signal is quantized– The quantized values are encoded as streams of bits

• Sampling: PAM (Pulse amplitude Modulation):• The analog signal is sampled over Ts where Ts is the sample

interval or period. Inverse of sampling interval is called sampling rate or sampling frequency.– According to the Nyquist theorem, the sampling rate must be at least 2 times

the highest frequency contained in the signal.

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Components of PCM Encoder

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Different Sampling Methods for PCM

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In ideal sampling Pulses from the analog signal are sampled

In natural sampling high speed switch is turned on for only the small period of time when the sampling occurs

The most common sampling method is sample and hold creates flat-top samples by using a circuit

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Nyquist Sampling Rate

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According to him one necessary condition is that sampling rate be at least twice the highest frequency in the original signal.

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Sampling Rate

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Quantization

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We want to digitize the human voice. What is the bit rate, assuming 8 bits per sample?

SolutionThe human voice normally contains frequencies from 0 to 4000 Hz. So the sampling rate and bit rate are calculated as follows:

Example 4.14

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Transmission Modes

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Parallel Transmission

• Use n wires to send n bits at one time synchronously• Advantage: speed• Disadvantage: cost Limited to short distances

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Serial Transmission

• On communication channel• Advantage: reduced cost• Parallel/serial converter is required• Three ways: asynchronous, synchronous, or isochronous

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Asynchronous Transmission

• Use start bit (0) and stop bits (1s)• A gap between two bytes: idle state or stop bits• It means asynchronous at byte level• Must still be synchronized at bit level• Good for low-speed communications (terminal)

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Synchronous Transmission

• Bit stream is combined into “frames”• Special sequence of 1/0 between frames: No gap• Timing is important in midstream• Byte synchronization in the data link layer• Advantage: speed high-speed transmission

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Isochronous In real-time audio and video, in which uneven delays between frames are not acceptable, synchronous transmission fails. For example, TV images are broadcast at the rate of 30 images per second; they must be viewed at the same rate. If each image is sent by using one or more flames, there should be no delays between frames. For this type of application, synchronization between characters is not enough; the entire stream of bits must be synchronized. The isochronous transmission guarantees that the data arrive at a fixed rate.