Thapar University4-1 Digital Transmission 1.Digital-to-Digital Conversion 2.Analog-to-Digital Conversion 3.Transmission Mode 12/4/2015

Thapar University4-2 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 12/4/2015

Thapar University4-3 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. 12/4/2015

Thapar University4-4 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: B min = c x N x 1/r The maximum data rate: N max = 1/c x B x r 12/4/2015

Thapar University4-5 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) 12/4/2015

Thapar University4-6 Lack of Synchronization 12/4/2015

Thapar University4-7 Line Coding Schemes 12/4/2015

Thapar University4-8 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) 12/4/ for positive 0 for bit 0

Thapar University4-9 Polar Scheme Two polarity: Voltage at the both side of time axis Problem of DC component is alleviated (NRZ,RZ) or eliminated (Biphaze) 12/4/2015

Thapar University4-10 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. 12/4/2015

Thapar University4-11 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 12/4/2015

Thapar University4-12 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 12/4/2015 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)

Thapar University4-13 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 12/4/2015

Thapar University4-14 Polar Biphase Minimum bandwidth is 2 times that of NRZ 12/4/2015 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 (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 (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 pro blem) –No net dc component –Lower bandwidth –Easy error detection

Thapar University4-18 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 12/4/2015

Benefits with respect to NRZ —No loss of sync if a long string of ones (zeros sti ll a problem) —No net DC component —Lower bandwidth —Easy error detection

Thapar University4-20 Multilevel Scheme To increase the number of bits per baud by encoding a pattern of m data elements into a pattern of n signal elements In mBnL (m is length of binary pattern, B is binary data, n is length of signal pattern, L is level in signaling) schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2 m ≤ L n 2B1Q (two binary, one quaternary) 8B6T (eight binary, six ternary) 4D-PAM5 (four-dimensional five-level pulse amplitude modulation) 12/4/2015

Thapar University4-21 2B1Q: for DSL (Digital Scriber Line) 12/4/2015 Uses data pattern of size 2 and encodes the 2 bit pattern as one signal element belonging to a four level signal m=2, n=1and L=4 Signal rate = N/4. using 2BIQ we can send data 2 times faster than by using NRZ-L (Redundancy reduced because 2 2 = 4 1 )

Thapar University4-22 8B6T Used with 100Base-4T cable (The 100 in the media type designation refers to the transmission speed of 100 Mbit/s. The "BASE" refers to baseband signalling, which means that only Ethernet signals are carried on the medium. T4 refer to the physical medium that carries the signal. ) Encode a pattern of 8 bits as a pattern of 6 (three-levels) signal elements 222 redundant signal element = 3 6 (478 ) (256) The average signal rate is theoretically, S ave = 1/2 x N x 6/8; in practice the minimum bandwidth is very close to 6N/8 12/4/2015

Thapar University4-23 4D-PAM5: for Gigabit LAN 12/4/2015 4D means data sent over 4 wires at the same time. It uses 5 voltage levels -2,-1,0,1,2. signal rate can be reduced Design to send data over four channels (four wires).Design to send data over four channels (four wires). Use to send 1 Gbps data over four copper cables that can handles 125 MbaudUse to send 1 Gbps data over four copper cables that can handles 125 Mbaud It has lot of redundancy in the signal pattern because 2 8 data pattern are matched to 4 4 =256 signal patterns.It has lot of redundancy in the signal pattern because 2 8 data pattern are matched to 4 4 =256 signal patterns. The extra signal patterns can be used for other purposes such as error detectionThe extra signal patterns can be used for other purposes such as error detection

Thapar University4-24 Multiline Transmission: MLT-3 No change at 0 bit MLT-3 when we need to send 100Mbps on a copper wire that cannot support more than 32MHz If the next bit is 0 there is no transition If next bit is 1 and current level is not 0 the next level is 0 If next bit is 1 and the current level is 0 the next level is the opposite of the last nonzero level. 12/4/2015

Thapar University4-25 Summary of Line Coding Schemes 12/4/2015

Thapar University4-27 Block Coding Block coding is normally referred to as mB/nB coding; it replaces each m-bit group with an n-bit group 12/4/2015

Thapar University4-28 4B/5B Solve the synchronization problem of NRZ-I 20% increase the signal rate of NRZ-I (Biphase scheme has the signal rate of 2 times that of NRZ-I Still DC component problem 12/4/2015

Thapar University4-29 4B/5B Mapping Codes 12/4/2015

Thapar University4-30 8B/10B 2 10 – 2 8 = 768 redundant groups used for disparity checking and error detection 12/4/2015

Thapar University4-31 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 12/4/2015

Thapar University4-32 B8ZS Commonly used in North America Updated version of AMI with synchronization 12/4/2015

Thapar University4-33 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. 12/4/2015

Thapar University4-34 Sampling: Analog-to-Digital Conversion Analog information (e.g., voice)  digital signal (e.g., …) Codec(Coder/Decoder): A/D converter 12/4/2015

Thapar University4-35 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. 12/4/2015

Thapar University4-36 Components of PCM Encoder 12/4/2015

Thapar University4-37 Different Sampling Methods for PCM 12/4/2015 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

Thapar University4-38 Nyquist Sampling Rate 12/4/2015 According to him one necessary condition is that sampling rate be at least twice the highest frequency in the original signal.

Thapar University4-39 Sampling Rate 12/4/2015

Thapar University4-40 Quantization 12/4/2015

We want to digitize the human voice. What is the bit rate, assuming 8 bits per sample? Solution The human voice normally contains frequencies from 0 to 4000 Hz. So the sampling rate and bit rate are calculated as follows: Example 4.14

Thapar University4-42 Transmission Modes 12/4/2015

Thapar University4-43 Parallel Transmission Use n wires to send n bits at one time synchronously Advantage: speed Disadvantage: cost  Limited to short distances 12/4/2015

Thapar University4-44 Serial Transmission On communication channel Advantage: reduced cost Parallel/serial converter is required Three ways: asynchronous, synchronous, or isochronous 12/4/2015

Thapar University4-45 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) 12/4/2015

Thapar University4-46 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 12/4/2015

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.