2.7 digital transmission

Chapter 2 : Data CommunicationsBENG 4522 Data Communications & Computer Networks

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2.7 Digital Transmission

Discussing the schemes and techniques that are used to transmit data digitally Digital-to-digital conversion techniques Analog-to digital conversion techniques Transmission modes

2.7.1 Digital-to-digital conversion How to represents digital data by using digital signals. 3 techniques used in the conversion

Line coding Block coding scrambling


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2.7.1.1 Line Coding

Line coding is the process of converting digital data to digital signals. Data (text, numbers, graphical image, audio, video etc) are stored in

computer memory as sequence of bits. Line coding converts a sequence of bits to a digital signal. At the sender, digital data are encoded into digital signal; at the receiver,

the data are recreated by decoding the digital signal.


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2.7.1.1 Line Coding

Signal elements vs. data elements In data communication, the goal is to send the data elements – smallest entity

that can represent the information or simply the BIT In digital data communications, a signal elements carries the data elements. Data elements are need to be sent, while signal elements are what can be sent. We defined a ratio r, which is the number of data elements carried by each

signal element.


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2.7.1.1 Line Coding

Signal elements vs. data elements Analogy : Suppose each data element is a person who needs to be carried from

one place to another. A signal elements can be thought as a vehicle that can carry the people.

r = 1 : each person is driving the vehicle

r > 1 : more than one person is traveling in a vehicle (carpool)

r = ½ : one person is driving a car and a trailer


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2.7.1.1 Line Coding

Data Rate vs. Signal Rate Data rate defines the number of data elements (bits) sent in 1s (unit = bps) Signal rate is the number of signal elements sent in 1s (unit = baud) Data rate = bit rate Signal rate = pulse rate = modulation rate = baud rate In data communications, the goal is to increase the data rate while decreasing

the signal rate (bring more people with fewer vehicles) Increasing the data rate = increases the speed of transmission Decreasing the signal rate = decrease the bandwidth requirement Relationship between data rate and signal rate :

S : number of signal elements; N : data rate (bps); c : case factor; r : ratio

)(1

baudr

NcS


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2.7.1.1 Line Coding

Data Rate vs. Signal Rate Ex : a signal is carrying data in which one data element is encoded as one signal

element (r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if the c is between 0 and 1 ?


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2.7.1.1 Line Coding Bandwidth

Most of nonperiodic digital signals encountered in the daily life have a bandwidth with finite values.

In other words, the bandwidth is theoretically infinite, but many of the components have such a small amplitude and can be ignored.

Means the effective bandwidth is finite. Baud rate determines the required bandwidth (the vehicles affects the traffic, not

the people !) More changes in the signal = injecting more frequencies into the signal. More frequencies = wider range of frequencies = wider bandwidth Thus bandwidth (frequency range) is proportional to the signal rate (baud rate) The minimum bandwidth can be defined as

rNcB

1min


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2.7.1.1 Line Coding

Bandwidth Thus the maximum data rate (Nmax) can be solved if the bandwidth of the

channel is given

rBc

N 1

max


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2.7.1.1 Line Coding

Design Consideration for Line Coding Scheme Baseline wandering DC components Self-synchronization Built-in error detection Immunity to noise and interference complexity


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2.7.1.1 Line Coding

Baseline Wandering In decoding a digital signal, the receiver calculates a running average of the

received signal power – called as a baseline. The incoming signal power is evaluated against this baseline to determine the

value of the data element. A long string of 0s and 1s can cause a drift in the baseline (baseline wandering)

and make it difficult for the receiver to decode correctly. A good line coding scheme is needed to prevent baseline wandering.


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2.7.1.1 Line Coding

DC Components When the voltage level in the digital signal is constant for a while, the spectrum

creates a very low frequencies. These frequencies around zero (DC components), present a problem for a

system that cannot pass low frequencies or a system that uses electrical coupling (via a transformer).

Ex : telephone line cannot pass frequencies below 200 Hz. Ex : a long-distance link may use one or more transformers to isolate different

parts of the line electrically. For these systems, a scheme with no DC component is necessary.


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2.7.1.1 Line Coding

Self-synchronization to correctly interpret the signals received from the sender, the receiver’s bit

intervals must correspond exactly to the sender’s bit intervals.


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2.7.1.1 Line Coding

Self-synchronization A self-synchronizing digital signal includes timing information in the data being

transmitted. It can be achieved if there are transitions in the signals that alert the receiver to

the beginning, middle or end of the pulse. If the receiver’s clock is out of synchronization, these points can reset the clock.

Built-in Error Detection It is desirable to have a built-in error-detecting capability in the generated code

to detect some or all the errors that occurred during transmission.


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2.7.1.1 Line Coding

Immunity to noise and interference It is desirable that the code is immune to noise and other interference.

Complexity A complex scheme is more costly to implement than a simple one. Ex : a

scheme that uses four signal levels is more difficult to interpret than one that uses only two levels


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2.7.1.1 Line Coding

Line coding scheme can be divided into five broad categories :


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2.7.1.1.1 Unipolar Sheme

In a unipolar scheme, all the signal levels are on one side of the time axis; either above or below.

NRZ (Non-Return-to-Zero) scheme

Designed so that positive voltage defines bit 1 and the zero voltage defines bit 0.

It is called NRZ because the signal does not return to zero at the middle of the bit.


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2.7.1.1.1 Unipolar Scheme

Unipolar NRZ (None-Return-to-Zero) is simple, but DC component : Cannot travel through system that does not allow a low frequency

component to passage (ex : 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


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2.7.1.1.2 Polar Scheme

In a polar scheme, the voltages are on both sides of the time axis. Ex : positive voltage level for bit 1, negative voltage level for bit 0.


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2.7.1.1.2 Polar Scheme : NRZ (NRZ-L & NRZ-I)

In a polar scheme, the voltages are on both sides of the time axis. Ex : positive voltage level for bit 1, negative voltage level for bit 0. Polar NRZ

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


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2.7.1.1.3 Polar Scheme : RZ

Provides synchronization for consecutive 0s/1s Signal changes during each bit Three values (+, -, 0) are used

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


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2.7.1.1.4 Polar Scheme : Biphase

Combination of RZ and NRZ-L/NRZ-I ideas Signal transition at the middle of the bit is used for synchronization Manchester (combine RZ & NRZ-L)

Used for Ethernet LAN Bit 1: negative-to-positive transition Bit 0: positive-to-negative transition

Differential Manchester (combine RZ & NRZ-I) Used for Token-ring LAN Bit 1: no transition at the beginning of a bit Bit 0: transition at the beginning of a bit

Minimum bandwidth is 2 times of that NRZ


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2.7.1.1.4 Polar Scheme : Biphase


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2.7.1.1.5 Bipolar Scheme

Three levels of voltage, called “multilevel binary” Bit 0: zero voltage, bit 1: alternating +1/-1 AMI (Alternate Mark Inversion) and pseudoternary

No DC component


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2.7.1.1.6 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 schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2m ≤ Ln

mBnL : m (length of binary pattern), B (binary data), n (length of the signal pattern), L (number of levels in the signaling) 2B1Q (two binary, one quaternary) 8B6T (eight binary, six ternary) 4D-PAM 5 (four-dimensional five-level pulse amplitude modulation)


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2.7.1.1.6 Multilevel Scheme (2B1Q) for DSL


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2.7.1.1.6 Multilevel Scheme (8B6T)

Used with 100Base-4T cable Encode a pattern of 8 bits as a pattern of 6 (three-levels) signal elements The average signal rate is theoretically, Save = 1/2 x N x 6/8; in practice the

minimum bandwidth is very close to 6N/8


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2.7.1.1.6 Multilevel Scheme 4D-PAM5: for Gigabit LAN


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2.7.1.1.7 Multiline Transition : MLT-3

The signal rate for MLT-3 is one-fourth the bit rate MLT-3 when we need to send 100Mbps on a copper wire that cannot

support more than 32MHz


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2.7.1.1 Summary of Line Coding Scheme

2.7 digital transmission

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