Chapter-3-1CS331- Fakhry Khellah Term 081 Chapter 3 Data and Signals.

Chapter-3-1 CS331- Fakhry KhellahTerm 081

Chapter 3

Data and Signals


Position of the physical layer & Services


Chapters

Chapter 3 Signals

Chapter 4 Digital Transmission

Chapter 5 Analog Transmission

Chapter 6 Multiplexing

Chapter 7 Transmission Media

Chapter 9 High Speed Digital Access


Chapter 3

Signals


To be transmitted, data must be transformed to electromagnetic signals.

Note


A Transmission System

Transmission: Communication of data by propagation and processing of signals through a communication channel

Signal: Electromagnetic energy that moves through the transmission medium

Transmitter Converts information into signal suitable for transmission Injects energy into communications medium or channel

Telephone converts voice into electric current Modem converts bits into electric current

Receiver Receives energy from medium Converts received signal into form suitable for delivery to user

Telephone converts current into voice Modem converts electric current into bits

Receiver

Communication channel

Transmitter


3.1 Analog and Digital

Analog Data: Continuous value data (sound, light, temperature)

Digital Data: Discrete value (text, integers, symbols) Analog Signals: Continuously varying electromagnetic

wave have an infinite number of values over a period of time

Digital Signals: Series of voltage pulses (square wave)

have only a limited number of values over a period of time. Maintains a constant level then changes to another constant level

Signals are classified into: Periodic Signals: Consists of repeated patterns over identical period of times

(cycles) Aperiodic Signals: change without a pattern that repeats over time


Figure 3.1 Comparison of analog and digital signals

How signals are represented on a graph


Periodic analog and digital signals


In data communication, we commonly use periodic analog signals and

aperiodic digital signals.

Note:Note:


3.2 Analog Signals

Sine WavePhaseExamples of Sine WavesTime and Frequency DomainsComposite SignalsBandwidth


Figure 3.2 A sine wave

Sine wave is the most fundamental form of a periodic analog signal (a simple signal )

Three main characteristics that describe Simple Sine Wave:

Amplitude

Frequency

Phase


Simple Signal Characteristics:

1- Amplitude

Peak amplitude is the absolute value of the highest/lowest signal value



2- Frequency

Period = The time needed to complete a full pattern (cycle)

Frequency: Number of completed (patterns) cycles per second


Frequency is the rate of change with respect to time.

Change in a short span of timemeans high frequency.

Change over a long span of time means low frequency.

Note


If a signal does not change at all, its frequency is zero.

If a signal changes instantaneously, its frequency is infinite.

Note


Frequency and period are the inverse of each other.

Note


The power we use at home has a frequency of 60 Hz. The period of this sine wave can be determined as follows:

Example 3.3

1 ms = 10-3 second


The period of a signal is 100 ms. What is its frequency in kilohertz?

Example 3.5

SolutionFirst we change 100 ms to seconds, and then we calculate the frequency from the period (1 Hz = 10−3 kHz).



3- Phase

Phase: the position of the signal relative to time zero (where the signal starts with respect to time zero)

measured in degrees or radians (1 degree is radian )

It describes the amount of the shift happened to the first cycle (its offset with respect to time zero)

360

2


Phase describes the position of the waveform relative to time zero.

Note:Note:


Mathematical description of sine wave

)2sin()( ftAts S instantaneous Amplitude

A peak Amplitude

F frequency

Phase


Figure 3.6 Sine wave examples


Figure 3.6 Sine wave examples (continued)


Medium Bandwidth meaning depends on the type of the transmitted signal through the medium:

Analog bandwidth is associated with Analog signals.it is the difference between the highest and the lowest (the range of ) frequencies that the medium can pass safely (expressed in Hz)Range of frequencies in a signal

High bandwidth Medium Less error in the transmitted data

Needs Expensive equipments Low bandwidth Medium more errors

cheaper Digital Bandwidth is associated with Digital signals

It is the maximum bit rate that a medium can pass expressed in bits per second (bps)

The Medium bandwidth


Example 4Example 4

A signal has a bandwidth of 20 Hz. The highest frequency is 60 Hz. What is the lowest frequency? Draw the spectrum if the signal contains all integral frequencies of the same amplitude.

SolutionSolution

B = fB = fhh f fll

20 = 60 20 = 60 ffll

ffll = 60 = 60 20 = 40 Hz20 = 40 Hz


3.3 Digital Signals1 = Positive voltage > 0

0 = Zero voltage

Most Digital Signals are Aperiodic Frequency is not appropriate


Figure 3.16 Two digital signals: one with two signal levels and the other with four signal levels


Figure 3.17 Bit rate and bit interval

Bit Interval (digital signal period): The time required to send (represent) one single bit (time units)

Bit Rate (digital bandwidth) : Number of bits sent per second (bps)


Example 6Example 6

A digital signal has a bit rate of 2000 bps. What is the duration of each bit (bit interval)

SolutionSolution

The bit interval is the inverse of the bit rate.

Bit interval = 1/ 2000 s = 0.000500 s = 0.000500 x 106 s = 500 s


Figure 3.12 Signal corruption

No transmission medium can pass all the signal frequencies safely

A medium may

Pass some frequencies

Block others

Weaken others


Impairments: Factors that make the received signal different from the transmitted one

3.4 Transmission Impairments


Attenuation Attenuation: Loss of energy due to resisting the medium (Signal strength falls off with distance)

Increases with signal frequency

Ex. A wire carrying electrical signal becomes warm after some time

Amplifiers (analog signals) and repeaters (digital signals) are used to handle attenuation

Attenuation affects analog signals


Distortion

Distortion : Signal changes in shape

Distortion will cause different bits to overlap

Usually occurs to composite signal due to different propagation delays of its components


Noise

Thermal Noise due to random motion of electrons in a wire which will create an extra signal

Induced Noise: caused by motors and electrical equipments.

crosstalk noise : Two wires beside each others (hearing another conversation in the background while talking with the phone)

Impulse noise: irregular pulses or noise spikes of short duration and high amplitude

caused by power lines or lightning

Very critical in case of digital signals (primary source of error in digital data communication)


Other Physical layer definitions: Propagation time= time required for a

bit to move between two nodes. Propagation time = distance of the link /

Propagation speed

Propagation speed : Speed of light in the medium.

Depends on the medium Light travels at 3x108 m/s Vacuum (free

space), lower in the air and much lower in a cable (2/3 in vacuum)

3-6 PERFORMANCE3-6 PERFORMANCE


Propagation Time


Transmission time = The time it takes the sender to transmit (put) all the bits of a frame into the link

Transmission time = Length of frame (message) in bits / link data rate (bandwidth) (bps)

Queuing Time = Time needed for each intermediate device to hold the message before it can be processed.

Latency (delay): Total message delivery time = Transmission time + Propagation time + processing time+ queuing time

Other Physical layer definitions – Transmission Time


What are the propagation time and the transmission time for a 2.5-kbyte message (an e-mail) if the bandwidth of the network is 1 Gbps? Assume that the distance between the sender and the receiver is 12,000 km and that light travels at 2.4 × 108 m/s.

SolutionWe can calculate the propagation and transmission time as shown on the next slide:

Example 3.46


Note that in this case, because the message is short and the bandwidth is high, the dominant factor is the propagation time, not the transmission time. The transmission time can be ignored.

Example 3.46 (continued)


What are the propagation time and the transmission time for a 5-Mbyte message (an image) if the bandwidth of the network is 1 Mbps? Assume that the distance between the sender and the receiver is 12,000 km and that light travels at 2.4 × 108 m/s.

SolutionWe can calculate the propagation and transmission times as shown on the next slide.

Example 3.47


Note that in this case, because the message is very long and the bandwidth is not very high, the dominant factor is the transmission time, not the propagation time. The propagation time can be ignored.

Example 3.47 (continued)


Figure 3.33 Concept of bandwidth-delay product

bandwidth-delay product = Length of the link in bits


We can think about the link between two points as a pipe. The cross section of the pipe represents the bandwidth, and the length of the pipe represents the delay. We can say the volume of the pipe defines the bandwidth-delay product

Example 3.48


The bandwidth-delay product defines the number of bits that can fill the link.

Note


Figure 3.31 Filling the link with bits for case 1


Figure 3.32 Filling the link with bits in case 2

5 bps

25 bits


Wavelength: the distance a simple signal can travel in one period (or the distance occupied by one cycle)

Depends on both the signal frequency and the medium Wavelength = Propagation speed x Period

= Propagation speed / frequency

Other Physical layer definitions - Wavelength

Chapter-3-1CS331- Fakhry Khellah Term 081 Chapter 3 Data and Signals.

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