Data Communications and Computer Networks Chapter 1

Network Architecture Models Logical and physical connections

Data Communications and Computer Networks Chapter 1

The Internet Model in ActionNote the flow of data from user to web browser and back

At each layer, information is either added or removed, depending on whether the data is leaving or arriving at a workstation

The adding of information over pre-existing information is termed encapsulation

Data Communications and Computer Networks Chapter 2

Introduction• Computer networks transmit signals

• Signals are the electomagnetic encoding of data

• Data and signals can be analog or digital

Data Communications and Computer Networks Chapter 2

Data and SignalsExamples of data include:

• computer files

• movie on a DVD

• music on a compact disc

• collection of samples from a blood gas analysis device

Data Communications and Computer Networks Chapter 2

Data and SignalsExamples of signals include:

• telephone conversation over a telephone line

• live television news interview from Europe

• Web page download over your telephone line via the Internet

Data Communications and Computer Networks Chapter 2

Analog versus DigitalAnalog is a continuous waveform, with examples such as music and video.

Data Communications and Computer Networks Chapter 2

Analog versus DigitalDigital is a discrete or non-continuous waveform with fixed voltage levels that represent data 1s and 0s.

Data Communications and Computer Networks Chapter 2

Analog versus DigitalIt is harder to separate noise from an analog signal than it is to separate noise from a digital signal.

Data Communications and Computer Networks Chapter 2

Analog versus DigitalNoise in a digital signal. You can still discern a high voltage from a low voltage. Regenerators are devices that automatically amplify and clean noise out of digital signals.

Data Communications and Computer Networks Chapter 2

Analog versus DigitalNoise in a digital signal. Too much noise - you cannot discern a high voltage from a low voltage. Here data will be lost in transmission.

Data Communications and Computer Networks Chapter 2

Signals Have Three Components• Amplitude

• Frequency

• Phase

Communicating Data

• Binary data (1s and 0s) are communicated by changing one or more of these components (amplitude, frequency, or phase) in predetermined ways at predetermined time intervals.

Data Communications and Computer Networks Chapter 2

AmplitudeThe amplitude of a signal is the height of the wave above or below a given reference point.

Data Communications and Computer Networks Chapter 2

FrequencyThe frequency is the number of times a signal makes a complete cycle within a given time frame.

Spectrum - The range of frequencies that a signal spans from minimum to maximum.

Bandwidth - The absolute value of the difference between the lowest and highest frequencies of a signal.

Attenuation - Loss of signal strength.

Data Communications and Computer Networks Chapter 2

Data Communications and Computer Networks Chapter 2

PhaseThe phase of a signal is the position of the waveform relative to a given moment of time or relative to time zero.

A change in phase can be any number of angles between 0 and 360 degrees.

Phase changes often occur on common angles, such as 45, 90, 135, etc.

Data Communications and Computer Networks Chapter 2

Data Communications and Computer Networks Chapter 2

Loss of Signal Strength• All signals experience power loss (attenuation) as they travel over a communications medium.

• Signals must be regenerated or amplified at regular intervals to prevent total signal loss.

• Attenuation is denoted as a decibel (dB) loss.

Data Communications and Computer Networks Chapter 2

Converting Digital Data into Digital SignalsThere are numerous techniques available to convert digital data into digital signals.

Let’s examine four techniques:

• NRZ-L

• NRZ-I

• Manchester

• Differential Manchester

Data Communications and Computer Networks Chapter 2

Self-Clocking Codes

• Big difference between NRZ and Manchester codes:– For long strings of 0-bits, NRZ codes generate signal that

does not change over long time period– Manchester codes always produce signal change during

every bit transmission.– Manchester codes are called self-clocking codes, because

they provide a guaranteed voltage change (a “clock signal”) in the middle of every bit received.

Data Communications and Computer Networks Chapter 2

Note how with a Differential Manchester code, every bit has at least one signal change. Some bits have two signal changes per bit (baud rate is twice the bps).

Self-Clocking Codes

• Why do we care about self-clocking codes?– Transmitter / receiver clocks are not perfectly synchronized

to tick at same rate (too expensive).– NRZ-L or NRZ-I cannot be used at high data rates or long

distances unless a separate clock signal is sent on another wire.

– Manchester codes can be used at high data rates or long distances, because receiver continuously gets feedback on sender clock rate.

Self-Clocking Codes

• Any disadvantage to Manchester codes?– Manchester codes must transmit two signal changes per

bit (2 baud per bit).– NRZ codes transmit only one signal change per bit (1

baud per bit).– Any transmission medium (copper wire, fiber optics,

etc.) has a maximum baud capacity that it can support. NRZ can send more bits per second.

Data Communications and Computer Networks Chapter 2

4B/5B Digital EncodingYet another encoding technique that converts four bits of data into five-bit quantities.

The five-bit quantities are unique in that no five-bit code has more than 2 consecutive zeroes.

The five-bit code is then transmitted using an NRZ-I encoded signal.

This system is self-clocking, yet provides 4 bits for every 5 signal changes (more efficient than Manchester)

Data Communications and Computer Networks Chapter 2

Data Communications and Computer Networks Chapter 2

Converting Digital Data into Analog SignalsThree basic techniques:

• Amplitude modulation

• Frequency modulation

• Phase modulation

Data Communications and Computer Networks Chapter 2

Amplitude ModulationOne amplitude encodes a 0 while another amplitude encodes a 1.

Data Communications and Computer Networks Chapter 2

Amplitude ModulationSome systems use multiple amplitudes.

Data Communications and Computer Networks Chapter 2

Frequency ModulationOne frequency encodes a 0, while another frequency encodes a 1.

Data Communications and Computer Networks Chapter 2

Phase ModulationOne phase change encodes a 0, while another phase change encodes a 1.

Data Communications and Computer Networks Chapter 2

Quadrature Phase ModulationFour different phase angles are used:

• 45 degrees

• 135 degrees

• 225 degrees

• 315 degrees

Data Communications and Computer Networks Chapter 2

Data Communications and Computer Networks Chapter 2

Quadrature Amplitude ModulationIn this technology, 12 different phases are combined with two different amplitudes.

Since only 4 phase angles have 2 different amplitudes, there are a total of 16 combinations.

With 16 signal combinations, each baud equals 4 bits of information. (2 ^ 4 = 16)

Data Communications and Computer Networks Chapter 2

Data Communications and Computer Networks Chapter 2

Converting Analog Data into Digital SignalsTo convert analog data into a digital signal, there are two basic techniques:

• Pulse code modulation

• Delta modulation

Data Communications and Computer Networks Chapter 2

Pulse Code ModulationThe analog waveform is sampled at specific intervals and the “snapshots” are converted to binary values.

Data Communications and Computer Networks Chapter 2

Pulse Code ModulationWhen the binary values are later converted to an analog signal, a waveform similar to the original results is created, as long as enough samples are taken

Data Communications and Computer Networks Chapter 2

Pulse Code ModulationThe more snapshots taken in the same amount of time, the better the resolution.

Data Communications and Computer Networks Chapter 2

Delta ModulationAn analog waveform is tracked, using a binary 1 to represent a rise in voltage, and a 0 to represent a drop.

Data Communications and Computer Networks Chapter 2

Converting Analog Data into Analog SignalsMany times it is necessary to modulate analog data onto a different set of analog frequencies. Broadcast radio and television are two very common examples of this.

In this situation a data signal is modulated by a carrier signal to produce a composite signal that can be broadcast in a particular range of frequencies near the carrier frequency. For example: music (analog data signal) is modulated by a 91.5 MHz sine wave, which produces a composite signal with components between 91.4 MHz and 91.6 MHz. This is transmitted. You retrieve the original signal by demodulating (tuning into) frequency 91.5 MHz on your radio to get back the music

Data Communications and Computer Networks Chapter 2

Data Communications and Computer Networks Chapter 2

Spread Spectrum TechnologyA secure encoding technique that uses multiple frequencies or codes to transmit data.

Two basic spread spectrum technologies:

• Frequency hopping spread spectrum

• Direct sequence spread spectrum

Data Communications and Computer Networks Chapter 2

Frequency Hopping Spread Spectrum

Data Communications and Computer Networks Chapter 2

Data CodeThe set of all textual characters or symbols and their corresponding binary patterns is called a data code.

There are two basic data code sets plus a third code set that has interesting characteristics:

• ASCII

• EBCDIC

• Baudot Code

Data Communications and Computer Networks Chapter 2

Data Communications and Computer Networks Chapter 2

Data Communications and Computer Networks Chapter 2

Data Communications and Computer Networks Chapter 2

Data and Signal Conversions in ActionLet us transmit the message “Sam, what time is the meeting with accounting? Hannah.”

This message first leaves Hannah’s workstation and travels across a local area network.

Data Communications and Computer Networks Chapter 2

Data and Signal Conversions in Action

Data Communications and Computer Networks Chapter 2

Data and Signal Conversions in Action

Data Communications and Computer Networks Chapter 2

Data and Signal Conversions in Action