Network ComponentsChapter 02

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

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• Digital signal– discrete, discontinuous voltage pulses– each pulse is a signal element– binary data encoded into signal elements

Digital Data, Digital Signal

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All signal levels are on one side of the time axis - either above or below

NRZ - Non Return to Zero scheme is an example of this code. The signal level does not return to zero during a symbol transmission.

Scheme is prone to baseline wandering and DC components. It has no synchronization or any error detection. It is simple but costly in power consumption.

Unipolar

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Unipolar NRZ scheme

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The voltages are on both sides of the time axis.

Polar NRZ scheme can be implemented with two voltages. E.g. +V for 1 and -V for 0.

There are two versions: ◦ NZR - Level (NRZ-L) - positive voltage for one

symbol and negative for the other◦ NRZ - Inversion (NRZ-I) - the change or lack of

change in polarity determines the value of a symbol. E.g. a “1” symbol inverts the polarity a “0” does not.

Polar - NRZ

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Polar NRZ-L and NRZ-I schemes

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In NRZ-L the level of the voltage determines the value of the bit.

In NRZ-I the inversion or the lack of inversion

determines the value of the bit.

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NRZ-L and NRZ-I both have a DC component problem and baseline wandering,

it is worse for NRZ-L. Both have no self synchronization &no error detection. Both

are relatively simple to implement.

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The Return to Zero (RZ) scheme uses three voltage values. +, 0, -.

Each symbol has a transition in the middle. Either from high to zero or from low to zero.

This scheme has more signal transitions (two per symbol) and therefore requires a wider bandwidth.

No DC components or baseline wandering.Self synchronization - transition indicates

symbol value.More complex as it uses three voltage level.

It has no error detection capability.

Polar - RZ

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Polar RZ scheme

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Manchester coding consists of combining the NRZ-L and RZ schemes.◦ Every symbol has a level transition in the middle:

from high to low or low to high. Uses only two voltage levels.

Differential Manchester coding consists of combining the NRZ-I and RZ schemes.◦ Every symbol has a level transition in the middle.

But the level at the beginning of the symbol is determined by the symbol value. One symbol causes a level change the other does not.

Polar - Biphase: Manchester and Differential Manchester

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Polar biphase: Manchester and differential Manchester schemes

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Comparison of analog and digital signals

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In data communications, we commonly use periodic analog signals and nonperiodic digital

signals.

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PERIODIC ANALOG SIGNALSPERIODIC ANALOG SIGNALS

Periodic analog signals can be classified as simple or composite. A Periodic analog signals can be classified as simple or composite. A simple periodic analog signal, a sine wave, cannot be decomposed simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. A composite periodic analog signal is into simpler signals. A composite periodic analog signal is composed of multiple sine waves.composed of multiple sine waves.Sine WaveWavelengthTime and Frequency DomainComposite SignalsBandwidth

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Frequency and period are the inverse of each other.

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Units of period and frequency

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A composite periodic signal

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The bandwidth of a composite signal is the difference between thehighest and the lowest frequencies contained in that signal.

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TRANSMISSION IMPAIRMENTTRANSMISSION IMPAIRMENT

Signals travel through transmission media, which are not perfect. Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise.causes of impairment are attenuation, distortion, and noise.

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Attenuation

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Noise

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In networking, we use the term bandwidth in two contexts.

❏The first, bandwidth in hertz, refers to the range of frequencies in a composite signal or the range of frequencies that a channel can pass.

❏The second, bandwidth in bits per second, refers to the speed of bit transmission in a channel or link.

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• has transition in middle of each bit period• transition serves as clock and data• low to high represents one• high to low represents zero• used by IEEE 802.

Manchester Encoding

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midbit transition is clocking onlytransition at start of bit period representing 0no transition at start of bit period representing 1

◦ this is a differential encoding schemeused by IEEE 802.5

Differential Manchester Encoding

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• phase of carrier signal is shifted to represent data• binary PSK

– two phases represent two binary digits• differential PSK

– phase shifted relative to previous transmission rather than some reference signal

Phase Shift Keying

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• bandwidth– ASK/PSK bandwidth directly relates to bit rate– multilevel PSK gives significant improvements

• in presence of noise:– bit error rate of PSK and QPSK are about 3dB

superior to ASK and FSK– for MFSK & MPSK have tradeoff between bandwidth

efficiency and error performance

Performance of Digital to Analog Modulation Schemes

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• digitization is conversion of analog data into digital data which can then:– be transmitted using NRZ-L– be transmitted using code other than NRZ-L– be converted to analog signal

• analog to digital conversion done using a codec– pulse code modulation– delta modulation

Analog Data, Digital Signal

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• Amplitude Modulation• Frequency Modulation• Phase Modulation

Analog ModulationTechniques

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• modulate carrier frequency with analog data• why modulate analog signals?

– higher frequency can give more efficient transmission

– permits frequency division multiplexing (chapter 8)• types of modulation

– Amplitude– Frequency– Phase

Analog Data, Analog Signals

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Bandwidth

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Bit rate and bit interval

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• NIC provides the physical interface between computer and cabling.

• It prepares data, sends data, and controls the flow of data. It can also receive and translate data into bytes for the CPU to understand.

• The following factors should be taken into consideration when choosing a NIC:1. - Preparing data2. - Sending and controlling data3. - Configuration4. - Drivers5. - Compatibility6. - Performance

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Network Interface Card (NIC)

An Ethernet NIC

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IRQ COMMON USE0 Timer

1 Keyboard

2 Secondary IRQ controller

3 COM 2 and 4

4 Com 1 and 3

5 LPT2 or MIDI device

6 Floppy disk drive

7 LPT1

8 Real-time clock

9 Free or sound card

10 Free or primary SCSI adapter

11 Free or secondary SCSI adapter

12 PS/2 Mouse

13 Floating-Point processor

14 Primary hard disk controller

15 Free or secondary hard disk controller

Classes of Transmission Classes of Transmission MediaMedia

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GUIDED MEDIAGUIDED MEDIAGuided media: Are those that provide a Guided media: Are those that provide a conduit from one device to another, include conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-twisted-pair cable, coaxial cable, and fiber-optic cable.optic cable.

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Coaxial Cable

• Was the predominant form of network cabling• Shielding: protective layer(s) wrapped around cable

to protect it from external interference• Less susceptible to interference and attenuation than

twisted-pair, but more susceptible than fiber-optic

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Coaxial Cable (continued)

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• Ethernet’s beginnings are in coaxial cable– First, it was run on a very thick, rigid cable, usually

yellow, referred to as thicknet (10Base5)– Later, a more manageable coaxial cable called

thinnet (10Base2) was used• 10Base5 is an IEEE designation

– 10 Mbps– Baseband– Maximum segment length is 500 meters

The Use of Coaxial Cable for Ethernet

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Coaxial Cable in Cable Modem Applications

Coaxial cable in LANs has become obsoleteThe standard cable (75 ohm, RG-6; RG stands for

“radio grade”) that delivers cable television (CATV) to millions of homes nationwide is also being used for Internet access

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Twisted-Pair Cable

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• 10BaseT– Maximum length is 100 meters

• UTP is now the most popular form of LAN cabling• The UTP cable used for networking usually includes

one or more pairs of insulated wires• UTP specifications govern the number of twists per

foot (or per meter), depending on the cable’s intended use

• UTP is used for telephony, but requirements for networking uses differ from the telephony ones

Unshielded Twisted Pair (UTP)

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UTP cabling is rated according to a number of categories devised by the TIA and EIA; since 1991, ANSI has also endorsed these standards◦ ANSI/TIA/EIA 568 Commercial Building Wiring

Standard for commercial environments includes: Category 1 (voicegrade) Category 2: up to 4 Mbps Category 3: up to 10 Mbps (16 MHz) Category 4 (datagrade): up to 16 Mbps (20 MHz) Category 5: up to 100 Mbps (100 MHz) Category 5e: up to 1000 Mbps (100 MHz) Category 6: up to 1000 Mbps (200 MHz)

UTP Cabling Categories

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• Shielding reduces crosstalk and limits external interference– Usually, wiring includes a wire braid inside cladding or

sheath, and a foil wrap around each wire pair• Enables support of higher bandwidth over longer

distances than UTP– No set of standards for STP corresponds to the

ANSI/TIA/EIA 568 Standard, yet it’s not unusual to find STP cables rated according to those standards

– Uses two pairs of 150 ohm wire (defined by the IBM cabling system), and was not designed to be used in Ethernet applications, but it can be adapted to

Shielded Twisted Pair (STP)

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Twisted-Pair Cable (continued)

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One of the skills required of a network technician is making a twisted-pair patch cable

To do this, you need:◦ Wire cutters or electrician’s scissors◦ Wire stripper◦ Crimp tool◦ RJ-45 plugs

There are two standards for the arrangement of wires: TIA/EIA 568A and TIA/EIA 568B◦ You must stick to one throughout your network

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Making Twisted-Pair Cable Connections

Making Twisted-Pair Cable Connections (continued)

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Making Twisted-Pair Cable Connections (continued)

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Fiber-Optic Cable

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Fiber-Optic Cable (continued)

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Fiber-Optic Cable (continued)

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• Installation of fiber-optic networks is more difficult and time-consuming than copper media installation

• Connectors and test equipment are considerably more expensive than their copper counterparts

• Two types– Single-mode: costs more and generally works with

laser-based emitters, but spans the longest distances– Multimode: costs less and works with light emitting

diodes (LEDs), but spans shorter distances

Fiber-Optic Cable (continued)

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Radio, satellite transmissions, and infrared light are all different forms of electromagnetic waves that are used to transmit data.•LANs use radio waves•WANs use microwave signals

Wireless Media

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•Land-based, line-of-sight transmission•Approximately 20-30 miles maximum between towers•Transmits data at hundreds of millions of bits per second•Popular with telephone companies and business to business transmissions

Terrestrial Microwave

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Similar to terrestrial microwave except the signal travels from a ground station on earth to a satellite and back to another ground station.

Satellites can be classified by how far out into orbit each one is (LEO, MEO, GEO, and HEO).

Satellite Microwave

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Data Communications and Computer Networks Chapter 3

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•LEO - Low Earth Orbit - 100 miles to 1000 miles. Used for pagers, wireless e-mail, special mobile telephones, spying, videoconferencing.•MEO - Middle Earth Orbit - 1000 to 22,300 miles. Used for GPS and government.•GEO - Geosynchronous Orbit - 22,300 miles. Used for weather, television, and government operations. •HEO – Highly Elliptical Orbit A fourth type of orbit used by the military for spying and by scientific organizations for photographing celestial bodies.

When satellite is far out into space, it takes photos. When satellite is close to earth, it transmits data.

Satellite Microwave

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Special transmissions that use a focused ray of light in the infrared frequency range. Very common with remote control devices, but can also be used for device-to-device transfers, such as PDA to computer.

Bluetooth is a Radio Frequency specification for short-range, point-to-multipoint voice and data transfer. Bluetooth can transmit through solid, non-metal objects. Its typical link range is from 10 cm to 10 m, but can be extended to 100 m by increasing the power.

Infrared & Bluetooth Transmissions

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This technology transmits data between workstations and local area networks using high speed radio frequencies.

Current technologies allow up to 54 Mbps data transfer at distances up to hundreds of feet.

Wireless LAN (IEEE 802.11)

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FACTORS UTP STP COAXIAL FIBEROPTICS

Cost Lowest Moderate Moderate High

Installation Easy Fairly easy Fairly easy Difficult

Bandwidth 1-155Mbps 1-155Mbps 2-10 Mbps 2 Gbps

Capacity per Segment

(typically 10 Mbps)

(typically 16 Mbps)

(typically 10 Mbps)

(typically 100Mbps)

Node Capacity per Segment

2 2 30 (10base2)100(10base5)

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Attenuation High (range of hundreds of meters)

High (range of hundreds of meters)

lower(range of few kilometers)

Lowest(range of tens of kilometers)

EMI Most Vulnerable to EMI and eavesdropping

Less Vulnerable than UTP but still vulnerable to EMI and eavesdropping

Less Vulnerable than UTP but still vulnerable to EMI and eavesdropping

Not effected by EMI or eavesdropping

Characteristics of Cable Media

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