Physical Layer

The Physical Layer EE424 EE 424 Communication Systems Abdel Fattah Sheta Part II

The Physical Layer

Outline 1. The theoretical basis of data communication 2. Guided Transmission Media • Twisted Pair • Coaxial C i l Cable C bl • Fiber Optics 3. Wireless Transmission Electromagnetic Spectrum Radio Transmission Microwave Transmission Infrared and Millimeter waves Light wave Transmission 4. Communication Satellite Geostationary Satellites – LEL – LEO 5. The Local Loop: Modem and DSL, ADSL, Wireless Local Loop

EE 424

Dr. Abdel Fattah Sheta

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The Physical Layer The Theoretical Basis for Data Communication • Fourier representation

Computation of series coefficients :

The Theoretical Basis for Data Communication – all transmission facilities attenuate different Fourier components by different amounts, thus introducing distortion. – Usually, the amplitudes are transmitted unattenuated from 0 up to some frequency fc with all frequencies above this cutoff frequency attenuated. – The range of frequencies transmitted without being strongly attenuated is called the bandwidth (0 - fc ). – The bandwidth is a physical property of the transmission medium and usually depends on the construction, thickness, and length of the medium

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The Physical Layer Transmission Impairments Many factors limit the ability of a transmission channel to carry information: • Fundamental bandwidth limitations • Amplitude distortion • Frequency or harmonic distortion • Phase or Delay distortion • Reflections • Noise

Bandwidth limited Signals Example: the transmission of the ASCII character ''b'' encoded in an 8-bit byte. bit pattern : 01100010

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The Physical Layer Bandwidth limited Signals

Bandwidth limited Signals Bit rate 1 bit will be transmitted in time 8 bits will take The fundamental harmonic The nth harmonic is

b 1/b 8/b b/8 nb/8

bits/s second seconds Hz Hz

If a filter of bandwidth = 3 KHz The number of highest harmonic passed through is 3000/(b/8) = 24,000/b

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The Physical Layer Bandwidth limited Signals

The Maximum Data Rate of a Channel Nyquist formula (for noiseless channel) Maximum data rate as a function of channel bandwidth (BW)

if BW=H then max. data rate is 2H for binary signals Bandwidth (H): several criteria, choice depends on application (3dB or 50% power; fraction of signal power) General C = 2H log2 (V) [bps], V = discrete levels

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The Physical Layer The Maximum Data Rate of a Channel Example 1: H=3 KHz channel, binary levels Max. bit rate = 2(3) log2 2 = 6 * 1 = 6 Kbps Example 2: 3 KHz, 8 signaling levels Max. bit rate = 2(3) log2 8 = 6 * 3 = 18 Kbps

Limit of Noisy Channel Capacity Shannon’s major result of a noisy channel [1948] is that: Max. Data rate

S N

= H log2 (1 + S/N)

bps

average signal strength (Watts), noise power (Watts).

By using appropriate channel coding in the presence of “white” noise only, we can approach the Shannon's capacity limit. The speed of the channel can be increased by: • Increasing the bandwidth • Increasing either the signal to noise ratio or designing more effective detectors that are able to reliable detect small signals in the presence of noise.

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The Physical Layer Limit of Noisy Channel Capacity Example # 1: H = 3 KHz & S/N ratio = 20 dB. Max. Bit rate = 3 * log2 (1 + 100) = 3 * 6.6583 = 19.975 Kb/s Example # 2: In telephone channel, If the filter is removed such that H = 250KHz with S/N = 20 dB. Max. Bit rate = 250 * log2 (1 + 100) = 250 * 6.6583 = 1.665 Mb/s

Limit of Channel Capacity For binary signals

Clearly the transmission rate depends on the length of the transmission path. Short lengths will support higher rates, while long lengths will be slower. The transmission rate is limited by skin effect since current travels in the outer layer of molecules of the wire, so larger physical wires have larger bandwidth.

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The Physical Layer Transmission Modes and Media Transmission modes: simplex, half duplex, half-duplex, full-duplex. Transmission media: Signals must be carried over some form of media and there are two fundamental types of media:

guided and unguided. 1. 2. 3. 4.

Twisted pair Coaxial cable Optical fiber Wireless

Twisted Pair (UTP & STP)

•

• • • • •

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Two parallel insulated copper wires twisted in a spiral form regularly around each other, and often combined with others into a cable Receiver detects information signal by the voltage difference in two wires. Interference is picked up by both wires, thus their difference will remain unaffected. Twisting reduces interference among adjacent pairs. pairs Twist: 5—15cm Thickness: 0.4—0.9mm


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The Physical Layer Twisted Pair (UTP & STP) • • • • • • • • • • •

cont.

Twisted Pair (TP) is used to reduce crosstalk wire pair acts as a single communication link usedd iin telephone t l h networks t k used within buildings inexpensive compared to other media easy to work with poor noise and interference immunity analog signal amplifiers required every about 5 km digital signal repeaters required every about 2 km interference reduced by sheathing UTP: ordinary telephone wire, cheapest media for LANs. STP: less prone to interference, more expensive, harder to work with

Twisted Pair (UTP & STP)

cont.

Unshielded Twisted Pair (UTP) • Ordinary telephone wire • Cheapest Ch t • Easiest to install • Suffers from external EM interference Shielded Twisted Pair (STP) • Metal braid or sheathing that reduces interference • More expensive • Harder to handle (thick, heavy)

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The Physical Layer Twisted Pair (UTP & STP) Twisted pair cable differential signals.

is

good

for

cont.

transferring

balanced

The most commonly used twisted pair cable impedance is 100 ohms. It is widely used for data communications and telecommunications applications in structured cabling systems. In most twisted pair cable applications the cable impedance is between 100 ohms and 150 ohms. When a cable has a long distance between the conductors, higher impedances are possible. Typical wire conductor sizes for cables used in telecommunications 26, 24, 22 or 19 AWG. AWG stands for American Wire Gauge. The noise pickup characteristics of twisted pair cable is determined by the following cable characteristics: • Number of twists/m (generally more twists/m gives better performance) • Uniform cable construction • Capacitance balance (less capacitance difference to ground, the better), • Cable diameter (less are between wires is better) • The amount of shielding (more shielding, the better).

Twisted Pair Classification Standards have been defined for twisted pair cable and described in terms of cable categories by: • Electronic Industry Association (EIA), • Telecommunications Industryy Association (TIA), ( ), and • American National Standards Institute (ANSI) The various categories provide a range of bandwidth from 1 MHz to 100 MHz . For 100 m length:

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The Physical Layer Twisted Pair Classification Category 1 It is mainly used to carry voice. CAT 1 was used primarily for telephone wiring prior to the early 1980s. It is not certified to carry data of any type Category g y2 Is used to carry data at rates up to 4Mbps. It is rated to 1MHz. Category 3 Is also known as voice-grade cable. It is used primarily in older Ethernet 10base-T LANs and is certified to carry data at 10Mbps. It is rated to 16MHz. Category 4 Is used primarily when implementing token-based or 10baseT/100b T/100base-T T networks. t k CAT4 is i certified tifi d att 16Mbps 16Mb and d consists it of four twisted wires. It is rated to 20MHz. Category 5 is the most popular Ethernet cabling category. It is capable of carrying data at rates up to 100Mbps and is used for 100base-T and 10base-T networks. It is rated to 100MHz.

Twisted Pair Classification

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The Physical Layer Attenuation Frequency Response

Coaxial Cable • Coaxial cable, named from the two cable axes that run the length of the wire, is a versatile and useful transmission medium. • The cable consists of a solid or braided outer conductor surroundingg either a solid or a stranded inner conductor. • The conductors are usually separated by a dielectric material, and the entire wire is covered with an insulating jacket. • Coaxial wire allows for greater shielding from interference and greater segment distances. • Coaxial 10base-5/2 has a transmission rate of 10Mbps. 10base-5 has a maximum segment length of about 500m/segment, whereas 10base-2 is about 180m/segment.

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The Physical Layer Coaxial Cable

Fiber Optic Cable •

• •

This consists of a central glass core, surrounded by a glass cladding of lower refractive index, so that the light stays in the core (using Total Internal Reflection) On the outside is a plastic jacket Many fibers may be bundled together surrounded by another plastic cover

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The Physical Layer Transmission of light waves in optical fiber

• • •

Core has slightly higher optical density (refraction index) than cladding Ratio of refraction indices define critical angle θc When incidence angle > θc light is reflected back into the core

How light travels in a fiber optic cable • The source of light is usually a Light Emitting Diode (LED) or a LASER. The light source is placed at one end of the optical fiber • Light g that hits the core of the fiber at a certain angle, g , known as the critical angle, is transmitted down through it by total internal reflection. • The detector, which is placed at the other end of the fiber, is usually a Photo Diode and it generates an electrical pulse when light falls on it.

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The Physical Layer Types of fiber Stepped Index Fiber: • The cladding has a lower refractive index than the core. Graded Index Fiber: • This is where the cladding has a lower refractive index than the core. The refractive index of the glass core changes as you move down the glass core. •

The light rays are redirected towards the central axis of the core as they travel through the fiber.

Types of fiber

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The Physical Layer Attenuation • •

The attenuation of light through glass depends on the wavelength of the light Attenuation of light through fiber in the infrared region

Fiber Cables

– multimode fibers : The core is typically 50 microns in diameter, about the thickness of a human hair – single-mode fibers : The core is 8 to 10 microns.

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The Physical Layer Fiber Optic Networks Passive star

Active repeaters

A fiber optic ring with active repeaters

A passive star connection in a fiber optics network

Comparison of Fiber Optics and Copper Wire Fiber can handle much higher bandwidths than copper Fiber not being affected by power surges, electromagnetic interference, or power failures Fiber is thin and lightweight Security in fiber is high Fiber is a less familiar technology requiring skills not all engineers have Fibers can be damaged easily by being bent too much Optical transmission is inherently unidirectional, two-way communication requires either two fibers or two frequency bands on one fiber fiber interfaces cost more than electrical interfaces

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The Physical Layer Wireless Transmission

• • • • •

The Electromagnetic Spectrum Radio Transmission Microwave Transmission Infrared and Millimeter Waves Lightwave Transmission

Radio Transmission

Radio waves are easy to generate and can travel long distances and penetrate buildings. Radio waves are omni-directional which basically means that they can transmit both ways. The transmitter and receiver do not have to be in direct line of sight

–

–

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Radio Transmission Properties At low frequencies (<100MHz) radio waves pass through obstacles well but the signal power attenuates (falls off) sharply in air At higher frequencies (>100MHz) radio waves tend to travel in straight lines and bounce of obstacles and can be absorbed by rain (e.g in the 8GHz range)


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The Physical Layer Radio Transmission – –

–

– –

At all frequencies, radio waves are subject to interference from motors and other electrical equipment In very low frequencies (VLF), low frequencies (LF) and medium frequency bands (MF) ((<1Mhz) 1Mhz) radio waves follow the ground. • The maximum possible distance that these waves can travel is approximately 1000km • AM radio stations use the MF band as they can penetrate buildings. Their main problem is their relatively low data rates In high frequency (HF) and very high frequency (VHF) bands (> 1MHz and <100MHz) ground waves are absorbed by the earth Waves that reach the outer atmosphere of the earth, the ionosphere, are refracted by it and sent back to earth These frequencies tend to be used by amateur radio operators (ham radio) and the military

The electromagnetic spectrum and its uses for communication.

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The Physical Layer The electromagnetic spectrum and its uses for communication ELF SLF ULF VLF LF MF HF VHF UHF SHF EHF IR

Extremely Low Frequency Super Low Frequency Ultra Low Frequency Very Low Frequency Low Frequency Medium Frequency High Frequency Very High Frequency Ult High Ultra Hi h Frequency F Super High Frequency Extremely High Frequency (submillimeter waves) Infared

3-30 Hz 30-300 Hz 300 Hz - 3 kHz 3 kHz - 30 kHz 30 kHz - 300 kHz 300 kHz - 3 MHz 3 MHz - 30 MHz 30 MHz - 300 MHz 300 MHz MH - 3 GHz GH 3 GHz - 30 GHz 30 GHz - 300 GHz 300 GHz - 3000 GHz 3000 GHz - 416,000 GHz

Microwave Transmission – –

– –

–

Above 100MHz, waves travel in straight lines and can be narrowly focused into a small beam using a special parabolic antenna The transmitters and receivers must be aligned correctly • Multiple transmitters and receivers can be set up in parallel without interfering with each other • Repeaters are needed to retransmit the signals due to the curvature of the earth. Typically, transmitting towers are 100 metres high and repeaters are needed every 80km Unlike radio waves, microwaves typically do not pass through solid objects Some Waves can be refracted due to atmospheric conditions and g to arrive than direct waves. These delayed y mayy take longer waves can arrive out of phase with the direct wave, causing destructive interference and corrupting the received signal This effect is called multipath fading

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The Physical Layer Microwave Transmission – – –

Because of increased demand for more spectrum (frequencies used to transmit), transmitters are using higher and higher frequencies However at around 8Ghz, the signal can be absorbed by water. Th f Therefore li links k hhave tto bbe shutdown h td when h it rains. i Microwave communication is widely used for long distance telephone communication and cell phones

Advantages of Microwave over Fiber Optics – No need to lay cables: 1. This causes less disruption to the areas where the microwave transmitters and receivers are placed 2 2. Thi also This l means that h microwave i communication i i is i less l expensive i than fiber optic cable

(a) In the VLF, LF, and MF bands, radio waves follow the curvature of the earth. (b) In the HF band, they bounce off the ionosphere.

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The Physical Layer Wide band • Most transmissions use a narrow frequency band to get the best reception • Wide band – Frequency hopping spread spectrum • change frequencies hundreds of times per second • security • avoids multipath fading • Example: 802.11,Bluetooth • Direct sequence spread spectrum spread the signal over a wide frequency band used in cell phones: second and third generation mobile phones

Infrared and Millimeter Waves • widely used for short-range • they are relatively directional, cheap, and easy to build • theyy do not pass p through g solid objects j • In this method infrared light is modulated by transmitter. • Transceivers must be in line of sight either directly or via reflection from a light colored surface such as a ceiling of a room. • Data rates of up to 20Mbps are possible. • No security or interference problems, as infrared transmission does not penetrate the walls. walls • License is not required. • point to point and potential eye damage if exposed to IR rays are the main disadvantages of infrared transmission.

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The Physical Layer Lightwave Transmission

Communication Satellites

• Communication satellite can be thought of as a bi microwave big i repeater t in i the th sky. k • Transponders – each of which listens to some portion of the spectrum, amplifies the incoming signal, and then rebroadcasts it at another frequency to avoid interference with the incoming signal

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The Physical Layer Communication Satellites

Communication Satellites (2)

The principal satellite bands.

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The Physical Layer Communication Satellites (3)

VSATs using a hub.

Low-Earth Orbit Satellites Iridium

(a)

(b)

(a) The Iridium satellites from six necklaces around the earth. (b) 1628 moving cells cover the earth.

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The Physical Layer Globalstar

(a) Relaying in space. (b) Relaying on the ground.

Major Components of the Telephone System

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The Physical Layer The Local Loop: Modems, ADSL, and Wireless

The use of both analog and digital transmissions for a computer to computer call. Conversion is done by the modems and codecs.

Modems • Analog and digital transmission • Sine wave carrier • Baud • Phase shift keying • Limits

(a) A binary signal (b) Amplitude modulation

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(c) Frequency modulation (d) Phase modulation


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The Physical Layer Modems

(a) QPSK.

(b) QAM-16.

(c) QAM-64.

Modems

(a) V.32 for 9600 bps.

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(b) V32 bis for 14,400 bps.


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The Physical Layer Limits • Base sampling rate – 2400 baud

• 35 kbps is the Shannon limit, 56 kbps? – eliminate one local loop – V.90 56-kbps down stream, 33-kbps upstream – V.92 48-kbps down stream, 48-kbps upstream

Digital Subscriber Line (DSL) Twisted pair telephone lines stopped at 56 Kb/s data rate. Voice conversation is not possible during the connection. At the same time cable TV company industry was offering up to 10 Mbps on shared cables, and satellite companies were planning to offer upward 50 Mbps xDSL xDSL is a term covers a number of forms of DSL technologies, including ADSL, HDSL, ….., and VDSL. DSL technology uses existing twisted-pair telephone lines to transport high-bandwidth data, such as multimedia and video, to service subscribers. Most DSL deployments are ADSL, mainly delivered to residential customers.

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The Physical Layer Bandwidth versus distanced over category 3 UTP for DSL • Goals – use existing Cat-3 lines – not interfere with current phone uses – always on – much better than 56kbps

• Remove the filters • Bandwidth

ADSL •

Dividing the available spectrum into three band -Voice service – upstream - down stream

•

Use DMT (Discrete MultiTone)

•

1.1 MHz = 256 channels * 4.3125

•

The remaining 250 = 248 + 1 channel for upstream control and 1 channel for downstream control.

•

Harmonics crosstalk and other effect keep practical system well below the theoretical limit

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The Physical Layer ADSL ADSL standard allows speeds of as much as 8.4 Mb/s down stream and 1 Mb/s . Within each channel a modulation scheme similar to V.34 V 34 is used although the sampling rate is 4 Kb/s instead of 2.4 Kbps baud. The line quality in each channel is constantly monitored and the data rate adjusted continuously as needed. Different channels may have different data rate. 15 bits/baud – 4000 baud – use 16 QAM – 224 channels for down stream

DSL Equipment • NID: Network Interface Device • ADSL Modem: 250 QAM modems • DSLAM: Digital g Subscriber Line Access Multiplexer p

ISDN require much greater changes to the existing switching equipment

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The Physical Layer HDSL Originally developed by Bellcore, high bit-rate DSL (HDSL)/T1/E1 technologies have been standardized by ANSI in the United States and by ETSI in Europe. The ANSI standard covers two-pair T1 transmission, with a data rate of 784 kbps on each twisted pair. ETSI standards exist both for a two-pair E1 system, with each pair carrying 1168 kbps, and a three-pair E1 system, with 784 kbps on each twisted pair. HDSL became p popular p because it is a better way y of pprovisioning g T1 or E1 over twisted-pair p copper lines than the long-used technique known as Alternative Mark Inversion (AMI). HDSL uses less bandwidth and requires no repeaters up to the CSA range. By using adaptive line equalization and 2B1Q modulation, HDSL transmits 1.544 Mbps or 2.048 Mbps in bandwidth ranging from 80ntrast to the 1.5 MHz required by AMI. (AMI is still the encoding protocol used for the majority of T1.) T1 service can be installed in a day for less than $1,000 by installing HDSL modems at each end of the line. Installation via AMI costs much more and takes more time because of the requirement to add repeaters between the subscriber and the CO. Depending on the length of the line, the cost to add repeaters for AMI could be up to $5,000 and could take more than a week. HDSL is i heavily h il usedd in i cellular ll l telephone t l h buildouts. b ild t Traffic T ffi from f the th base b station t ti is i backhauled b kh l d to the CO using HDSL in more than 50 percent of installations. Currently, the vast majority of new T1 lines are provisioned with HDSL. However, because of the embedded base of AMI, less than 30 percent of existing T1 lines are provisioned with HDSL. HDSL does have drawbacks. First, no provision exists for analog voice because it uses the voice band. Second, ADSL achieves better speeds than HDSL because ADSL's asymmetry deliberately keeps the crosstalk at one end of the line. Symmetric systems such as HDSL have crosstalk at both ends.

VDSL

Very-High-Data-Rate Digital Subscriber Line (VDSL) transmits high-speed data over short reaches of twisted-pair copper telephone lines, with a range of speeds depending on actual line length. The maximum downstream rate under consideration is between 51 and 55 Mbps over lines up to 1000 feet (300 m) in length. Downstream speeds as low as 13 Mbps over lengths beyond 4000 feet (1500 m) are also common. Upstream rates in early models will be asymmetric, just like ADSL, at speeds from 1.6 to 2.3 Mbps. Both data channels will be separated in frequency from bands used for basic telephone service and Integrated Services Digital Network (ISDN), enabling service providers to overlay VDSL on existing services. Currently, the two high-speed channels are also separated in frequency. As needs arise for higher-speed upstream channels or symmetric rates, VDSL systems may need to use echo cancellation.

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The Physical Layer ISDN Digital Subscriber Line ISDN digital subscriber line (IDSL) is a cross between ISDN and xDSL. It is like ISDN in that it uses a single-wire pair to transmit full-duplex data at 128 kbps and at distances of up to RRD range. Like ISDN, IDSL uses a 2B1Q line code to enable transparent operation through the ISDN "U" interface. Finally, the user continues to use existing CPE (ISDN BRI terminal adapters, bridges, and routers) to make the CO connections. The big difference is from the carrier's point-of-view. Unlike ISDN, ISDL does not connect through the voice switch. A new piece of data communications equipment terminates the ISDL connection and shuts it off to a router or data switch. This is a key feature because the overloading of central office voice switches by data users is a growing problem for telcos. The limitation of ISDL is that the customer no longer has access to ISDN signaling or voice services. But for Internet service providers, who do not provide a public voice service, ISDL is an interesting way of using POTS dial service to offer higher-speed Internet access, targeting the embedded base of more than five million ISDN users as an initial market.

Wireless Local Loops

Architecture of an LMDS system.

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The Physical Layer Trunks and Multiplexing • Frequency division multiplexing • Wavelength division multiplexing • Time division multiplexing – aside on compression • SONET

Frequency Division Multiplexing

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The Physical Layer Wavelength Division Multiplexing

• FDM for optical

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Physical Layer

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