datornätverk a – lektion 7
DESCRIPTION
Datornätverk A – lektion 7. Kapitel 7: Transmissionsmedia Kapitel 8: Kretskoppling Bredbandsinfrastruktur Kapitel 9: Bredbandsaccesstekniker: DSL, Cable Modem. Chapter 7. Transmission Media. Figure 7.2 Classes of transmission media. Figure 7.3 Twisted-pair cable (TP). - PowerPoint PPT PresentationTRANSCRIPT
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Datornätverk A – lektion 7
Kapitel 7: Transmissionsmedia
Kapitel 8: Kretskoppling
Bredbandsinfrastruktur
Kapitel 9: Bredbandsaccesstekniker: DSL, Cable Modem.
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Chapter 7
TransmissionMedia
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Figure 7.2 Classes of transmission media
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Figure 7.3 Twisted-pair cable (TP)
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Figure 7.4 UTP (Unshielded TP) and STP (Shielded TP)
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Table 7.1 Categories of unshielded twisted-pair cablesTable 7.1 Categories of unshielded twisted-pair cables
Category Bandwidth Data Rate Digital/Analog Use
1 very low < 100 kbps Analog Telephone
2 < 2 MHz 2 Mbps Analog/digital T-1 lines
3 16 MHz 10 Mbps Digital LANs
4 20 MHz 20 Mbps Digital LANs
5 100 MHz 100 Mbps Digital LANs
6 (draft) 200 MHz 200 Mbps Digital LANs
7 (draft) 600 MHz 600 Mbps Digital LANs
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Figure 7.5 UTP connector
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Figure 7.7 Coaxial cable
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Table 7.2 Categories of coaxial cablesTable 7.2 Categories of coaxial cables
Category Impedance Use
RG-59RG-59 75 Cable TV
RG-58RG-58 50 Thin Ethernet
RG-11RG-11 50 Thick Ethernet
Ledningens karaktäristiska impedans är den impedans (eller resistans) ledningen skulle ha om den vore oändligt långt.
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Figure 7.8 BNC connectors
BNC-Bayonet Neill Concelman
T-koppling
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Ledningsreflektioner
• Se animering.• Ledningsreflektioner kan orsaka s.k. stående vågor, som
innebär att vissa frekvenser släcks ut i vissa punkter på ledningen.
• Reflektioner kan uppstå:○ om ledningar med olika karaktäristisk impedans kopplas samman, ○ om en ledning får t.ex. en kläm- eller fuktskada, ○ om långa ledningar parallellkopplas (t.ex. om man stoppar in en
ledning mellan T-kopplingen och datorn som är längre än halva våglängden), eller
○ om en ledning inte avslutas med en termineringsresistor som har samma resistans som ledningens karaktäristiska impedans.
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Ledningsreflektioner (Forts)
• Viktigt vid bussnät baserade på koaxialkablar.
• Vid bussnät är nätverkskortet högohmigt, dvs avbrott. I ledningens båda ändar behövs en terminering på samma impedans som ledningen karaktäristiska impedans, t.ex. 50 Ohm vid Ethernet koaxialkabel.
• Vid TP-kablar innehåller Ethernetkortet en termineringsresistor, dvs det är lågohmigt. Därmed behövs ingen extra termineringsresistor. Men därför är det inte möjligt att ansluta flera datorer till samma TP-kabel, utan att det uppstår reflektioner. Datorerna måste anslutas till ett nätnav (hub) eller en växel (switch), och således bilda ett fysiskt eller logiskt stjärnnät.
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Kabeltyper för Ethernet
• 10BASE5=Tjock Ethernet, 10Mbps, 500m avstånd, koaxial.
• 10BASE2=Tunn Ethernet, 10Mbps, 200m, koaxial.
• 10BASE-T, 10Mbps, 100m, TP=Tvinnad parkabel, hubnät.
• 100BASE-T=Fast Ethernet, 100Mbps, 100m, TP, hubnät.
• 1000BASE-T, 1000Mbps, TP, hubnät.
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Kontaktdon för Ethernet
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Figure 7.13 Modes
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Figure 7.15 Fiber-optic cable connectors
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Figure 7.16 Optical fiber performance
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Figure 7.9 Coaxial cable performance
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Figure 7.2 Classes of transmission media
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Figure 7.17 Electromagnetic spectrum for wireless communication
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Våglängd och frekvens
8
300[ ]
Antal MHz
där ljusets hastighet 3 10 / ,
är våglängden i meter, och
är radiovågornas frekvens i Hz.
c cf m
f
c m s
f
Ju högre frekvens desto kortare våglängd.
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Vågutbredning av radio- och mikrovågor
• Exempel: Radio-LAN använder ofta frekvensen 2.4GHz,dvs våglängden 300/2400 =0.125m.
• Radioskugga kan uppstå bakom föremål med storlek några våglängder (några dm i vårt exempel).
• Radiovågor dämpas kraftigt av metallnät, t.ex. armeringsjärn, med mindre hål än en halv våglängd (ca 6 cm i vårt fall). Metallnätet utgör då Faradays bur.
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Figure 7.19 Wireless transmission waves
2 – 300GHzKort avståndeller line-of-sight
30kHz – 2GHz 300 – 400GHz
Line-of-sight
•Broadcasting (radiooch TV),•Mobiltelefoni
•Mikrovågslänkar (tvåparabolantenner på två huseller master)•Satellitkommunikation•Radio-LAN•Korthållskommukation(t.ex. Bluetooth)
•Fjärrstyrning•IRDA
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Figure 7.20 Omnidirectional antennas
Används ofta vid radiovågor
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Radio waves are used for broadcast communications, such as radio and
television, and paging systems.
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Figure 7.21 Unidirectional antennas
Används ofta vid mikrovågslänkaroch satellitkommunikaton
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Infrared signals can be used for short-range communication in a closed area
using line-of-sight propagation.
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Chapter 8
Circuit Switchingand
Telephone Network
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Telephone network – Local Switch
• The telephone network uses switches○ Every subscriber ( telephone jack in a house) has a twisted-pair wire
connected to the closest telephone exchange. They are called local switches or local exchanges.
subscriber
switch
• This cannot provide connection to subscribers connected to another local switch.
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Switches
• What is a switch?○ A central device usually used with a star topology
○ Can be built in hardware and/or software
○ Used to provide temporary connections between any two devices connected to the switch
○ A network of switches can be made if a very large number of devices spread in a large geographic space need to be connected
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A Circuit Switch
• Device with a number of inputs and outputs
• Creates temporary physical connection between an input and output link
Subscribers connected to the same swich• The local switch can connect
each telephone with each other
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Figure 8.2 A circuit switch
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Circuit Switching
• Three phases of the connection:○ Circuit establishment
○ Data transfer
○ Circuit disconnect
• The bandwidth is guaranteed during the connection○ The bandwidth cannot be used by anyone else, even if it is not
needed at certain moment (no flexibility)
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Figure 8.4 Crossbar switch
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Figure 8.5 Multistage switch
Space switching
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Figure 8.6 Switching path
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Figure 8.7 Time-division multiplexing, without and with a time-slot interchange
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Figure 8.8 Time-slot interchange
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Figure 8.10 TST switch (Time-Space-Time)
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Characteristics of the Switches
• Space switches○ The advantage is that if a cross point is available, the connection is
almost instantaneous○ The disadvantage is the need for many cross points which is
expensive
• Time switches○ Advantage is that it does not need cross points○ Limited by the maximum data rate of one line.○ Introduces a fixed delay.
• Combined switches combine the advantages of both types
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Figure 8.11 A telephone system
Accessnät(Spridningsnät)
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Hierarchy of the Telephone Network
subscriberlines (local loops)
local exchanges (toll offices)
International gateway exchange
National tandem exchanges
regional tandemexchanges
local tandemexchangeslocal
network
Internationalnetwork
trunk network
Tandem offices
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Chapter 9
Bredbandsaccess-tekniker
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Bredbands-infrastruktur
Ethernet-LANanvänds ofta i flerfamiljshus
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9.1 DSL Technology9.1 DSL Technology
ADSL
Other DSL Technologies
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ADSL is an asymmetric communication technology designed for residential users; it is not suitable
for businesses.
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The existing local loops can handle bandwidths up to 1.1 MHz.
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ADSL is an adaptive technology. The system uses a data rate based on the
condition of the local loop line.
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Figure 9.2 Bandwidth division
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Figure 9.1 DMT
Dicrete Multi-tone Modulation
- Många långsamma modulatorer, var och en på olika underbärvågsfrekvens.Syfte: - Vid störningar på vissa frekvenser kan antal bit per symbol minskas endast på dem. - Långa symboler ger mindre känslighet för intersymbol-interferens.
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ADSL Frequency Spectrum
• Divides the bandwidth into 256 x 4.3K channels
• 1 (ch 0) POTS, 5 (ch 1-5) not used, 1 upstream control, 1 downstream control
• Typical 6-30 for upstream, rest for downstream
• Each 4.3K channel 4K baud sample, V.34 QAM modulation, up to 15 bits per baud4K * 15 = 60 Kbps per channel
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Figure 9.3 ADSL modem
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Figure 9.4 DSLAM
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Other DSL Technologies
• SDSL (Symmetric DSL) divides frequencies evenly
• HDSL (High-rate DSL) provides DS1 bit rate in both directions
Short distances
Four wires
• VDSL (Very high bit rate DSL) provides up to 52 Mbps
Very short distance
Requires Optical Network Unit (ONU) as a relay
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9.2 Cable Modem9.2 Cable Modem
Traditional Cable NetworksHFC NetworkSharingCM and CMTSDOCSIS
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Figure 9.5 Traditional cable TV network
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Communication in the traditional cable TV network is unidirectional.
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Figure 9.6 HFC network
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Communication in an HFC cable TV network can be bidirectional.
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Figure 9.7 Coaxial cable bands
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Downstream data are modulated using the 64-QAM modulation technique.
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Upstream data are modulated using the QPSK modulation technique.
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Figure 9.8 Cable modem
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Figure 9.9 CMTS
CMTS = Cable Modem Terminating System