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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).

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Datorn tverk a lektion 7

Datornätverk A – lektion 7

Kapitel 7: Transmissionsmedia

Kapitel 8: Kretskoppling

Bredbandsinfrastruktur

Kapitel 9: Bredbandsaccesstekniker: DSL, Cable Modem.


Chapter 7

TransmissionMedia


Figure 7.2Classes of transmission media


Figure 7.3Twisted-pair cable (TP)


Figure 7.4UTP (Unshielded TP) and STP (Shielded TP)



Figure 7.5UTP connector


Figure 7.7Coaxial cable


Table 7.2 Categories of coaxial cables

Ledningens karaktäristiska impedans är den impedans (eller resistans) ledningen skulle ha om den vore oändligt långt.


Figure 7.8BNC connectors

BNC-Bayonet Neill Concelman

T-koppling


Ledningsreflektioner
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.


Ledningsreflektioner forts
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.


Kabeltyper f r ethernet
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.




Figure 7.15Fiber-optic cable connectors


Figure 7.16Optical fiber performance


Figure 7.9Coaxial cable performance


Figure 7.2Classes of transmission media


Figure 7.17Electromagnetic spectrum for wireless communication


V gl ngd och frekvens
Våglängd och frekvens

Ju högre frekvens desto kortare våglängd.


V gutbredning av radio och mikrov gor
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.


Figure 7.19Wireless transmission waves

30kHz – 2GHz

2 – 300GHz

Kort avståndeller line-of-sight

300 – 400GHz

Line-of-sight

  • Fjärrstyrning

  • IRDA

  • Mikrovågslänkar (tvåparabolantenner på två huseller master)

  • Satellitkommunikation

  • Radio-LAN

  • Korthållskommukation(t.ex. Bluetooth)

  • Broadcasting (radiooch TV),

  • Mobiltelefoni


Figure 7.20Omnidirectional antennas

Används ofta vid radiovågor


Note:

Radio waves are used for broadcast communications, such as radio and television, and paging systems.


Figure 7.21Unidirectional antennas

Används ofta vid mikrovågslänkaroch satellitkommunikaton


Note:

Infrared signals can be used for short-range communication in a closed area using line-of-sight propagation.


Chapter 8

Circuit SwitchingandTelephone Network


Telephone network local switch

switch

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

  • This cannot provide connection to subscribers connected to another local switch.


Switches
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


A circuit switch

Subscribers connected to the same swich

A Circuit Switch

  • Device with a number of inputs and outputs

  • Creates temporary physical connection between an input and output link

  • The local switch can connect each telephone with each other


Figure 8.2A circuit switch


Circuit switching
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)


Figure 8.4Crossbar switch


Figure 8.5Multistage switch

Space switching


Figure 8.6Switching path


Figure 8.7Time-division multiplexing, without and with a time-slot interchange


Figure 8.8Time-slot interchange


Figure 8.10TST switch (Time-Space-Time)


Characteristics of the switches
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


Figure 8.11A telephone system

Accessnät

(Spridningsnät)


Hierarchy of the telephone network
Hierarchy of the Telephone Network

International

network

International gateway exchange

National tandem exchanges

regional tandem

exchanges

trunk network

Tandem offices

local tandem

exchanges

local

network

local

exchanges (toll offices)

subscriber

lines (local loops)


Chapter 9

Bredbandsaccess-

tekniker


Bredbands infrastruktur
Bredbands-infrastruktur

Ethernet-LAN

används ofta

i flerfamiljshus


9.1 DSL Technology

ADSL

Other DSL Technologies


Note:

ADSL is an asymmetric communication technology designed for residential users; it is not suitable for businesses.


Note:

The existing local loops can handle bandwidths up to 1.1 MHz.


Note:

ADSL is an adaptive technology. The system uses a data rate based on the condition of the local loop line.


Figure 9.2Bandwidth division


Figure 9.1DMT

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.


Adsl frequency spectrum
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


Figure 9.3ADSL modem



Other dsl technologies
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


  • 9.2 Cable Modem

    Traditional Cable Networks

    HFC Network

    Sharing

    CM and CMTS

    DOCSIS


    Figure 9.5Traditional cable TV network


    Note:

    Communication in the traditional cable TV network is unidirectional.


    Figure 9.6HFC network


    Note:

    Communication in an HFC cable TV network can be bidirectional.


    Figure 9.7Coaxial cable bands


    Note:

    Downstream data are modulated using the 64-QAM modulation technique.


    Note:

    Upstream data are modulated using the QPSK modulation technique.


    Figure 9.8Cable modem


    Figure 9.9CMTS

    CMTS = Cable Modem Terminating System


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