Datorn

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Datornätverk A lektion 6

• 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
cables
Category Bandwidth Data Rate Digital/Analog Use
1 very low lt 100 kbps Analog Telephone
2 lt 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 cables
Category Impedance Use
RG-59 75 W Cable TV
RG-58 50 W Thin Ethernet
RG-11 50 W 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

• 10BASE5Tjock Ethernet, 10Mbps, 500m avstånd,
  koaxial.
• 10BASE2Tunn Ethernet, 10Mbps, 200m, koaxial.
• 10BASE-T, 10Mbps, 100m, TPTvinnad parkabel,
  hubnät.
• 100BASE-TFast 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
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 300GHz Kort avståndeller line-of-sight
30kHz 2GHz
300 400GHz
Line-of-sight

• Fjärrstyrning
• IRDA

• Mikrovågslänkar (tvåparabolantenner på två
  huseller master)
• Satellitkommunikation
• Radio-LAN
• Korthållskommukation(t.ex. Bluetooth)

• Broadcasting (radiooch TV),
• Mobiltelefoni

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Figure 7.20 Omnidirectional antennas
Används ofta vid radiovågor
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Note
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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Note
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 SwitchingandTelephone 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

• 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

• 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
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)
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Chapter 9
Bredbandsaccess- tekniker
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Bredbands-infrastruktur
Ethernet-LAN används ofta i flerfamiljshus
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9.1 DSL Technology
ADSL Other DSL Technologies
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Note
ADSL is an asymmetric communication technology
designed for residential users it is not
suitable for businesses.
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Note
The existing local loops can handle bandwidths up
to 1.1 MHz.
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Note
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 Modem
Traditional Cable Networks HFC Network Sharing CM
and CMTS DOCSIS
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Figure 9.5 Traditional cable TV network
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Note
Communication in the traditional cable TV network
is unidirectional.
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Figure 9.6 HFC network
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Note
Communication in an HFC cable TV network can be
bidirectional.
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Figure 9.7 Coaxial cable bands
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Note
Downstream data are modulated using the 64-QAM
modulation technique.
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Note
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
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Optical Hierarchies

• SONET (Synchronious Optical NETwork)
• A standard for TDM used in United States
• SDH (Synhronous Digital Hierarchy)
• A standard for TDM in Europe
• Both use synhronous communication
• Digital telephony systems use clocking for
  synchronous data delivery
• Synchronous network moves data at a precise rate

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Note
SONET and SDH are synchronous TDM systems
controlled by a master clock.
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Figure 9.10 A SONET
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Figure 9.11 Frame format
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Table 9.1 SONET rates
STS OC Rate (Mbps) SPE (Mbps) User (Mbps)
STS-1 OC-1 51.84 50.12 49.536
STS-3 OC-3 155.52 150.336 148.608
STS-9 OC-9 466.56 451.008 445.824
STS-12 OC-12 622.08 601.344 594.432
STS-18 OC-18 933.12 902.016 891.648
STS-24 OC-24 1244.16 1202.688 1188.864
STS-36 OC-36 1866.23 1804.032 1783.296
STS-48 OC-48 2488.32 2405.376 2377.728
STS-192 OC-192 9953.28 9621.604 9510.912
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Figure 9.12 Data rate
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Figure 9.13 VT types
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Figure 9.14 STS multiplexing