LongDistance Digital Connection Technologies

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Long-Distance Digital Connection Technologies
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Digital Telephony

• Phone companies have long used digital signals
  for voice
• Has higher quality
• Has greater capacity
• Can travel longer distances
• Amplifiers degrade signal, audible for analog

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Digital Telephony Continued

• Hardware must change voice (analog) signal to
  digital signal
• Analog to digital converter (ADC) used
• Finds digital voltage closest to analog level
• Results in continuous stream of numbers

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Digital Telephony Continued

• Need frequencies up to 4000Hz
• Nyquist Theorem says sampling at 8000Hz needed
• Sample every 125µs (µs millionth of a second))
• Each sample has captures 256 signal levels, needs
  8 bits
• So we get 8000 8-bits samples per sec 64 Kbps
• Called Pulse Code Modulation (PCM)

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Synchronous

• Telephone network uses synchronous communication
  for voice
• Facilities for voice different from data
• Voice is not bursty
• Must be sent continuously
• Termed synchronous or clocked (isochronous)
• Precise data rate
• Does not vary under load, same amount always send

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DSU/CSUs

• Phone company uses synchronous signally, LANs use
  asynchronous
• Need to convert from one to other
• Need Data Service Unit (DSU) for LAN side and
  Channel Service Unit (CSU) for phone side
• Usually one piece of equipment

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DSU/CSU
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CSU

• CSU provides line termination and diagnostics
• Surge suppression
• Prevents too many 1s excessive current
• Connection testing
• Has loopback for testing
• Everything sent out is actually simply echoed back

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DSU

• Handles data
• Translates between LAN and phone frame formats

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Telephone Standards

• Phone companies have standard for underlying data
  circuits
• T series of hardware circuits
• T not universal
• Europe and Japan have different standards
• E for Europe

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Data Rates
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Data Rates Continued

• A single voice channel can do 64kbps
• 8000 8-bit samples per second
• T1 is 24 of these
• T2 4 x T1
• T3 7 x T2
• Multiplexing technology called Digital Signal
  (DS) DS1 for T1

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Data Rates Continued

• Might want something less than T1
• Can purchase fractional T circuits
• Might want something between T standards
• Can inverse multiplex See Fig 12.4

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Inverse Multiplexing
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Highest Rate Circuits STS and OC

• For higher rates use Synchronous Transport Signal
  (STS)
• STS for copper, OC (Optical Carrier) for fiber

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STS and OC
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SONET

• Broad set of standards for digital transmission -
  protocols
• Termed Synchronous Optical NETwork in America and
  Synchronous Digital Hierarchy (SDH) in Europe
• Specifies framing, multiplexing, and
  synchronization

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SONET Continued

• For STS-1 each frame 810 octets
• 9 rows and 90 columns
• Frame size varies with speed of underlying medium
• Frame size equal to number of octets per 125µs
  810 channels at 64 Kbps each
• Multiplexing multiple slower circuits into one
  faster circuit easy

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Getting Communication from Service Provider to
Subscriber Residence - Local Loop

• Term local loop or local subscriber line used for
  line to house
• Connected to central office (CO) via twisted pair

Service Provider Central Office (LEC)
CO Telephone Switching system
Subscriber Residence
Copper Wire Loop
Telephone
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ISDN

• One of first efforts to supply digital to home
  and office called Integrated Services Digital
  Network (ISDN)
• Used twisted pair
• Has different channels
• Customer typically has 2BD for 128Kbps

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ISDN Continued

• B channels are data, D is control (16Kbps)
• 2BD called Basic Rate Interface (BRI)
• Also supplies voice traffic
• Possible through special phones

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xDSL

• Digital Subscriber Line (DSL)
• Downstream is bps toward subscriber
• Upstreams is bps away from subscriber
• Asymmetric (ADSL)
• Has larger downstream bps than upstream as
  appropriate for Internet traffic behavior
• This is what typical user wants
• Not good for servers

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ADSL

• Maximum downstream 6.144Mbps
• Maximum upstream 640Kbps
• Does not require changes to wiring in local loop
  - rides on local loop with its own modems tapping
  in (Figure 12.7)
• Local loop traffic (voice) can continue
  unaffected (64 Kbps reserve up and dwon for
  telephone leaving 576 Kbps upstream for DSL)

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ADSL Continued

• Most local loops have excess bandwidth higher
  frequencies unused
• But there are a variety of wiring and
  interference issues
• Varies by location
• ADSL is adaptive ADSL modems find usable good
  frequency
• Modems on either end probe line to determine
  characteristics

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ADSL Continued

• Uses Discrete Multi Tone modulation (DMT)
• Channel divided into multiple sub-channels each
  with own modem
• 255 for downstream
• 31 for upstream
• 2 for control

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Other DSL

• Symmetric DSL
• High-Rate DSL (Frequencies lt 4KHz reserved for
  phone, so HDSL provides 1.544 Mbps both ways)
  only good at short distances
• Very high-rate DSL (VDSL) uses fiber from CO to
  concentration points

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Cable Modems

• Coax better than twisted pair
• Higher bandwidth
• Better shielding
• Most already wired for cable TV
• Cable has more capacity than TV channels there
  is unused bandwidth
• Uses frequency DM and then TDM to get to
  individual users
• Each user cannot have own frequency
• Sharing limits bandwidth per user

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Cable Modems Continued

• Rates of up to 36Mbps
• Wasnt originally designed to have any upstream
  capability heavy upstream a problem
• Originally used dual path
• Downstream cable
• Upstream dialup voice modem
• Clumsy interface
• New approaches in progress but likely need
  infrastructure changes

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Hybrid Fiber Coax (HFC)

• Hybrid Fiber Coax
• Has both fiber and coax
• Fiber used in trunks
• Existing coax used in feeder circuits to homes
• Have to replace trunks

CATV Office
CM
Concentration Point
OM
OM
CM
Fiber Trunk over distance
CM
Local neighborhood cable connections
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Satellites

• Broadcast from satellites with unique
  addressing scheme for reception
• Dual path for the upstream dial-up
• Subscriber needs dish antenna
• Good for remote locations