Computer%20Networks

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Computer Networks

• Digital Access Technologies

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How Computer Networks are Built?

• LANs (Local Area Networks) are relatively cheap
  and easy to built.
• WANs are expensive and difficult to built
• PSTN (Public Switched Telephone Network) is an
  older brother to the computer networks.
• Computer still need to use infrastructure built
  by PSTN, esspecially when it comes to WAN

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Core, Distribution and Access Network

• Core Network
• Combination of switching centers and
  transmission systems connecting switching
  centers.
• Distribution Network
• Network in between the access and core network
• Access Network
• The portion of public network that connects
  individuals or companies to some access node
  through which they can reach the core network
  (directly or via some distribution network)

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Example
Access
Core
Distribution
Fiber-optic
OC12 DPT
ISP1
HFC
HFC
Regional Core
OC48 SONET
Switch
ISP2
HFC
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Various Access Options

• Access at the customer premises
• Narrow band
• PSTN based access
• ISDN based access
• Broad band
• xDSL
• Cable modem
• Fiber to the curb
• Distribution network
• E1, E2, E3, SDH (T1, T2, T3, SONET)

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PSTN Based Access

• PSTN (Public Switched Telephone Network) is
  acronym for the telephone network that uses
  circuit switching
• When using PSTN access to a computer network, the
  modems are needed at both ends of the connection
• Circuit switching is used although inconvenient
  for data traffic
• Data are transmitted in bursts and therefore the
  bandwidth is not used all the time during the
  connection
• The bit rate is limited with the bandwidth
  (usually 4KHz for voice) and the Shannons
  theorem
• Different modulation techniques (QAM, multiple
  PSK) can improve the bit rate
• Due to the known S/N for voice channel these
  techniques cannot do much more then 33600bps

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Local Loop Based Access

• Local loop describes connection from telephone
  office to home
• Also known as local subscriber line
• Most local loops use analog signals
• Sometimes called POTS (Plain Old Telephone
  Service)
• Legacy infrastructure is copper

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Access through Dial-up or Leased Line

• Dial-up
• The connection goes through the switch (telephone
  exchange (the line is released after
  disconnection)
• The customer is charged by the time it uses the
  connection
• Leased line (Dedicated line)
• The connection does not go through the switch
  (the line is dedicated all the time to the
  connection)
• The customer is charged with fixed monthly rate

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Connecting to ISP through Local Loop
Up to 56 Kbps from ISP to the subscriber
Up to 33.6 Kbps
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ISDN Based Access

• ISDN stands for Integrated Services Digital
  Network
• Provides digital service on existing local loop
  copper
• Establishes a digital pipe between the customer
  and the telephone company
• Allows access to multiple services through a
  single access (digital telephone, digital
  terminal, digital facsimile machine)
• Attempt to replace the analog telephone system
  with digital one (for voice and data traffic)
• Obsolete for many reasons
• Too expansive
• Charged by time
• Almost equivalent to analog modems (64 Kbps)

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ISDN Architecture
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ISDN Standardized Channels

• BRA/BRI (Basic Rate Access/Basic Rate Interface)
• 2 B D
• 2 x 64 Kbps 16 Kbps 144 Kbps (not including
  overhead)
• designed to operate using the average local
  copper pair
• PRA/PRI (Primary Rate Access/Primary Rate
  Interface)
• 23 B D
• 23 x 64 Kbps 64 Kbps 1.536 Mbps (not
  including overhead)
• Designed to operate using DS-1/E1
• In Europe 30 B D
• Optional backup D channel.

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Access with xDSL

• xDSL stands for a family of DSL (Digital
  Subscriber Loop) technologies
• ADSL (Asymmetric DSL) is most popular for
  residential access
• Higher speed into home than out of home
• More bits flow in ("downstream") than out
  ("upstream")
• The maximum speed depends on the length and
  quality of the copper in the subscriber loop

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ADSL - Configuration
Network Interface Device
Access Multiplexer

• Takes advantage of higher frequencies on most
  local loops
• Can be used simultaneously for POTS (Plain Old
  Telephone Service)

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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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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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Access with Cable Modem

• Cable TV already brings high bandwidth coaxial
  cable into the houses
• Cable modems encode and decode data from cable TV
  coaxial cable
• Instead of a traditional cable box, the splitter
  is installed in the home (directs the TV bands to
  the TV set and the Internet access bands to the
  PC)
• Bandwidth dedicated to the Internet is
  multiplexed among all users
• Usually the rate is asymmetrical (500 Kbps to 1
  Mbps from PC to Internet and 3 to 10 MHz in the
  oposite direction

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Cable Network Configuration
Traditional cable TV network
Hybrid fiber-coax (HFC) cable TV network
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Cable Frequency Spectrum

• TV band 54 550 MHz, 6MHz per channel
• Downstream 550 750 MHz
• 6MHz QAM-64 (6bit) 36 Mbps (gross), 27 Mbps
  (net)
• Total effective downstream bandwidth 200 / 6 27
  891 Mbps
• Upstream 5 42 MHz
• 6MHz QPSK (2bit) 12 Mbps (gross), 9 Mbps (net)
• Total effective upstream bandwidth 37 / 6 9
  54 Mbps

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Access through Fiber to the Curb
Infrastructure with fiber can be build especially
for Internet access
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Distribution with Optical Hierarchies

• SONET (Synhronious 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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Optical Hierarchies - Characteristics

• SONET and SDH are based on the principal of
  direct synchronous multiplexing.
• Provide advanced network management and
  maintenance features.
• Both SONET and SDH can transport signals for all
  the networks in existence today and it has the
  flexibility to accommodate any networks defined
  in the future.
• Can be used in the three traditional
  telecommunications areas long-haul networks,
  local networks and loop carriers. It can also be
  used to carry CATV video traffic.