CSCI 45508556 Computer Networks

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CSCI 4550/8556Computer Networks

• Comer, Chapter 12
• Long Distance Digital Connection Technologies

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Introduction

• Previous technologies cover short distances
  (e.g. campus, building, a few city blocks)
• They can be extended over short distances.
• We also need to cover longer distances - e.g.,
  San Francisco to Boston
• The name for long distance technology is WAN -
  Wide Area Network
• There are two categories
• Long distance between other networks
• Local loop

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Digital Telephony

• The telephone system spans long distances.
• The use of digital telephony improves long
  distance service
• Better quality than analog technology
• More logical connections in the same wire than if
  analog technology is used.

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Digitizing Voice

• Problem how to encode an analog audio signal as
  digital data
• Solution
• Sample the audio signal at periodic intervals
• Convert the samples to digital using an A-to-D
  converter
• Send the digital data samples over wire (or
  fiber, or )
• Regenerate the audio using a D-to-A converter

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Example

• The continuous line shows one possible analog
  signal (e.g. voice).
• The vertical lines show possible digital
  equivalents resulting from periodic sampling of
  the analog signal.

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Sampling Parameters

• We want to carry signals up to 4000Hz (recall the
  telephone system bandwidth)
• To do this, we select a sampling rate of 8000Hz
  (twice the maximum frequency in the source)
• Each sample is in the range 0-255 (so we can use
  8 bits per sample).
• The standard for this is called Pulse Code
  Modulation (PCM).

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Synchronous Communication

• Converting these samples back to audio requires
  that the data be available on time.
• Digital telephony systems use clocking for
  synchronous data delivery.
• The samples must not be delayed as network
  traffic increases.

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Using Digital Telephony for Data Delivery

• So, digital telephony can handle synchronous data
  delivery.
• Can we use that for arbitrary data delivery?
• An Ethernet frame is not 8-bit PCM synchronous
  data.
• Thus, to send Ethernet frames we need to convert
  data formats...

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Conversion for Digital Circuits

• To use digital telephony for data delivery
• Lease a point-to-point digital circuit between
  sites and
• Convert between local and PCM formats at each
  end.
• The conversion uses a Data Service Unit / Channel
  Service Unit (DSU/CSU) at each end of the line.
• CSU - manages control functions
• DSU - converts data

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Using a DSU / CSU
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Telephone Standards

• Several standards exist for data transmission
  rates in telephone systems.
• These are called the T-series standards, and are
  similar throughout the world.

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Intermediate Capacity

• The price for a leased line does not go up
  linearly with speed.
• For example, the cost for a T3 line is less than
  the cost for 28 T1 lines.
• However, if all you need is 9 Mbps, the cost for
  a T3 line is greater than the cost for 6 T1
  lines.
• Solution combine multiple T1 lines with an
  inverse multiplexor.

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Higher Capacity Circuits
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About the Terminology

• T-standards define the underlying bit rate
  Digital Signal Level standards (DS standards)
  define
• how to multiplex calls
• The effective bit rates
• A T1 line transmits data at DS-1 rate
• Synchronous Transport Signal (STS) standards
  define high speed connections over copper,
  Optical Carrier (OC) standards are for fiber
• The C suffix indicates concatenated
• OC-3 three OC-1 circuits at 51.84 Mbps
• OC-3C one 155.52 Mbps circuit

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SONET

• Synchronous Optical Network (SONET) defines how
  to use high-speed connections
• Framing STS-1 uses 810 bytes per frame
• Encoding Each sample travels as one octet in
  payload
• Payload changes with data rate
• STS-1 transmits 6,480 bits in 125 microseconds
  ( 810 octets)
• STS-3 transmits 19,440 bits in 125 microseconds
  (2,430 octets)

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Getting To Your Home

• The term local loop describes the connection from
  a telephone end office to your home.
• This is sometimes called POTS (Plain Old
  Telephone Service).
• The legacy infrastructure is copper wires.
• Other available connections to your home include
  cable TV, wireless, and electric power.

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ISDN (Integrated Services Digital Network)

• Provides digital service (like T-series) on
  existing local loop copper wiring.
• Three separate circuits, or channels, in a
  typical home connection
• Two B channels, 64 Kbps each equivalent to two
  voice circuits
• One D channel, 16 Kbps used for control
  functions
• Often written as 2BD called Basic Rate
  Interface (BRI)
• ISDN has been slow to catch on, because
• It is/was expensive.
• It is charged by time used.
• It is now (almost) equaled by analog modems.

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DSL (Digital Subscriber Line)

• DSL is a family of technologies.
• It is sometimes called xDSL.
• It provides high-speed digital service over the
  existing local loop.
• One common form is ADSL (Asymmetric DSL).
• It has a higher speed into a home than out of it.
• More bits flow in (downstream) than out
  (upstream).
• ADSL maximum speeds
• 6.144 Mbps downstream
• 640 Kbps upstream

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ADSL Technology

• Uses existing local loop copper
• Takes advantage of higher frequencies on most
  local loops
• Can be used simultaneously for POTS

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Adaptive Transmission

• Individual local loops have different
  transmission characteristics
• Different maximum frequencies
• Different interference frequencies
• ADSL uses FDM
• 286 frequencies
• 255 downstream
• 31 upstream
• 2 control
• Each frequency carries data independently
• All frequencies are outside the audio range
• The bit rate adapts separately to the quality in
  each frequency

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Other DSL Technologies

• SDSL (Symmetric DSL) provides divides frequencies
  evenly.
• HDSL (High-rate DSL) provides DS1 bit rate in
  both directions
• Useful over short distances
• Requires a four wire circuit (POTS is two wires)
• VDSL (Very high bit rate DSL) provides up to 52
  Mbps
• Very short distance
• Requires an Optical Network Unit (ONU) as a relay
  

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Cable Modem Technologies

• Cable TV already brings high bandwidth coaxial
  cable into your home.
• Cable modems encode and decode data from cable TV
  coaxial cable
• One modem in the cable TV center connects to the
  network
• One modem in your home connects to your computer

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Features of Cable Modems

• The available bandwidth is multiplexed among all
  users.
• It is a multiple access medium (like Ethernet)
  your neighbor can see your data!
• Not all cable TV coaxial cable plants are
  bidirectional thus not cable TV systems can
  support networking as well as television.

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Upstream Communication

• Cable TV requires sending in one direction only.
• The signal is broadcast from a central location.
• Amplifiers boost the signal as it travels through
  the network
• Amplifiers are unidirectional, so how can data
  travel successfully opposite to television
  signals?
• Solutions
• Retrofit the system with bi-directional
  amplifiers.
• Use an alternate upstream path - e.g., dialup.

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Alternatives

• In addition to POTS, ISDN, xDSL and cable TV
  coaxial cable, there are other alternatives for
  connecting residences to networks.
• Satellite systems, either per subscriber or per
  neighborhood
• Fiber to the curb
• Fiber to each subscribers home is much too
  expensive
• Fiber to the neighborhood is a potential, then
  use short-distance copper local loops to provide
  connectivity to fiber cable.

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Summary

• WAN links between sites use digital telephony
• Based on digitized voice service
• Several standard rates
• Requires conversion via DSU/CSU
• Local loop technologies
• ISDN
• xDSL
• Cable modem
• Satellite
• Fiber to the curb