4Chapter DSL technology

• Broadband


  An 'always-on' data connection that is able to support interactive services including Internet access and has the capability of the minimum download speed of 512 kilobits per second (kbps) to an individual subscriber from the Point Of Presence (POP) of the service provider
• Broadband access technologies


  • Wired line
  • Wireless
• Wired line broadband access

  • DSL (Digital Subscriber Line)
  • Cable Modem
  • Optical Fibre Technologies
  • PLC (Power Line Communication)
• DSL (Digital Subscriber Line)

  Uses the existing twisted-pair telephone lines as the access media
• xDSL technologies


  • ADSL (Asymmetric Digital Subscriber Line)
  • VDSL (Very High-Speed Digital Subscriber Line)
  • RADSL (Rate Adaptive Digital Subscriber Line)
  • HDSL (High Data-Rate Digital Subscriber Line)
  • SDSL (Symmetric Digital Subscriber Line)
• ADSL (Asymmetric Digital Subscriber Line)


  A form of DSL that has the distinguishing characteristic that the data can flow faster in one direction (used for download streaming) than the other (used for upload streaming)
• How ADSL works


  1. Split the available bandwidth on the twisted copper wire into 3 channels: high speed downstream, medium speed upstream, POTS channel

  2. Use Frequency Division Multiplexing or Echo Cancellation to separate upstream and downstream data
• ADSL modulation


  Uses CAP (Carrier less Amplitude Phase Modulation) and DMT (Discrete Multi Tone), with DMT being the most widely used
• Common elements in ADSL

  • CPE (Customer Premises Equipment) containing Splitter and ADSL Modem
  • PC
  • Central Office Premises Equipment containing DSLAMs, MDF, PSTN
  • Aggregator and ATM core
• Factors determining ADSL connectivity

  • More distance from DSLAM to customer reduces data rate
  • Signal attenuation and Signal to Noise Ratio
  • Line impedance
• Data rate, wire size, and distance are related in ADSL