[1] xDSL Technology, www.tuketu.com/dsl/xdsl.htm
[2] Digital Subscriber Line, www.cisco.com
[3] HowStuffWorks “How VDSL Works”, www.howstuffworks.com/vdsl.htm
[4] HowStuffWorks “How DSL Works”, www.howstuffworks.com/dsl.htm