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1 Elementary Knowledge
As the basis for subsequent chapters, this chapter introduces the elementary knowledge
of VDSL and ADSL, including the basic principles of VDSL and ADSL, ATM
technology and ADSL, together with common access modes of VDSL.
1.1 Overview
With rapid development of Internet and gradual finalization of backbone access layer
networks, the so-called user-oriented “last mile” access has become more and more
important. To adapt to new trends and meet new demands, multiple broadband access
technologies have emerged. In terms of transmission media, there are multiple wired
access technologies, such as copper wire access technology, fiber access technology,
and Hybrid Fiber Coaxial (HFC) access technology, and wireless access technologies.
The copper wire access technology, applied most widely, includes two wiring modes.
One is to lay Category 5 twisted pairs and establish a switching Ethernet; the other is to
fully employ precious resources of the original copper wires (telephone subscriber
lines), and adopt various technologies of high-speed modulation and coding for
broadband access. The popular copper wire technologies at present are described as
follows.
1. LAN access technology
The transmission media of Ethernet/fast Ethernet include twisted pairs, coaxial
cables or fibers. At present, twisted pair networks with the optimal
cost-performance ratio have been widely applied. The Ethernet technology
transmits directly baseband digital signals over transmission media, for example,
up to a 100-meter transmission distance over Category 5 cables. Its
disadvantages include short transmission distance, excessively scattered
equipment, high-cost initial investment, inconvenient management, and
necessary cable redistribution.
