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WHAT IS VDSL?

VDSL

VDSL standardization work began in 1995 within ETSI, ITU and T1E1.4. In 1997, a group of service providers led by BT formed the FSAN (Full Service Network Access) organization and defined the first end-to-end VDSL conditions and architecture. However, VDSL did not materialize immediately as the debate over the modulation method (QAM or DMT) could not be finalized.

In 2003, 11 major silicon manufacturers collectively announced their support for DMT line coding modulation for VDSL-based standards and the ITU eventually published the G.993.1 standard, known as VDSL or VDSL1. This was a unique standard in that it used DMT in the main body and supported QAM addition.
VDSL2

The idea for the VDSL2 (G.993.2) standard was adopted by the ITU in January 2004. It was finalized as a concept at the Geneva meeting in May 2005 and approved as a standard in May 2006.

Basically the DMT modulation code was the same as ADSL and ADSL2+. It had spectral compatibility with legacy services and backward compatibility with ADSL.

BAND PLAN

Vdsl2 is a worldwide standard for the provision of forward-looking services. It has many configuration profiles and bandplans according to the needs of local service providers. It has 8.5, 12, 17.7 and 30 MHz bandwidth options and can be used symmetrically (plan 997) and asymmetrically (plan 998).

  • 8.5 MHz (Residential and Long Reach)
  • 12 MHz (Operating)
  • 17 MHz (Fiber to the Node)
  • 30 MHz (Indoor)

VDSL PROFILES

In order to simplify the configuration of network equipment, 8 different profiles with different power, bandwidth and communication speeds were created for equipment such as remote DSLAM, central office, multi-dwelling units.

  • Parameter 8a 8b 8c 8d 12a 12b 17a 30a
  • Bandwidth MHz 8.5 8.5 8.5 8.5 12 12 17.7 30
  • Tones D/S 1,971 1,971 1,971 1,971 2,770 2,770 4,095 2,098
  • Spacing KHz 4.312 4.312 4.312 4.312 4.312 4.312 4.312 8.625
  • TX Power D/S dBm +17.5 +20.5 +11.5 +14.5 +14.5 +14.5 +14.5 +14.5 +14.5
  • Max. Net Data Mbps 50 50 50 50 68 68 100 200

PERFORMANCE

Below is a performance comparison of VDSL2 30a, 17a and 8b profiles. The analysis was performed using 26 AWG Gaugekablo.

  • profile-30a: The Profile 30a VDSL2 (30 Mhz) configuration allows data transfer of over 100 Mbps over very short distances. Since the frequency spectrum is wide, the rate drops very quickly as the distance increases, so it is suitable for multi-dwelling units and direct-to-home fiber applications.
  • profile-17a: Profile 17a canfiguration is used for fiber to the curb and fiber to the cabinet applications.
  • profile-8b: Profile 8b is the most suitable profile for long distance data transmission. It is used in remote DSLAMs and fiber to the cabinet applications.

OFDM (ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING)

It is often used as coded OFDM (COFDM) or discrete multi-tone modulation (DMT). It is a method of frequency division multiplexing and digital multicarrier modulation. The idea of OFDM was introduced to overcome the bandwidth inefficiency of FDM by using subchannels that orthogonalize parallel data. It uses a large amount of low-space orthogonal subcarriers to carry the data. The data is divided into many parallel data channels. Each subcarrier is a channel. Each subcarrier is modulated with conventional low symbol rate modulation schemes such as QAM or PSK.

In OFDM, at the transmitter side, the data is converted from prioritized to parallel and modulated into subcarriers using Discrete Inverse Ouriertranfotm (IFFT). The subcarriers are orthogonal to each other, then the data is converted back to series and a cyclic prefix is added. The orthogonal and cyclic prefixing prevents inter-symbol interference and inter-channel interference. The cyclic prefix is chosen longer than the channel’s maximum delay spread to avoid inter-symbol crosstalk. The acquired data is converted to analog form and sent to the channel.

ADVANTAGES

  • Can easily adapt to harsh channel conditions without complex equalization.
  • Resistant to narrow-band co-channel interference.
  • Resistant to inter-symbol interference (ISI)
  • High spectrum efficiency
  • Efficient sampling using FFT
  • Low sensitivity to timing error
  • No need for tuned bottom channel filters

DISADVANTAGES

  • Sensitivity to Doppler frequency shift
  • Sensitivity to frequency synchronization problems
  • High peak to average power ratio (PAPR)