VDSL2 Overview and Tutorial

VDSL2 (Very high bit rate Digital Subscriber Line 2), a performance-improved version of the original VDSL now known as VDSL1, is standardized by the ITU in G.993.2.  A relatively new local loop access technology, VDSL2 provides broadband access rates up to 100 Mbps over twisted pair copper.  VDSL2 is important mainly because it leverages all that expensive copper local loop in the ground to provide a compelling broadband Internet service.   It is well suited to deployments on the curb (FTTC) or in a building (FTTB) and can be used for Fiber to the Node (FTTN).  The diagram below shows a typical VDSL FTTC network architecture.

VDSL2 faces a rather large challenge, however, because many copper local loops are too long to make effective use of all the capabilities of VDSL2, which relies on higher frequencies that are filtered out on longer local loops.  VDSL2 chipsets provide a fallback mode to ADSL2+ because of this.  ConvergeDigest offers the graph below showing a comparison of ADSL2+ and VDSL2 performance over distance.  According to this graph, it  is obvious that VDSL2 provides only marginal benefits over ADSL2+ beyond 4000 feet and no benefit beyond 7000 feet.

VDSL2 will often be deployed at a Feeder Distribution Interface (FDI) because of easy access to local loops at this point, though some of the local loops can be a bit long for optimal VDSL2 performance.  A picture of an FDI (just about a mile from my house) is shown below.

When VDSL2 is deployed at an FDI, this architecture is called Fiber to the Node or FTTN. A typical FTTN network architecture is shown in the drawing below.

VDSL2 is implemented with what are called profiles, which specify what subset of VDSL2’s total capabilities are to be used in a particular installation.  Differerent profiles are optimized for different situations.  Some profiles are for Central Office (CO) VDSL2 deployment, some are for OutSide Plant (OSP) cabinet deployments of VDSL2, and some are for in-building deployments of VDSL2, especially the higher rate ones.  VDSL2 profiles 8a, 8b, 8c, and 8d use 8 Mhz to provide data rates up to 50 Mbps.  Profiles 12a and 12b use 12 Mhz to provide bandwidths up to 68 Mbps. Profiles 17a (17 Mhz) and 30a (30 Mhz) provide bandwidths up to 100 Mbps, but are intended only for very short distances such as those common with in-building (FTTB) VDSL2 installations (see below).

VDSL2 is expected to be more successful than VDSL1, which has been around a while but has seen only limited deployments.  The following table, from RubyTech, compares VDSL1 and VDSL2.

VDSL and VDSL2 Comparison

Sunrise Telecom offers this screenshot below from their MTT – ACM II to show how VDSL2 is susceptible to bridge taps.  The screenshot also shows how other technologies like ADSL and ADSL2+ do not require the same range of frequencies as VDSL2 does, though VDSL2 can operate at higher levels of attenuation.  A bridge tap 15 feet in length created the notch shown and centered at about 9.7 MHz.

Sunrise Telecom MTT - ACM II Screenshot

The following companies are prominent manufacturers of VDSL2 chipsets.

• Broadcom
• Conexant
• Infineon
• Ikanos

Ericsson provides more technical information in this article on VDSL2.

© 2009, The Product Group LLC. All rights reserved.

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1. cyberdoyle says:

   September 26, 2009 at 3:17 pm

   7000 feet = 1.32575758 miles
   not very far then huh? suppose stopgap for cities then, but not much good for rurals. shame. will have to carry on lobbying for Fibre to the home.
2. admin says:

   September 26, 2009 at 4:12 pm

   Those numbers are a bit optimistic as well. For VDSL2 to be useful, it has to be fed with fiber and deployed very close, and rural areas do not typically have the houses in enough proximity to make this economically feasible.