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VDSL - Frequently Asked Questions
Why VDSL?
- With downstream speeds of up to a blazing 52 Mbits/s, VDSL (Very high speed Digital
Subscriber Line), is the next step up the speed ladder beyond ADSL. However, the price
paid for VDSL's increased speed is a shorter distance range. Additionally, VDSL comes in
two variants, a symmetrical version and an asymmetrical version. Here is one of the key
differences from ADSL, which usually only comes in a single asymmetrical version although
some telcos may offer a 384 Mbits/s or 500 kbits/s symmetrical version.
VDSL technology
will provide the final delivery of data over the existing copper drop, most likely fed by
a high capacity fibre optic link from a central site but occasionally directly from a
central office or switch.
Over short ranges, VDSL can offer up to 52 Mbits/s downstream capacity compared to the
ADSL capacity of up to 8 Mbits/s; upstream, the asymmetrical versions of VDSL offers a
slower data rate but still proportionately higher than for ADSL.
Why Develop VDSL?
- ADSL is a great, always-on, service; however, the maximum speed available downstream is
around 8 Mbits/s with a lesser upstream speed. VDSL extends this downstream speed to a
potential 52 Mbits/s with a proportionately lower upstream speed but at the price of a
shorter distance range than ADSL.
Ideal applications for VDSL include:
- provision of multiple TV channels within apartment blocks;
- ultra-high speed data access;
- campus style data distribution over short distances;
- video conferencing;
- streaming video;
- combined data and video over the same line;
- the final drop for a full service network architecture.
When Will VDSL Become Available?
- Today, it is impossible to tell when VDSL will become widely available. However, with
the current rate of technical progress it seems likely that there will be major trials
taking place before the end of 1999 and limited commercial services introduced before the
end of 2000.
Since VDSL comes in two forms - asymmetrical and symmetrical - unlike ADSL
which is usually only asymmetrical, one needs to look separately at these two different
flavours of service. Most early uses of the symmetrical service seem likely to be in the
corporate sector while early users of the asymmetrical services are likely to be in the
multi-dwelling or campus environment. In the former case, VDSL can offer a cheap way to
use the existing copper infrastructure to transmit data to / from the final drop, while in
the latter case, the service is more likely to centre on a mixture of Internet / remote
LAN access and multiple TV delivery.
What Will VDSL Do?
- VDSL will open up new possibilities in two key areas:
- corporate data distribution - symmetric;
- very high speed downstream capacity for VoD etc. for users, typically in multi-dweller
apartment blocks.
In both cases, VDSL will use the existing copper telephone lines for the final drop;
these lines will be fed by high capacity fibre optic connections from the host or remote
site. Over short distances, VDSL will offer a speed increase of around 5 times over ADSL.
Over longer distances, VDSL will offer a speed increase of 2 to 3 times but VDSL's
distance range will always be less than that for ADSL.
ADSL Forum Objectives With VDSL?
- There is a high degree of synergy between ADSL and VDSL; the same companies are working
on both technologies and the same individuals within those companies are also involved.
Thus, it is sensible that the ADSL Forum should address the development and deployment of
VDSL; this will make the best use of resources, people and time.
Additionally, the VDSL
development time-line runs behind that for ADSL; as a result, as the work-load on ADSL
falls off, so the work-load on VDSL will continue building. It is also important to ensure
that there is the maximum re-use of technology and solutions passing from ADSL to VDSL.
The overall objective of the ADSL Forum adopting VDSL is to consider the architectural
and VDSL systems aspects of a hybrid fibre / copper VDSL-based broadband access solution.
What Are the Differences Between ADSL and VDSL?
- Speed is the main differentiation between ADSL and VDSL, the latter being significantly
faster over short distances. Both ADSL and VDSL offer high speed data transmission over
existing copper telephone lines. ADSL, as its name implies, offers asymmetrical data
transmission with a ratio of about 10:1, downstream to upstream capacity. ADSL also offers
a maximum range of up to 18,000ft (say 5.5 km) albeit at a relatively slow downstream data
rate of around 2 Mbits/s; below about 9,000ft (say 2.7 km) a data rate of 6+ Mbits/s is
achievable.
VDSL, although not yet standardised, will certainly offer both symmetrical
and asymmetrical versions; within these two versions, there will be a variety of speeds.
Like ADSL, there is a trade-off between speed and range - the higher the speed, the
shorter the range. The ultimate form of asymmetrical VDSL will offer up to 52 Mbits/s
downstream and up to 6.4 Mbits/s upstream, only over some 1,000 ft or 300 m, but as the
data rate requirement reduces, then the range would increase. On a symmetrical basis,
speeds of up to 34 Mbits/s in both directions are possible, with the same range
restrictions as for the asymmetrical version.
Because VDSL gains in speed but loses in distance or range, there is a need for a high
capacity feed to bring the service to within say a maximum of 2 km from the user - this
feed is a fibre optic link terminating in an ONU (Optical Network termination Unit). At
the ONU, the signal is transferred from the fibre to the final copper drop to the
subscriber or user.
VDSL technology will provide the final delivery of data over the existing copper drop,
most likely fed by a high capacity fibre optic link from a central site but occasionally
directly from a central office or switch.
What Is The Current Status of VDSL Standards?
- Today, there are no accepted VDSL standards. However, ETSI and ANSI, the European and
American standards bodies, are both actively working towards a standard for VDSL; both
bodies are actively co-operating in this effort. In addition, DAVIC (the Digital Audio
VIsual Communication forum) had been actively working to define a specific physical layer
implementation of VDSL.
There is also an active group of telcos working under the
nomenclature of FSAN (Full Services Access Network) which has developed a VDSL
requirements specification. While this specification does not amount to a standard, such
an influential group of telcos will undoubtedly have a major influence on the formal
standards bodies. Also, since the FSAN group incorporates many of the major purchasers of
VDSL, their specified requirements amount - de facto - to the minimum standard their
vendors much achieve.
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