Telecom operators are today, responding to the competitive threat of cable
and satellite services by offering high-speed applications and converged
services to residential and business customers. These services generally require
minimum duplex bit rates in the range of 5—6 Mbps, both upstream and
downstream (symmetric).
While there are several digital subscriber line (DSL) technologies available,
none of them meet these high-bandwidth requirements. But help may be at hand.
With symmetric data rates of over 50 Mbps at 500—800 m, asymmetric rates of 20
Mbps at 2 km and long-reach (LR) capabilities that support 4 Mbps at 4 km, very
high-bit-rate DSL (VDSL) could well be the answer.
Operators may employ a number of DSL technologies, where each technology
comes with its own advantages and limitations. ADSL, for example, provides
typical downstream speeds of 1—4 Mbps and serves customers within a 5 km
radius from the central office (CO). ADSL2+ systems may offer as much as 24 Mbps
downstream rates at up to 1.5 km from the CO; however ADSL2+ is still limited by
narrow upstream bands that allow between 750 kbps and 1 Mbps data rates.
VDSL provides bandwidth flexibility based on distance and the band plan |
Single-pair high-bit-rate DSL (SHDSL) dramatically improves on ADSL, with
symmetric transmission rates up to 2.3 Mbps. However, the technology is still
geared toward data-oriented applications.
Some integrated communications solutions, including voice and
videoconferencing, can be delivered, but bandwidth resources fall short of
providing toll-quality video solutions. This is where VDSL comes into its own.
Not simply an upgrade to ADSL, VDSL gives bandwidth flexibility based on
distance and the band plan used:
- VDSL services include video and other revenue-generating, value-added
services. It is not just for fast Internet access, and it requires a much
higher, assured quality-of-service level. - VDSL offers much wider frequency bands than ADSL, along with high
frequencies with high attenuation and crosstalk that require powerful
equalization. - VDSL is typically deployed from a fiber trunk that reaches the curbside or
an in-building wiring cabinet. VDSL runs in elevator shafts, on in-building
wiring and in other communication-unfriendly places, and therefore must be
immune to long bursts of noise. - VDSL is typically deployed in short loops, ranging from 500 m in
fiber-rich countries and up to 1,500 m elsewhere - New LR-VDSL solutions successfully compete with ADSL in medium range
loops, providing 4 Mbps over 4 km of copper wires.
VDSL Operation
The VDSL lines bridge the gap between very high-speed backbone
infrastructures and customer premises equipment and/or integrated access
devices. Furthermore, with the ability to provide Ethernet over VDSL, telecom
operators can now connect a business LAN or an entire campus to a 10/40-Gbit
Ethernet wide-area network on the way to achieving an all-Internet Protocol
infrastructure. The variety of deployment scenarios of VDSL systems, including
fiber extension in fiber-to-the-curb (FTTC) or fiber-to-the-building (FTTB)
applications as well as the emerging long-reach VDSL services.
The different VDSL standards |
In new deployments and new networks, which represent a large portion of the
systems installed since VDSL technology became available in 1998, equipment and
service providers are free to choose the transport protocol. Ethernet is the
frequent choice, providing: connectivity, economies of scale for end users, and
creating a requirement for a simple, robust, plug-and-play VDSL solution.
On a per-link basis, Ethernet-over-VDSL implementations have reached pricing
parity with ATM-over-ADSL, making the technology very attractive for new
deployments.
Alternatively, when VDSL is deployed as an upgrade to existing DSLAM-based
systems, equipment providers and carriers find the higher per-link costs for ATM
transport cceptable. Thus, the choice of transport protocol is driven by
application requirements, not technology.
VDSL Standards
The VDSL standard-as defined in ETSI's TS 10127002, ANSI's T1.424, and
ITU's G.993.1-defines a four-band division structure. In China, however, a
three-band division has been proposed. These are illustrated in the figure
alongside. For service flexibility, the standardization bodies have defined band
allocation plans. Every country can have different service needs, but within
each region, spectral compatibility and band plan conformance must be kept, in
order to commit to the service offered.
VDSL Services
Triple-play Voice, Data and Video: One of the main advantages of VDSL over
competing Broadband access technologies is its ability to provide a number of
video (or television) channels simultaneously using single-pair copper wires. In
conjunction to this, VDSL still leaves ample bandwidth for high-speed data
transmission as well as analog voice, and, VoIP) services.
Lab test rate/reach results of a LR-VDSL system |
LAN-extension and Ethernet at Home: For the business user, Ethernet over VDSL
services, which extend local area networks (LANs) up to 1,200 m, offers a
cost-effective solution for connecting remote locations in a campus or
industrial park environment. EoVDSL can also be used for in-building
applications, turning every telephone jack into an Ethernet LAN connection. This
saves the time and money associated with deploying new CAT-5 cabling.
You simply hook up your modem to the wall-and use a standard 10BaseT
connector to plug in your computer. It's all plug-n-play from there. Business
centers, hotels, hospitals, and even residential high rises can now become
Ethernet ready within the matter of a few short hours.
LR-VDSL: Flexible LR-VDSL system solutions are an economical way for
delivering a wide range of upgradeable services, while preserving profit
margins, and giving service providers a break from the continuous upgrade cycle.
Using LR-VDSL, service providers can offer their customers modest 4/0.6 Mbps
services up to 4 km, gradually increasing up to fiber-like speeds at 50 Mbps
symmetric or even 100 Mbps asymmetric data rates over several hundred meters.
LR-VDSL is a future-proof technology. Emerging fiber-to-the-home (FTTH) and
FFTB services are expected to provide very high bit rates directly to the
customer premises.
Once fiber is deployed, VDSL will most likely be employed to fully exploit
the newly available bandwidth. When this happens, LR-VDSL enabled systems will
simply go on to support full-scale VDSL services saving the need to replace the
CO as well as the customer-side equipment.
It's All About Cost
One might say, "If it's so good, it must cost a lot." The truth
is that it doesn't. Already the price-per-port for VDSL systems is comparable
with ADSL. As the technology advances, costs continuously come down. Higher
density solutions increase the number of subscribers that a system can support,
without inflicting on footprint and higher levels of integration reduce the
overall bill of material.
Combined with VDSL's very high bandwidth and new long-reach capabilities,
operators gain a powerful tool that will enable them to better serve their
private as well as business users and successfully compete with rival broadband
access service providers.
Thomas Simonis managing
director, Infineon Technologies India