BROADBAND: Vroom, Vroom...VDSL

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.

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.

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.

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