Print

Comment

Email

Digg

Del.icio.us

Reddit

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