The Magic Is Created Behind the Scenes

VDSL2 Sets the Stage for the Video Promise

VDSL2 (G.993.2) is one of the most highly anticipated standards to emerge from the International Telecommunications Union (ITU-T). It is also one of the most complex, with numerous profiles and bandplans detailing everything from geographic-specific requirements to variations of reach and bandwidth.

This article offers insight into the history of the standard, its bandplans, profiles, architectures, and new features. The major driving force behind VDSL2’s success is IPTV. Interestingly, the same intent was behind DSL when it was developed in the late 1980s. Its original function was to deliver video over copper. However, the bandwidth required was more than early generations of ADSL could offer, so the emphasis was put on data communications over DSL.

High-speed Internet access was a killer app for several years. Now we have moved into the next phase of broadband: HD IPTV. With the introduction of VDSL2, there is finally a standardized way to make this a reality for service providers.

Developing the Script

With the introduction of VDSL2, the industry is seeing a major transition from high-speed Internet, which is primarily a data-only service, to moving customers beyond the PC into other realms of entertainment and non-traditional web interfaces. This will open new lines of revenue and unlimited opportunity for carriers and service providers. The addition of IPTV will also help level the playing field, allowing carriers and service providers to compete with the cable companies that are driven to steal providers’ market share by delivering voice services.

1.VDSL

Efforts to standardize VDSL began in 1995 with several simultaneous projects initiated in the ETSI, ITU, and T1E1.4 organizations. In 1997, a group of service providers from around the world joined together in an organization known as the Full Service Access Network, or FSAN. This group, which was led by British Telecom, developed the first VDSL end–to-end requirements specification. However, the industry was not able to capitalize on FSAN architecture due to on-going debate of which VDSL modulation code (QAM or DMT) should be used. Separate camps from QAM and DMT proponents limited VDSL’s standardization. Therefore, VDSL had limited deployment in Korea and Japan, primarily in multiple-dwelling unit (MDU) applications.

Then in 2003, 11 major silicon suppliers collectively announced support for the DMT line coding for standards-based VDSL, and the ITU finally ratified G.993.1, which is known as "VDSL" or "VDSL1". The standard was unique in that it supported DMT in the main body of the specification and QAM in a normative annex. It was also agreed that any new features and functionality to VDSL1 would be added to a second-generation standard, VDSL2, and that VDSL2 would only contain the DMT linecode.

2. VDSL2

The VDSL2 standard (G.993.2) was initiated in by the ITU in January 2004, with input from the North American ANSI and ETSI standards. It reached consent at the Geneva meeting in May 2005 and was approved in May 2006. The underlying DMT modulation code is the same as ADSL and ADSL2+, providing spectral compatibility with existing services and enabling backward-interoperability with ADSL.

ANNEXES

As in legacy ADSL and ADSL2+, VDSL2 includes regional bandplan annexes that specify PSD Mask for numerous regional bandplans, and are designed to provide coexistence with other services. Annex A specifies bandplans for the North American region and enables VDSL2 to be deployed with traditional POTS telephony or in an all-digital mode (similar to Annex J in ADSL2). Annex B specifies bandplans for Europe and enables VDSL2 deployment with underlying POTS and ISDN services. Annex C allows VDSL2 to coexist with TCM ISDN services, found primarily in Japan.

BAND PLANS

VDSL2 is a true worldwide standard which has been developed to support the next generation of advanced services. VDSL2 has numerous configuration profiles and bandplans to meet regional service provider requirements. The frequency bandwidth has increased to 30 MHz, with configuration options at 8.5 MHz, 12 MHz, 17.7 MHz, and 30 MHz. VDSL2 also defines asymmetric (Plan 998) and symmetric (Plan 997) bandplans for the transmission of upstream and downstream signals as well as the North American Plan 998 Extension (Figure 1).

PROFILES

To simplify the task of configuring network equipment, the VDSL2 standard defines profiles tailored for different regional deployment architectures such as central office, remote DSLAMs, digital loop carriers, and MDUs. There are 8 profiles which define power options from 11.5 dBm to 20.5 dBm, bandwidth up to 30 MHz, and a minimum data rate for each profile (Figure 2).

The Scene Is Set

The rate and reach charts in Figures 3, 4, and 5 show the expected downstream and upstream performance of VDSL2 using the 30a, 17a, and 8b configuration profiles. The rates are in the presence of crosstalk over 26 AWG Gauge wire. Figure 3 shows the VDSL2 (30MHz) configuration, which achieves data rates up to 100 Mbps over very short loops.

Because the rate declines rapidly, this configuration is ideally suited for MDU and fiber-to-the-home applications. Figure 4 defines the expected performance for a 17a configuration which is ideally suited for a fiber-to-the-curb or fiber-to-the-cabinet application. Figure 5 shows the data rate for the 8b profile, which is ideally suited for longer reach applications such as remote DSLAMs or fiber-to-the-cabinet deployments.

Seven of This Star’s Best Features

Similar to any other star that takes the stage, VDSL2 has more than a few properties that give it the "wow" factor. Seven are listed below:

1. Improved Rate and Reach Using Upstream "0" Band
Whereas VDSL1 systems typically operate to approximately 3 Kft, the goal of VDSL2 was to provide loop reach out to 9 Kft. In order to achieve this, VDSL2 had to be redesigned with certain features from ADSL systems, which provide service over very long loops.

Traditionally, VDSL1 systems use a technique called digital duplexing for maximizing bandwidth utilization in the upstream and downstream directions. But, digital duplexing works well only on shorter loops, and does not work well when the first "Upstream 0" (US0) Band between 20 and 138 kHz is used.

As a result, the VDSL1 initialization procedure had to be completely redesigned for VDSL2 to include new Channel Discovery and Transceiver Training phases. The Channel Discovery phase enables the VDSL2 transceivers to identify the loop conditions (e.g., loop length and noise) and select the best mode of operation (e.g., US0 enabled, FFT size, etc.).

2. Impulse Noise Protection (INP)
The copper loop plant is susceptible to short impulses caused by external sources. These impulses cause large bursts of errors, which could have a significant impact on the video picture quality. In order to remove errors, VDSL2 has an impulse noise immunity (INP) capability. INP provides the ability to correct any impulse noise less than 250 microseconds (INP=2, 8 milliseconds latency). VDSL2 can even provide impulse noise protection values up to INP=16, which corresponds to the ability to correct any impulse noise that is less than 3.75 milliseconds.

3. Packet Transfer Mode – Transmission Convergence (PTM-TC)
In addition to classical ATM transport, VDSL2 supports the transport of packet-based services (e.g., Ethernet packets, IP packets, etc.) In particular, VDSL2 specifies a Packet Transfer Mode – Transmission Convergence (PTM-TC) function that is based on the Ethernet in the First Mile (EFM) IEEE802.3ah standard.

4. Common Management Interface With ADSL2/2+
For many service providers, VDSL2 is the next-generation technology offering after ADSL2/2+ systems. As a result, it is predicted that many of the VDSL2 devices sold will be capable of operating in an ADSL2/2+ mode. For this reason, VDSL2 was designed to provide a management interface that is virtually identical to that of currently deployed ADSL2 systems.

5. Dual Latency for Improved QoS
It is well known that different user applications have different physical layer QoS requirements with regard to data rate, latency, bit error rate, and INP. Since VDSL2 is specifically targeting to Triple Play services, the ITU standard specifies a Dual Latency operation mode. Dual Latency provides improved QoS features by enabling simultaneous transport of applications with different physical layer requirements. For example, Dual Latency can be used to transport a video channel with a high INP while simultaneously transporting a voice channel with very low latency.

6. Loop Diagnostics Modes Based on ADSL2/2+
After years of ADSL deployments, service providers have learned that DSL fault identification is challenging. To tackle the problem, ADSL2/2+ transceivers were enhanced with extensive diagnostic capabilities defined as dual-ended line testing or DELT. This solved many of the operator deployment problems. As a result, when the VDSL2 specification was initiated, service providers required that the VDSL2 standard contain the same diagnostic capabilities as in ADSL2/2+. These diagnostic capabilities provide raw data which can be interpreted by advanced diagnostics software for trouble resolution during and after installation, performance monitoring while in service, and upgrade qualification.

7. ADSL-Backward Compatibility
VDSL2 solutions will most commonly be multi-modal, interoperating with ADSL, and ADSL2+, as well as VDSL2 chipsets. This will allow service providers the flexibility to evolve their networks to support advanced services such as IP Video using a single customer premises solution.

Taking the Stage

To accommodate the bandwidth requirements of Triple Play and interactive services, service providers are extending fiber deeper into the access network. Typically, this is not fiber-to-the-home, but a hybrid approach whereby fiber is fed to a remote DSLAM or remote node, shortening the copper connection to the home to under 5 Kft. This approach leverages the existing copper infrastructure and bridges the bandwidth gap between fiber and copper while avoiding the cost and time of deploying fiber all the way to the premises.

Figure 6 shows a comparison of downstream data rates for ADSL2+, versus VDSL2 profile 8b, a common profile used in North America, for a service area between 1,000 and 5,000 feet. For customers beyond 5,000 feet, the data rates for ADSL2+ are superior to VDSL2. Remote DSLAMs will support multi-mode operation which requires central office equipment to be backward compatible and interoperable with ADSL, ADSL2+, and VDSL2 customer premises equipment.

From an OSP perspective, handheld test devices must be multi-mode ADSL2+ / VDSL2 capable, allowing technicians to use one test device to verify ADSL2+ or VDSL2 sync and performance across the entire network. In addition, adding DSLAM emulation capability to handheld testers will allow technicians to test from the remote node to determine the best VDSL2 profile, band plan, and parameter value for a particular subscriber line (Figure 2). DSLAM emulation will also allow the technician to test the benefit of upgrading a customer’s line to a newer ADSL2+ or VDSL2 DSLAM port.

In summary, the VDSL2 standard holds the promise of the next generation of broadband services over DSL. By maximizing bandwidth and utilizing its new features and functionality, it is an ideal access technology for delivering video. Carriers and service providers will now have a true competitive advantage in the broadband Triple Play space when video is combined with their extensive knowledge of the voice and data space.

Carriers realized years ago that in order to effectively compete in this market, they must not only include IPTV in their service offering, but also make the package more attractive to subscribers in order to attain video market domination, maximize revenues, and reduce churn through service bundles.

Peter LeBlanc is vice president of sales and marketing for Aware’s DSL business. He has more than 12 years experience in the DSL industry, including 3 terms as Board of Director of the Broadband Forum (formerly DSL Forum). For more information, please visit: www.Aware.com.