VDSL2 (Very-High-Bit-Rate Digital Subscriber Line 2), G.993.2, is the newest and most advanced standard of xDSL broadband wireline communications. Designed to support the wide deployment of Triple Play services such as voice, video, data, high definition television (HDTV) and interactive gaming, VDSL2 enables operators and carriers to gradually, flexibly, and cost efficiently upgrade existing xDSL-infrastructure. RubyTech introduced the industry’s first ully standard compliant VDSL2 solution. Allowing symmetric data rates of up to 100Mbits/s over short distances and ADSL-like long reach performance, VDSL2 does not just meet today’s requirements for high speed telecommunication networks, but offers a future proof technology that will allow carriers to deliver more services to a growing number of subscribers for years to come.
Why we need VDSL2
Looking at today’s xDSL infrastructure the access technologies are still the bottleneck between CO (Central Office) and subscriber. Central Office and Remote DSLAMs (Digital Subscriber Line Access Multiplexer) are supplied with Gigabit Ethernet links and even faster technologies while on the subscriber side we find 54Mbit/s and 100Mbit/s wired and wireless Local Area Networks (LAN/WLAN). But the link in between with either ADSL, ADSL2/2+ or VDSL still limits the rate. Furthermore, the combination of video/HDTV, data, and voice on a single medium, dubbed “Triple Play”, requires more sophisticated QoS (Quality of Service) features than are available in either VDSL1 or ADSL. Lastly, Triple Play services, with a minimum of three TV channels, several VoIP connections, and sufficient speed for Internet access, require high data rates of at least 30 – 40 Mbit/s, about twice of what can be achieved under real field conditions with today’s ADSL2+ solutions. VDSL2 was developed and standardized in record time to address the shortcomings of existing access technologies. It serves as the ideal xDSL technology for eliminating last-mile bottlenecks and to enable global mass deployment of advanced Triple Play services.
Key drivers for end-users to implement VDSL2 for broadband services are: Availability of multi-channel video, HDTV educational and entertainment services; Reduced telephony toll charges through the use of VoIP; Increased speed of basic Internet data access; and competitive pricing in regions where VDSL services are sold for the same price as ADSL with increased bandwidth (mainly Korea and Japan). Telecom carriers will benefit from offering bundled services that can successfully compete with anything cable operators have to offer. In addition, with the migration towards combined ADSL2+/VDSL2 networks (see below), a single network can be maintained to provide the entire xDSL services portfolio while reducing deployment, operating and maintenance costs.
Last but not least VDSL2 efficiently solves the last mile problem with cost efficient hybrid FTTH (Fiber-to-the-Home) networks: by bringing a fiber connection as close as possible to the home, to the DSLAM or DLC via existing tubes deployed in the groundwork and by connecting to high speed VDSL2 to bridge over the last few hundred meters.
Work on the VDSL2 standard began shortly after with the intention of enabling Triple Play services, including multi-channel HDTV and the VDSL2 standard, G.993.2, was approved and announced by the ITU-T on May 27, 2005. A fully compliant VDSL2 solution, provides ultra high symmetric data rates up 100Mbit/s over 1.2 - 1.5 km and ADSL-like bit-rates at distances of more than 4-5 km, thus covering over 90 percent of the households within a Telco network and in some countries 100 percent. Figure 1 shows VDSL2 downstream rate/reach performance in comparison with VDSL1 and ADSL2+.
Unlike its predecessor, which allowed choosing either DMT (Discrete Multitone) or QAM (Quadrature Amplitude Modulation) technology, VDSL2 only uses the DMT line-code. DMT is a method of separating a DSL signal so that the usable frequency range is separated into multiple small frequency bands, or tones. It uses up to 4096 tones which are spaced 4kHz or 8kHz apart. Each tone can be used for either downstream or upstream however the VDSL2 standard has defined upstream and downstream bands as well as eight different profiles for several application areas.
DSL1 in comparison to VDSL2
The new VDSL2 standard offers, compared to VDSL1, several new features. These added functionalities explained below enhance bit-rate and service coverage while ensuring high levels of QoS for voice and video transmission - hence facilitating the convergence of all broadband services onto a single network.
Higher bit-rates: A frequency spectrum of up to 30 MHz as opposed to 12MHz with VDSL1 and 17MHz in pre-standard solutions results in 100 Mbit/s symmetric datarates - much higher than the maximum of 70/30 Mbit/s in early VDSL1 solutions. In addition, on-chip bonding which means using two separate channels for one connection (also referred to as channel-bundling) increases loop reach performance and even supports VDSL2 bit rates for longer loops. Unlike the shared medium cable modem, VDSL2 provides a point-to-point connection which keeps the data rates per subscriber stable.
ADSL-like long reach (LR) performance: ADSL-like long reach performance is one of the key advantages of VDSL2. LR-VDSL2 enabled systems are capable of supporting speeds of around 1-4 Mbit/s (downstream) over distances of 4 to 5 km, gradually increasing the bit rate up to symmetric 100Mbit/s as loop-length shortens. This means that VDSL2-based systems, unlike VDSL1 systems, are not limited to short loops or MTU/MDUs only, but can also be used for medium range applications. LR-VDSL2 is implemented by using frequencies down to 25kHz in combination with the ADSL PSD mask supporting 20dBm transmit power. The upstream band in this case is allocated up to 138kHz/276kHz, also referred to as “Upstream Band-0” or “Extended Upstream Band-0”. The VDSL2 downstream band DS0 operates as an extended ADSL band, reaching as high as 3.75MHz as opposed to 1.1MHz and 2.2MHz used in ADSL and ADSL2+ respectively. An example of a VDSL2 bandplan is shown in figure 2. LR-VDSL functionality is ensured using Echo Cancellation and Time Domain Equalization. VDSL1 systems currently use digital duplexing as a method to compensate delay effects for longer subscriber loops. However this technique is limited by loop reach, has no effect above 3kft (about 1km) and would lead to a connection loss. Standard compliant VDSL2 systems therefore must implement Echo Cancellation and Time Domain Equalization to reach required performance for medium and long loop applications above 1-1.2km.
Advanced QoS for delay-critical applications: QoS is of particular concern for the continuous transmission of video and voice. Therefore the VDSL2 standard defines an inherent Pre-emption mechanism giving higher priority to the delay-critical voice and video packets over other data packets such as email messages, webpages etc.
Pre-emption essentially means that a high priority packet (such as voice) will always have a “right of way” over lower priority packets. Pre-emption can be explained using the example in figure 3: data packets with different priority (voice, data, video, gaming) have to be transmitted through the VDSL2 chip. In order to provide the desired quality (no echo, no delay for on-line gaming) certain delay requirements have to be met. In example A, where a 100Mbit/s connection is assumed, there is no delay interference between the red high priority voice packets and the yellow low priority data packets. In example B however, where a lower speed 1Mbit/s link is assumed, delay becomes an issue as the packet transmit time increases due to the lower speed. The transmit time of packet #2 violates the timing requirements of the voice packet and would therefore cause jitter in the voice packet transmission which means echo in a voice link. This will be the case with any regular system using first-in-first-out (FIFO) technique. A Pre-Emption enabled system however will stop the transmission of the low priority data packet until the transmission of the high priority voice packet is completed. Through this feature disturbing echo in a voice call can be avoided.
Optimised network utilization: Through the implementation of powerful Trellis/Viterbi coding - not available in VDSL1 - and flexible ADSL-like framing, VDSL2 provides efficient utilisation of line-rate, especially in the downstream mid-range and long reach. Trellis/Viterbi coding gives an improved coding gain of 2dB. Therefore VDSL2 achieves a higher bit-rate compared to VDSL1 under equal signal-to-noise conditions.
ADSL backwards compatibility and interoperability: Due to its DMT nature and ADSL-like functionalities mentioned above, including Framing, Interleaving, and Trellis/Viterbi coding, VDSL2 includes all technical prerequisites to address the ADSL backwards compatibility issues ideally. A fully VDSL2 standard compatible chip solution would be able to perform as both a VDSL2 as well as an ADSL/2/2+ device. This enables operators a smooth, gradual and efficient network transition towards VDSL2 with only a single technology and allows them to cover all xDSL service applications with a single network. At the same time subscribers can still use their ADSL modems as long as they wish to and receive advanced Triple Play services by simply upgrading their CPEs (Customer Premises Equipment). This results in lower infrastructure and maintenance costs and a seamless upgrade to VDSL2.
However, technical functionality is only one part of the equation. The other important and difficult factor is interoperability with the installed ADSL infrastructure. This is not provided by the standard but is mostly determined by the ADSL know-how and field experience of the particular silicon vendor. Major ADSL players have invested years of effort into interoperability and only continuous participation at interoperability events and partnerships between industry leaders ensure that end customers can use any equipment without worrying about technical compatibility. Therefore, only the combination of technical backwards compatibility and interoperability to ADSL, ADSL2, and ADSL2+ will allow service providers to install VDSL2 as the ultimate DSL technology and utilise the financial advantages through lower operating and maintenance costs.
VDSL2 was developed and standardized in record time to address the shortcomings of existing access technologies. It serves as the ideal x DSL technology for eliminating last-mile bottlenecks, enabling global mass deployement of Triple Play services and allows carriers to efficiently compete with cable providers. VDSL2 provides ultra high data rates of up to symmetric 100Mbit/s that are already a mandatory requirement in regions like Korea and Japan for service upgrade. Additionally, with its enhanced longreach capabilities, a VDSL2 system can provide ADSL-like bit-rates at distances over 4-5 km hence covering more than 90% of the households within a Telco network and in some countries even 100 percent. Thanks to its ADSL2+ backwards compatibility VDSL2 enables a smooth, gradual and flexible upgrade of existing networks according to market demand and various regional network requirements.