VDSL: the future broadband platform? The most advanced DSL technology yet developed could support a nation's drive for broadband - Advertisement

Telecommunications International, August, 2002 by Stephen McClelland

Around the world, the drive is on to deliver technologies that will support future broadband services to both residential and business mass markets. In Japan, delivering mass-market broadband networks has become a national priority. Under plans by the Japanese government, a national programme is underway to stimulate the creation of an IT-based society and provide a world-leading communications infrastructure. One target is to accelerate the deployment of new backbone and access networks that could see 30 million households with high speed Internet access, and perhaps as many as 10 million households with ultra high speed Internet access by mid-decade. Broadband infrastructure will make it possible to deliver high performance applications from telcos, content providers, ISPs and ASPs. More sophisticated and varied possibilities will be available as new features such as IPv6 become widely implemented.

Providing for this mass market could mean the introduction of several different technology platforms in the country, including fibre to the building (FTTB) or home (FTTH), cable TV (CATV), fixed wireless access (FWA) and xDSL. Of these, xDSL is widely expected to dominate the marketplace, accounting for over 50% of subscribers. A key market segment is the huge number of multi-tenanted buildings with business and residential occupants in urban areas; this market segment may be very successfully served by VDSL (Very High Bit Rate Digital Subscriber Line) technology, the most advanced DSL technology platform available.

Leading vendors such as NEC are developing VDSL to meet the anticipated market needs. NEC believes it has now achieved a robust and state-of-the-art VDSL solution that has already been implemented in pilot projects in actual locations to demonstrate its utility. NEC experts say its current VDSL technology is the most high performance of any yet available in the marketplace. The NEC VDSL Broadband Access System makes a 51.2Mbps downlink rate (and a 6.4Mbps uplink rate) possible for asymmetric communication applications over 300 metre distances. Alternatively, symmetric data rates of 28.8Mbps for both uplink and downlink over the same distance are possible on a duplex basis. This VDSL technology implementation therefore is more than 800 times faster than current typical ISDN performance. VDSL speeds available through existing copper telephone lines effectively compare with those offered by optical fibre and are significantly greater than ADSL technologies that generally offer a maximum of 6Mbps downlink capability.

As a technology compatible with existing copper wire infrastructure, VDSL data transmission takes place in frequency bands well above those used by PSTN voice (generally the band up to 4kHz), and even those used by other DSL technologies such as ADSL. Utilizing extremely high bandwidth - near to radio waves, in fact - permits high data rate throughput over reasonable distances (around 300 metres). It performs impressively over longer distances too, offering, for example, 12Mbps at 1km.

However, a critical issue in any proposed VDSL deployment remains assured compatibility with existing high speed protocols and applications established on copper cable infrastructures. In practice, VDSL and any other ultra high bandwidth technologies will need to co-exist with other communications media and applications. VDSL data traffic exists in very high frequency bands and can be relatively easily separated from PSTN voice traffic by the use of a splitter, so voice and VDSL data on the same line is practicable. However, any high speed technology has to contend with cross talk and interference, from ISDN and other DSL transmissions in lower frequency bands, and radio interference in higher frequency bands. This interference has the potential to limit practicability and performance of the technology. NEC has developed a unique modulation scheme called Discrete Multi-Tone, a technique that uses multiple carriers with different frequencies in the VDSL frequency bands. Effectively, a single VDSL channel is divided into up to 255 different wavelengths, and because many carriers are used in the modulation, the data transmission is stable, high quality and offers a high data rate performance. Cross talk interference is largely suppressed near both the ADSL and radio bands. Overall, NEC experts say that DMT modulation techniques should offer superior speed and noise robustness compared to other modulation schemes such as CAP.

CONFIGURATION

Operation of the NEC VDSL system utilizes two units: the VF100T/VF100TD modem, and the VC1200T model, a VDSL concentrator for central hub connection to outlying VDSL modems. Individual concentrators have a capacity of 12 VDSL lines, and offer security features between individual VDSL ports and other ports through MAC filtering and VLAN functionality. The VC1200T is also configured with 10BASE-T and 100BASE-TX boards for uplink and downlink interfaces. All units feature internal POTS splitters for voice band separation. By utilizing existing 2-wire copper telephone cables, installation is eased and facilities can be shared, which is a significant advantage in multi-tenanted buildings. Network management is also made as simple as possible and is available through standard PC systems. The Ethernet-based interfaces enable interconnection to many other systems and units. The VDSL implementation is scalable and, in cascade configuration, can support up to 96 lines (effectively providing for up to 8 systems, each with 12 remote lines). Actual data rates may be adjusted to match those of external system demand; the VDSL system can be incremented in units of 64kbps up to 1.5Mbps capacity, and in 1Mbps increments above 1.5Mbps.