VDSL

VDSL (Very high speed Digital Subscriber Lines) is based on the same underlying technology as ADSL2þ. It is the newest and most sophisticated DSL technology at the time of writing and was developed to overcome the shortcomings of previous versions of ADSL access technologies. It eliminates last mile bottlenecks and supports huge data rate capacities that allow service providers to comfortably offer IPTV subscribers a whole range of services including video-on-demand and Multichannel high definition TV broadcasting. VDSL was also designed to support the transmission of ATM and IP based traffic over the copper loop that is very useful for providers who want to migrate their legacy ATM networks over to an IP based infrastructure. There are different members of the VDSL family:
VDSL1 This avor of DSL was ratified in 2004. It operates at an upper limit of 55 Mbps on the downstream and 15 Mbps over the upstream channel. It does however have a very short range and is typically installed inside multiple dwelling units (MDUs).
VDSL2 is an enhancement to VDSL1 and is defined in the ITU-T Recommendation G.993.2. It can be subclassified into VDSL2 (Long Reach)and VDSL2 (Short Reach)VDSL2 (Long Reach) Owing to the fact that DSL is highly dependent upon the length of the local loop, a version of VDSL was created to deliver VDSL services to as many customers as possible, while enjoying the high speed capacities of the VDSL broadband access platform. In fact, VDSL with enhanced long reach capabilities can provide IPTV subscribers with 30 Mbps broadband access at distances of between 4000 and 5000 ft (1.2 1.5 km) away from the CO. These long reach capabilities are enabled through theuse of relatively high power levels when transmitting the data. A frequency spectrum of 30 MHz is used to achieve the high throughput as opposed to the 12 MHz frequency range used by VDSL1. Advanced error correction mechanisms also help to improve the reliability of end-to-end VDSL2 connections.
VDSL2 (Short Reach) Based on the DMT modulation scheme, the technology uses 4096 tones that are separated by 4 and 8 KHz frequency bands. The VDSL2 standard uses channel bonding techniques to allow it to operate at over 12 times the speed of the original ADSL standard, namely, a massive 100 Mbps on the downstream path over relatively short distances approximately 1200 ft (350 m). Although the data rates do not typically reach 100 Mbps on the upstream path, the rates do exceed the upstream data rates of ADSL2þ based networks. These performance levels make the assumption that no interference exists on the copper cable and the quality of the cable is quite good. The ability to provide IPTV subscribers with 100 Mbps access service enable operators to start offering a wide variety of advanced interactive services to their customers.
New VDSL2 features such as advanced quality of service (QoS), dual latency(the ability to segregate delay sensitive data such as IPTV multicast traffic), andimproved coding techniques are all particularly suited to the delivery of triple-playapplications. One final benefit of VDSL that solidifies its position as the ultimateDSL technology is its backward compatibility and interoperability with previous versions of ADSL networks. This enables IPTV providers to smoothly andefficiently migrate to VDSL based next generation networks.
There are two main approaches used by IPTV service providers when integrating VDSL2 into their existing networking infrastructure. The first approach is to add new VDSL2 equipment to the regional office and allow the DSLAM run in parallel to the existing ADSL and ADSL2 DSLAM systems. The various avors of DSL will subsequently continue in operation. The second approach is to locate the VDSL2 equipment closer to the IPTV subscriber. Possible locations for the new euipment include street cabinets and underground chambers for new housingestates. Table 2.2 provides a feature comparison between the various avors of DSLtechnologies used to carry IPTV signals.
The main advantage of DSL for IPTV systems is the fact that it utilizes theexisting wires that already run into most houses around the world. On the negativeside, all DSL systems have to make a trade-off between distance and bandwidthcapacity. In other words the DSL access speed reduces as the distance between theIPTV subscriber and the CO increases.Cable TV operators have mad e significant investments in the recent past to upgradetheir networks to support advanced communication services such as IPTV. Tounderstand the delivery of IPTV content over a cable TV network and to put thetechnology in context, it is first necessary at least on a high level, to review thebasic principles of hybrid fiber networks and traditional digital TV transmissiontechnologies.