For many years, telcos have touted video as the "killer" application that would reduce churn and improve the bottom line. However, it was not until recently that competitive pressures and technological advancements have combined to make Internet protocol television (IPTV) a reality.

Telcos are facing increasingly stiff competition from cable multiple system operators (MSOs) and direct broadcast satellite operators, which are siphoning voice revenues with bundled, triple play offerings. However, new technologies – such as MPEG-2, MPEG-4, H.264/AVC, and WMV9 compression – are giving telcos an advantage by enabling multiple streams of video across available bandwidth. Advancements in access technologies also are improving the prospects for delivering video services. VDSL2 technology is expanding the bandwidth to individual subscribers, enabling delivery of multiple high definition television (HDTV) streams to a single residence.

VDSL2 technology provides an optimum conduit for telcos to deliver IPTV and triple play services cost-effectively and efficiently. Capable of delivering 100 Mbps symmetric data rates over existing "last mile" copper infrastructure, VDSL2 allows telcos to accelerate deployment of premium services. However, simply providing a high-bandwidth pipe is not enough to deliver broadcast quality video to which customers are accustomed.

This article will discuss:

• how voice and video quality can be enhanced with features such as dual-latency and dynamic rate repartitioning;
• how broadband links can become more robust with seamless rate adaptation, impulse noise protection and dynamic spectrum management; and
• how, when armed with these capabilities, VDSL2 can bring IPTV and triple play services into prime time.

With the advent of VDSL2, the link capacity is significantly increased from 24 Mbps downstream and 2 Mbps upstream rates in legacy ADSL2+ technology to 100 Mbps symmetric rates in VDSL2. With the increased bandwidth and MPEG 2/4 compression, a VDSL2 link is capable of delivering multiple streams of IPTV/HDTV, an Internet connection and multiple VoIP channels to residential customers. The sweet spot of carrier bandwidth requirements seems to be converging at 30 Mbps downstream and 5 Mbps upstream, in order to deliver up to three HDTV channels (6-8 Mbps/channel, MPEG4), 3-5 Mbps for Internet surfing, and the remaining bandwidth for VoIP connections.

If triple play services are delivered over a single bearer channel in the physical layer, a compromise must be made between the quality of voice and the protection of video against impulse noise impairment. Voice is intolerant to increased latency or delays through the system because excessive delays cause echoes and degrade quality. On the other hand, video needs to be protected from transient impulse noise. Impulse noise can be eliminated by providing adequate buffering and performing error correction on the data stream. However, this buffering introduces latency, which is not conducive to the voice traffic. Therefore, there are conflicting requirements for voice and video traffic when they are transmitted over a single bearer channel.

In order to alleviate this problem, dual latency (Figure 1) can be used, where the voice traffic and video traffic are directed over two separate bearer channels within the physical layer. The voice bearer channel is characterized by minimum latency "fast path," while the video bearer channel uses buffered data and the "interleaved path" for error correction. With the separation of voice and video traffic over the fast and interleaved path respectively, the problem of conflicting latency requirements is eliminated. Next, the port bandwidth must be divided between the two paths either statically or dynamically. In the static approach, the bandwidth is allocated to each path once at start up and remains fixed during the entire operation period. This approach, although simplistic in nature, is not efficient since the unused bandwidth in each path would be wasted as a result of the fixed allocation. To circumvent this issue, the dynamic rate repartitioning (DRR) feature of VDSL2 can be used to dynamically re-allocate bandwidth between the paths depending on the use. DRR monitors bandwidth use of each path and reallocates unused bandwidth to the other path where it could be used more efficiently. The reallocation of bandwidth is achieved seamlessly without disturbing active user applications.

Figure 1: Dual Latency Paths

Link robustness and stability is another critical factor in a successful deployment and adoption of triple play services over VDSL2. Different types of noise impairments can affect integrity of a VDSL2 link and cause disruption of services. One such impairment is the impulse noise, which can be mitigated by using interleaved path and performing forward error correction as previously described.

Another type of noise is known as crosstalk, which occurs when pairs of wires in a bundle couple electromagnetically with each other. This interaction produces noise that varies slowly with time and causes significant problems at high frequencies. Severe crosstalk noise can cause either link drops or acute performance degradation resulting in service disruptions. Crosstalk consists of near-end crosstalk (NEXT) and far-end crosstalk (FEXT) (Figure 2). NEXT is the crosstalk that couples between a receive path and a transmitting path at the same end of two different subscriber loops within the same binder.

FEXT is the crosstalk noise detected by the receiver located at the far end of the cable from the transmitter that is the noise source.

Figure 2: Near End and Far End Crosstalk

Two leading techniques for combating crosstalk are seamless rate adaptation (SRA) and dynamic spectrum management (DSM). In SRA, the receiver monitors the signal-to-noise-ratio (SNR) of the channel, then determines that a data rate change is necessary to compensate for changes in channel conditions, and sends a message to the transmitter to initiate a change. The message contains necessary parameters, such as numbers of bits modulated and transmit-power of each sub-channel. In response, the transmitter sends a "sync flag," which is used as an indicator for designating the exact time at which the new data rate and transmission parameters are to be used. The receiver detects the sync signal and both the transmitter and the receiver seamlessly transition to the new data rate without disruption to the end user.

The second approach – DSM – has recently had a lot of industry discussion. DSM optimizes channel capacity by adapting the transmit spectra of all VDSL2 lines to the actual time-variable crosstalk interference. DSM has expanded in scope to include techniques for crosstalk mitigation and cancellation by jointly processing signals from multiple DSL lines. DSM is divided into four levels, depending on the degree of coordination between multiple DSL lines.

DSM Level 0 is static spectrum management where the performance of an individual pair is optimized without considering the performance of the neighboring pairs. Spectrum compatibility of each pair is achieved by individual control of the transmit power in a binder. In DSM Level 1, each pair in the binder manages its power in such a way that crosstalk with its neighbors is avoided. The power level is determined by its own line conditions and performance requirements, without any coordination of other pairs in the binder. Similar to DSL Level 1, Level 2 adapts transmit spectra for crosstalk avoidance, but the power allocation is based on its own line condition as well as the neighbors in the binder. Thus, DSM Level 2 allows the optimal spectrum allocation such that total capacity of the pairs in the binder is maximized. Finally, DSM Level 3 reduces or eliminates crosstalk by jointly processing the actual signal of multiple pairs in a binder.

In summary, increased competition and churn and eroding bottom line are forcing telcos to accelerate deployment of the bundled premium services. Armed with the VDSL2 technology featuring high-bandwidth and advanced quality of service and link robustness features, telcos are ready to take IPTV and triple services into prime time.