Telcos are facing increasingly stiff competition from cable MSOs (multiple-system operators) 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 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 100Mbit/s symmetric data rates over existing last-mile copper infrastructure, VDSL2 allows them 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.
With the advent of VDSL2, the link capacity is significantly increased from 24Mbit/s downstream and 2Mbit/s upstream rates in legacy ADSL2+ technology, to 100Mbit/s 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 30Mbit/s downstream and 5Mbit/s upstream, in order to deliver up to three HDTV channels: 6 to 8Mbit/s/channel for MPEG4, 3 to 5Mbit/s 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 and video traffic are directed over two separate bearer channels within the physical layer. The voice-bearer channel is characterised 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 DDR (dynamic-rate-repartitioning) 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 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 NEXT (near-end crosstalk) and FEXT (far-end crosstalk; 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 that is located at the far end of the cable from the transmitter that is the noise source.
Figure 2: Near- and far-end crosstalk.
Two leading techniques for combating crosstalk are SRA (seamless tate adaptation) and DSM (dynamic spectrum management). In SRA, the receiver monitors the SNR (signal-to-noise-ratio) 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 second approach, DSM, has recently had a lot of industry discussion. DSM optimises channel capacity by adapting the transmit spectra of all VDSL2 lines to the actual time-variable crosstalk interference. DSM Level 0 is static spectrum management, where the performance of an individual pair is optimised without consideration of the performance of the neighbouring 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 neighbours 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 on the neighbours in the binder. Thus, DSM Level 2 allows the optimal spectrum allocation, such that total capacity of the pairs in the binder is maximised. 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, plus an eroding bottom line, are forcing telcos to accelerate deployment of the bundled premium services. Armed with the VDSL2 technology, telcos are ready to take IPTV and triple-play services into prime time.
