Virtual noise
Increased noise
Cable multiple system operators (MSOs) and direct broadcast satellite (DBS) operators continue to chip away at the domain of telecommunications companies (telcos). However, advances in technologies, such as VDSL, are helping gird telcos against that invasion, offering a perfect way to cost-effectively and efficiently deliver Internet protocol television (IPTV) and premium triple-play services.
VDSL2 can deliver 100Mbps symmetric data rates over existing “last mile” copper infrastructure. Still, simply providing a high-bandwidth is not enough to deliver the broadcast quality video expected by customers. To ensure enhanced quality of experience (QoE), telcos must guarantee reliability of the VDSL2 line with “always-on” capability.
Maintaining A Robust Link
Several factors contribute to achieving carrier-grade level of service availability, including reliable power systems, resilient network architectures, well-planned maintenance practices, and robust links. Of these factors, maintaining robust links is the most challenging task for telcos.
On top of that, telcos face a number of infrastructure challenges that impact the integrity of VDSL2 links. These include unshielded twisted pair, bundles of wires from the central office (CO) or remote terminal (RT), unprotected outside plant, and crosstalk.
Of these, crosstalk is particularly detrimental to VDSL2 networks in always-on, triple-play deployments. Severe crosstalk noise resulting from the on/off switching of DSL lines can cause link drops or acute performance degradation, which results in service disruptions.
The most effective way to deliver reliable triple-play services is to deploy an intelligent and automated technology that enables telcos to ensure link integrity, as well as dynamically adapt to optimum data rates in the presence of sudden and large noise changes.
Today, a number of solutions address the impact of crosstalk on link uptime:
• Upstream and downstream power back-off (UPBO/DPBO): Introduced in the G.993.2 VDSL2 standard, this technique is commonly used for spectral management. With UPBO/DPBO, stationary noise limits are accounted for in network design. Telcos can impose restrictions related to transmit power spectral density (PSD) to minimize crosstalk on neighbouring lines.
• Virtual noise (static excess margin): This technique, introduced in the G.993.2 standard, improves line stability. Virtual noise is added to the line over the tones likely to be impacted by crosstalk when a neighbouring line switches on. Noise margin adapts to accommodate the virtual noise (Fig. 1). Virtual noise is very effective if there’s prior knowledge of crosstalk. This technique is not dynamic. In fact, it often results in conservative bit loading, and could cause excessive power to be used, thus introducing crosstalk to other users in a binder.
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• Seamless rate adaptation (SRA) and bitswap: These techniques are used dynamically. SRA handles small noise changes by modifying the bit distribution among all carriers and the overall data rate. Bitswap is designed to minimise minor, long-term changes in noise by adjusting the number of bits applied to specific tones, while keeping constant the total number of bits allocated.
• Rapid rate adaptation (RRA): RRA is an automated technique that can intelligently and dynamically adapt the data rate in the presence of NEXT or FEXT without interrupting service for retraining.
• Dynamic spectrum management (DSM): DSM mitigates crosstalk on DSL pairs sharing the same binder by shaping the spectra of the transmit signals or by partially cancelling crosstalk within the binder. While this technique is effective in minimising crosstalk, it requires coordination by all users sharing a binder.
The Benefits Of RRA
With RRA, telcos are able to offer IPTV and real-time triple-play services with guaranteed always-on bandwidth, higher data rates, and predictable quality of services (QoS). RRA offers telcos two distinct benefits:
• Reduced operating expenses (OPEX): RRA increases reliability and availability of the link and minimises service degradation by adapting to the optimum data rate without requiring any retrains. This feature helps reduce the cost of customer service and truck-rolls, maximising OPEX savings.
• Improved reliability: RRA maintains optimum data rates without requiring any link drops, service degradations, or line retrains.
RRA, an automated, intelligent software-based solution, allows telcos to enhance link robustness, reliability, and availability under severe and time-varying noise environments in both CO and customer-premises-equipment (CPE) applications. RRA minimises disconnections and subsequent retrains on the DSL link in the presence of sudden, large wideband noise changes by rapidly and dynamically adjusting the data rate to a level that corresponds with the increased noise (Fig. 2).
RRA, which is highly resilient to dynamic crosstalk in the binder from on/off switching of xDSL lines, will allow for throughput enhancements. RRA outperforms other techniques, such as SRA.
One of the primary technological strengths of RRA in minimising impact of noise-induced disruptions is a series of automated and intelligent algorithms that continuously monitors link quality. Due to possible use of large constellations and high noise level, accurate error calculation is difficult. An intelligent triggering mechanism should be used to measure the link quality, ensuring that it’s not responding to anomalies such as transient impulse noise. The algorithms take into account a number of optimisation techniques and metrics for link degradation detection.
RRA also provides robust communication channels. Under severe noise conditions, the existing overhead message channel (EOC channel) cannot be relied upon when the SNR margin is significantly degraded and possibly negative. The robust communication channel should be protected against noise environment.
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The VTU receiver selects a group of tones with higher noise immunity by either using an intelligent profile-based band-selection algorithm, error-control coding, higher SNR margin setting, or a combination of one or more of these techniques.
Other operation advantages possible with RRA include:
• Short message exchange format: Under a degraded SNR environment, it’s undesirable to have to exchange the long bits and gain table from the receiver to the remote transmitter. Applying a flat reduction in bit loading per group will reduce the amount of data exchange.
• Fast adaptation time: For RRA to be effective in the presence of sudden and strong noise, the link needs to be stabilised by changing to a new rate, which must be done quickly to avoid dropping the link.
• Support of traffic-sensitive applications, based on QoS priority: Telcos can avoid pixelisation in traffic-sensitive applications (e.g., video) by giving that application the highest priority. Then telcos can ensure that video services will maintain their needed bandwidth, while reducing bandwidth for lower priority traffic, such as voice or data.
Achieving Reliable Service
To succeed in the triple-play market, telcos need to ensure that their customers experience no downtime, particularly with video and IPTV services. With RRA, telcos can be confident that they will have a stable, guaranteed, always-on, and predictable QoS, particularly in a progressive FEXT environment.
RRA capability is critical for a successful deployment of premium and enhanced services. When fully integrated with the available QoS classifications and service differentiations, it results in unprecedented service quality and reliability in an end-to-end, IP-based VDSL2 access network.
Dr. Manouchehr Rafie, PhD, is director of products technologies, Access Products Group, Ikanos Communications.