PhyR™ Retransmission System Overcomes ADSL Challenges in Rural India Date:   Monday , April 19, 2010 As one of the fastest growing economies in the world today, India has its own unique set of challenges, especially when it comes to connecting broadband transmissions. For this reason, broadband has become a national priority for the government, and a recognized necessity to help drive growth throughout the country. With this challenge, the Government of India (Dept of Telecom and Ministry of Communications and IT) has set broadband communications as a priority with the hope of having 20 million broadband-enabled homes by 2010. Additionally, the State-run telecommunications provider (BSNL), has an ongoing tender to support nearly 8 million customer premises equipment (CPE) over the next 3 years, providing broadband services, information and employment in order to connect rural India. Connectivity in rural India will allow the Business Process Outsourcing (BPO) industry to now consider setting up call centers in previously remote locations, supporting various services on broadband that includes tele-medicine, tele-education, entertainment, voice over internet protocol (VoIP) and more. So what does that mean for broadband services, such as DSL? In general, rural locations are typically longer reach markets for DSL, since the existing copper infrastructure may not be robust enough, which could, in turn, increase susceptibility to impulse noise. As a result, the focus in recent years has been on improving the robustness of DSL connections in rural areas, such as in India, against non-stationary or impulsive noise (that leads to residual errors and restrains). A considerable effort had been invested in standardization, but with the exception of a new slightly improved interleaving scheme (extensions of some framing parameters and interleaving memory leading to even higher coding overhead); no conclusive progress has been made up until now. To address these broadband challenges in rural areas, Broadcom Corporation has developed an innovative data retransmission system that is implemented above the DSL physical layer and called PhyR™ (pronounced ‘Fire’) technology. This new retransmission technology has been tested in the field, deployed by several telcos worldwide and proven to provide ten times more protection against impulsive noise when compared to a traditional Reed Solomon (RS) + interleaving scheme, and with much less overhead and physical constraints. PhyR technology overcomes the limitations of high bit error rates in the DSL standard, enabling telecom service providers and carriers with a cost-effective, scalable solution that is available today and ready to roll out across existing central office (CO) and CPE hardware via a simple remote firmware upgrade. What does PhyR do specifically for India? PhyR technology improves the robustness of DSL connections in rural India, against non-stationary or impulsive noise. Extending the DSL reach allows carriers, such as BSNL, MTNL, Bharati and others, to obtain more customers by providing additional service coverage in rural areas or urban areas where you have legacy wiring and infrastructure that may not be provisionable today. Improving the DSL reach also helps carriers to increase data rates and bandwidth for existing customers and helps to reduce total support calls by significantly lowering the number of cyclic redundancy checks (CRCs) and errors. The Broadcom® PhyR solution is an uncomplicated, easily provisioned, firmware upgrade to the company’s xDSL chipsets and is based on physical layer retransmission technology (or PMS-TC). This technology has enabled carriers and equipment providers to dramatically improve voice, data and video services to their customers, while delivering as much as a ten-fold improvement in noise resilience. As a result, service providers have been able to offer higher quality, more advanced triple-play services for xDSL networks that equate to better revenue opportunities, with less support required and lower infrastructure costs. Beyond increasing service reach and xDSL experience in existing deployments, PhyR also serves to improve the quality of IPTV services. As the demand for IPTV systems improves in India, those equipped with PhyR technology will enable users to enjoy improved network performance, better service coverage, fewer errors and an overall heightened viewing experience. Traditionally, video services provisioned over copper loops had been susceptible to noise sources in the ambient environment, limiting the coverage area over which services could be made available, or may even reduce video quality by inducing ‘macro-blocking’ or corrupted images. Quality IPTV deployments require carriers to provide a level of impulse noise protection and margin that dictates an achievable data rate and the loop length over which voice, video and data, or IPTV services, will be delivered. While traditional approaches for increasing noise protection have an improved effect on residual errors, they unfortunately have an adverse effect on the serviceable reach and data rate, thereby limiting the service coverage area. In quick review, Broadcom’s PhyR technology provides a number of key advantages: As much as a ten times higher impulse noise resilience Significantly lower residual BER or packet-loss. An extended network service area (higher rate, longer reach, lower delay). Simplified network provisioning (no per user ‘tuning’). Firmware upgrade to central office and customer premise equipment. Transparency to network and upper layer applications (significantly reduces the burden on networks that use higher layer retransmission schemes for improving network efficiency) How Does PhyR Work? A retransmission buffer stores the transmitted data, packed in a retransmit unit. When a data unit is received, its frame check sequence (FCS) is checked, and a first retransmission request is immediately launched if it is found to be corrupted. Even if corrupted, the data units are pushed into the receive buffer. If the retransmitted data unit arrives while the corrupted one is still present in the receive buffer, the corrupted data is replaced. If the retransmitted data unit does not arrive on time, the corrupted data will be further processed by the receiver data path. It should also be noted that: * The end-to-end delay is kept constant, even when retransmission takes place. This makes the end-to-end behavior very similar to a standard interleaving scheme. * Retransmission does not prevent the use of forward error correction (FEC) at the receiver side to correct a data unit prior to retransmission and provide additional coding gain (i.e., complementary). * The roundtrip delay is quite small, typically 3 to 4 ms. As 2 ms maximum per direction is mandated by the standard today, a link running retransmission operates at comparable (and typically much lower for decent INP) delay than an interleaved-based system. * By using the PhyR retransmission scheme, an effective Impulsive Noise Protection (INP) as high as 16 with a maximum delay of 4 ms at 24 Mbps can be achieved with the same buffer size as today’s ADSL2+ interleaved system without compromising the effective data rate (the same gain applies to VDSL2). At this time, let’s review the benefits of a retransmission technique when compared to legacy approaches. PhyR—Pay Per Fault The first advantage of a retransmission technique is that the real overhead is only present when errors are present. If errors are present only 1 percent of the time, the associated cost is only present 1 percent of the time and not 100 percent, as with an RS + interleaved approach. There is some steady-state overhead associated with the retransmission of a transport data unit’s ID or CRC, but both are negligible. PhyR—No Margin Cheating Less obvious, but also essential, is that PhyR technology stops the ‘organized cheating’ of overbooked coding gains and impulse protection. The probability to correct an impulse at 0 dB margin is a function of the overhead allowed for retransmission (allowing for 1 percent of bandwidth lost by retransmission). As a result, PhyR corrects 99.99 percent of the impulses at a 0 dB noise margin, and much better than the interleaved approach at 3 dB noise margin. PhyR-enabled modems, therefore, provides much less sensitivity to additional noise in low margin conditions and are the only ones capable of remaining in showtime when repetitive impulse noise (REIN) is applied during these conditions, dramatically improving modem robustness against extreme stress in the field. PhyR Offers 10-10 BER PhyR also provides an answer for the lower BER required by IPTV services. Indeed, retransmission even provides some coding gain. For example, when you compare residual BER at a 0 dB margin of an interleaved scheme versus a retransmission scheme, the interleaved approach results in one error every 10 seconds at 40 Mbps, versus retransmission results of only one error every 100 hours. Note that in this case, the data rate lost (due to retransmission) is only about 6 Kbps—and completely negligible in comparison to 40 Mbps. PhyR— ‘Set and Forget’ Provisioning A major benefit of having these PhyR retransmission schemes without any fixed penalty is that they can be blindly provisioned to everyone. There is no need to tune the configuration on a per line basis, so everyone can receive a low delay and high rate service, with only the few bad lines effectively spending some controlled part of the data rate bandwidth into retransmissions. This removes all of the need for the provisioning nightmare of current implementation or even dual latency (gamers, VoIP and IPTV requirements are met with a single, low latency path). PhyR—DSL just as Ethernet or Fiber Another key advantage of implementing a retransmission protocol at the physical layer is that no additional and dedicated complexity is required at upper layers in the system (e.g., on a network processor or the backbone network) to address the poorer BER characteristic of DSL. This makes DSL provisioning similar to other physical layers used in access provider network (e.g., Ethernet or GPON) and helps access providers’ operation and planning. PhyR is mostly transparent and significantly reduces the burden on a network that uses retransmission on the application layer, thereby improving the efficiency and scalability of the network. In essence, physical and application layer retransmission schemes are complementary and work constructively together to increase triple-play service protection against packet losses. Conclusions The Broadcom PhyR retransmission scheme is available today and provides a number of key advantages that extend service reach with a higher rate, lower delay including as much as a ten times (10x) higher impulse-noise resilience without impacting link delay and error propagations. PhyR significantly lowers residual BER or packet-loss even in REIN conditions. This repositions DSL for increased service coverage, higher datarates, significantly lower service calls, scalability to IPTV services all of which are key to the proliferation of xDSL in the Indian market. This retransmission scheme also enables much simpler network provisioning (‘set and forget’, no per user tuning). The scheme is transparent to both network and upper layer applications and significantly reduces the burden on networks using higher layer retransmission schemes to improve network efficiency (Microsoft IPTV, TCP/IP). Most importantly, PhyR is easy to implement on any existing Broadcom-powered ADSL2+ or VDSL2 central office and customer premises equipment via a simple firmware upgrade (deployed easily through remote upgrade functions). The author of the article is Shankar Bala, Product Line Manager, Broadcom Corporation