Four DSL Challenges and Solutions That Can Help
by: Hawkee Ngu

While they’re not on the growth curve of a decade ago, DSL technologies represent a huge, embedded global customer base that increasingly demands sophisticated Triple Play services delivered at a high-quality experience.

The challenge for service providers: How to meet high expectations for video, broadband Internet connectivity, and digital voice offerings, while dealing with networks that may concurrently rely on multiple flavors of DSL technology. Service providers also want to keep an eye on capital and operating costs.

Intense competition from cable and mobile networks for broadband customers mandates that DSL service providers rigorously test to verify and maintain the quality of customer connections. And that’s no easy feat with issues such as packet loss, CRC errors, and other impairments. Each and every one of them can impact the video stream, seriously hindering service and playing into the hands of DSL’s broadband competitors.

DSL service providers require test and measurement solutions that are effective, flexible, help control costs, boost workforce productivity, are easy to use, and cut the training time for techs who may not be familiar with all DSL technologies.

ADSL Yesterday/VDSL2 Today

DSL technologies have evolved significantly since ADSL1 burst onto the scene more than 10 years ago. Today’s more advanced ADSL2+ technology supports downstream rates of up to 15 Mbps at 4,000 feet. ADSL2+ supplies standard definition IPTV and Internet access, and is typically coupled with traditional phone service.

VDSL2, the latest iteration of DSL, supports downstream rates of up to 50 Mbps for a typical FTTN architecture, delivering applications such as multiple HD channels, VoIP, and speedy Internet access. Furthermore, newer DSL technologies such as VDSL2 bonding allow service providers to double bandwidth. SHDSL.bis also is an up-and-coming technology for business services and mobile backhaul applications requiring high symmetrical bandwidth.

Many service providers are moving toward VDSL2 technology to support advanced Triple Play services. As they migrate to VDSL2 while continuing to use legacy ADSLx in some areas, telcos need test and measurement solutions that verify both flavors of the technology.

Four Steps to Ensure xDSL Performance

For DSL-supported IPTV services, problems usually are associated with the quality of the copper pair.

Below are 4 challenges and potential solutions technicians can employ in the field to help ensure optimal xDSL performance:

Challenge #1: Measuring achievable data rates with sufficient noise margins. Service providers need to determine if the DSL layer metrics meet strict service guarantees while also addressing impairments such as noise.

Solution: The bread-and-butter method for DSL service verification is the modem synchronization test. For today’s Triple Play environment, the test set must support all flavors of VDSL2 and ADSLx. Data rates for upstream and downstream must be checked to ensure it meets service guidelines. The SNR Margin for both directions must be sufficient to ensure stable service. Typical minimum guidelines for VDSL2 at any location specify a minimum SNRM of 9 dB. Additional metrics such as CRC errors and FEC events also need to be checked. For a typical synchronization test that lasts 3 minutes, there should be zero (0) CRC errors and a low FEC rate. Technicians should run DSL tests at different access points: Near the DSLAM, whether it is CO or FTTN supplied, and at the subscriber site. Each access environment is different, and service providers must ensure testing has been done to meet existing pass/fail thresholds.

Noise impairments typically degrade the data rate and SNR Margin, and cause transmission errors. While it’s impossible to eliminate all noise impairments, it’s up to the service provider to enforce best practices for noise mitigation. Technicians must ensure the cable shield is properly secured to earth ground to minimize AC power influence. The copper pair also must be properly twisted and balanced to reduce the impact of crosstalk and ingress that may cause video to freeze, become pixilated or tiled, and audio to disappear. Longitudinal and capacitive balance tests, as well as a ground test, help verify and ensure the operator has done all it can to maximize the quality of the copper pair.

Challenge #2: Impairments such as packet loss and excessive jitter in packet switching networks affect DSL services.

Solution: The typical causes of packet loss and excessive jitter are network congestion or physical layer related faults. The field technician’s role is to tackle the copper plant-related issues. For example, DSL Layer CRC errors can cause packet loss. A dropped packet results in seven lost MPEG-2 Transport packets, which can result in a visible impairment of the video stream.

The best approach for dealing with these impairments is systematic testing of DSL networks. Service providers should verify IPTV services using test gear that emulates the IPTV set-top box and provides video stream measurements. Packet metrics are used to determine if the network connection meets acceptable standards. The Broadband Forum’s TR-126 specification defines quality of experience (QoE) guidelines for Triple Play services, including a Packet Loss Rate of less than 10-6 and Jitter less than 50 mS. Should such measurements not meet these guidelines, the first step should be to verify the physical layer.

Challenge #3: DSL services also suffer from bridge taps. With this impairment, a piece of cable connected in parallel to the main line can degrade the frequency response, which results in diminished or unusable carrier tones. Short bridge taps, on the order of a few feet long, are notorious for affecting VDSL2 connections, given these services operate at higher frequencies. The closer this bridge tap is located to either end of the subscriber loop, the greater the impact. Even inside wiring that is multiplied from one jack to another can impact VDSL2 data rates as the pig tail on the jack may act like a short bridge tap.

Solution: Technicians can conduct a VDSL2 band insertion loss/frequency response sweep, which delivers a point-to-point view of the network, identifying if a bridge tap exists in the network, including its length. This test is particularly effective in detecting short bridge taps. After that, a TDR test can be conducted to locate this impairment.

By testing for bridge taps, technicians get data on the overall behavior of the cable delivering DSL services. Detecting and removing bridge taps minimizes future service problems, which in turn reduces future, costly truck rolls.

Challenge #4: Technicians should carry out proof-of-performance testing to determine if the at-home network is capable of supporting such advanced services as IPTV. Qualifying the physical layer for service helps identify small problems that may escalate into a major service disruption.

Solution: If service providers don’t constantly verify the capabilities of their DSL service, they risk losing customers to a competing operator. Customers notice minor delays in video, Web access, and downloading files. Operators must detect these issues early before they worsen into bigger problems.

To ensure customers receive the best possible service, service providers must test the performance of the copper pair delivering DSL offerings. They need to determine if the copper cables can sufficiently support VDSL2 frequencies.

For in-home wiring test and measurement, technicians can conduct a PING test, HPNA analysis, coaxial and twisted pair loss measurement, and test the RF signal.

Empower the Tech!

Given that large and small telcos are operating not only ADSLx and VDSL2 configurations but also multiple iterations of each technology, test and measurement solutions that gauge the status of these services must be flexible, cost-effective to deploy, and simple to use.

Service providers can empower their technicians by giving them a single device containing multiple capabilities for testing both ADSLx and VDSL2 and identifying impairments such as bridge taps, eliminating the need to equip these field professionals with multiple test devices. Reducing the number of test devices in the field also cuts capital costs for the service provider.

An intuitive, easy-to-use user interface speeds the training time for technicians who may be dealing with unfamiliar or new DSL configurations. In addition, the test set must make the job easier for technicians by automatically detecting the DSL flavor they are evaluating. This takes the guesswork out of testing DSL and simplifies the installation and maintenance process.

Streamlining test procedures also is important. For example, Sunrise Telecom’s SunLite UDSL features one-button test capabilities for obtaining key modem status measurements and quick deployment for any DSL configuration.

The complicated nature of services accessed via DSL -- coupled with intense competition from cable and mobile broadband for DSL customers -- mandates rigorous testing. Without constant verification of all DSL configurations, service providers risk losing customers to rivals.

About the Author - Hawkee Ngu, HW Engineering Manager, is a 10-year hardware design veteran at Sunrise Telecom Inc. His primary focus is on DSL technologies and system designs. Recent DSL technologies Ngu has been working with include: ADSL, ADSL 2+, SHDSL, SHDSL.bis, UDSL, and VDSL 2, by various chip vendors. For more information, visit www.sunrisetelecom.com.