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Whitepaper

MELT - The Cornerstone of Service Quality Assurance for Next Generation NetworksXiaoling Li, Alberto Canella, Gerhard Noessing Lantiq

Oct. 2014

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MELT - The Cornerstone of Service Quality Assurance for Next Generation NetworksXiaoling Li, Alberto Canella, Gerhard Noessing Lantiq

Abstract

The network architecture is evolving towards a so called All Digital Loop (ADL) environment. This trend poses challenges for Line Testing as metallic access to the line via traditional POTS function, or dedicated test head is no longer available. In this paper, an integrated DSL-friendly hardware solution is proposed for Metallic Line Testing (MELT) and wetting current functionality, which provides cost effectiveness, high density, power optimality, and supports parallel measu-rement.Starting with state-of-the-art line testing for access networks, the importance of MELT as an indispensable part of a complete, reliable line testing solution is emphasized. The underlying network model requires to determine the MELT parameters. These parameters and typical error cases are explained in detail. The network maintenance requirements for useful MELT integ-ration are discussed and an overview of the MELT standardization is given. Finally, the MELT solution, which is ITU G.996.2 standard-compliant, is presented.

Introduction

In the traditional Access Networks architecture, dual systems coexist side by side in the CO (Central Office). One is classical narrowband PSTNs (public switched telephone networks) and the other is wideband data networks, either ATM (Asynchronous Transfer Mode) or IP (Internet Protocol) based. Narrowband voice services come in two flavors – POTS (Plain Old Telephone Service) and ISDN (Integrated Services Digital Network), and appear on the copper line at the low frequency end with narrow bandwidth. A DSLAM (Digital Subscriber Line Access Multip-lexer) allows data services to be connected to the copper line using a much wider frequency range, above POTS or ISDN frequencies. Typical technologies for wideband data transmission are ADSL (Asymmetric Digi-tal Subscriber Line), VDSL (Very High Speed Digital Subscriber Line), or G.fast . To allow both voice and data services to coexist on the same single copper line, a splitter is placed on the CO side, between the POTS and DSL service and the copper line. The splitter ensures that the voice services only use the lower frequency band, while data services are confined to the higher frequency band. A second splitter separates the two signals in the same way on the CPE (Customer Premises Equipment) side.Maintaining these two parallel worlds is very expensive. The equipment on the POTS/ISDN side is typically older, they were built with old technologies which consumes much higher power, and are approaching the end of its lifetime, thus need more intensive maintenance effort. This leads to a high operating cost for the network operator. On the DSL side, the rapidly growing demand for speed and bandwidth is forcing the distance to the customer to be reduced. Consequently, DSL deployments are being moved outside the CO and positioned closer to the customer using

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so-called street cabinets, which contain access equipment such as MDUs (Multi-Dwelling Unit).This reflects the major trend of architectural transformation in the telecommunication networks, towards IP technology, as carrier of various services. The final destination of this transforma-tion is the so called NGN (Next Generation Network), in which one single network transports all information and services by encapsulating these into packets. NGN implies the migration from the dual system of voice parallel to xDSL setup in CO, to a converged setup where access equipment integrate voice ports or VoIP within the data network, making it possible to remove the legacy voice switching infrastructure. This helps network operators to greatly simplify their network and save operating costs. Access equipment will move ever closer to the customer. The integrated voice service is only translated back just before or directly at the customer premises, for which purpose Home Gateways or ATAs (Analog Telephony Adapter) are required. There will be access network based solutions still offering traditional POTS/ISDN in terms of Voice Enabled MDUs or MSANs (Multi Service Access Nodes), but the preferred and least ex-pensive solution will be a pure digital connection to the customer, also known as an “All Digital Loop” (ADL). While an All Digital Loop offers numerous advantages to network operators, chal-lenges also emerge. One of them is how to provide certain testing and maintenance capabilities parallel to the broadband data transmission, when direct metallic test access from traditional POTS is no longer available.

2 Line Testing Principles

2.1 Line Testing in Traditional Network Architecture

When transmission faults occur or services do not exhibit the expected quality, the customer typically files a complaint to the operator’s service hotline. The customer service usually runs several line tests during the complaint call. This can be achieved by a test server connecting a special test head to the affected line via a matrix of relays on the POTS linecard. The test head performs a sequence of measurements to identify the errors that may exist. Alternatively, more advanced POTS hardware may itself be able to perform such tests with almost the same accuracy. In this case, an expensive test head along with the matrix of relays are no longer re-quired. Either method can determine the line status. For instance, whether the line provides the expected transmission quality and is correctly terminated, if the remote modem is connected, or if the line is suffering from a short or open circuit (one or both wires), or the line is affected by other failures (see section 4.1). Based on the test results the customer service can then locate where the error occurred, directly at the CO, between the CO and customer premises, or at customer premises. Then actions can be taken accordingly, e.g., a service technician may need to be dispatched to the faulty location (the so-called Truck-Roll).

2.2 Line Testing for Next Generation Networks

The majority of faulty transmission lines occurring when a new DSL connection is set up, af-fected from trivial metallic errors such as open wires, short-circuits, leakage or incorrect termi-nations. Only MELT offers the possibility to prevent these kinds of errors in an ADL scenario.

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There is indeed no POTS hardware, and consequently no (POTS) metallic test head is generally available.

Integrating a test head into each and single street cabinet based access equipment is also not a viable alternative. While in CO-based equipment the test head would be shared among many customer lines, this solution would be far too expensive for small access equipment in a street cabinet. If test functions are waived completely, then sending a technician to every fault once an error occurs would be inevitable, which implies high operating expenses for the network operator.This poses a major problem to the network operator. Attempts to test for metallic errors using only the DSL connection to the line have been of limited success. A special test signal is typi-cally sent to the line from the CO-sided DSL port. Certain parameters and conditions of the line under test can then be determined by analyzing the returning reflection of the signal. These tests only require a single-sided connection to the line and are known as Single-Ended Line Testing (SELT). Other line testing methods require DSL modems on both sides of the line and are consequently called Dual-Ended Line Testing (DELT). However, as described in the Figure 2, DSL signals are coupled to the line via transformers, therefore there’s no metallic access (re-sistive coupling) to the loop and it is practically impossible to find galvanic errors using SELT or DELT. Inevitably, a dedicated hardware with MELT capability is required to detect such errors, i.e. providing independent metallic access to each single wire.This does not mean that SELT and DELT are not useful for examining line conditions. SELT and DELT have clear advantages when it comes to spectral measurements. However, a complete and powerful line testing package needs SELT and DELT to be complemented by a MELT so-lution.

Figure 1: Transformation from Traditional Network Structure towards NGN

Traditional Voice & DataNetwork Structure

Next Generation Network(IP based)

ATM-DSLAM

+ splitter

VoiceClass 5switch

ATM/IP PSTN

Data Voice

DSL and VoiceSubscriber

Video,Data &Voice

MSANs(incl. xDSL

and PacketizedVoice)

IP

IPTV, DSL andVoice Subscriber

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In order to integrate the MELT functionality within Next Generation Networks (NGN), additional hardware for metallic line access needs to be placed directly on the DSL linecard. Additional costs per channel should be kept to a minimum, and the MELT hardware should have no in-fluence on the running DSL services. Further detailed requirements for MELT integration are discussed in section 4.

3 Network Model

The whole subscriber loop can be characterized by a number of groups of electrical parameters. For our purposes here, it is necessary to define what the MELT algorithms have to measure and how each line parameter interacts with the others. Figure 2 shows the proposed network model, consisting of different terminations, a line (cable) model and foreign voltage sources, all of which can be coupled to the line under test. The main goal of the MELT algorithms is the reliable, accurate and fast measurement of the electrical parameters of the network model and detection possible faults.The MELT algorithms must also be able to recognize how the line under test has been termi-nated on the CPE side and return the main parameters of that termination, with the obvious assumption that the transmission line satisfies a given level of quality in terms of isolation, con-ductivity, noise, etc.

3.1 Fault Terminations

In the case of an ADL, the line is terminated by a DSL modem and, for some countries, by an xDSL signature as well. POTS/ISDN subscriber lines are of course terminated by one or more on-hook or off-hook telephone, respectively ISDN NTBA. The detection of legacy equipment at the end of an ADL would lead to a failure indication, as they should obviously be connected to the voice enabled CPE like a VoIP (Voice over IP) Home Gateway. Some country-specific master socket terminations also have to be recognized, e.g. the Germans PPA (“Passiver Pruefabschluss” - passive test termination), or BTs NTE5 line Box, or FTs ITD.The detection of other generic electrical failures such as short-circuits or open loop conditions must also be sup-ported by the termination detection algorithms.

3.2 Foreign Voltages

Under normal conditions a given subscriber loop is well isolated from external voltage sources or from other loops, so that no interference from DC or AC signal sources affects the voice or data transmission. However, in the event of road work, incorrect customer installations, wiring errors, the rebuilding of the network infrastructure etc., a power line might be coupled into a sub-scriber loop. Resistive, capacitive as well as inductive coupling are of course generally possible. The MELT algorithms must be able to measure the amplitude and frequency of the coupled AC or DC foreign voltages. Regular monitoring is very important to prevent hazardous voltage levels ensuring the safety of the technicians and customers. The second target is to identify the root

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of the problem. For example, the measurement of a 50 Hz (or 60 Hz) foreign voltage would indicate a coupling to a power network, while a 162/3 Hz disturbance may indicate coupling to a railway power line.Finally, the MELT algorithms should be able to indicate which wire has been coupled to the ex-ternal voltage source and, in the case of electrical coupling, to return the coupling impedance as well.

4 Network Maintenance Requirements

Within a network infrastructure it is essential to monitor each subscriber loop, to detect abnor-mal line terminations or failures, to give support during line installation and to periodically apply a wetting current. It is very important that these functions are always available at the network operator’s disposal, so that the expense of network maintenance can be minimized.

4.1 Detecting Abnormal Termination and Error Cases

In order to ensure customer satisfaction, it is important to provide a fast and accurate response to service complaints. If a customer calls the service desk, it is important to already gather infor-mation about the line status during the first few seconds of the call. Therefore, the time required for the measurements must be short enough to allow the information to be available on time. Then the service operator can decide whether it is necessary to send out a service technician and whether an appointment with the customer should be arranged.Therefore, the following information has to be provided quickly and accurately: error location, error classification and termination classification.The termination classification is already a first indication of whether the line can be operated

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by the customer or not. If a signature (e.g. passive test termination or xDSL signature) can be detected, the line up to the customer can be operated. Other terminations such as electronic or mechanical ringers, ISDN NTBA or DSL splitter may also provide indication that the line up to the subscriber is operational. On the other hand, an error may already exist if a legacy equipment is connected to an ADL as explained in section 3.1. A further possible error is a short circuit between the tip and ring wires. It is necessary to distin-guish between a short circuit and an off-hook telephone. It is assumed that a short circuit has a linear characteristic while an off-hook telephone displays non-linear behavior. Further typical errors are any kind of ground faults or broken wires. A single break in either the tip or ring wire is detectable via the capacitive unbalance between tip and ring, while a break in both wires is detected via a missing termination. The distance to the line break can be calculated via the measured capacitance or via a SELT measurement.In the case of a foreign voltage, it is also important to recognize hazardous potentials and power contacts by comparing the measured amplitudes with specified thresholds.

4.2 MELT Parallel to DSL

To ensure quality of service it is important to perform periodic line monitoring and to react to changes in the line status before the customer complains. Therefore it is essential to be able to perform a MELT measurement without any influencing and interrupting the running DSL service. A MELT solution that involves relays switching may not be able to perform a measurement without influencing the DSL performance.

4.3 Accuracy and Duration

As previously mentioned, the duration of the measurement must be short enough to provide the results during the service desk call. A reasonable measurement time would be below 30 seconds, but it is also important that the results are of good quality.When using a capacitance measurement to estimate loop length, the accuracy for the measure-ment of small capacitance values should be as close as possible to +/-1 nF. This corresponds to a resolution of about +/-15 meters for length estimates. The range for capacitance measure-ments is from 1 nF up to 5 µF. This should be sufficient for all digital loop applications including ADSL, VDSL and SHDSL (Single-Pair High-Speed Digital Subscriber Line).The range of resistance measurements is from 1 Ohm up to 10 MOhm. The measurement of high leakage resistance values can provide information about the condition of the cable (e.g. wet cable) while the measurement of small resistance values can provide length information in the event of a short circuit.

4.4 Technical Support Features

The installation of new lines can also be difficult for an ADL scenario. While a POTS linecard could generate some tones, a DSL system will not be able to generate audible tones.

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Therefore, a metallic test unit should also be able to generate so called “pair identification tones” to indicate to a service technician which line should be connected. Furthermore, a DC voltage of a given polarity should be applied to identify the tip and ring line, or to distinguish between tip and ring lines.

4.5 Wetting Current Generation

Wetting current (also called sealing current) helps to prevent oxidation on line splices and relay contacts, thus helping to maintain good electrical connections.On standard POTS lines, the feeding current serves this purpose to keep contacts clean. Ho-wever, there is no feeding current in a ADL scenario. In these cases, a device to provide wetting current from the CO side is recommended. A DC termination (e.g. 10 kOhm resistor or active DC termination) is required on the CPE side.

5 ITU G.996.2 Standardization

Proprietary line testing solutions from various chip and system vendors have existed for a long time. However, such solutions are difficult to integrate into the Network Management Systems (NMS) of network operators since no standardized interfaces exist. With the progress of xDSL deployment, it was soon recognized that a standardized way of testing the line would be neces-sary. The Loop Diagnostics Mode as part of the ITU ADSL2 (G.992.3), ADSL2+ (G.992.5), VDSL2 (G.993.2) Standards is considered to be a first attempt at establishing an early DELT. The first attempts to standardize SELT within ITU Q4/SG15 started in 2002 with the G.996.2 recommendation, at that time called “G.selt”. In 2006, BT et al. published a table of requested line testing parameters as a summary of previous ITU contributions, with an assessment of which of the three line testing flavors (SELT, DELT or MELT) would be the most suitable for different kinds of error cases. In the same year, G.selt finally migrated to G.996.2 and is now regarded as a global line testing standard combining SELT, DELT and MELT.

6 MELT Solution

A MELT solution, implemented by means of a chipset placed onto a DSL linecard, requires very little board space and provides metallic access via high ohmic resistors directly connected to the tip and ring wires, as in Figure 2. This is a DSL-friendly solution which, in contrast to a relay cou-pling of the MELT circuitry, does not change the impedance seen by the DSL when switching between active and idle. It allows simultaneous MELT measurements or wetting current cycles during a running DSL connection without interfering with the DSL. The Lantiq chipset offers a 16 channel high voltage SLIC with integrated multiplexer and low voltage AD/DA converters on a single mixed-signal chip. A line test controller processes the digitized measurement signals. One line test controller supports up to 8 multiplexers, thus enab-ling up to 128 channels per linecard.

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In addition, the MELT solution offers low power consumption in idle mode, fast test times (less than 20 seconds for a complete test sequence), full wetting functionality, and moreover pair identification tone generation. It comes with a complete software package for various host con-troller platforms. Last but not the least, it fulfils the ITU K.20 protection requirements and fully complies with the G.996.2 standard.

Lantiq

VINETIC™-LTC

Lantiq

Smart SLIC-T16MLT16

µController

xDSLDFE

Lantiq

xDSL AFE

Lantiq

xDSL AFE

DSL/MELTAPI

LD

LD

LD

LD

Figure 3: Block Diagram of Lantiq MELT Solution

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Contact

If you are interested in getting further information about MELT, please do not hesitate to contact:

Xiaoling Li

Product Marketing, Voice and Telecom Product Line

xiaoling.li@lantiq.com

Lantiq Deutschland GmbH

Lilienthalstraße 15, 85579 Neubiberg, Germany

www.lantiq.com