MELT: Metallic Line Testing for All-Digital Loops MELT is Lantiq’s Multichannel Line Testing Solution for “All Digital” Local Loops with significantly reduced system costs. MELT is designed to provide integrated line testing on all xDSL access equipment. The unique, extremely cost effective and reliable Testing solution with built-in ‘wetting current’ function offers sophisticated line and loop testing features for real-time monitoring of voice quality and network stability with test head accuracy and zero impact on the DSL performance. Compared to traditional test approaches the Lantiq MELT chipset provides a significant system cost advantage of up to 90 percent. MELT now comes with Tollgrades LoopCare Operations Support System (OSS) Software. Tollgrade Communications and Lantiq partner to offer broadband carriers a best in class, seamless and standardized line testing solution for Next Generation Networks (NGN). The solution combines Lantiq’s MELT chipsets with Tollgrade’s LoopCare Operations Support System (OSS) software and allows telcos to migrate their field proven and mature test expertise to the requirements of new IP-Networks. More: http://www.lantiq.com/MELT

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Whitepaper
MELT - The Cornerstone of
Service Quality Assurance for
Next Generation Networks
Xiaoling Li, Alberto Canella, Gerhard Noessing Lantiq
Oct. 2014

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MELT - The Cornerstone of Service Quality Assurance for
Next Generation Networks
Xiaoling 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.
Traditional Voice & Data
Network Structure
ATM/IP PSTN
Data Voice
ATM-DSLAM
+ splitter
Voice
Class 5
switch
DSL and Voice
Subscriber
Figure 1:
Transformation from Traditional Network Structure towards NGN
Next Generation Network
(IP based)
Video,
Data &
Voice
MSANs
(incl. xDSL
and Packetized
Voice)
IP
IPTV, DSL and
Voice Subscriber
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.

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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.
Ringer
Load
Rring
CPE Terminations Cable Foreign Voltage
Cring
ETSI xDSL
Signature
6.8V
33k
Csig
Ctg
Ctr
Rtr
Crg
Rtg
Off-Hook
Telephone
Vzener
Rzener
PPA
470k
xDSL Modem
Cdsl
Rppa
4 Network Maintenance Requirements
Sources
Zbat_tip
UtgDc_source
UtgAc_source
Rline/2
UtrDc_source
UtrAc_source
Rline/2
UrgDc_source
UrgAc_source
Rrg
Central Office
xDSL
Components
Zbat_tipring
Zbat_ring
Cco
xDSL
tip
ring
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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xDSL
DFE
Lantiq
xDSL AFE
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-T16
MLT16
μController
Lantiq
xDSL AFE
DSL/MELT
API
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