A Brief Discussion on the Line Test Solution of "Next Generation Network"

As telecommunications system operators gradually embrace the "Next Generation Network" (NGN), this Internet communication protocol (IP)-based network will replace the old "public switched telephone networks" (PSTN) and transmit voice and data at the same time. The most common scenario is to evolve to fully digital lines, and transmit voice signals in a "packaged" manner through user terminal devices, using Internet Voice (VoIP: Voice over Internet Protocol) Digital Subscriber Loop (DSL) data streams to transmit. In order to prepare for these networks, system operators need to introduce new testing and diagnostic measures to find interference and problems in the lines. It is for this reason that the highly integrated "Metallic Line Testing" (MELT: Metallic Line Testing) chipset solution can play a role in its effectiveness and flexibility.

【Figure 1】In the face of the transformation of traditional ATM-DSLAMs networks to flexible and cost-effective IP networks, a new line test solution is needed in this all-digital loop (ADL)

Traditionally, access networks are divided into two major blocks: telephone networks (PSTN) and broadband data networks (such as asynchronous transfer mode ATM networks and IP networks). However, with the trend of network aggregation, network infrastructure is moving towards integration and unification. During this transition period, phased solutions are still necessary, in order to provide traditional "general old telephone services" and "integrated service digital networks" (POTS/ISDN), as well as voice calls, through digital subscriber line access multiplexers (DSLAM). With this pure digital connection network, it can be called "all digital loop" (ADL). The benefits it brings, such as reducing network complexity and saving costs, are obvious, but it is a big challenge to enable the monitoring of user loops and the transmission of bandwidth data in parallel. In addition, every link and step in all transition stages must be able to meet various supporting measures, otherwise many elusive problems will arise. For example, telecom operators will use splitters to maintain the original PSTN/POTS structure and gradually transition to the NGN architecture. In such a situation, a flexible MELT solution must also be able to comply with the network architecture that is parallel to POTS to help operators control all their networks.

2a

2b

Figure 2: Line test method for existing network infrastructure (2a), and line test method for the next-generation all-digital network (2b, excluding PSTN/POTS)

In traditional networks, testing is usually done periodically during certain engineering stages of POTS lines or when specific errors occur. In the past, the method used was to connect an external test server to a central test head via a POTS linecard relay matrix. This was later replaced by a more advanced POTS chipset with a complete built-in test mechanism, which saves the purchase of expensive test heads with relay matrices. In either case, the problem with the line can be found within 30 seconds, and it can be determined whether its transmission quality meets the requirements, or the condition of its signal endpoints, such as short circuit, open circuit (Translator's note: the line is not connected to the terminal device), or broken circuit (Translator's note: the line is broken).

SELT, DELT, and MELT

In today's DSL connection environment, traditional testing methods are not feasible. After all, the hardware such as relay matrix and central test head in POTS no longer exists. For example, DSLAM, which may be located on the roadside, cannot be achieved in the existing test architecture. Due to various factors such as these, operators have to face many challenges. Not only that, but there are not enough methods available to test DSL lines, which is also a headache for operators. The actual method is to send DSL test signals through the line from the transmission port in the central room, and then analyze the signal results by mastering or comparing certain specific parameters of the reflected signal. These tests are called Single-Ended Line Testing, or SELT for short. Other line testing methods require DSL modems to be installed at both ends, which is called Dual-Ended Line Testing, or DELT for short. Since DSL signals are coupled through transformers, it is difficult to find asymmetric errors and grounding problems whether using SELT or DELT. Therefore, in order to effectively debug and provide independent and direct access to individual lines, hardware with integrated MELT function is necessary. In broadband measurements, SELT and DELT are both very useful methods, which can detect frequency-independent parameter results such as line attenuation, crosstalk and external noise. However, these methods can only truly detect all physical parameters in the line when combined with MELT solutions.

The MELT solution strengthens the existing SELT detection of ADSL and VDSL, allowing system operators to fully grasp the transmission status of user loops, including: line distance and transmission rate, bridge tap detection, crosstalk analysis and other related information and data. In addition, the MELT solution can provide information on potential overlaps of electromagnetic waves, even known radio frequency interference (RFI), and use this to deduce where interference may occur in the line.

If system operators want to apply MELT functions to next-generation networks, they must integrate the corresponding hardware directly into the DSL line card. The additional circuit cost and circuit board space should be minimized as much as possible, and the impact of MELT hardware on DSL services should also be minimized.

Network testing requirements

In network infrastructure construction, it is very important to monitor the abnormalities and errors of each user loop, provide necessary support during installation, and regularly transmit moisture-proof currents (provide low-power signals to prevent the line from aging or oxidation due to moisture). These feed current signals can make the transmission quality of POTS lines "lively and energetic", but this is not necessary in the xDSL environment. [page]

An effective MELT solution must also be able to locate and identify errors. Typical errors include short circuits between the Tip and Ring wires, and poor insulation due to moisture. Other common errors include various grounding problems or cable damage. In addition, the MELT algorithm must be able to measure the amplitude and frequency of external AC and DC voltages coupled, such as those at construction sites, or abnormal installation and wiring errors. Among these external voltages, the most important thing is to find potential sources that may cause fatal effects by comparing them with the corresponding voltage amplitude limit values.

As for ensuring the quality of service (QoS), it is necessary to continuously monitor the line and respond quickly when errors occur. Therefore, the system must be able to perform MELT measurements without affecting DSL operation. This is especially true in a fully digital network without a splitter. A MELT solution that needs to handle relay switching cannot perform measurements without affecting DSL operation performance.

standardization

Exclusive testing solutions from chip vendors and system suppliers have been in the market for many years. As the penetration rate of xDSL telecommunications services increases, the industry calls for standardized solutions. First, in 2002, the industry tried to standardize SELT1 with the G.selt standard, but it has not been fully completed to this day. Before 2006, there was still a lack of parameter items that must be tested. Later, there was some explanation of the matching of the three test methods (i.e. SELT, DELT, MELT) and their suitability for finding different errors. G.selt later became G.lt and became a global standard that merged SELT, DELT, and MELT. The functional reference model established from SELT has been adopted by DELT and MELT and has become the functional reference for general line testing.

Lantiq, a communications solutions provider, is a company that actively participates in MELT standardization and works with many first-class system operators and system suppliers to promote related work. Today, MELT has become part of the ITU 996.2 standard (G.lt), a global standard of the International Telecommunication Union (ITU). This standard is used in "single-ended detection, double-ended detection and metal detection of XDSL transceivers".

In the market, the first introduction to field practice was in April 2010, when German network operators began to bring these MELT functions into all future xDSL installations.

Innovative MELT chipset

One of the features of Lantiq's MELT chipset is that it can be placed directly into the DSL line card. The DSL line is accessed through high-ohmic resistors, in other words, the impedance of this method does not change during the exchange process, compared to the relay-based connection. This allows the MELT solution to measure or transmit moisture-proof current without affecting the DSL signal and service. The coupling resistor is connected to the output buffer and measurement amplifier through the high-voltage multiplexer. The output buffer transmits the measurement signal to the line and reflects the processing results of these signals through the measurement amplifier.

The cost-effective and reliable multi-channel MELT solution provided by Lantiq has highly efficient line and loop detection functions to facilitate real-time sound quality and network stability monitoring, and its accuracy exceeds that of external test heads, and can operate simultaneously with DSL connections. In addition to such integrated performance, the MELT solution can also meet the extremely limited space constraints of DSL line cards.

[Figure 3] In Lantiq's MELT solution, its chipset can be directly installed in the DSL line card

Compared to today's line detection solutions, which use a switching matrix to control external test heads or test cards, Lantiq's integrated MELT solution will provide up to 90% cost advantages. At the same time, its extremely low power consumption meets the European "Code of Conduct" (CoC) requirements for energy efficiency and is also compatible with the International Telecommunication Union's ITU 996.2 test standard (G.lt).

The chipset supports all DSL technologies (ADSL, VDSL, and SHDSL) from various manufacturers, and has many additional features compared to traditional relay-based PSTN line detection, such as line card production detection. The solution also provides moisture-proof current transmission (granularity of 16 ports) to meet various requirements such as line reliability and long-term regional loop operation.

At the same time, for detection heads that require customized functions, the pre-defined DC pairing identification tones can still be selected and used in the integrated MELT solution provided by Lantiq.

The MELT solution consists of a 128-channel controller (VINETICTM-LTC128) and a 16-channel high-voltage detection multiplexer (Smart SLIC-TTM-16/MLT-16), and has an integrated multiplexer and AD/DA converter. The line detection controller is responsible for processing the digital detection signal and calculating the corresponding network mode. The Smart SLICTM-T16/MLT16 can be connected to up to sixteen DSL physical lines, and can also transmit moisture-proof current and generate matching identification tones. Since VINETICTM-LTC128 can support eight Smart-SLICTM-T16/MLT16, it will be possible to perform 128 channel detection work with a single line card.

【Figure 4】The latest MELT chip (MLT16) also supports a special transitional solution for those who still use POTS splitters. Since this MELT has the characteristics of high input impedance, it will avoid the access interference to the DSL line caused by the high-pass filter when using the splitter.

With Lantiq's MELT solution, a whole set of tests on a single channel can be completed in just 30 seconds. Lantiq also provides a complete software package that supports different host controllers and meets the protection requirements of ITU K.20. Because this solution builds the functions of MELT and SELT on the same line card, it will make various related measurement tasks easier.

Fully integrated line detection

In order to fully experience and evaluate these new MELT technologies, the company has provided MELT Easy-Boards, a design kit that supports ADSL, VDSL, and SHDSL. This will allow system developers to quickly and easily enjoy the advantages of highly integrated MELT chipsets and significantly reduce overall costs through extremely small space requirements (no relays or switching matrices are required). Line detection and moisture-proof current can be performed without limiting any DSL operation, and regular and parallel detection can fully ensure the operation of the network. In the latest generation of VDSL2 and ADSL2+ chipsets, Lantiq has also gone a step further and integrated the line detection controller into the DSL chip.