**Measurement and Modeling Techniques for the Fourth Generation Broadband Over Copper**

Diogo Acatauassu1, Igor Almeida1, Francisco Muller1, Aldebaro Klautau1, Chenguang Lu2, Klas Ericson2 and Boris Dortschy<sup>2</sup> <sup>1</sup>*Federal University of Pará* <sup>2</sup>*Ericsson AB* <sup>1</sup>*Brazil* <sup>2</sup>*Sweden*

#### **1. Introduction**

262 Advanced Topics in Measurements

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> Digital subscriber lines (DSL), the broadband data transmission technologies that use the copper cable as channel, are the most used Internet media around the world with more than 300 million users (Oksman et al., 2010), Much of the DSL success is related to the cost-benefit for both operators and served consumers. As the transmission channel is the common copper twisted-pair telephone cable, there is no need for large investments in infrastructure, because the telephone network is largely consolidated and active in almost all the world.

> Over the years, DSL systems suffered a step-by-step evolution, being divided in different generations according to the increase of its data rates and technology improvements (Odling et al., 2009). A lot of elements were evolved in this process. Two of them can be highlighted. The first one was the development of modern signal processing techniques to avoid crosstalk effects in DSL transmissions, such as the so-called dynamic spectrum management (DSM) techniques (Cendrillon & Moonen, 2005; Moraes et al., 2010; Oksman et al., 2010; Song et al., 2002). The second one was the reduction of the used copper cable length, resulting in a consequent reduction in the attenuation imposed on the transmission signals. This fact allowed the increase of the used bandwidths, and consequently the data rates.

> The standards of the first and the second generations DSL, matching the integrated services digital network (ISDN) (ITU-T, 1991) and asymmetric DSL (ADSL) family (ITU-T, 1999a; 2003; 2005a), were developed to operate over several kilometers copper cables achieving maximum data rates up to 20 Mb/s. The standards of the third generation DSL, matching the very high speed DSL (VDSL) family (ITU-T, 2005b; 2006), were developed to operate over hundred meters copper cables, achieving maximum data rates up to 100 Mb/s. This case, when users do not live near to the service provider's central office, fiber to street cabinets (FTTC) are inevitable to make the copper loop sufficiently short (van den Brink, 2010).

**2. State of art and standardization of the fourth generation DSL**

cable is illustrated in Figure 1.

**1st Generation**

**2nd Generation**

**3rd Generation**

**4th Generation**

Fig. 1. Comparison of DSL Generations.

van den Brink, 2010).

DSL systems.

Recently, the International Telecommunication Union (ITU) started the fourth generation broadband over copper standardization under the working name G.Fast, where Fast means Fast Access to Subscriber Terminals (van den Brink, 2010). It came due to the desire of Telcos, industries and universities, which observed the potential of this technology, based mainly by the economic point of view. As the costs for installation, digging and putting optical fiber into the ground for every broadband subscriber are so significant, the required investment becomes disproportional to the current market demand (Odling et al., 2009; van den Brink,

Measurement and Modeling Techniques for the Fourth Generation Broadband Over Copper 265

A comparison between the fourth generation DSL proposal and older DSL generations, in terms of desirable data rates, transmission bandwidth and typical length of the used copper

Some initial available works describing the research on the fourth generation broadband over copper can be find in (Acatauassu et al., 2009; Magesacher et al., 2006; Odling et al., 2009;

**< 6 Km copper cables**

**< 6 Km copper cables**

**FTTC < 1 Km copper cables**

**50-300m copper cables** **Up to 20 Mb/s**

**2.2 MHz**

**Up to 10 Mb/s**

**1.1 MHz**

**Up to 100 Mb/s**

**Up to 30 MHz**

**Up to 1 Gb/s**

**Up to 300 MHz**

**FTTH**

A key point of any communication system, does not matter if wireline or wireless, is to characterize the transmission channel. Copper cables have been used by communication systems for more than a century (Chen, 1998), and play the hole of transmission channel in

Twisted-pair copper cables are usually bundled together in a cable sheath with 25 to 100 twisted-pairs. Each wire is also coated with some type of insulating material such as a paper-based (PULP) or a plastic-based (PIC) material. The rate of twisting is usually in the range of 12 to 40 turns per meter. They are also characterized by the diameter of the copper wire (gauge). The ETSI (European Telecommunications Standards Institute) defines the gauges in millimeters, with diameters ranging from 0.9mm to 0.32mm. The ANSI (American National Standards Institute) defines the gauges using the American Wire Gauge (AWG) designation, with typical values ranging from 19 AWG to 26 AWG (Chen, 1998; Yoho, 2001). The electrical characteristics of twisted-pair copper cables are defined using the classical transmission line model, which can be described by an equivalent circuit built up with four frequency-dependent parameters: series resistance R, series inductance L, shunt capacitance C and shunt conductance G. The RLGC parameters are known as primary parameters (Chen,

**(G.fast)**

2010), so there is a gap that can be bypassed by using the existing telephone wiring.

**Voice band modems, ISDN, ADSL** 

**ADSL2, ADSL2+** 

**VDSL, VDSL2**

**Next Standard**

**3. Twisted-pair copper cables characterization theory**

The fourth generation DSL systems will try to explore the copper twisted-pair cable to the maximum, reducing it to few meters and joining it to hybrid optical fiber access architectures, such as fiber to the home (FTTH) (Magesacher et al., 2006; Odling et al., 2009; van den Brink, 2010). The copper cable length reduction process will allow unused frequencies, far above the 30 MHz VDSL2. Recent works described that this frequency values can achieve 100 MHz (Magesacher et al., 2006; Odling et al., 2009; van den Brink, 2010), even reaching 300 MHz, depending on the quality of the used cable (van den Brink, 2010). Results of capacity simulations showed that this transmission channels can achieve near 1 Gb/s, and the use of DSM techniques to avoid crosstalk effects can help ensuring these data rates (Acatauassu et al., 2009; Magesacher et al., 2006; Odling et al., 2009).

It is important to notice that the use of FTTH does not necessary mean that fiber is deployed all the way to a point inside the home. The costs for installation, digging and putting the fiber into every subscriber house are so significant that the required investment is disproportional to the current market (Odling et al., 2009; van den Brink, 2010; Vergara et al., 2010) making it impracticable. So, there will be, in most cases, a lack for the last 50-300m transmission channel, which can be easily used by the existing telephony wiring. To know the behavior of short copper cables, transmitting data in unexplored frequencies, is essential to the development and implementation of the fourth generation DSL systems, and is the focus of this chapter.

The text describes a measurement campaign, that was performed in order to obtain the direct transfer functions and far-end crosstalk transfer functions of 50m, 100m and 200m copper cables. The description of the measurement techniques includes the used equipments, the experimental setup and the reference parameters used during the experiments. Moreover, this chapter introduces a simple procedure for fitting a well-know copper cable model, commonly used for performance evaluation of current DSL standards. The obtained fitted model was compared to the results of the short copper cable measurements, and showed good performance, indicating it can be used in frequency domain-based simulations. In fact, as another contribution, this chapter describes some preliminary simulations results in order to evaluate the fourth generation DSL systems performance, in terms of achievable data rates, optimization of transmission parameters and verification of the rates degradation due to the effects of uncanceled crosstalk.

The text is organized as follows, Section 2 briefly describes the state of art and the standardization effort held by the International Telecommunication Union for the fourth generation DSL systems. Section 3 describes the well-known theory behind the twisted-pair copper cable characterization, and shows some cable reference models used for performance evaluation of the current DSL standards. Section 4 describes a measurement campaign, performed in order to characterize the quality of short copper cables in terms of direct transfer functions (which are related to the loop attenuation) and crosstalk transfer functions (which describe signal leakages of twisted-pairs inside the cables). Continuing, Section 5 describes a modeling technique based on fitting the simplified twisted-pair copper cable model, which is commonly used for performance evaluation of current DSL standards. The data obtained by the developed fitted model is then compared to the results obtained by the short cable measurements. Section 6 shows some preliminary simulations in order to evaluate the performance of the fourth generation DSL systems, including the verification of the achievable data rates and optimization of transmission parameters, and, finishing, Section 7 describes the conclusions of the chapter.

#### **2. State of art and standardization of the fourth generation DSL**

Recently, the International Telecommunication Union (ITU) started the fourth generation broadband over copper standardization under the working name G.Fast, where Fast means Fast Access to Subscriber Terminals (van den Brink, 2010). It came due to the desire of Telcos, industries and universities, which observed the potential of this technology, based mainly by the economic point of view. As the costs for installation, digging and putting optical fiber into the ground for every broadband subscriber are so significant, the required investment becomes disproportional to the current market demand (Odling et al., 2009; van den Brink, 2010), so there is a gap that can be bypassed by using the existing telephone wiring.

A comparison between the fourth generation DSL proposal and older DSL generations, in terms of desirable data rates, transmission bandwidth and typical length of the used copper cable is illustrated in Figure 1.

Some initial available works describing the research on the fourth generation broadband over copper can be find in (Acatauassu et al., 2009; Magesacher et al., 2006; Odling et al., 2009; van den Brink, 2010).

Fig. 1. Comparison of DSL Generations.

2 Will-be-set-by-IN-TECH

The fourth generation DSL systems will try to explore the copper twisted-pair cable to the maximum, reducing it to few meters and joining it to hybrid optical fiber access architectures, such as fiber to the home (FTTH) (Magesacher et al., 2006; Odling et al., 2009; van den Brink, 2010). The copper cable length reduction process will allow unused frequencies, far above the 30 MHz VDSL2. Recent works described that this frequency values can achieve 100 MHz (Magesacher et al., 2006; Odling et al., 2009; van den Brink, 2010), even reaching 300 MHz, depending on the quality of the used cable (van den Brink, 2010). Results of capacity simulations showed that this transmission channels can achieve near 1 Gb/s, and the use of DSM techniques to avoid crosstalk effects can help ensuring these data rates

It is important to notice that the use of FTTH does not necessary mean that fiber is deployed all the way to a point inside the home. The costs for installation, digging and putting the fiber into every subscriber house are so significant that the required investment is disproportional to the current market (Odling et al., 2009; van den Brink, 2010; Vergara et al., 2010) making it impracticable. So, there will be, in most cases, a lack for the last 50-300m transmission channel, which can be easily used by the existing telephony wiring. To know the behavior of short copper cables, transmitting data in unexplored frequencies, is essential to the development and implementation of the fourth generation DSL systems, and is the focus of this chapter. The text describes a measurement campaign, that was performed in order to obtain the direct transfer functions and far-end crosstalk transfer functions of 50m, 100m and 200m copper cables. The description of the measurement techniques includes the used equipments, the experimental setup and the reference parameters used during the experiments. Moreover, this chapter introduces a simple procedure for fitting a well-know copper cable model, commonly used for performance evaluation of current DSL standards. The obtained fitted model was compared to the results of the short copper cable measurements, and showed good performance, indicating it can be used in frequency domain-based simulations. In fact, as another contribution, this chapter describes some preliminary simulations results in order to evaluate the fourth generation DSL systems performance, in terms of achievable data rates, optimization of transmission parameters and verification of the rates degradation due to the

The text is organized as follows, Section 2 briefly describes the state of art and the standardization effort held by the International Telecommunication Union for the fourth generation DSL systems. Section 3 describes the well-known theory behind the twisted-pair copper cable characterization, and shows some cable reference models used for performance evaluation of the current DSL standards. Section 4 describes a measurement campaign, performed in order to characterize the quality of short copper cables in terms of direct transfer functions (which are related to the loop attenuation) and crosstalk transfer functions (which describe signal leakages of twisted-pairs inside the cables). Continuing, Section 5 describes a modeling technique based on fitting the simplified twisted-pair copper cable model, which is commonly used for performance evaluation of current DSL standards. The data obtained by the developed fitted model is then compared to the results obtained by the short cable measurements. Section 6 shows some preliminary simulations in order to evaluate the performance of the fourth generation DSL systems, including the verification of the achievable data rates and optimization of transmission parameters, and, finishing, Section 7 describes the

(Acatauassu et al., 2009; Magesacher et al., 2006; Odling et al., 2009).

effects of uncanceled crosstalk.

conclusions of the chapter.
