INTERFERENCES ON THE TWISTED PAIR IN THE EMPLOYMENT OF THE XDSL SYSTEM

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1 46th International Symposium Electronics in Marine, ELMAR June 2004, Zadar. Croatia INTERFERENCES ON THE TWISTED PAIR IN THE EMPLOYMENT OF THE XDSL SYSTEM Romeo Svaha, Natalia MatoSiC and Wmton AfriC Croatian Telecom natalia.matosic~l~t.hr Abstract: This paper describes main interferences on the twisted pair important in planning and implementing the XDSL technology. First part of this paper describes fundamental types ofthe interferences which have influence on the implementation of the XDSL technology. Second part ofthis paper provides basic characteristics of the other interferences which can appear on the subscriber line. End of this paper gives conclusion emphasizing what should be applied and included in the procedure ofprojecting stationa y access networks with XDSL technology. Key words: Copper pair, twisted pair cable, XDSL technology, interference INTRODUCTION The copper twisted pair is one of the most unpredictable and changeful transmission media, which new communication technology, xdsl, has to deal with. Transceivers, which work on the copper twisted pair, are one of the most complexes. They have to deal with different heavy conditions that appear on the transmission media. These conditions are not exactly known. One of the main characteristics of the copper pair happens during transmission signal process across copper pair signal is signal distortion due to the interference conditions. Some of the telecommunication cables electrical parameter of the symmetric pair, like shirt and armature, can be significantly different than is a normal value. Due to these reasons cable can not fulfill its purpose. Therefore during exploitation period of the cable, periodical measures of the cable and twisted pair characteristics have to be done. In the area under electrical field influence, where cable shirt as a system for protection is used, and where there are errant current, periodical measurements of the electrical parameters on the cable shirt (Pb, AI) and on the cable armature (Fe) are required. Proficiency of the exploitation measurement equipment and right choice of the measurement methodology are very important for development and maintenance of the digital system. The wrong choice of the measurement methodology can caused significant errors during measurement and bringing incorrect conclusions. This article gives a survey of the communication channel interferences which are important for the xdsl technology implementation. 178

2 46th International Symposium Electronics in Marine. ELMAR2004, 1618 June 2004, Zadar. Croatia 1. INTERFERENCES ON THE TWISTED PAIR Distortions of the signal increase with its bit rate (frequency). In order to obtain higher bit rate in the communication cannel the process of the avoiding bit errors should be considered carefully. Bit error rate in the communication channel can be caused by some of the following parameters: the electrical collusion between lines, difference of the potential points, usage of the transmission media, variation of the power supply and other. Twisted pair is a physical line which consists of two copper wires. Each of these wires is isolated. Commonly used insulators are made of the polyvinyl, paper or polyurethane. Twisted pair is usually situated with others pairs in the cable. Subscriber cable usually consists of fifty pairs. In the access network to the subscriber is not only one uninterrupted pair, but few interconnected pairs. These interconnected pairs can have different electrical characteristic. On the twisted pair in the POTS network there are five different types of the interference: noise, attenuation, crosstalk, echo and inter symbol interference. Decrease of the bit rate is the consequence of all of these interferences Noise Noise has influence on the all of the communication model elements. It is the major problem in the data transmission across twisted pair. The notion noise includes the appearance of the spontaneously fluctuations of the signal. These fluctuations are caused due to crosstalk, interference, distortions of the signal or some other reasons. The noise can be divided into the noise on the internal channel, the noise which is generated in the channel, and external noise which incomes from the surrounding. The external noise is caused by external influence on the channel, such as neighborhood sources of the static electricity, atmospheric occurrences, sun eruptions, disturbances of the electrical installations and neighborhood equipments. Due to the wrongful influence which noise gives on the channel redundancy coding will be used. f"(1/2) model Frequency (KHz)... J transmission function Fig. 1. Frequency characteristic ofthe attenuation, oftwistedpair 0.5 mm, on the 4 km length (model and real transmission function) A special kind of the noise is Additive white Gaussian noise (AWGN). AWGN has various different origins: thermal noise and shot noise. A thermal noise is the arbitrary movement of electrons in conductor while intensity of this movement depends on the temperature. 179

3 46th International SvmDosiurn Electronics in Marine. ELMAR June Zadar. Croatia The source of shot noise is the energetically discrete movement of electrons and wholes in semiconductors. The main problem of Impulse noise is its irregularity in occurrence and intensity. The main parameters are impulse length, interarrival time and the impulse amplitude. A general model is very difficult to describe because of the time length which has large deflection from location to location, and variation of the interarrival time Attenuation Attenuation of the signal is decreasing of the signal during transmission across the lines. Attenuation is the consequence of the transmission loop function, i.e. skin effect in the conductor. Due to frequency increase conductor intersection decreases and the consequence is resistance and attenuation increase. Also, due to frequency increase some losses appear in the conductor isolator. Higher frequency signal are more attenuated than lower frequency signals. As signal is transmitted in wide frequency specter, it changes its shape. Attenuation increases proportionally with second root of the signal frequency, and also with length of the transmission line. Fig. 1 shows attenuation frequency characteristic Crosstalk Crosstalk is the interference that occurs between twisted pairs in the same cable or between neighboring cables [2]. Crosstalk is mixing of the signals between neighboring transmission lines. Crosstalk happens due to capacities and inductivities connections between neighboring pairs. Signals that arise as a consequence of the crosstalk are designed as: x.(t) for crosstalk on the near end, (NEXT) and x+(t)for crosstalk on the far end, (FEXT). For different use of the XDSL technology first or second type of the crosstalk is a dominant. For example: For ADSL technology FEXT crosstalk is dominant, for FTTC and VDSL technology FEXT crosstalk is dominant, and for HDSL technology NEXT crosstalk is dominant. In the crosstalk interference analysis different models are considered. In the DSL system analysis WCS (Worst Case Scenario) model is very often used. This model considers the worst possible case in the cable. That is a case where all interference sources generates HDSL signal. Cable usually consists of 50 pairs, so there are 49 sources of the crosstalk. All of these sources are active in same time. If in the cable, twisted pairs are not separately shielded, there is always some process of crosstalk. Pairs are typically unshielded (example, 10BaseT type). Let consider case when only one pair gives influence to the observed pair. Denoted with V,(f), X.(d,f) i Xm(x,f) corresponding Furrier transform of the signals v,(t), x.(t) i XU, (t). Let suppose that the pairs are concluded with its characteristic impedance, spectral characteristic of the NEXT and FEXT can be described as in the equations (1) and (2): d (1) X(d,f) = j2nfv.(f)~c.(x,f)~~~~~'"dx d X+(d,f) = j2nfv.(f)ey(')d jcm(x,f)dx

4 46th IntematiOnal Symposium Electronics in Marine. ELMAR2004, 1618 June 2004, Zadar, Croatia Measurement on the real twisted pairs in the major part of the cases shows correspondences with previous equations. Some cases give multiple null points in the spectrum that are consequence of the unbalance points in the cable. Traditionally, the telecommunications industry has characterized crosstalk in terms of power sums. For a given pair in a binder group, the powersum NEXT is formed from the sum of the pairtopair NEXT coupling powers of the other pairs in the binder group to this given pair. For a typical 25pair binder group in a 1000ft long PIC cable [4], the 25 power sums are shown in Fig. L Frequency(r4 Hz) 1.4. Echo Fig. 2. Power sum NEXTloss Echo is the phenomenon of returning sent signal from the receiving side of the line. In the bidirectional lines echo causes interference on the receiving side in the opposite direction. Also, echo causes considerable problems in the receiver due to erroneous understanding of the signal. Echo as the phenomena is the consequence of the unbalanced line. Elimination of the echo phenomena can be obtained by using two approaches: Echo Suppressors and Echo Cancellers. The intensity of echoes depends on the amount of discontinuity and the distance between the echocausing discontinuity and the sender to which the echo is traveling back Inter symbol Interference The problems caused by the nonideal channel transfer function have only been analyzed in the frequency domain, but some problems also appear in the time domain. Inter symbol interference happens because of amplitude phase distortion and causes noise increase and deceleration of the transmission process. Propagation speed, which is frequency dependant, leads to a dispersive behavior of the channel that results in spreading of a time sample into neighboring time slots. Inter symbol interference increases with transmission bit rate and causes errors increase in the receiver, which can be reduced by using equalization equipment [3]. This paper does not deal with other interferences.. 181

5 461h International Symposium Electronics in Marine, ELMAR2004, 1618 June Zadar, Croatia 2. ADSL TECHNOLOGY IN THE CROATIAN TELECOM From the beginning of year 2004, for the purpose of the high speed Internet access, (ADSL Full 8 Mb/s downstream) in the HT company intensive measurements of the quality on the subscribers cables have been performed. The purpose of these measurements was cables quality testing in order to implement different types of the wideband services [l]. For twisted pair s measurements, measurement instrument (equipment) ALT2000 was used. Noise single 5ided test LOOP UnderTest DMM Remote management Master Fig. 3. Return loss measurement This equipment enables application of different measurements methods and can be used for wide types of the measurement on the twisted pair, such as: noise, return loss, attenuation, longitudinal balance, impulse noise, spectral analysis, and so on. Using this instrument the following measurements can be performed: XDSL, POTS, ISDN, Modem, El, T1, HDSL, SDSL and G.SHDSL systems lines and equipments. Slave Return 1088 For pairs on the cable 05 STI frequency (KHr) m 4s zm 245 Eo 95 38) 425 4m ex 850 ~ 8 6 MI 785 w rn sze ES ma nm Line mask I Measurement results :, : j Fig. 4. Return loss measurement result on the cable 05 ST I for pairs

6 46th International Symposium Eledronics in Marine, ELMAR June 2004, Zadar. Croatia The following measurements in Croatian Telecom have been performed: return loss, longitudinal balance and bit rate evaluation. An example of the measurement of the return loss is shown on the Fig. 3 Measurement date: February, 16,2004. Cable type: 05 ST1 TK OOV 5OOx4x0,4 (Pb,1972.), number of the pairs , cable length 890 m. Relation: Trg brace RadiCa, Streets Kralja Tomislava TonEiCeva Marmontova Bana JelaEiCa and Obala Ante TrumbiCa Figure 4. shows irregularity between sending and reflecting signals (reflected signal fiom the opposite end of the line). Return loss is the consequence of the mismatched line terminations. Measurement of the return loss gives result which described total quality of the cable installation. 3. CONCLUSION Procedure of the projecting existing and particularly new stationary access networks must include protocol of absolutely necessary measurements on twisted pairs. Planning of the new modern XDSL broadband networks is not only determined by measurement protocol but also conditions of the copper network quality and defmed exploitation. Material quality measurement is the basic preparation part in the telecommunication cable production. These measurements are defined by electrical parameters of cable constructive elements which should be within predefined technical limits. In this paper the general environmental conditions for highspeed digital data transmission in the telephone loop plant has been investigated. The loop plant with its elements has been described and their effects for digital data transmission have been pointed out. The impairments for ADSL and VDSL in the telephone loop plant have been treated and as well as numerical calculations for the strength of NEXT, FEXT, AWGN and residual echo noise. The theoretical limits of digital data transmission over telephone loops for infinite and finite complexity, as well as for practical implementations with operational security margins, have been computed. There it had been shown that the achievable ranges for ADSL are highly dependent on the population of services in the same binder group and NEXT has been shown to be the dominant impairment. REFERENCES: [l] G.H. Im i J.J. Werner: Bandwidth efficient digital transmission over unshielded hvistedpair wiring; IEEE J. Select. Areas Comrnun., vol. 13, pp , Dec [2 ] James T. Aslanis, Jr. and John M. Cioffi, Achivable Information Rates on Digital Subscriber Loops: Limiting Information Rates with Crosstalk Noise, IEEE Transactions on Communications, vo1.40 no.2,veljaea [3 ] [4 3 S.Galli, C.Valenti, and K.J. Kerpez : A FrequencyDomain Approach to Crosstalk Identification in xdsl Systems, IEEE 5. Select. Areas Commun., vo1.19, No. 8,1947,Aug

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