Overview. IUB, SES P. K. Agyapong, Line Qualification 2

Transcription

1 Line Qualification Presented by Patrick Kwadwo Agyapong International University Bremen Under the supervision of Prof Dr Werner Henkel 26 th November 2004

2 Overview Introduction and Focus Importance of Testing and Prequalification Methodologies for xdsl Loop Prequalification xdsl Testing Needs Measurement Techniques on the Line SELT Conclusion IUB, SES P. K. Agyapong, Line Qualification 2

3 Introduction Digital Subscriber Line (DSL) technology was developed to exploit the existing outside plant (OSP) to allow high-speed data and voice services. Advantages of using the OSP: Ubiquitous No major investments to upgrade the OSP IUB, SES P. K. Agyapong, Line Qualification 3

4 Need for xdsl Testing Testing is needed to Aid in prequalification Identify disturbers in the binder group Verify acceptance criteria for new installations Resolve post-installation issues. Deploying xdsl services is dependent on the quality and design of the OSP, which was, in many cases, designed decades ago to deliver POTS. Prequalification in the current context is the testing of loops to determine if the loop is capable of supporting xdsl transmission prior to attempting to provide service. IUB, SES P. K. Agyapong, Line Qualification 4

5 Methodologies for xdsl Loop Prequalificaion 1. Loop-record examination 2. Single-Ended Measurements 3. Double-Ended Measurements IUB, SES P. K. Agyapong, Line Qualification 5

6 Loop Record Examination Loop records historically were kept on paper, called plats, and more recently are manually entered into a computer database. Problems associated with keeping the records accurate and up to date. A significant percentage of existing loops have incomplete records. IUB, SES P. K. Agyapong, Line Qualification 6

7 Double-Ended Measurements IUB, SES P. K. Agyapong, Line Qualification 7

8 Single-Ended Measurements IUB, SES P. K. Agyapong, Line Qualification 8

9 xdsl Testing Needs Load coil detection Measurement of line length Detection of bridge taps Wide band noise measurements Metallic tests and longitudinal balance IUB, SES P. K. Agyapong, Line Qualification 9

10 Load Coils IUB, SES P. K. Agyapong, Line Qualification 10

11 Load Coils ctd Originally added to subscriber loops longer than 18,000 feet to improve voice quality by low pass filtering action Since xdsl transmission techniques rely on frequencies above the voiceband, the transmission of any xdsl on a loop with load coils is impossible. Thus, it is essential for prequalification to include the capability to determine if load coils are present in a circuit. IUB, SES P. K. Agyapong, Line Qualification 11

12 Line Length and Gauge All xdsls are sensitive to the line length and the gauge of wire between the CO and the customer s premises. The transmission rate of all xdsls is approximately inversely proportional to loop distance. The wire gauge is also important; typically the distance supported for 26AWG is only two-thirds of the distance supported for 24AWG. IUB, SES P. K. Agyapong, Line Qualification 12

13 Line Length and Gauge ctd IUB, SES P. K. Agyapong, Line Qualification 13

14 Bridge Taps IUB, SES P. K. Agyapong, Line Qualification 14

15 Bridge Taps ctd A bridge tap is any unterminated portion of a loop not in the direct talking path between the CO or DLC and the customer s premise equipment (telephone or modem). The length of a bridge tap is the primary factor that determines if xdsl transmission will be impaired not simply the presence of a bridge tap. The nearer the bridged-tap to the modem, the more adverse it s effects. IUB, SES P. K. Agyapong, Line Qualification 15

16 Wideband Noise Measurements Crosstalk noise caused by the presence of other transmission in the same binder group adversely affects some xdsls. The systems have been standardized such that they are usually most disturbed by systems that use the same kind of bandwidth. The ability to recognize signature frequencies will aid in loop conditioning. IUB, SES P. K. Agyapong, Line Qualification 16

17 Wideband Noise Measurements ctd Line qualification should include wideband noise measurement to help isolate xdsl transmission problems. For 3-Pair HDSL (2B1Q), one would need 2048/2/3 khz as a spectrum to do measurements. Noise measurements up to 1.1 MHz will be required to determine if a loop can support ADSL transmission. The identification of T1 circuits or other DSLs can be determined by looking for their characteristic frequencies. IUB, SES P. K. Agyapong, Line Qualification 17

18 Metallic Tests and Longitudinal Balance An unbalanced circuit caused by unbalanced longitudinal currents or power-line harmonics can cause crosstalk noise, causing bit errors that result in slower xdsl transmissions. The longitudinal balance measurements of the loop are important to ensure efficient delivery of xdsl services. Standard metallic fault testing should be included in prequalification The general quality of the outside plant can be determined with the standard tip-to-ring (T-R), tip-to-ground (T-G), and ring-to-ground (R- G) parameters including DC voltage and resistance along with AC voltage, resistance, and capacitance. IUB, SES P. K. Agyapong, Line Qualification 18

19 Gaining Access to the Copper Loop for Testing Switches DSLAM External MTAUs IUB, SES P. K. Agyapong, Line Qualification 19

20 Performance Requirements for Metallic Access IUB, SES P. K. Agyapong, Line Qualification 20

21 Subscriber Loop Structure Twisted pair cables can be characterized in terms of a two-port system Characteristic Impedance, transfer function, and insertion loss are the important characteristics of any subscriber loop. IUB, SES P. K. Agyapong, Line Qualification 21

22 Measurement Techniques on the Line 1. Time Domain Reflectometry (TDR) 2. Sequence Time Domain Reflectometry (STDR) 3. Frequency Domain Reflectometry (FDR) 4. One port scattering parameter, S_11 or echo response 5. Power Spectral Density (PSD) of noise in the line IUB, SES P. K. Agyapong, Line Qualification 22

23 Time Domain Reflectometry (TDR) Systematic method of sending a RF pulse into a system and measuring the reflections at the input port to determine information about the system content, such as length to an impedance change, values of the reflection coefficients and impedances at the discontinuities. Works because of impedance mismatches. IUB, SES P. K. Agyapong, Line Qualification 23

24 TDR ctd IUB, SES P. K. Agyapong, Line Qualification 24

25 What can be obtained from a TDR Distance to opens/shorts Location and length of bridged taps Presence of load coils Essential property of TDR is the ability to distinguish discontinuities and the time separation between them. Separation in time allows the determination of distances to mismatches, whereas the shape of the reflected wave reveals both the nature and magnitude of the mismatch. IUB, SES P. K. Agyapong, Line Qualification 25

26 What can be obtained from TDR ctd T is the transit time from monitoring point to the mismatch and back again. Propagation delay along the cable is typically 0.5us/km. Distance to the mismatch can be obtained by IUB, SES P. K. Agyapong, Line Qualification 26

27 What can be obtained from TDR ctd A bridge tap terminated with an open will exhibit a first echo resembling a short and a second echo resembling an open. The first echo has negative polarity because the bridged tap is in parallel with the remaining portion of the line. The positive echo is reflected from the open at the end of the bridged tap. The length of the bridged tap can be found by computing the distance between the negative and positive echoes. TDR detects only one load coil. Other methods needed to determine multiple load coils IUB, SES P. K. Agyapong, Line Qualification 27

28 What can be obtained from TDR ctd Bridge tap detection with a TDR IUB, SES P. K. Agyapong, Line Qualification 28

29 What can be obtained from TDR ctd End of loop IUB, SES P. K. Agyapong, Line Qualification 29

30 TDR Displays for Typical Loads IUB, SES P. K. Agyapong, Line Qualification 30

31 TDR Two parameters are available for adjustment namely the transmit level of the pulse the duration of the pulse Also the types of disturbers present in the binder may necessitate modifying the transmit signal. To assist with weak reflections and mitigating the effects of loop noise, typically a train of impulses is used and the reflections averaged. IUB, SES P. K. Agyapong, Line Qualification 31

32 Sequence Time Domain Reflectometry (STDR) Uses concepts from direct sequence spread spectrum communications to enable the TDR functionality to be incorporated into a DSL transceiver integrated circuit. Offers improved spectral compatibility, interference immunity and fault resolvability. IUB, SES P. K. Agyapong, Line Qualification 32

33 STDR Theory of Operation Digital signaling with a sequence of digits (information bit is spread or modulated with a pseudo-random noise (PN) sequence). PN sequence removed at the receiver through a correlation operation thereby mitigating various forms of narrowband and multiple access interference. Sequence is chosen such that one period of its periodic autocorrelation function closely resembles the Kronecker delta function. This type of signal can be used for TDR, because using this sequence signal has the equivalent effect of transmitting an impulse down the line. IUB, SES P. K. Agyapong, Line Qualification 33

34 STDR Theory of Operation ctd Proof Consider a discrete time system Let s(n) be a periodic extension of a sequence m(n) of length M and h(n) be the impulse response of the echo channel Assuming the receiver is matched to m(n) then the correlator output is given by : c(n) = (h(n) * s(n)) * m(n) = h(n,m) Where * is the discrete convolution operator and h (n, M) is the periodic extension of h (n) given by IUB, SES P. K. Agyapong, Line Qualification 34

35 STDR Theory of Operation ctd Proof ctd o By choosing M greater than the round trip propagation delay of the discrete time equivalent channel, a TDR reflectogram will correspond to one period of h (n, M). IUB, SES P. K. Agyapong, Line Qualification 35

36 STDR Theory of Operation ctd Typical sequences, which satisfy the Kronecker Delta include maximal length or m-sequences, gold sequences and Kasami sequences. m-sequences have a sequence length of (2^m) - 1. Advantage of m-sequences is that they exist for a wide range of periods, M, thus providing flexibility in trading fault identification with complexity. IUB, SES P. K. Agyapong, Line Qualification 36

37 STDR Implementation IUB, SES P. K. Agyapong, Line Qualification 37

38 Frequency Domain Reflectometry (FDR) Originally developed as a microwave design and measurement tool Has a very high dynamic range (signal-to-noise ratio) in frequency domain ( >100dB) Measures voltage versus frequency IUB, SES P. K. Agyapong, Line Qualification 38

39 FDR IUB, SES P. K. Agyapong, Line Qualification 39

40 Steps for FDR A sweep signal (sinusoid) is transmitted over the loop to identify frequencies that either resonate or are dead. A portion of the transmitted sweep signal on the transmission line will be reflected back to the transmitter if the load is not a perfect impedance match The reflected energy will be the same frequency as the incident (sweep) signal, but different in phase. The resulting signal (incident + reflected) will appear as standing waves on a frequency sweep. IUB, SES P. K. Agyapong, Line Qualification 40

41 What does the FDR tell us? Peaks in the measured receive signal correspond to frequencies that create standing waves which provide information about the length of the cable. The reflection is such that the peaks of the individual cycles can be translated to distance to the fault (impedance mismatch) through the following equation where D = distance to the fault, c = speed of light, NVP = relative velocity of propagation along the cable and f = frequency of 1 cycle of the standing wave. IUB, SES P. K. Agyapong, Line Qualification 41

42 What does the FDR tell us? ctd The peak-to-peak value of the reflection determines the magnitude of the fault and is calculated by the equation: Magnitude Return Loss (db) = 20log(Reflected Peak-to-Peak) Analysis can be made either in the frequency domain or in the time domain, by performing an IFFT IUB, SES P. K. Agyapong, Line Qualification 42

43 Comparison of TDR and FDR IUB, SES P. K. Agyapong, Line Qualification 43

44 Comparison of TDR and FDR ctd IUB, SES P. K. Agyapong, Line Qualification 44

45 One-Port Scattering Parameter Measurements A transmission line is completely characterized by its transfer function and characteristic impedance, which are also called the secondary transmission line parameters Line qualification requires estimating channel capacity, which will be most accurate if the transfer function of the line and the noise PSD at CO and CP are measured directly -> One port scattering parameter IUB, SES P. K. Agyapong, Line Qualification 45

46 One-Port Scattering Parameter IUB, SES P. K. Agyapong, Line Qualification 46

47 Measuring the S_11 Scattering Function Subscriber loop can be characterized by four independent variables called S-parameters. S_11 and S_22 are forward and reverse reflection parameters respectively S_21 and S_12 are the forward and reverse transmission parameters IUB, SES P. K. Agyapong, Line Qualification 47

48 Measuring the S_11 Scattering Function ctd We are interested in measuring the reflection of the forward wave, i.e. the S_11 scatter function. In this case, the subscriber loop is attached only to one port (measuring port of the test equipment). Hence a_2 = b_2 = 0 and S_11 = b_1/a_1 IUB, SES P. K. Agyapong, Line Qualification 48

49 S_11 Scattering function Estimating the transfer function and channel capacity Models for the scattering parameter have been proposed for various loop configurations/topologies. From knowledge of S_11 and by using a five-parameter model of a twisted-pair telephone line (VUB0 Line Model), the transfer function of the actual line could be estimated. The channel capacity can then be estimated using the computed transfer function as well as noise PSD at CO and CP More of this in reference [6]. The focus of the paper was to determine channel capacity rather than impairment location IUB, SES P. K. Agyapong, Line Qualification 49

50 One Port Scattering Parameter Determination of Loop Topology Similar to FDR, but instead of looking for individual frequencies, a complete echo response measurement is utilized. From the echo response, the input impedance of the loop can be determined, from which the loop topology can be determined. IUB, SES P. K. Agyapong, Line Qualification 50

51 Insertion Loss Measurements Double Ended measurement Frequency Response Based on the fact that a signal will attenuate as it travels along the line The loss for a particular frequency increases with distance and higher frequency signals typically have higher attenuation than lower frequency signals. IUB, SES P. K. Agyapong, Line Qualification 51

52 Insertion Loss Measurements ctd A signal generator and receiver are required at either end of the line The generator transmits a signal at a specific frequency and power level The receiver measures the signal at the same frequency and reports the received power level. Analysis can then be made based on the received power level about the nature and length of the line. A lot of parameters can be deduced from such measurements. IUB, SES P. K. Agyapong, Line Qualification 52

53 Power Spectral Density (PSD) of noise on the line In this measurement, the transmitter is quiet, and the modem measures the loop noise present. IUB, SES P. K. Agyapong, Line Qualification 53

54 Single-Ended Loop Testing (SELT) ADSL2 standard has provisions for dual-ended loop testing (DELT) using coordinated transmission and measurement between the CPE and CO modems. Single-ended loop testing provides an alternative, where measurements can be carried out at only one end of the loop. Aim of current work is to provide SELT through the line card. Currently, SELT is not standardized. Work on G.selt is ongoing in the ITU. An overview of the G.selt functional model agreed so far is given. IUB, SES P. K. Agyapong, Line Qualification 54

55 SELT ctd A functional model, composed of three layers: 1. Data Gathering (Measurement) 2. Data Analysis 3. Data Distribution IUB, SES P. K. Agyapong, Line Qualification 55

56 SELT ctd Data Gathering (Measurement) Some of the standard measurement techniques that can be used for SELT include: TDR FDR One-Port Scattering Parameter PSD of noise on the line IUB, SES P. K. Agyapong, Line Qualification 56

57 SELT ctd Data Analysis The data analysis layer is responsible for using the SELT measurements to estimate a variety of parameters and DSL capabilities associated with the loop, including: Loop Topology Loading Coils Loop Attenuation/Insertion Loss Crosstalk profile Far-end loop noise Achievable downstream and upstream data rates IUB, SES P. K. Agyapong, Line Qualification 57

58 SELT ctd Data Distribution The final step for SELT is the distribution of the estimated parameters into the OSS. It is necessary to integrate the control of and the results for SELT into a form that can be used by relevant personnel. Today, this level of integration does not exist for DSL. However, it does exist for MTAU equipment, and it is expected that the technology coud be reused. IUB, SES P. K. Agyapong, Line Qualification 58

59 Comparison of Line Testing Technologies IUB, SES P. K. Agyapong, Line Qualification 59

60 Comparison of Line Testing Technologies ctd IUB, SES P. K. Agyapong, Line Qualification 60

61 Conclusion and Outlook The measurements undertaken in single ended measurements are not highly accurate and more work need to be done to make them reliable. The need for loop qualification cannot be overemphasized. Some telcos like DT still rely on loop records. With the exception of the PSD measurement, all other line measurement techniques contain identical information about the loop. Thus all the measurement techniques can provide identical information about the loop. Most of the testing technologies are not automated and incorporated in the xdsl modem. Current trends in research seek solutions where several automated measurements can be undertaken at once quickly and cost-effectively. Single-Ended Loop Testing (SELT) is a promising candidate in this direction. For more information check [1] IUB, SES P. K. Agyapong, Line Qualification 61

62 References 1. Peter J.W. Melsa & Krista S. Jacobsen, Single-Ended Loop Testing Expectations and Results, Texas Instruments, SPAY011A-March 2003 White Paper 2. William W. Jones & Keith R. Jones, Sequence Time Domain Reflectometry (STDR) For Digital Subscriber Line Provisioning and Diagnostics, March Stefano Galli & David L. Waring, Loop Makeup Identification Via Single Ended Testing: Beyond Mere Loop Qualification, IEEE Journal on Selected Areas in Communications, Vol. 20, No. 5, June 2002 ( ) 4. Jean Schmitt, Why use FDR or TDR measurement techniques for checking subscribers loops providing xdsl services?, Acterna IPMS, 04/04/2003, FDR and TDR Measurements V1r4 5. Agilent Technologies Time Domain Reflectometry Theory, Application Note Bostoen et al, Estimation of the Transfer Function of a Subscriber Loop by Means of a One- Port Scattering Parameter Measurement at the Central Office, IEEE Journal on Selected Areas in Communications, Vol. 20, No. 5, June 2002, ( ) IUB, SES P. K. Agyapong, Line Qualification 62

63 Thank you for your attention