A comparison of loop simulation technologies: Passive Line Simulation and Active Line Simulation

1. A comparison of loop simulation technologies:Passive Line Simulation andActive Line Simulation Jack Douglass February 2003 La Meridian, Dallas, TX dsl2003.066.02 Sparnex n.v. jackdouglass@sparnex.com

2. Introduction • Currently deployed ADSL technology • Operating frequency — 4kHz to 1.104MHz • Reliable loop reach — approximately 16kft. • Market demands and new DSL technologies • ADSL+, Long Reach DSL and G.shdsl.bis • New bandwidth requirements —2.2 MHz or more • Reliable operating distances up to 18kft. • DSL technology is now approaching theoretical limit of the telephone lines • The DSL Industry needs a flexible, accurate, reliable and stable Line Simulator • Wider bandwidth capability — greater than 2MHz • Simulate real world line conditions in the lab • Guaranteeing consistent and comparable DSL test results

3. Example of Telephone Line Distribution Twisted copper wire pairs (Loop) between Telephone Exchange or Central Office (CO) and the customer premises

4. Loop is Made of Segments of Twisted Pair Copper Wires The Loop is created by connecting segments of wire together

5. Examples of “Typical or Standard Loops" There are different sets of “Typical or Standard Loops” for every DSL technology, country and region

6. Twisted Pair Copper Wire Parameters • Wire gauge(s) (AWG) or diameter(s) (mm) • Segment(s) Length (m or ft) • Bridged Tap(s) (length, gauge/diameter and location) • Type of insulation (i.e., Paper, PE, PVC, PET, PIC) • Complex Impedance • DC Resistance (ohms) • Line Attenuation Characteristics (Attenuation Distortion) • Group Delay (Envelop Delay Distortion) • Temperature Characteristics • Propagation Delay • White Noise Characteristics (Thermal noise or shot noise) It should be noted that there are many other impairments such as crosstalk (FEXT and NEXT), RFI, impulse noise, dynamic line conditions, and longitudinal balance that affect the performance of DSL modems and need to be considered when testing a DSL modem or system, but they are beyond the scope of this presentation.

7. DSL Industry Needs a Flexible, Accurate, Reliable and Stable Line Simulator • Chip designers, modem venders and DSL service providers need to have a flexible, accurate, reliable and stable way of simulating the characteristics of twisted copper wire pairs in the lab, so that they can: • Accurately simulate any line condition for DSL testing (interoperability, compliance or performance) • Achieve consistent test results from simulator to simulator, one day to the next and from one test lab to another • Accurately reproduce real field lines for any country, region or zone in the lab, so that they can provide the best performing modem for the network • Provide products that push the theoretical limit and meet the market demands for higher data rates and longer operating distances • Provide products that utilize wider bandwidths —2.2 MHz or more

8. Example Basic Cell of a Passive Loop Simulator • Basic Cell (Lumped Elements R, L, and C) of passive loop simulator • A resistor, in some cases, may be added across the capacitor • Characteristics are fixed and focused on primary parameters of R, L and C

9. Passive Line Simulators typically use Line Card(s) to create Line Segments with Fixed Lengths and Characteristics • In a Passive Simulator a loop is typically created by concatenating dedicated line card(s) with fixed lengths and characteristics. • Only one parameter (length or insertion loss) can be varied. • Simulation of physical layer is limited to the available line cards in the simulator.

10. Inherent Limitations of Passive Line Simulation • Focuses on primary parameters of Resistance (R), Inductance (L) and Capacitance (C) • Secondary parameters immerge as a byproduct • Accuracy of the secondary parameters is of primary importance to modem’s operation and performance • attenuation distortion, group delay (envelop delay distortion), propagation delay, complex impedance and DC resistance. • Component tolerances inherently make it to difficult achieve consistent characteristics from simulator to simulator. • Component characteristics vary with the environmental conditions and age • Room temperature differences may cause Day to Day variations of the simulated line characteristics

11. Inherent Limitations of Passive Line Simulation (Continued) • Passive Line Simulation may inherently have unrealistic precursor energy that does not occur on actual lines • Internal cross coupling of signals between Cells • Signals bypasses the intended simulated line • Modem signal cannot travel any faster than the speed of the line (speed of light) • High frequency group delay characteristics of the lumped elements does not match real line characteristics • Precursor energy can be observed in the impulse response of passive line simulation • Adaptive equalizers in modems must use additional taps to deal with this precursor energy • Additional taps may not be required on real lines

12. Typical Impulse Response of a Passive Line Simulator Typical Impulse Response of passive line simulator obtained by numerical inverse LaPlace transformation of an insertion loss network. Unrealistic precursor energy—caused by internal cross coupling between cells and high frequency group delay—leads the pulse

13. Attenuation Characteristics of a typical Passive Line Simulator The ripples in the response are often caused by interference from unwanted signals that bypasses the simulated line inside the simulator

14. Typical Attenuation and Group Delay Characteristics of a Passive Line Simulator • Typical Attenuation and Group Delay (Envelope Delay Distortion) Characteristics of a passive line simulator. • Significant deviation from target characteristics exists above 1 MHz.

15. Inherent Limitations of Passive Line Simulation (Continued) • Bandwidth limitations at high frequencies • Cell design always approximates the line characteristics • Bandwidth characteristics do not reflect real world loops at higher frequencies • Current Passive simulators are relatively accurate out to about 1MHz • Long line are less accurate, because they have more cells • Ideally, the bandwidth accuracy of a line simulator should be at least twice that of the bandwidth requirements of the DSL technology being evaluated • Modem performance may be affected in an unrealistic way

16. Inherent Limitations of Passive Line Simulation (Continued) • Only a relatively small number of loops can be practically implemented in a passive simulator • Each loop is created by switching in and out dedicated hardwired line segments • Every country, region and zone has it own set of “Standard or Typical Loops” that are used for network specific DSL tests. • Hundreds of different loops are specified in the xDSL literature, standards, and technical recommendations • New network specific loops will be specified in the future • It is not practical or economical to have different lines cards or simulators for every scenario • There is no way to easily duplicate the characteristics of a real line in the field in the lab

17. Basic Component of an Active Loop Simulator • Illustration of an Active Filter -- Implemented Filters are more complex • Many stages of active filters are combined in an Active Simulator to create the desired loop characteristics. • Individual parameters can be digitally adjusted to simulate any line characteristic

18. Active Line Simulators use Line Segments with Fully Programmable Characteristics and Lengths • In an Active Simulator a Loop is created by concatenating Line Segments with fully programmable characteristics and lengths • An infinite number of loops and line types can be simulated without changing hardware. • Every segment can be independently programmed according any DSL Standard or County specific network

19. Advantages of Active Line Simulation • Active line simulators focuses on controlling the characteristics of the secondary parameters • Attenuation distortion, group delay (envelop delay distortion), propagation delay, complex impedance and DC resistance • Primary importance to modem’s operation and performance. • Line characteristics are consistent and stable over time and from simulator to simulator • Can be reproduced in any lab and on any simulator • Characteristics are consistent regardless of temperature or ageing of passive components • Hardware tolerance is dominated by network analyzer accuracy during calibration, more than the circuit accuracy

20. Consistent and Stable Attenuation Characteristics Attenuation Distortion Characteristics of three line settings simulated on five different un-calibrated Active Line Simulators. All fifteen cures are superimposed on each other.

21. Consistent and Stable Impedance Characteristics Impedance Characteristics of three typical line settings on five different un-calibrated Active Line Simulators. All fifteen curves are superimposed on each other.

22. Advantages of Active Line Simulation (Continued) • Line parameters can be controlled and programmed • Gauge/diameter, length, insulation material (dielectric), complex impedance, resistance, attenuation distortion, group delay (envelop delay distortion), temperature and propagation delay • One parameter can be varied while holding all other parameters constant. • Almost any line in the condition can be reproduced in the lab by inputting the measured line characteristics • Characteristics of real field lines can be accurately simulated in the lab • Line characteristics be stored in a library and reactivated in seconds • Simulate line characteristics in the frequency range from 5kHz to 4.5MHz.

23. Inherent Limitations of Active Line Simulation • The Noise Floor in an Active Line Simulator is inherently higher than the noise floor in a Passive Line Simulator • Passive simulators are able to achieve lower noise floors • Inherently the resistance of the passive circuits is very low — results in low thermal noise • Noise floor level of approximately –125dBm/Hz • Inherent group delay, impedance and limited bandwidth characteristics of Passive line simulators may have a much greater impact on Modem performance than the -125dBm/Hz Noise floor of an Active Simulator • Except in situations—on very long loops—where crosstalk from other disturbers and noise sources (e.g., RFI) are not dominant

24. Background for -140dBm/Hz Noise Level • A noise floor of -140dBm/Hz is specified in many DSL test standards • Level is based on arbitrary and committee driven levels. • Final draft of ETSI TS 101-524 SDSL specifies a white noise level between –140dBm/Hz and –120dBm/Hz • Source for value of -140dBm/Hz • 1989 Bellcore technical report • Based on what was considered (by some industry experts) to be a reasonable limit for contemporary analog-to-digital converters • Actual background noise level on real telephone lines varies • Measuring or estimating the actual background noise is difficult • Dependent many environmental conditions • Most noise studies on real cable have been limited by the measurement capabilities of the test equipment • Test equipment accuracy is typically between -120dBm/Hz and -140dBm/Hz • An acceptable level of background noise level depends on the capabilities of the DSL system under test and the noise environment in which it has been designed to operate • A noise floor -125dBm/Hz is considered, by many engineers and operating companies, to be a reasonable and acceptable level for DSL testing • Alien (PSD) Crosstalk Noise levels and Impulse Noise levels are higher than the Background Noise level of -125dBm/Hz

25. Alien (PSD) Crosstalk Noise Levels Typical Alien Crosstalk Noise Levels induced by the PSD of various types of interferes are greater than the -125dBm/Hz noise floor of active simulators

26. Typical Impulse Noise Related Spectrum Typical impulse noise related spectrums are greater that the -125dBm/Hz noise floor of active simulators

27. Inherent Limitations of Active Line Simulation (Continued) • Active simulators are not designed for POTS and Voiceband testing applications • Characteristic below 3kHZ are not controlled • DC can be passed for remote feeding • Active Simulators are specifically designed for testing high speed DSL modems and systems that operate above the voiceband (3 kHz)

28. Voiceband Characteristic of an Active Line Simulator • Low Frequency Insertion Loss for lines set to 30, 60 and 90 dB at 1MHz. • Active simulators are not designed for POTS and voiceband testing, since they do not have controlled characteristics below 3 kHz (DC can be passed for remote feeding)

29. Impulse Response of an Active Line Simulator Impulse Response of an Active Line Simulator does not contain precursor energy

30. Attenuation Characteristics of a typical Active Line Simulator Measured and target characteristics closely match

31. Attenuation and Group Delay Distortion Characteristics of an Active Line Simulator • Standard Line Library Characteristics of an Active Line • Line Characteristics optimized up to 1 MHz ( )

32. Curve fitted Attenuation and Group Delay Distortion Characteristics of a Active Line Simulator • Optimized Characteristics — Active line simulators can use Curve Fitting to optimize line characteristics. • Line characteristics optimized to 2 MHz. ( )

33. Typical Technical Specification for an Active Line Simulator • Accuracy of the line configuration attenuation • for 0.5 to 85 dB attenuation @ 1 MHz ± 0.4 dB • for 85 to 99 dB attenuation @ 1 MHz ± 0.7 dB • Attenuation range at 1 MHz 0.5 to 99 dB • Hardware resolution of attenuation setting @ 1MHz ± 0.15 dB • Useful operating frequency range 5kHz to 3MHz • Crosstalk attenuation between ports (20 kHz to 2 MHz) 110 dB • Deviation of group delay (5kHz to 3MHz) 10%

34. Typical Technical Specification for an Active Line Simulator (Continued) • Maximum common mode signal voltage 5 V • Output signal unbalance @ 100 kHz -65 dB • Maximum DC voltage at ports • any wire to ground ± 250 V • between wires ± 400 V • Maximum DC current in ports ± 100 mA • DC loop resistance range 46 to 1316 • Deviation of DC resistance ± 3 % / 5 ohms • Hardware resolution of DC resistance setting 5 ohms

35. Conclusion • Inherently the typical Passive Line Simulator has characteristics that don’t match real world line conditions • Limited bandwidth, group delay and impedance • May result is a false indication of real world performance • Inherently the passive R-L-C concept cannot answer the demand for more accurate line simulation

36. Conclusion (Continued) • Active line simulators can provide the chip designers, modem venders and DSL service providers with the flexible, accurate, reliable and stable line characteristics that they need for: • Designing products that push the theoretical limit and meet the market demands for higher data rates and longer operating distances. • Accurately and reliably simulating any line condition for DSL testing (interoperability, compliance or performance). • Accurately reproducing real field lines characteristics for any country, region or zone in the lab. • Achieving consistent test results from simulator to simulator, one day to the next and from one test lab to another. • Designing products that utilize wider bandwidths — up to 4.5MHz • Simulating line conditions anywhere in the world, because all parameters are individually programmable and adjustable