DCM555 - Data Communications Lab 8 Time Division Multiplexing (TDM) Part 1 - T1/DS1 Signals Name: St. #: Section: (Note: Show all of your calculations, express your answer to the appropriate number of significant digits (at least three) and use the correct units in your answer. Place your answer in the space provided.) Lab Objectives to gain experience using industry standard telephony test equipment (Wandel & Goltermann DSA-15) to generate and examine T1/DS1 signals in the time and frequency domains to generate and examine the Alternate Mark Inversion (AMI) encoding in relation to clock recovery to examine Binary Eight Zero Substitution (B8ZS) and arbitrary data patterns on the T1/DS1 signal to observe the effect of improper transmission line termination on the T1/DS1 signal Components Required none Equipment Required Wandel & Goltermann DSA-15 (DS-1 Signaling Analyzer) 12VDC AC adaptor 1 - T1/DS1 balanced patch cord Agilent 54641D 350MHz Digital Oscilloscope Lab Exercise 1. Connect the 12VDC AC adaptor to the lab bench power bar. Connect the power cable to the back panel of the Wandel & Goltermann DSA-15 where indicated on Figure 1. Power Cable Jack Tx Output Jack Figure 1 2. When the power cable is connected, the DSA-15 may begin to beep repeatedly. This is an indication that the gel cell battery is undercharged. When the beeping stops, the battery is charged sufficiently to turn the power on. 1 of 9
3. On the DSA-15 front panel, turn on the power. Connect the T1/DS1 balanced patch cord to the Transmit (Tx) Output Jack at the back panel (see Figure 1). 4. The T1/DS1 patch cord is a Shield Twisted Pair cable (STP). The cable plug has three electrodes: tip, ring and ground/shield, as shown in Figure 2. The signal is carried on the balanced line formed by tip and ring conductors. Connect the alligator clips from a test lead as shown in Figure 2 with black to the tip and red to the ring, carefully avoiding any shorts between tip, ring and ground. Ring Tip Black Lead Ground/Shield Red Lead Figure 2 5. Use the BNC end of the test lead to the Channel 1 input of the Agilent 54641D oscilloscope. Set the scope input impedance to an equivalent input impedance of 50Ω by connecting the 50Ω terminator to the BNC TEE connector as shown in Figure 3. This terminates the bantam Tx line and prevents most of the reflections. Set the scope for 1V/div and 5µs/div. (NOTE: Strictly speaking the bantam cable should be terminated in 100Ω, but 50Ω is available, and serves as a reasonable approximation.) Also, adjust the scope to trigger on Ch. 1 at 1V on the rising edge, and Save/Recall > Formats > TIF. Oscilloscope DSA-15 Ch.1 or Ch. 2 Figure 3 BNC Tee Connector 50Ω Terminator NOTE: Never set the input impedance of the lab oscilloscope to 50Ω, or you will get ZERO mark for your lab (because someone else could damage the scope)! 6. The DSA-15 control panel is menu driven. The arrow keys can be used to navigate through most menus. Selections from the secondary menu appearing along the bottom of the LCD display can be made using the function keys (F1, F2, etc.). The screen contrast, brightness and lighting can be controlled using the three keys to the left of the ON key. (See Figure 4.) From the Main Menu, select "Bit Error RatioTesting", then Patterns > BERT slots pattern. Ensure that the QRSS pattern is selected. 2 of 9
Function Keys Main Menu Arrow Keys Contrast Adjust Figure 4 Brightness Adjust 7. The "Troubleshooter" function generates a T1/DS1 signal on the Tx output terminal. When the QRSS pattern is selected, the twenty four T1 channels are populated with pseudo-random binary data (PRBS). Push the Main Menu button, then Select "Troubleshooter". This initiates an "Auto-configuration" of the T1/DS1 signal. Recall that the DS1 line code format is Return-to-Zero Alternate Mark Inversion (RZ-AMI). Push the Run/Stop button to stop capturing and examine the waveform to confirm that this is so. 8. It is standard practice to measure pulse widths from the half-way point of the pulse. This pulse width is called Full Width at Half Maximum (FWHM) as shown in Figure 5. 100% 50% Full Width at Half Maximum (FWHM) 0% Figure 5 All pulse-related times (including bit times) should be measured from the half way point of the pulse. Stop capture by pressing the scope Run/Stop button and change the time base to 200ns/div to zoom in on the waveform. Use the scope cursors to measure the pulse width and the bit time and record them below. Note that in order to measure the bit time, you will have to scan across the waveform time base to find two successive "1" RZ-AMI symbols. Save the time domain waveform. Pulse width = Bit Time = 3 of 9
9A. Commence capture by pressing the scope Run/Stop button. Set the time base to 20µs/div. Push the Math button on the scope and select FFT. Push the Settings softkey and make the following selections: Source = Ch. 1 Span = 5MHz Centre Frequency = 2.5MHz Push the More FFT softkey and make the following selections: Scale = 10dBV Offset = 27~ 30dBmV Window = Hanning Push the Ch. 1 button twice to turn it off so that we can examine only the frequency domain of the DS1 signal. Push the Acquire button and select Averaging. Select # of Averages as 32. Use the x cursors to determine the first two spectral null frequencies. Record them here. 9B. Describe what is unusual about the spectral nulls for the DS1 signal: What is the significance of the peak in the first spectral null? (Hint: Measure the peak s frequency.) Save the frequency domain display. 10. Push the Acquire button and select Normal. Turn off the FFT display and turn the Ch. 1 time domain display back on with 2V/div and 5µs/div settings. Remove the 50Ω terminator now so that the actual load is the scope termination of 1MΩ. Stop capturing and zoom in on the DS1 waveform and scan across. Describe what you see? What is causing the waveform to do this? Save the time domain waveform. 11. Set the time base to 20µs/div then turn off the time domain. Turn the frequency domain back on again, selecting the Averaging Acquisition Mode as before. Describe the difference between this spectrum and that in Step 9: Turn the time domain back on. Connect the 50Ω terminator back so that the actual load is now roughly 50Ω and change the Acquisition Mode back to Normal. 4 of 9
12. You are now going to manually control the data in the T1/DS1 signal. Go to the Main Menu and select "Bit Error RatioTesting" (BERT). Use the down arrow to "Patterns" and select it with the F1 key. The first menu item is "BERT slots pattern". As mentioned before, this determines the data which fills out the twenty four T1/DS1 channels. 13. Change the "BERT slots pattern" from "QRSS" to "1:7" (function key F2). Select the function key for "next" and then for "Run". Based on the observed waveform, what does the "1:7" pattern mean? Use the cursors to measure the period of this pattern (i.e. the time between successive starts). Save the time domain waveform. Pattern Duration = 14. Return to the Main Menu and select "Bit Error RatioTesting" then "Patterns" again. This time use the "more" option to select the "1111" pattern, followed by "next" then "run". Based on the observed waveform, what does the "1111" pattern mean? Save the time domain waveform. 15. Once again, return to the Main Menu and select "Bit Error RatioTesting" then "Patterns". Use the "more" option to select the "0000" pattern, followed by "next" then "run". Based on the observed waveform, what does the "0000" pattern mean? 16. Return to the Main Menu and select "Bit Error RatioTesting" then "Interface". In this sub-menu, change the "Line Code" to "B8ZS", followed by "next", "next" and then "run". Observe the waveform on the oscilloscope. Describe what do you see? Use the cursors to measure the period of this pattern (i.e. the time between successive starts). Save the time domain waveform. 5 of 9 Pattern Duration =
Change to Framed Tester so that you can set Time slots Tx 17. Return to the Main Menu and select "Bit Error RatioTesting" then "Interface". In this sub-menu, change the "Tester Mode" to "Framed Tester". Then change the "Line Code" back to AMI, followed by "next". This places us in the "BERT Patterns Menu" again, but now the menu has changed due to the new Tester Mode. Use the "more" option to scroll through the different types of BERT patterns. This is data which can be inserted into the T1/DS1 signal. List a few of the different types of BERT patterns: Change to Framed Tester under Interface sub-menu so that you can set Time slots Tx 18. Change the "BERT slots pattern" to the "1111" pattern and ensure the Idle slots pattern is "0000 0000". Use the arrow keys to navigate down to the "Time slots Tx" menu item. Use the function key to select "Edit". A menu of all twenty four channels appears. Edit this field so that only Channel 1 contains the "BERT" pattern, and all the rest contain the "Idle" pattern. Push the "Exit" button when done. The "Time slots Tx" menu should display "1 + Idle". Select "next" and then "Run". Set the scope time base to 50µs/div to see the big pattern and also zoom in. Based on the observed waveform, describe the pattern for which you have configured the test set: On the 50µs/div time base setting, measure and record the time interval and frequency of "1" bursts. If more than eight 1 s in a burst are observed, is there any frame bit just before every frame? Pattern Duration = Pattern Frequency = 19. Change the time base to 1µs/div. Push the Run/Stop button several times and observe the data captured. How many 1's are displayed? Observe repeated captures using the Run/Stop button. Is this number constant? Explain: 20. Using the same method outlined in Step 18, change the "BERT slots pattern" to "USER1", then Edit the pattern to some arbitrary 8-bit binary pattern using the numeric keypad. Make a note of the binary pattern here: BERT 8-bit Binary Pattern = Push the "Exit" key when done. Then go to "Time slots Tx" and edit the twenty four channel field to enable transmission of bytes of your USER1 patterns in various TDM channels. It's best to place your BERT USER1 patterns in closely spaced channel positions, such as 1, 4 and 7. Make a note of your positions here: BERT Channel Positions = 6 of 9
Run the Troubleshooter and verify the occurrence of your BERT patterns on the scope. Also verify the position of the patterns by using the time base cursor keys. Comment on your observations: 21. Return the test set to the QRSS pattern by selecting Main Menu > Bit Error Ratio Testing > Interface > Unframed Tester, then Exit. Finally select Patterns > BERT Slots pattern > QRSS. Write-up Exercise 22. Compile the TIF images captured in this lab into an MS-Word document. Clearly and completely label each image, including all setting information such as V/div and time base settings. Print this out and attach it to your lab report. 23. In transmission lines, what does STP mean? Give two reasons why STP cable used for T1 cabling: i ii 24. Based on the measured bit time in Step 8, calculate the bit rate: Calculated bit rate = Based on the pulse width measured in Step 8, where should the 1st spectral null be located? Predicted 1st spectral null frequency = Based on the spectrum from Step 9, where was the 1st deep spectral null located? 7 of 9 Observed 1st spectral null frequency = 25. What was the spectral peak frequency measured in Step 9B and what is its significance? 26. What happened to the signal when the termination was changed to 1MΩ in Step 10? What caused this?
Comparing spectra from Steps 9 and 10, what effect did terminating in an effective open circuit (1MΩ) have on the observed spectral peak? Practically, describe the likely effect of improper termination on the T1/DS1 line: 27. What does the time measured in Step 13 correspond to? 28. Describe and explain the difference between the waveforms observed in Steps 14 and 15: Describe the problem which would result from a transmission like that in Step 15: 29. Describe and explain the pattern observed in the waveform in Steps 16 (B8ZS): Describe the advantage of using B8ZS in the T1/DS1 transmission: 30. In Step 18, when only data for Channel 1 was transmitted, how many "1" bits were observed? Did this number change? If so, explain why this happened: 31. What is the significance of the frequency measured in Step 18? 8 of 9
32. What is "Bit Error Ratio Testing" (BERT). Why is this necessary on a T1 line? 9 of 9