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Network Loss Planning

Network Loss Planning. Received Volume Control Subscribers must have a received signal level within an appropriate range. i.e. Not too loud and not to quiet. Stability or Oscillation Control: “Singing”

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Network Loss Planning

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  1. Network Loss Planning • Received Volume Control • Subscribers must have a received signal level within an appropriate range. • i.e. Not too loud and not to quiet. • Stability or Oscillation Control: “Singing” • Manage reflections that can result if there’s a poor mismatch of the 2-wire line impedance and the hybrid balance impedance. • Singing can result. • Talker Echo • Talker should not hear his/her own voice reflected back (with a significant enough delay).

  2. Volume Objectives • Reference Equivalent (RE) or Overall R. E. (ORE) • A measure of perceived loudness of the signal. • ITU in Geneva used group of telephone users to judge loudness. • Measured by adjusting an attenuator in a simulated network. • Rated “highest tolerable volume”, “preferred volume” and “lowest tolerable volume”. • Results showed that attenuator settings of <6dB were too loud and >21dB were too faint.

  3. Mouth to Interface Loss Interface to Interface Loss Interface to Ear Loss Overall Loudness Rating (OLR) • New standard circa 1990. • Loss accumulated from speaker’s mouth and listener’s ear. • OLR = SLR + CLR + RLR SLR – Send Loudness Rating CLR – Circuit Loudness Rating RLR – Receive Loudness Rating OLR Good/Excellent Poor/Bad 5-15dB 90% 1% 20dB 80% 4% 25dB 65% 10% 30dB 45% 20%

  4. Amplifier transmit receive 2-wires 2-wires Hybrid Hybrid 2-wires 2-wires 2-wires 2-wires receive transmit Amplifier Reflection (ZB ZL) Stability • Long distance connections all have 2-W to 4-W to 2-W conversion (as do most local connections). • If there’s a poor mismatch of the 2-W line impedance with the hybrid balance impedance, signal energy passes across the hybrid reflecting from one 4-W direction into the other.

  5. Minimum return loss seen at the hybrid in any frequency in the voice-band Ideal loss Loss in practice (~3.5 db splitting loss) Stability (2) • Reflection at the hybrid re-inserts the signal with “balance return loss” (BRL or BS) into the return side of the 4-W loop. Additional 3+dB loss at hybrid when converting 4-W signal to 2-W signal, and another 3+dB going from 2-W to 4-W (6db total). Total trans-hybrid loss of returned signal:

  6. Net Gain of one side of 4-W loop (total amplifier gain minus line losses) G 3dB BS+6dB 3dB T 2-W to 2-W total attenuation Stability (3)

  7. Stability (4) • Total round-trip closed loop loss (“singing margin”): Generally found to be adequate if: • Otherwise, singing may result. • out of control runaway oscillation in the loop. • can continue even after the original impulse ceases.

  8. Stability (5) • Loss in a 4-W circuit may depart from its nominal value for a number of reasons: • Variation in line losses and amplifier gain with time, temperature, etc. • Gain or loss will differ at different frequencies (usually tested at 800 Hz and/or 1600 Hz). • Measurement errors. • Circuit errors.

  9. Variance of trans-hybrid loss Variance of gain or loss in each 4-W section Number of 4-W sections Recall: 3 standard deviations from the mean is equivalent to 0.1% Recall: We have singing if m = 2(T+BS) < 6dB 3Tot 0.1% 6dB 2(T+BS) Stability (6) • Define new term for variance of round trip loss: What if we want only 0.1% chance of singing?

  10. Stability (7) Typical values: This is basis for a generally accepted approximation or rule of thumb for 99.9% chance of stability (i.e. no singing):

  11. Stability (8) • Example: We have a long distance circuit with 6 4-W sections. What is the minimum attenuation (T) required for a 99.9% chance of not singing?

  12. Talker Listener Be+6dB T Talker Echo: Listener Echo: Echo-Delay • If the reflection at the hybrid is strong enough, telephone users will hear it. • Talker echo is when talker hears his/her own voice. • Listener echo is when listener hears talker’s voice twice. Loss = Be + 2T Loss = 2Be + 2T

  13. B(f) Return Loss at Frequency f Be (echo) BS (stability) Frequency Echo-Delay (2) • Recall Bs: • Balance Return Loss • Minimum return loss seen at any voice-band frequency • What is Be? • Hybrid Echo Return Loss • Average return loss in voice-band. Why Be and not BS?

  14. Echo-Delay (3) • Subjective annoyance of echo depends on relative echo level and on the delay. • The stronger the echo and the longer the delay, the more troublesome the echo. One-Way Delay Loss to Satisfy 50% 10 ms 11.1 dB 20 ms 17.7 dB 30 ms 22.7 dB 40 ms 27.2 dB 50 ms 30.9 dB

  15. Echo-Delay (4) • The echo objective is for 99% of all connections to have acceptable or better echo effects. • Factors to consider: • Be = Expected hybrid echo return loss. • Be = Standard deviation of Be • T = Nominal 2W-2W loss in connection. • T = Standard deviation of T • Ē(t) = E = Expected echo attenuation for delay t at which 50% of users find echo tolerable. • E = Standard deviation of E

  16. Mean margin against objectionable echo M has a standard deviation: 2.33M 1% 0 E Echo-Delay (5) • For the connection to be acceptable, we want: What if we want a 99% chance of acceptable echo? Recall: 2.33 std. dev. from the mean is equivalent to 1%

  17. In-Class Example

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