1. 1 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering EE 489 Telecommunication Systems Engineering Introduction to Analog Telephony Concepts

2. 2 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Network Types

3. 3 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Trade-off Between Switching and Transmission Costs in Network Architecture

4. 4 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Trade-off Between Switching and Transmission Costs in Network Architecture

5. 5 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Network Types

6. 6 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Concept of a “Homing Plan”

7. 7 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering PSTN Hierarchy Local Exchanges, or Central Offices (C.O.’s) Primary Switching Centres Regional Exchanges National Exchanges International Exchanges Also connected to submarine cables and satellite links.

8. 8 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering World Numbering Plan zone codeWorld is divided into zones, with each zone assigned a single digit zone code. country codeEach country within a zone is assigned a country code (usually 2-3 digits). –1 st digit is the country’s zone code. numbering plan areas area coderouting codeRegions or “numbering plan areas” (NPAs)within countries are assigned a 1-4 digit “area code” or “routing code”. –NPA size and shape driven by numerous factors: Size and shape Present and future numbering capacity Political boundaries Population demographics Local exchanges are assigned codes (3 digits in N.A.), followed by several digits assigned to each phone (4 digits in N.A.)

9. 9 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering World Numbering Plan (2) World Numbering Zones CodeZone 1N.A. & Carib. 2Africa 3 & 4Europe 5S.A. & Cuba 6South Pacific 7Former USSR 8N. Pac., E. Asia 9Far & Mid East 0Spare Sample Country Codes CodeCountry 20Egypt 231Liberia 33France 351Portugal 44UK 55Brazil 593Ecuador 60Malaysia 886Taiwan 966Saudi Arabia

10. 10 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Network Architecture Customer terminals 20% Outside plant, cables 29% Switching equipment 25% Multiplexing and Transmission equipment 15% Buildings, land, other 11%

11. 11 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Telephone System Early telephone system –Powered by self-contained local battery –Ringing created by cranking generator Today’s telephone system –Powered through the line by battery at the central office (-48V) off hook –Circuit is closed when handset is lifted from the cradle (“off hook”) Transmitter – carbon granule microphone –Air pressure of sound waves impact on diaphragm, varying pressure on carbon granules –Resistance of electrical current passing through carbon granules varies the current (analog) Receiver –Varying electrical current passing through windings on magnet, moves a diaphragm. Same as in a music loudspeaker.

12. 12 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Telephone System (2) PSTNPOTS“PSTN”, or “POTS” simplified circuit model of any connection: coil (Z B ) central battery speech current transmission bridge coil The coil is a “transmission bridge coil” with a high impedance (Z B ) preventing the speech current from shorting out at the central battery.

13. 13 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Establishing A Call (Conventionally) 1.Calling customer takes phone off hook which closes the circuit to the C.O. (“looping the circuit”). 2.C.O. detects the “loop” and indicates readiness with dial tone. 3.Calling customer hears dial tone and dials number. The network converts (“translates”) the phone number to a physical equipment address 4.The network checks on the called party status and decides on a routing for the connection. 5.If connection possible, the called party is alerted. –Large 20 Hz alternating current is applied to line (“ringing current”). 6.“Ring tone” is returned to the caller. 7.The called party picks up the handset and closes his/her loop. 8.Exchange detects second loop and “trips” or stops ringing, then establishes call. 9.One party opens loop by hanging up, and exchange clears connection.

14. 14 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Subscriber Line Signalling

15. 15 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Loop and Disconnect Signalling pulse dialling “pulse dialling” : Line is rapidly disconnected and reconnected in sequence with one pulse for digit value “1”, two pulses for digit value “2”, etc. Each pulse lasts 0.1 second. Inter-digit pause (IDP) must be >0.5 second. –If not, current digit may combine with previous digit. Ten digit phone number typically takes 6-15 seconds total. This is the kind of signalling old “rotary dial” phones produced.

16. 16 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Dual-Tone Multi-Frequency Signalling DTMF signallingtone signalling DTMF signalling” or “tone signalling”. Faster than pulse dialling (1-2 seconds for ten digit number). –Reduces call set-up time. Each digit produced by combination of 2 pure frequency tones. –Reduces chances of error or interference.

17. 17 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Address Signalling Dial Pulsing Dual-Tone Multi- frequency (DTMF)

18. 18 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering DTMF Receiver

19. 19 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering The Concept and Implications of “two- wire” (2W) to “four-wire” (4W) conversion Or…why your phone only needs one twisted pair and why you sometimes hear an echo

20. 20 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 2-W to 4-W Conversion For short distances, two-way communication is possible on a single pair of wires (bi-directional transmission). Problems occur however when amplification (in past) or digital regeneration (nowadays) is needed. –Amplifiers or regenerators in the network are uni-directional. Hybrid Transformer  A Hybrid Transformer is used to convert a 2-wire circuit at the phone/terminal end to a 4-wire system in the switching network:

21. 21 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 2-W to 4-W Conversion (2) 2-wires Hybrid Amplifier or Regenerator 2-wires receivetransmit Amplifier 2-wires receivetransmit

22. 22 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 2-Wire to 4-Wire Conversion Any telephone call undergoes 2W-4W conversions: - from the phone (4W) to the subscriber line (2W) - from the subscriber line (2W) to the network interface (4W) Overall structure of any phone connection “2W” “4W”

23. 23 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 2-W to 4-W Conversion (3) Z BBalance network has a balance impedance of Z B. Z B =Z LineIf Z B =Z Line then half the signal goes to the line and half goes to the balance network with little or no coupling (reflection) to the local receiver. Z B  Z Line sidetoneBut by design, we use Z B  Z Line to create “sidetone”. –Reflections from the C.O. return to the station set. –Talker hears his/her own voice. –Useful because acts (almost subconsciously) as a signal to the talker that the line is live. –No sidetone makes the line feel “dead” and unnatural (IP telephony often sounds like this since there’s no sidetone). –Today’s electronic phones have a small sidetone network within them to create sidetone.

24. 24 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 2W to 4W conversion: The “Hybrid Coupler” Circuit

25. 25 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 2W to 4W conversion: The “Hybrid Coupler” Circuit

26. 26 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Electronic Hybrid Coupler

27. 27 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Concept of Hybrid Return Loss Echo return loss (ERL) = average attention of power reflected at the 2W-4W interface Singing Return Loss (SRL) = minimum attenuation to reflected power at any frequency coming back from the 2W-4W interface

28. 28 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Introduction to the “Subscriber Loop” Plant Wire network from the central office to the station sets. Largest portion of capital expenditure (50%?) and workforce requirements (30%-40%?). Prime candidate for replacement by optical fibre but costs often prohibitive. Main goal is to design and work with length limits. –Limited by resistance and attenuation along the line.

29. 29 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Subscriber “Loop” Plant (N. America)

30. 30 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Subscriber “Loop” Plant (UK)

31. 31 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Three Main Design Goals/Methods (A) (D.C.) Resistance Limit Requirement(A) (D.C.) Resistance Limit Requirement –Keep total line resistance below a target level by choosing the appropriate wire gauges. –Historically 1300  limit but now ~1700 . (B) (A.C.) Attenuation Limit Requirement(B) (A.C.) Attenuation Limit Requirement –Keep total signal loss below a target maximum level. –North America usually uses 8 dB maximum loss at 1000 Hz. –Elsewhere usually uses 7 dB maximum loss at 800 Hz. “Uni-Gauge” Design Method“Uni-Gauge” Design Method –In principle could mix and match wire gauges in loop makeup to satisfy (A) and (B) at minimum cost of the copper used. –Actual practice has been to keep to a single size wire (often 26 gauge) as much as possible (better economically) and add battery boost, range extenders, amplifiers, or “load coils” as needed.

32. 32 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering More on Resistance Design How do we determine the target resistance? 20mA minimum –We need a high enough current at the customer premises to operate the station set (20mA minimum in North America). –Use V=IR, with a known battery voltage of –48V. –48V  20mA x R  R  2400  total –Budget  400  for the battery feed bridge at the C.O. –Budget  300  for other miscellaneous wire resistances (e.g. subset wiring, etc.). 1700  –  The subscriber loop’s wire resistance must not exceed 1700 .

33. 33 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering American Wire Gauge (AWG) Data Loop Resistance Data Loop Attenuation Loss Data (@ 1kHz) N.B.: Each change of 3 gauge numbers is a factor of 2 in wire area (cross section), this a factor of 2 in resistance / unit length

34. 34 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Subscriber “Loop” Plant

35. 35 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Extending Loop Length: Load Coils Amplifiers can be used to add gain (  7dB each is common). Line loading“Line loading” by adding inductive coils at fixed intervals. –Decreases velocity of signal propagation. –Increases impedance. –Acts as high frequency filter but generally outside speech band. Transmission line theory says that attenuation is lowest when: where  We can change l by adding coils periodically along the line. Line Loading “Line Loading” reduces attenuation.

36. 36 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Inductive Loading - Lumped “Load Coils”

37. 37 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Effect on Transfer Function of Twisted Pair

38. 38 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Example Calculation of Cutoff Frequency

39. 39 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Inductive Loading - “Load Coils”

40. 40 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Inductively Loaded Line Notation Example #1: 19H88 “19H88” means 19 gauge wire loaded every 1830 m (H) with 88mH inductors. Example #2: 26B66 “26B66” means 26 gauge wire loaded every 915 m (B) with 66mH inductors. Some Cable Conductor Properties:

41. 41 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Psophometric and “C Weighting” Noise Filters

42. 42 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Example

43. 43 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 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).

44. 44 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 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 21dB were too faint.

45. 45 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 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 Mouth to Interface Loss Interface to Interface Loss Interface to Ear Loss OLRGood/ExcellentPoor/Bad 5-15dB90% 1% 20dB80% 4% 25dB65% 10% 30dB45% 20%

46. 46 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 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. 2-wires Hybrid Amplifier 2-wires receivetransmit Amplifier 2-wires receivetransmit Reflection (Z B  Z L )

47. 47 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Stability (2) balance return lossBRLB SReflection at the hybrid re-inserts the signal with “balance return loss” (BRL or B S ) into the return side of the 4-W loop. Minimum return loss seen at the hybrid in any frequency in the voice-band 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: Ideal loss Loss in practice (~3.5 db splitting loss)

48. 48 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Stability (3) 3dB B S +6dB G Net Gain of one side of 4-W loop (total amplifier gain minus line losses) T 2-W to 2-W total attenuation

49. 49 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Stability (4) singing marginTotal round-trip closed loop loss (“singing margin”): Generally found to be adequate if: singingOtherwise, singing may result. –out of control runaway oscillation in the loop. –can continue even after the original impulse ceases.

50. 50 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 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.

51. 51 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Stability (6) Define new term for variance of round trip loss: Variance of trans-hybrid loss Variance of gain or loss in each 4-W section Number of 4-W sections What if we want only 0.1% chance of singing? 2(T+B S ) Recall: We have singing if m = 2(T+B S ) < 6dB Recall: 3 standard deviations from the mean is equivalent to 0.1% 6dB 0.1% 3  Tot

52. 52 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 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):

53. 53 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering 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?

54. 54 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Echo-Delay Phenomena If the reflection at the hybrid is strong enough, telephone users will hear it. Talker echoTalker echo is when talker hears his/her own voice. Listener echoListener echo is when listener hears talker’s voice twice. TalkerListener B e +6dB T Talker Echo: Loss = B e + 2T Listener Echo: Loss = 2B e + 2T

55. 55 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Echo-Delay (2) Recall B s : –Balance Return Loss –Minimum return loss seen at any voice-band frequency What is B e ? –Hybrid Echo Return Loss –Average return loss in voice-band. Frequency B(f) Return Loss at Frequency f B e (echo) B S (stability) Why B e and not B S ?

56. 56 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Echo-Delay (3) SubjectiveSubjective 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 DelayLoss to Satisfy 50% 10 ms11.1 dB 20 ms17.7 dB 30 ms22.7 dB 40 ms27.2 dB 50 ms30.9 dB

57. 57 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Echo-Delay (4) The echo objective is for 99% of all connections to have acceptable or better echo effects. Factors to consider: B e = Expected hybrid echo return loss.  Be = Standard deviation of B e 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

58. 58 EE489 – Telecommunication Systems Engineering –University of Alberta, Dept. of Electrical and Computer Engineering Echo-Delay (5) For the connection to be acceptable, we want: Mean margin against objectionable echo M has a standard deviation: If we want 99% chance of acceptable echo in all connections then: Recall: 2.33 std. dev. from the mean is equivalent to 1% 1% 0 2.33  M E