EE 489 Telecommunication Systems Engineering Introduction to Analog Telephony Concepts

1. EE 489 Telecommunication Systems Engineering Introduction to Analog Telephony Concepts

2. Network Types

3. Trade-off Between Switching and Transmission Costs in Network Architecture

4. Trade-off Between Switching and Transmission Costs in Network Architecture

5. Network Types

6. Concept of a “Homing Plan”

7. Also connected to submarine cables and satellite links. PSTN Hierarchy International Exchanges National Exchanges Regional Exchanges Primary Switching Centres Local Exchanges, or Central Offices (C.O.’s)

8. World Numbering Plan • World is divided into zones, with each zone assigned a single digit zone code. • Each country within a zone is assigned a country code (usually 2-3 digits). • 1st digit is the country’s zone code. • Regions 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. World Numbering Plan (2) • World Numbering Zones CodeZone 1 N.A. & Carib. 2 Africa 3 & 4 Europe 5 S.A. & Cuba 6 South Pacific 7 Former USSR 8 N. Pac., E. Asia 9 Far & Mid East 0 Spare • Sample Country Codes CodeCountry 20 Egypt 231 Liberia 33 France 351 Portugal 44 UK 55 Brazil 593 Ecuador 60 Malaysia 886 Taiwan 966 Saudi Arabia

10. Network Architecture Customer terminals 20% Outside plant, cables 29% Switching equipment 25% Multiplexing and Transmission equipment 15% Buildings, land, other 11%

11. 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) • 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. Telephone System (2) • “PSTN”, or “POTS” simplified circuit model of any connection: speech current coil (ZB) central battery The coil is a “transmission bridge coil” with a high impedance (ZB) preventing the speech current from shorting out at the central battery.

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

14. Subscriber Line Signalling

15. Loop and Disconnect Signalling “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. Dual-Tone Multi-Frequency 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. Address Signalling Dial Pulsing Dual-Tone Multi-frequency (DTMF)

18. DTMF Receiver

19. 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. Send Amplifier 4-wire portion 2-W Line (ZLine) Balance Network (ZB) Receive Amplifier 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.  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. Amplifier or Regenerator transmit receive 2-wires 2-wires Hybrid Hybrid 2-wires 2-wires 2-wires 2-wires receive transmit Amplifier 2-W to 4-W Conversion (2)

22. 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) “4W” “4W” “2W” “4W” “2W” Overall structure of any phone connection

23. 2-W to 4-W Conversion (3) • Balance network has a balance impedance of ZB. • If ZB=ZLine 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. • But by design, we use ZBZLine 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. 2W to 4W conversion: The “Hybrid Coupler” Circuit

25. 2W to 4W conversion: The “Hybrid Coupler” Circuit

26. Electronic Hybrid Coupler

27. 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. 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. Subscriber “Loop” Plant (N. America)

30. Subscriber “Loop” Plant (UK)

31. Three Main Design Goals/Methods • (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 • 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 • 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. More on Resistance Design • How do we determine the target resistance? • 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.). •  The subscriber loop’s wire resistance must not exceed 1700 .

33. American Wire Gauge (AWG) Data Loop Resistance Data 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 Loop Attenuation Loss Data (@ 1kHz)

34. Subscriber “Loop” Plant

35. where Extending Loop Length: Load Coils • Amplifiers can be used to add gain (7dB each is common). • “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:  We can change l by adding coils periodically along the line. “Line Loading” reduces attenuation.

36. Inductive Loading - Lumped “Load Coils”

37. Effect on Transfer Function of Twisted Pair

38. Example Calculation of Cutoff Frequency

39. Inductive Loading - “Load Coils”

40. Some Cable Conductor Properties: Inductively Loaded Line Notation Example #1: “19H88” means 19 gauge wire loaded every 1830 m (H) with 88mH inductors. Example #2: “26B66” means 26 gauge wire loaded every 915 m (B) with 66mH inductors.

41. Psophometric and “C Weighting” Noise Filters

42. Example

43. 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. 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.

45. 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%

46. 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.

47. 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:

48. 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)

49. 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.

50. 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.