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Introduction to Telephony

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1 Introduction to Telephony
3 1 3

2 Agenda Basic Analog Telephony Basic Digital Telephony
Consolidated Transport Networking 2

3 Telephony Equipment Telephone set Key system
Optimizes use of telephone sets to lines Mechanical to electronic 2 to 10+ sets typically PBX (Private Branch Exchange) Advanced features and call routing 10s to 100s of telephone sets Central office switch 3

4 Analog Telephony—POTS Basics
Tip Ring Tip—0 VDC potential in USA Ring—48VDC potential in USA Sleeve—Earth Ground Sleeve 4

5 Basic Call Progress: On-Hook
Telephone Switch Local Loop Local Loop -48 DC Voltage DC Open Circuit No Current Flow 5

6 Basic Call Progress: Off-Hook
Closed Circuit Telephone Switch DC Current Dial Tone Local Loop Local Loop 6

7 Basic Call Progress: Dialing
Off-Hook Closed Circuit Telephone Switch Dialed Digits Pulses or Tones DC Current Local Loop 7

8 Basic Call Progress: Switching
Off-Hook Closed Circuit Telephone Switch Address to Port Translation DC Current Local Loop Local Loop 8

9 Basic Call Progress: Ringing
Off-Hook Closed Circuit Telephone Switch Ring Back Tone DC Current DC Open Cct. Ringing Tone Local Loop Local Loop 9

10 Basic Call Progress: Talking
Off-Hook Closed Circuit Telephone Switch Voice Energy DC Current Voice Energy DC Current Local Loop Local Loop 10

11 Analog Telephony—Signaling
Supervisory Addressing Call progress Dial tone Busy Ringback Congestion Reorder Receiver off-hook howler No such number Special recordings 11

12 Off-Hook Signaling Loop Start (almost all telephones)
Seizure is detected when current flows through local loop, due to off-hook Ground Start (PBXs) Seizure is detected when one wire is grounded Seizure can be initiated in both directions 12

13 Analog Telephony— Supervisory Signaling
Loop start Current flow sensed Ground start Momentary ground ring lead Switch Switch 13

14 Loop (Local or Station)
Loop Start Station PBX or Central Office Loop (Local or Station) + Switch DC Current + Switch Supervision =Two states—ON HOOK or OFF HOOK Call Information—Busy, Ringing (ring back), Fast Busy (network availability), others AC Ringing + Switch 14

15 E&M Signaling PBXs, switches State On-Hook Off-Hook E-Lead Open Ground
Separate signaling leads for each direction E-Lead (inbound direction) M-Lead (outbound direction) Allows independent signaling State On-Hook Off-Hook E-Lead Open Ground M-Lead Ground Battery Voltage 15

16 Signaling and Addressing
Dial Pulse DTMF ISDN Analog Transmission “In-Band” Signaling 0–9, *, # (12 Digits) Digital Transmission “Out-of-Band” Message-Based Signaling Dial Pulse Address Signaling opens and closes the tip and ring connectors at specific intervals interrupting current. # of interruptions = digit dialed Pulse speed is nominally 10pps +/- 1pps Pulse period is nominally 0.1 second (open) Interdigit interval is ~700ms Dialed Digits toggle the A bit in DS1 Dual Tone Multifrequency Address Signaling uses pairs of frequencies transmitted over the loop. Permits end-to-end signaling after the connection is made, i.e. IVR/Audiotex 60ms Break / 40ms Make Tones are digitized and decoded ISDN signaling is message based 16

17 Pulse Dialing Off-Hook Dialing Inter-Digit Next Digit Make
(Circuit Closed) Break (Circuit Open) 700 ms US:60/40 Break/Make Pulse Period (100 ms) 17

18 Dual Tone Multifrequency (DTMF)
Tone Dialing Dual Tone Multifrequency (DTMF) 1209 1336 1477 1633 697 1 2 3 A Timing: 60 ms Break 40 ms Make 770 4 5 6 B 852 7 8 9 C 941 * # D 18

19 Network Call Progress Tones
Frequency (Hz) On Time Off Time Dial Busy Ringback, Normal Ringback, PBX Congestion (Toll) Reorder (local) Receiver Off-hook No Such Number 200 to 400 Continuous 0.5 O.5 2 4 1 3 Continuous, Freq. Mod 1Hz 19

20 Voice Channel Bandwidth
Output Voltage or Energy Voice Signal Frequency (K-Hertz) .2 1 2 3 4 Tone Dialing Signals Systems Control Signals 20

21 Local Access Network Feeder Route Boundary Central Office 40,000 to
50,000 Lines Serving Area Boundary 21

22 Switching Systems Off-Hook Indicator Tip Ring
Manual control—Switch/cord boards Off-Hook Indicator Tip Ring Patch Cord Pairs Manual Ring 22

23 PSTN Network Hierarchy
1 1 1 Class Name 1 Regional Center 2 Sectional Center 3 Primary Center 4C Toll Center 4P Toll Point 4X Interm. Point 5 End Office 5R EO w/ RSU R Remote Sw. Unit 2 3 4C 4P 5 4C 4P 3 5 4X 5 5R 4C 4P 5 5R 5 4X R 23

24 Types of Voice Circuits
Serving Area 415-NXX-XXX Serving Area 510-NXX-XXX Class 5 Class 5 Switch Switch OPX Off-Premises Ext. FX Foreign Exchange ARD Auto Ring Down 24

25 Echo in Voice Networks Listener Talker Delay Talker Echo Listener Echo

26 Normal Signal Flow Receive Direction Central Office 2-Wire Local Loop
2w-4w Hybrid Transmit Direction 2- to 4-wire hybrid combines receive and transmit signals over the same pair 2-wire impedance must match 4-wire impedance 26

27 How Does Echo Happen? Central Office Receive 2-Wire Local Loop
Echo is due to a reflection Central Office Receive Direction 2-Wire Local Loop 2w-4w Hybrid Rx and Tx superimposed Transmit Direction Impedance Mismatch at the 2w-4w Hybrid Is the Most Common Reason for Echo 27

28 Echo Is Always Present Echo as a problem is a function of the echo delay, and the magnitude of the echo Echo Is Unnoticeable (dB) Echo Path Loss Echo Is a Problem Echo Path Delay (ms) 28

29 Ways to Defeat Echo Increase the loss in the echo path Echo suppresser
Can often be the solution Disadvantage: Static setting, reduces the signal strength of the speaker Echo suppresser Acts like a noise gate, effectively making communications half-duplex 29

30 Echo Canceller Most effective means for removing echo Central Office
+ Echo Canceller Block Diagram Adaptive Filter 30

31 Summary Analog voice technology dates back to the 1900’s
Information exchange based on voltage, current flow, grounding, etc. 31

32 Agenda Basic Analog Telephony Basic Digital Telephony
Consolidated Transport Networking 32

33 Digital Loop Digital Network
Digital Telephony Digital Trunking Switch Switch Analog Loop Digital Network POTS Switch CB Digital Loop Digital Network Switch ISDN 33

34 Pulse Code Modulation—Nyquist Theorem
Digital Telephony Pulse Code Modulation—Nyquist Theorem Voice Bandwidth = 300 Hz to 3400 Hz Analog to Digital Conversion requires three operations: Sampling Quantizing Encoding Nyquist Theorem—When a signal is sampled instantaneously at the transmitter at regular intervals and at a rate at least twice the highest frequency in the channel, then the samples will contain sufficient information to allow an accurate reconstruction of the signal at the receiver Filters used to block signals above 4kHz and below 300Hz to avoid a condition called aliasing. Aliasing happens when a digital point can represent more than one signal. Analog Audio Source Sampling Stage = Sample 8 kHz (8,000 Samples/Sec) Codec Technique 34

35 Pulse Code Modulation— Analog to Digital Conversion
A—Law (Europe) Quantizing Noise Stage 1 µ—Law (USA–Japan) Quantizing Stage 35

36 Digital Telephony—T1 and E1/J1
T1 ITU-T G.733 E1/J1 ITU-T G.732 Sampling Frequency 8 kHz 8 kHz Channel Bit Rate DS0—64 kbps DS0—64 kbps Time Slots per Frame 24 32 Channels per Frame 24 30 Bits per Frame 24 x = 193 32 x 8 = 256 Framing D4/Super Frame (12) Extended Super Frame (24) E1: Multiframe (16) J1: CRV in Bit 1 of frame Framing Indicator 193rd Bit of Frame 2.048 kbps Word of 7 Bits in the 0 Channel of Odd Frames System Bit Rate 8,000 x 193 = Mbps 8,000 x 256 = Mbps Signaling “Robbed Bit” Channel Associated Signaling D4/Super Frame Extended Super Frame LSB/Channel LSB/Channel Frame 6 and 12 Frames 6, 12, 18, 24 E1: CCS in TS 16 CAS in TS 16—2 Ch Every Other Frame J1: TS0 36

37 DS1 Framing Format 24 Frames per Extended SuperFrame
193rd Bit of each frame used for frame synchronization. D4 Framing is 12 frames D4 framing pattern is: ESF is 24 frames, with framing, CRC and an FDL channel ESF Framing pattern is 001011, in frames 4, 8, 12, 16, 20 and 24 Channel Associated Signaling robs the LSB of every byte in frames 6, 12, 18 and 24 for ABCD bits Common Channel Signaling (ISDN) uses TS 24 24 Frames per Extended SuperFrame 24 Time Slots—125m sec

38 Extended Super-Frame Format
S Bits Bit Use in Each Channel Time Slot Signaling—Bit Use Options Frame Number Fe DL BC Traffic Signaling T 2 4 16 1 m 2 C1 3 m 4 5 m 6 C2 Bits 1–7 Bit 8 * A A A 7 m 8 9 m 10 C3 11 m 12 1 Bits 1–7 Bit 8 * A B B 13 m 14 C4 15 m 16 17 m 18 C5 Bits 1–7 Bit 8 * A A C 19 m 20 1 21 m 22 C6 23 m 24 1 Bits 1–7 Bit 8 * A B D 38

39 E1 Frame Format 16th Time Slot ABCD Signaling Bits for
Contains Multiframe Alignment Signal ABCD Signaling Bits for Time Slots 1 and 17 Time Slot 0 Synchronization Bits 2 to 8 16 Frames per Multiframe ABCD Signaling Bits for Time Slots 15 and 31 32 Time Slots—125usec

40 Digital Signaling Schemes
Channel Associated Signaling Extended Super Frame Bit Frame A th B th C th D th Audio Address Signaling (DTMF) Supervision On/Off Hook Address Signaling (Dial Pulse) 40

41 Digital Signaling Schemes
Common Channel Signaling E-1 Time Slot 0 Time Slot 16 Supervision On/Off Hook Address Signaling (Dial Pulse) Audio Address Signaling (DTMF) 41

42 Digital Telephony—Synchronization
Bit synchronization Primary reference source Ones density (except for J1/CMI) Time slot synchronization Bits/byte/channel Frame alignment Basic rule 193rd bit pattern Bit Level Synchronization—Operate the transmitter and receiver at same bit rate so bits are not lost Ones density is no more than 15 0s in a row (old) or 80 0s most important 1s density must be at least 12.5% For Voice PCM network robbed bit signaling renders the 8th bit in all channels unreliable, being used for signaling or not…so 8th bit is “discarded or ignored” and forced to “1” state for data transmission causing 56kb/s channels. Frame Level Synchronization—Phase alignment between the transmitter and receiver so the time slots are lined up for information retrieval Time Slot Synchronization—Phase alignment between the transmitter and receiver so the time slots are lined up for information retrieval To help explain the critical nature of network synchronization in a T1 system. Consider this: the receiver must be able to detect each bit as it appears on the link interface at the receiver. Each time slot is only 648nsec in duration Each bit is only 348nsec in duration 42

43 Digital Telephony— Synchronization
One Multiframe (ESF) 3 ms 1 12 24 1 Frame, 125us, 193bits 24 Time Slots 1 12 24 1 Channel Time Slot, 5.18us 648ns 43

44 Synchronization—Traditional Network Clocking Strata
Master Clock Stratum PRS 1 .00001ppm Timing Toll Office Timing Timing One per LATA 2 Just in case you care Stratum 1 Short Term Accuracy = +/-1 x Long Term Stability = Unquestionable LTS is expressed as total over 20yrs free running Sources = Atomic Clocks, GPS, Loran-C Stratum 2 STA = +/-1 x LTS = +/-1.6 x 10 -8 Must track or lock to any stratum 2 clock Stratum 3 STA = < 256 frame slips/day LTS = +/-4.6 x 10 -6 Must track or lock to any stratum 3 clock Stratum 4 STA = Undefined LTS = +/-32 x 10 -6 Must track or lock to any stratum 4 clock Locking all switches to a single clock ensures that they all send and receive bits at the same rate. Accurate synch can reduce buffer sizes, which will reduce jitter. page 474 of orange book Timing End Office End Office Bits Distribution DCS 3 PBX PBX 4 44

45 Digital Telephony— Analog Emulation and Pair Gain
Backbone to largest interoperable network in the world Signaling information exchange based on ~30 year old concepts Twiddling bits based on ~100-year old signaling 45

46 Digital Telephony Summary
Analog telephony emulation Voice encoding Limited signaling Loop consolidation 46

47 Agenda Basic Analog Telephony Basic Digital Telephony
Consolidated Transport Networking 47

48 Consolidated Transport Networking
C A L L P R O C E S S I N G Remote/Branch Access Frame, Packet, Cell,CES, DS0 Gateway/Integrated Switching Trunking Cell, CES Gateway Campus/ Desktop Packet 48

49 Consolidated Transport Network Solutions
PBX trunking PBX trunk pathing Intelligent voice network switching Branch/remote office access Virtual switch access Tie line and OPX transport Alternate “packet” routes 49


51 Network Synchronization— Adaptive Clocking
Transmit Clock Outbound Frames In-Bound Cells Reassembly FIFO Queue PBX1 ATM Network PBX2 IWF 1 IWF 2 51

52 Network Synchronization— Synchronous Residual Time Stamp
PRS1 PRS1 Transmit Clock PRS2 PBX1 ATM Network PBX2 Receive Clock IWF 1 IWF 2 52

53 Network Synchronization— Synchronous Clocking
Traceable to a Single Reference Source PRS2 PRS PBX1 PBX2 ATM Network IWF 1 IWF 2 53

54 Network Synchronization
PSTN PBX2 PBX2 PBX Trunk Pathing PSTN PSTN Network Synchronization PBX1 PBX4 PRS2 PSTN PBX2 CES/IWF CES/IWF PSTN PSTN PVCs PBX1 PBX4 Synchronization: Develop a synch map Ordered list Synch 1st choice Synch 2nd choice Synch 3rd choice Synch Residual Time Stamp 4th choice Adaptive for demos ONLY! Ensure your ordered list in each node reflects the correct Clock Source if link to PRS fails 4 choices per node: static config CES/IWF CES/IWF PSTN PBX3 54

55 PBX Trunk Pathing—Considerations
PBX Trunking PBX Trunk Pathing—Considerations N2 connections Tandem “hops” dependent on topology Dedicated point-to-point circuits Point-to-point signaling Platform availability 55

56 PBX Trunking— Intelligent Voice Switching

57 PBX Trunking—Intelligent Voice Network Switching
Efficient trunk groups Efficient WAN utilization PBX to network signaling Tandem switch replacement Dynamic setup of virtual circuits Compression Voice activity detection 57

58 Consolidated Transport— Branch/Remote Office Access
Explosive branch office “data” networking Similar voice and data traffic patterns Technology advancements Technology Advancements include: Emergence of Useful and Interoperable Standards DSP performance/price and availability Access line pricing dropping significantly 58

59 Satellite Access— Consolidated Transport

60 Branch/Remote Office Virtual Switch Access
PBX Access WAN KTS/ PBX 60

61 Branch/Remote Office Access Tie Line and OPX Transport

62 Consolidated Transport— Alternate Routes
Access WAN PBX PBX Backbone Network PSTN 62

63 Fixed vs. Usage Billed Service
CTN Bandwidth Tie Trunk DDD “A” Plan Billing Cycle Cost ($) Z 5 10 15 20 25 30 35 Usage/Billing Cycle (Hours/Month) 63

64 Consolidated Transport - FAX Services
PBX1 PSTN PBX Trunk Network PBX4 PSTN Internet F Access WAN $ From Pitney Bowes Survey - How much PSTN $ Attributed to intra-company FAX FAX Relay - technology Demodulation? Rate Matching? Routing Options? F F F PSTN PSTN F H-PBX1 H-PBX2 64

65 Agenda Basic Analog Telephony Basic Digital Telephony
Consolidated Transport Networking 65

66 Thank You! Q&A 66


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