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Introduction to Telecommunications

Explore key concepts of telecommunications including switches, trunks, lines, and equipment. Learn about the Public Switch Telephone Network (PSTN), telephony equipment, call progress, signaling, and more.

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Introduction to Telecommunications

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  1. Introduction to Telecommunications • Lecture 1 • Paul Flynn

  2. switches trunks lines station equipment (e.g. telephone sets, MODEMs, FAX machines, …) transmission equipment (e.g. repeaters, multiplexers, DCS) network signaling and control (e.g. SS7 and IN) SS7 & IN switch switch switch PBX repeater PC MODEM The Public Switch Telephone Network(PSTN) Composed of the following major network elements:

  3. Telephony Equipment • Telephone set • PBX (Private Branch Exchange) Advanced features and call routing 10s to 100s of telephone sets • Telephone Exchange

  4. Basic Call Progress: On-Hook Telephone Exchange Local Loop 2 wires Local Loop -48 DC Voltage DC Open Circuit No Current Flow

  5. Basic Call Progress: Off-Hook Off-Hook Closed Circuit Telephone Exchange DC Current Dial Tone Local Loop Local Loop

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

  7. Basic Call Progress: Switching Off-Hook Closed Circuit Telephone Exchange Address to Port Translation DC Current Local Loop Local Loop

  8. Basic Call Progress: Ringing Off-Hook Closed Circuit Telephone Exchange Ring BackTone DC Current DC Open Cct. Ringing Tone Local Loop Local Loop

  9. Basic Call Progress: Talking Off-Hook Closed Circuit Telephone Exchange Voice Energy DC Current Voice Energy DC Current Local Loop Local Loop

  10. Analog Telephony—Signaling • Supervisory • Addressing • Call progress

  11. Off-Hook Signaling • Loop Start (almost all telephones) Seizure is detected when current flows through local loop, due to off-hook

  12. Loop start Current flow sensed Analog Telephony— Supervisory Signaling Switch Switch

  13. Loop Start Station PBX or Central Office Loop (Local or Station) + Switch – + DC Current Switch – AC Ringing + Switch –

  14. Signaling and Addressing Dial Pulse DTMF ISDN Analog Transmission “In-Band” Signaling 0–9, *, # (12 Digits) Digital Transmission “Out-of-Band” Message-Based Signaling

  15. 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)

  16. Tone Dialing Dual Tone Multifrequency (DTMF) 1209 1336 1477 1633 697 1 2 3 A Timing: 60 ms Break 40 ms Make 4 5 6 B 770 852 7 8 9 C 941 * 0 # D

  17. Network Call Progress Tones Tone Frequency (Hz) On Time Off Time Dial Busy Ringback, Normal Ringback, PBX Congestion (Toll) Reorder (local) Receiver Off-hook No Such Number 350 + 440 480 + 620 440 + 480 440 + 480 480 + 620 480 + 620 1400 + 2060 + 2450 +2600 200 to 400 Continuous 0.5 O.5 2 4 1 3 0.2 0.3 0.3 0.2 0.1 0.1 Continuous, Freq. Mod 1Hz

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

  19. Local Access Network Feeder Route Boundary Telephone Exchange 40,000 to 50,000 Lines Serving Area Boundary

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

  21. Digital Telephony Digital Trunking Exchange Exchange Analog Loop Digital Network Exchange POTS L.E. Digital Loop Digital Network Switch ISDN

  22. = Sample 8 kHz (8,000 Samples/Sec) Digital Telephony Pulse Code Modulation—Nyquist Theorem Voice Bandwidth = 300 Hz to 3400 Hz Analog Audio Source Sampling Stage Codec Technique

  23. µ—Law (USA–Japan) Pulse Code Modulation—Analog to Digital Conversion A—Law (Europe) Quantizing Noise 100100111011001 Stage 1 Quantizing Stage

  24. Digital Telephony Summary • Analog telephony emulation Voice encoding Limited signaling Loop consolidation

  25. Voice Coding and Compression • Speech coding schemes • Digitizing voice • Voice compression

  26. Voice Compression Technologies Unacceptable Business Quality Toll Quality 64 * PCM (G.711) (Cellular) Bandwidth (Kbps) 32 * ADPCM 32 (G.726) 24 * ADPCM 24 (G.726) 16 * ADPCM 16 (G.726) * LDCELP 16 (G.728) 8 * CS-ACELP 8 (G.729) * LPC 4.8 0 Quality

  27. Digitizing Voice: PCM Waveform Encoding • Nyquist Theorem: sample at twice the highest frequency Voice frequency range: 300-3400 Hz Sampling frequency = 8000/sec (every 125us) Bit rate: (2 x 4 Khz) x 8 bits per sample = 64,000 bits per second (DS-0) • By far the most commonly used method CODEC PCM 64 Kbps = DS-0

  28. Waveform Coders 1110010010010110 Filtering Encoding Sampling Quantizing Waveform ENCODER Waveform DECODER

  29. Voice Compression • Objective: reduce bandwidth consumption Compression algorithms are optimized for voice Unlike data compression: these are “loose” • Drawbacks/tradeoffs Quantization distortion Tandem switching degradation Delay (echo)

  30. Digital Speech Interpolation (DSI) • Voice Activity Detection (VAD) • Removal of voice silence • Examines voice for power, change of power, frequency and change of frequency • All factors must indicate voice “fits into the window” before cells are constructed • Automatically disabled for fax/modem

  31. Voice Activity Detection - 31 dbm B/W Saved Voice Activity (Power Level) Hang Timer No Voice Traffic Sent SID Buffer SID - 54 dbm Pink Noise Voice “Spurt” Silence Voice “Spurt” Time

  32. Bandwidth Requirements Voice Band Traffic Result Bit Rate Encoding/ Compression G.711 PCM A-Law/u-Law 64 kbps (DS0) G.726 ADPCM 16, 24, 32, 40 kbps G.729 CS-ACELP 8 kbps G.728 LD-CELP 16 kbps G.723.1 CELP 6.3/5.3 kbps Variable

  33. Voice Quality Anything Above an MOS of 4.0 Is “Toll” Quality Delay(msec) Compression Method MOS Score 64K PCM (G.711) 4.4 0.75 32K ADPCM (G.726) 4.2 1 16K LD-CELP (G.728) 3–5 4.2 8K CS-ACELP (G.729) 4.2 15 3.6 8K CS-ACELP (G.729a) 15

  34. Voice Network Transport • Voice Network Transport is typically TDM circuit-based: T1/E1 DS3/E3 SONET (OC3, OC12, etc.) • But can also be packet-based: ATM Frame Relay IP

  35. Voice Transport and Delay Cumulative Transmission Path Delay CB Zone Satellite Quality High Quality Fax Relay, Broadcast 0 100 200 300 400 500 600 700 800 Time (msec) Delay Target

  36. switches • trunks • lines • station equipment (e.g. telephone sets, MODEMs, FAX machines, …) • transmission equipment (e.g. repeaters, multiplexers, DCS) • network signaling and control (e.g. SS7 and IN) SS7 & IN switch switch switch PBX repeater PC MODEM The Public Switch Telephone Network(PSTN) Composed of the following major network elements:

  37. Structure of the tel. network Telephone networks

  38. Fiber to the curb (FTTC) Telephone networks

  39. Station number Exchange code Area code (geographic region) International Country Code European Numbering Plan • defines format for dialed numbers in Europe • As of 1/95 • 00 + 353 + 71 - 91 - 55363 • (N digits are 2-9, X digits are 0-9) • international calls are initiated by dialing 00, followed by country and city code (if any) • phone number shortage crisis – Why? • International preliminary code

  40. Network Architecture • ISDN is an access specification to a network Common Channel Signalling Network and Database Telco Switch PRA Circuit Switched Services Telco Switch ISDN PBX BRA NT1 Dedicated Circuit Services Public Packet Network

  41. Public ISDN and SS7 SignalingNetwork BRI BRI Transmission Network Switch Switch PBX1 PBX2 PRI PRI DSS1 Signaling System 7 DSS1 DSS1 is a Public ISDN Protocol

  42. ISDN and SS7 “The Bridge Between the Islands” Voice Transmission STP Exchange Exchange SSP SSP SS7 Signaling Network SCP Voice and Signaling Voice Transmission ISDN— PRI STP STP STP STP Exchange PBX1 SSP SCP STP

  43. Convergence

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