Telephony & Internet Telephony

1. Telephony & Internet Telephony Shivkumar Kalyanaraman Based upon slides of Henning Schulzrinne (Columbia)

2. Telephony

3. Telephone Network: What is It? • Specialized to carry voice traffic • Aggregates like T1, SONET OC-N can also carry data • Also carries • Telemetry, video, fax, modem calls • Internally, uses digital samples • Switches and switch controllers are special purpose computers Pieces: 1. End systems 2. Transmission 3. Switching 4. Signaling

4. Telephone Network: What is It? • Single basic service: two-way voice • low end-to-end delay • guarantee that an accepted call will run to completion • Endpoints connected by a circuit, like an electrical circuit • Signals flow both ways (full duplex) • Associated with reserved bandwidth and buffer resources

5. Public Telephony (PSTN) History • 1876 invention of telephone • 1915 first transcontinental telephone (NY–SF) • 1920’s first automatic switches • 1956 TAT-1 transatlantic cable (35 lines) • 1962 digital transmission (T1) • 1965 1ESS analog switch • 1974 Internet packet voice • 1977 4ESS digital switch • 1980s Signaling System #7 (out-of-band) • 1990s Advanced Intelligent Network (AIN)

6. Telephone System Overview • Analog narrowband circuits: home-> central office • 64 kb/s continuous transmission, with compression across oceans • -law: 12-bit linear range -> 8-bit bytes • Everything clocked a multiple of 125 s • Clock synchronization framing errors • AT&T: 136 “toll”switches in U.S. • Interconnected by T1, T3 lines & SONET rings • Call establishment “out-of-band” using packet-switched signalingsystem (SS7)

7. Telephony: Multiplexing • Telephone Trunks between central offices carry hundreds of conversations: Can’t run thick bundles! • Send many calls on the same wire: multiplexing • Analog multiplexing • bandlimit call to 3.4 KHz and frequency shift onto higher bandwidth trunk • Digital multiplexing: convert voice to samples • 8000 samples/sec => call = 64 Kbps

8. Telephone Network Design • Fully connected core • simple routing • telephone number is a hint about how to route a call • But not for 800/888/700/900 numbers: these are pointers to a directory that translates them into regular numbers • hierarchically allocated telephone number space

9. Telephone Network Design

10. Telephone Pieces: End Systems

11. Telephone Pieces: End Systems • Transducers: key to carrying voice on wires • Dialer • Ringer • Switch-hook

12. Last-Mile Transmission Environment • Wire gauges:19, 22, 24, 26 gauge(smaller better) • Diameters: 0.8, 0.6, 0.5, 0.4 mm (larger better) • Various forms of noise: (twisting reduces noise) • Bridged-tap noise: bit-energy diverted to extension phone sockets • Crosstalk • Ham radio • AM broadcast • Insertion loss: -140 dBm noise floor • 100 million times more sensitive than normal modems • Bandwidth range = 600 kHz • Notch effects in insertion loss due to bridged-taps • Transmission PSD = -40dBm => 90 dBm budget

13. 2-wire vs 4-wire: Sidetones and Echoes • Both trans & reception circuits need two wires • 4 wires from every central office to home • Alternative: Use same pair of wires for both transmission and reception • Signal from transmission flows to receiver: sidetone • Reverse Effect: received signal at end-system bounces back to CO (esp if delay > 20 ms): echo • Solutions:balance circuit (attenuate side-tone) + echo-cancellation circuit (cancel echoes).

14. Dialing • Pulse • sends a pulse per digit • collected by central office (CO) • Interpreted by CO switching system to place call or activate special features (eg: call forwarding, prepaid-calls etc) • Tone • key press (feep) sends a pair of tones = digit • also called Dual Tone Multifrequency (DTMF) • CO supplies the power for ringing the bell. • Standardized interface between CO and end-system => digital handsets, cordless/cellular phones

15. Telephone Pieces: Transmission Muxing • Trunks between central offices carry hundreds of conversations • Can’t run thick bundles! Instead, send many calls on the same wire • Multiplexing (a.ka. Sharing) • Analog multiplexing • Band-limit call to 3.4 KHz and frequency shift onto higher bandwidth trunk • obsolete • Digital multiplexing • first convert voice to samples • 1 sample = 8 bits of voice • 8000 samples/sec => call = 64 Kbps

16. Transmission Multiplexing (contd) • How to choose a sample? • 256 quantization levels, logarithmically spaced (why?) • sample value = amplitude of nearest quantization level • Two choices of levels ( law and A law) • Time division multiplexing • Trunk carries bits at a faster bit rate than inputs • n input streams, each with a 1-byte buffer • Output interleaves samples • Need to serve all inputs in the time it takes one sample to arrive => output runs n times faster than input • Overhead bits mark end of frame (why?)

17. Transmission Multiplexing • Multiplexed trunks can be multiplexed further • Need a standard! (why?) • US/Japan standard is called Digital Signaling hierarchy (DS)

18. Telephone Pieces: Switching

19. Telephone Pieces: Switching • Problem: • each user can potentially call any other user • can’t have (a billion) direct lines! • Switches establish temporarycircuits • Switching systems come in two parts: switch and switch controller

20. Switching System Components

21. Switch: What does it do? • Transfers data from an input to an output • many ports (up to 200,000 simultaneous calls) • need high speeds • Some ways to switch: • 1. space division switching: eg: crossbar • if inputs (or crosspoints) are multiplexed, need a schedule (why?)

22. Crossbar Switching Elements

23. Switching (Contd) • Another way to switch • time division (time slot interchangeor TSI) • also needs a service schedule (why?) • To build larger switches we combine space and time division switching elements

24. Telephone pieces: Signaling • A switching system has a switch and a switch controller • Switch controller is in the control plane • does not touch voice samples • Manages the network • call routing (collect dialstring and forward call) • alarms (ring bell at receiver) • billing • directory lookup (for 800/888 calls)

25. Signaling • Switch controllers are special purpose computers • Linked by their own internal computer network • Common Channel Interoffice Signaling (CCIS) network • Earlier design used in-band tones, but was hacked • Also was very rigid (why?) • Messages on CCIS conform to Signaling System 7 (SS7)

26. Signaling (contd) • One of the main jobs of switch controller: keep track of state of every endpoint • Key is state transition diagram

27. Telephony Routing of Signaled Calls • Circuit-setup (I.e. the signaling call) is what is routed. • Voice then follows route, and claims reserved resources. • 3-level hierarchy, with a fully-connected core • AT&T: 135 core switches with nearly 5 million circuits • LECs may connect to multiple cores

28. Telephony Routing algorithm • If endpoints are within same CO, directly connect • If call is between COs in same LEC, use one-hop path between COs • Otherwise send call to one of the cores • Only major decision is at toll switch • one-hop or two-hop path to the destination toll switch. • Essence of telephony routing problem: which two-hop path to use if one-hop path is full (almost a static routing problem… )

29. Features of telephone routing • Resource reservation aspects: • Resource reservation is coupled with path reservation • Connections need resources (same 64kbps) • Signaling to reserve resources and the path • Stable load • Network built for voice only. • Can predict pairwise load throughout the day • Can choose optimal routes in advance • Technology and economic aspects: • Extremely reliable switches • Why? End-systems (phones) dumb because computation was non-existent in early 1900s. • Downtime is less than a few minutes per year => topology does not change dynamically

30. Features of telephone routing • Source can learn topology and compute route • Can assume that a chosen route is available as the signaling proceeds through the network • Component reliability drove system reliability and hence acceptance of service by customers • Simplified topology: • Very highly connected network • Hierarchy + full mesh at each level: simple routing • High cost to achieve this degree of connectivity • Organizational aspects: • Single organization controls entire core • Afford the scale economics to build expensive network • Collect global statistics and implement global changes => Source-based, signaled, simple alternate-path routing

31. Telecommunications Regulation History • FCC regulations cover telephony, cable, broadcast TV, wireless etc • “Common Carrier”: provider offers conduit for a fee and does not control the content • Customer controls content/destination of transmission & assumes criminal/civil responsibility for content • Local monopolies formed by AT&T’s acquisition of independent telephone companies in early 20th century • Regulation forced because they were deemed natural monopolies (only one player possible in market due to enormous sunk cost) • FCC regulates interstate calls and state commissions regulate intra-state and local calls • Bells + 1000 independents interconnected & expanded • FCC rulemaking process: • Intent to act, solicitation of public comment etc…

32. Deregulation of telephony • 1960s-70s: gradual de-regulation of AT&T due to technological advances • Terminal equipment could be owned by customers (CPE) => explosion in PBXs, fax machines, handsets • Modified final judgement (MFJ): breakup of AT&T into ILECs (incumbent local exchange carrier) and IXC (inter-exchange carrier) part • Long-distance opened to competition, only the local part regulated… • Equal access for IXCs to the ILEC network • 1+ long-distance number introduced then… • 800-number portability: switching IXCs => retain 800 number • 1995: removed price controls on AT&T

33. Telecom Act of 1996 • Required ILECs to open their markets through unbundling of network elements (UNE-P), facilities ownership of CLECs…. • Today UNE-P is one of the most profitable for AT&T and other long-distance players in the local market: due to apparently below-cost regulated prices… • ILECs could compete in long-distance after demonstrating opening of markets • Only now some ILECs are aggressively entering long distance markets • CLECs failed due to a variety of reasons… • But long-distance prices have dropped precipitously (AT&T’s customer unit revenue in 2002 was $11.3 B compared to 1999 rev of $23B) • ILECs still retain over 90% of local market • Wireless substitution has caused ILECs to develop wireless business units

34. US Telephone Network Structure (after 1984)

35. Exchange Area Network

36. IP Telephony, VoIP etc

37. IP Telephony: Overview… • IP Telephony: Why ? • Adding interactive multimedia to the web • Being able to do basic telephony on IP with a variety of devices • Basic IP telephony model • Protocols: SIP, H.323, RTP, Coding schemes, MGCP, RTSP • Future: Invisible IP telephony and control of appliances

38. Telephone Service Penetration in the US AT&T Divestiture

39. Trends: Price of Phone Calls: NY - London AT&T Divestiture

40. Trends: Data vs Voice Traffic Since we are building future networks for data, can we slowly junk the voice infrastructure and move over to IP?

41. Trends: Phone vs Data Revenues

42. Private Branch Exchange (PBX) Post-divestiture phenomenon... 7040 212-8538080 External line 7041 Telephone switch Corporate/Campus Private Branch Exchange Another switch 7042 7043 Corporate/Campus LAN Internet

43. IP Telephony: PBX Replacement Another campus Corporate/Campus 7040 8151 External line 8152 7041 PBX PBX 8153 7042 8154 7043 Internet LAN LAN

44. Voice over Packet Market Forecast – North America

45. Invisible Internet Telephony • VoIP technology will appear in . . . • Internet appliances • home security cameras, web cams • 3G mobile terminals • fire alarms • chat/IM tools • interactive multiplayer games

46. IPtel for appliances: “Presence”

47. Speech Coders Waveform Coders Source Coders Time Domain: PCM, ADPCM Frequency Domain: e.g. Sub-band coder, Adaptive transform coder Linear Predictive Coder Vocoder Taxonomy of Speech Coders • Waveform coders:attempts to preserve the signal waveform not speech specific (I.e. general A-to-D conv) • PCM 64 kbps, ADPCM 32 kpbs, CVSDM 32 kbps • Vocoders: • Analyse speech, extract and transmit model parameters • Use model parameters to synthesize speech • LPC-10: 2.4 kbps • Hybrids: Combine best of both… Eg: CELP (used in GSM)

48. Speech Quality of Various Coders

49. Applications of Speech Coding • Telephony, PBX • Wireless/Cellular Telephony • Internet Telephony • Speech Storage (Automated call-centers) • High-Fidelity recordings/voice • Speech Analysis/Synthesis • Text-to-speech (machine generated speech)

50. Pulse Amplitude Modulation (PAM)