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 (NYSF)
• 1920s 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
• ATT 136 tollswitches in U.S.
• Interconnected by T1, T3 lines SONET rings
• Call establishment out-of-band using
  packet-switched signaling system (SS7)

7
Telephony Multiplexing

• Telephone Trunks between central offices carry
  hundreds of conversations Cant 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 gauges19, 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
• Cant 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
• cant have (a billion) direct lines!
• Switches establish temporary circuits
• 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 interchange or 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
• ATT 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 ATTs 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 ATT due to
  technological advances
• Terminal equipment could be owned by customers
  (CPE) explosion in PBXs, fax machines,
  handsets
• Modified final judgement (MFJ) breakup of ATT
  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 ATT

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
  ATT 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 (ATTs 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
ATT Divestiture
39
Trends Price of Phone Calls NY - London
ATT 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
Internet
Corporate/Campus LAN
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
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)
51
Pulse Code Modulation (PCM)
PCM PAM quantization
52
Quantization
53
Companded PCM

• Small quantization intervals to small samples and
  large intervals for large samples
• Excellent quality for BOTH voice and data
• Moderate data rate (64 kbps)
• Moderate cost used in T1 lines etc

54
Companding
55
How it works for T1 Lines

• Companding blocks are shared by all 16 channels

56
Adaptive Gain Encoding
Automatic Gain control (AGC), but accounting for
silence periods
57
Time Waveform of Voiced/Unvoiced Sound
High correlation (0.85) between samples, cycles,
pitch intervals etc
58
Differential PCM
Exploits sample-to-sample correlation (0.85)
differences require fewer bits feedback avoids
cascading quantization errors
59
Delta Modulation

• Used in first-generation PBXs (switching was more
  sensitive to
• Digital conversion cost and less sensitive to
  quality or data rate)

60
Adaptive Predictive Coding
Adapt both the prediction coefficients (alphas)
and the estimates Based upon past or present
samples 20 dB prediction gain
61
Subband Coding
Frequency domain analysis of input instead of
time-domain Analysis adjust quantization based
upon energy level of each band Eg G.722 coder
7kHz voice w/ 64 kbps
62
G.722 (7 kHz) audio Codec
63
Recall Taxonomy of Speech Coders

• Waveform coders attempts to preserve the
  signal waveform not speech specific.
• 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

64
Vocoders
Encode only perceptually important aspects of
speech w/ fewer bits than waveform coders eg
power spectrum vs time-domain accuracy
65
LPC Analysis/Synthesis
66
Speech Generation in LPC
67
Multi-pulse LPC
68
CELP Encoder
69
Example GSM Digital Speech Coding

• PCM 64kbps too wasteful for wireless
• Regular Pulse Excited -- Linear Predictive Coder
  (RPE--LPC) with a Long Term Predictor loop.
• Subjective speech quality and complexity (related
  to cost, processing delay, and power)
• Information from previous samples used to predict
  the current sample linear function.
• The coefficients, plus an encoded form of the
  residual (predicted - actual sample), represent
  the signal.
• 20 millisecond samples each encoded as 260 bits
  13 kbps (Full-Rate coding).

70
Speech Quality of Various Coders
71
Speech Quality (Contd)
72
VoIP Camps
Circuit switch engineers We over IP
Convergence ITU standards
Conferencing Industry
Netheads IP over Everything
H.323
SIP
Softswitch
BICC
ISDN LAN conferencing
I-multimedia WWW
Call Agent SIP H.323
BISDN, AIN H.xxx, SIP
IP
IP
IP
any packet
73
Internet Multimedia Protocol Stack
74
IP Telephony Protocols SIP, RTP

• Session Initiation Protocol - SIP
• Contact office.com asking for bob
• Locate Bobs current phone and ring
• Bob picks up the ringing phone
• Real time Transport Protocol - RTP
• Send and receive audio packets

75
Internet Telephony Protocols H.323
76
H.323 (contd)

• Terminals, Gateways, Gatekeepers, and Multipoint
  Control Units (MCUs)

77
H.323 vs SIP
Typical UserAgent Protocol stack for Internet
Terminal Control/Devices
Terminal Control/Devices
Q.931
H.245
RTCP
RAS
RTCP
SIP
SDP
Codecs
Codecs
RTP
RTP
TPKT
TCP
UDP
Transport Layer
IP and lower layers
78
SIP vs H.323

• Binary ASN.1 PER encoding
• Sub-protocols H.245, H.225 (Q.931, RAS,
  RTP/RTCP), H.450.x...
• H.323 Gatekeeper

• Text based request response
• SDP (media types and media transport address)
• Server roles registrar, proxy, redirect

- Both use RTP/RTCP over UDP/IP - H.323 perceived
as heavyweight
79
Light-weight signaling Session
InitiationProtocol (SIP)

• IETF MMUSIC working group
• Light-weight generic signaling protocol
• Part of IETF conference control architecture
• SAP for Internet TV Guide announcements
• RTSP for media-on-demand
• SDP for describing media
• others malloc, multicast, conference bus, . . .
• Post-dial delay 1.5 round-trip time (with UDP)
• Network-protocol independent UDP or TCP (or AAL5
  or X.25)

80
SDP Session Description Protocol

• Not really a protocol describes data carried by
  other protocols
• Used by SAP, SIP, RTSP, H.332, PINT. Eg
• v0
• og.bell 877283459 877283519 IN IP4 132.151.1.19
• sCome here, Watson!
• uhttp//www.ietf.org
• eg.bell_at_bell-telephone.com
• cIN IP4 132.151.1.19
• bCT64
• t3086272736 0
• kclearmanhole cover
• maudio 3456 RTP/AVP 96
• artpmap96 VDVI/8000/1
• mvideo 3458 RTP/AVP 31
• mapplication 32416 udp wb

81
SIP functionality

• IETF-standardized peer-to-peer signaling protocol
  (RFC 2543)
• Locate user given email-style address
• Setup session (call)
• (Re)-negotiate call parameters
• Manual and automatic forwarding
• Personal mobility different terminal, same
  identifier
• Call center reach first (load distribution) or
  reach all (department conference)
• Terminate and transfer calls

82
SIP Addresses Food Chain
83
SIP components

• UAC user-agent client (caller application)
• UAS user-agent server à accept, redirect, refuse
  call
• redirect server redirect requests
• proxy server server client
• registrar track user locations
• user agent UAC UAS
• often combine registrar (proxy or redirect
  server)

84
IP SIP Phones and Adaptors
1

• Are true Internet hosts
• Choice of application
• Choice of server
• IP appliances
• Implementations
• 3Com (3)
• Columbia University
• MIC WorldCom (1)
• Mediatrix (1)
• Nortel (4)
• Siemens (5)

Analog phone adaptor
2
3
Palm control
4
5
4
85
SIP-based Architecture
86
Example Call

• Bob signs up for the service from the web as
  bob_at_ecse.rpi.edu

• sipd canonicalizes the destination to
  sipbob_at_ecse.rpi.edu

• He registers from multiple phones

• sipd rings both ephone and sipc

• Bob accepts the call from sipc and starts talking

• Alice tries to reach Bob
• INVITE ipBob.Wilson_at_ecse.rpi.edu

ecse.rpi.edu
87
PSTN to IP Call
88
IP to PSTN Call
89
Traditional voice mail system
Bob can listen to his voice mails by dialing some
number.
90
SIP-based Voicemail Architecture
vm.office.com
The voice mail server registers with the SIP
proxy, sipd
Alice calls bob_at_office.com through SIP proxy.
SIP proxy forks the request to Bobs phone as
well as to a voicemail server.
91
Voicemail Architecture
v-mail
vm.office.com
After 10 seconds vm contacts the RTSP server for
recording.
vm accepts the call.
Sipd cancels the other branch and ...
rtspd
...accepts the call from Alice.
Now user message gets recorded
92
SIP-H.323 Interworking ProblemsEg Call setup
translation
H.323
SIP
Q.931 SETUP
INVITE
Destination address (Bob_at_office.com)
Q.931 CONNECT
200 OK
Terminal Capabilities
Media capabilities (audio/video)
Terminal Capabilities
ACK
Open Logical Channel
Media transport address (RTP/RTCP receive)
Open Logical Channel

• H.323 Multi-stage dialing

93
MGCP and Megaco

• Media Gateway Controller Protocol (RFC 2705)
• Controlling Telephony Gateways from external call
  control elements called media gateway controllers
  (MGC) or call agents
• Gateways Eg RGW physical interfaces between
  VoIP network and residences
• Call control "intelligence" is outside the
  gateways and handled by external call control
  elements
• Goal scalable gateways between IP telephony and
  PSTN
• Successor to MGCP H.248/Megaco

94
MGCP Architecture
Goal large-scale phone-to-phone VoIP deployments
RGW Residential Gateway TGW Trunk Gateway
95
Summary

• Telephony and IP Telephony
• Protocols SIP, SDP, H.323, MCGP
• Example operation and services
• Calls, voice mail etc
• Future Integration with Web and long-term
  replacement for current telephone systems