Voice over Packet

1
VoiceoverPacket

• Yaakov (J) Stein
• Chief ScientistRAD Data Communications

2
Voice over Packet

• What is this course all about?
• NOT course on Voice over IP (although we may use
  VoIP as an example)
• Voice means telephony voice (not high-quality
  or communications-quality)
• Packet means any cell or packet-based data
  network (FR,ATM,IP,etc)
• Most of the course is about the over
• all the mechanisms needed to carry
  voice on a packet network
• Everything in common to VoIP, VoATM, VoFR, VoDSL,
  VoCATV, etc

3
Course Outline -1-
TOC

• Introduction
• PSTN Review history and terminology
• Packet networks IP, FR, ATM
• The case for VoP
• PSTN Emulation
• Digital Voice Processing
• Speech production mechanisms pitch, formants,
  LPC, cepstrum
• Speech perception mechanisms hearing,
  psychophysics
• Simple Voice DSP gain, AGC, simple-VAD
• Complex Voice DSP correlation, pitch, U/V, LPC,
  LSP
• Speech Compression
• Simple coders G.711, ADPCM
• ABS CELP coders G.723.1, G.728, G.729
• Other coders MELP, MBE, STC, WI

4
Course Outline -2-
TOC

• Other features
• Echo cancellation
• Modem/fax relay
• Qos
• Paying for QoS
• Speech quality measurement PSQM, PESQ, E-model
• VoX
• VoIP
• VoFR
• VoATM
• VoDSL
• VoCATV
• TDMoIP
• VoCATV

5
PSTN Review
6
The PSTN circa 1900
PSTN Review
pair of copper wires local loop
manual routing at local exchange office

• Analog voltage travels over copper wire
  end-to-end
• Voice signal arrives at destination severely
  attenuated and distorted
• Routing performed manually at exchanges
  office(s)
• Routing is expensive and lengthy operation
• Route is maintained for duration of call

7
Multiplexing
PSTN Review

• 1900 25 of telephony revenues went to copper
  mines
• standard was 18 gauge, long distance even heavier
• two wires per loop to combat cross-talk
• needed method to place multiple conversations on
  a single trunk
• 1918 Carrier system (FDM)
• 5 conversations on single trunk
• later extended to 12 (group)
• still later supergroups, master groups,
  supermaster groups
• 1963 T-carrier system (TDM)
• T1 24 conversations per trunk
• later T3 28 T1s
• still later SDH rates with 1000s of conversations
  per trunk

channels
timeslots
8
PSTN Topology
PSTN Review
Local Exchange
local loop
Local Exchange
Long distance network
trunk circuit
subscriber line

• Many local telephone exchanges had sprung up
• Bell Telephone acquired them
• and interconnected them for long distance

9
Switching / routing
PSTN Review

• Originally
• All switching was manual
• All routing was unprincipled
• Both were expensive and performed once per call
• 1879 Connolly McTigthe invent automatic
  switching
• 1888 Strowger invents dial telephone and
• automatic telephone exchange
• 1892 first public automatic exchange (La Porte,
  Indiana)
• 1902 first rotary dial phone
• 1917 Blauvelt invents large city numbering plan
• 1930 tandem crossbar switch
• 1953 centralized automatic message accounting
  (call billing)
• 1963 touch-tone dialing
• 1970 Erna Hoover invents computerized telephony
  switch

10
Old US PSTN
PSTN Review
Class 1
Regional centers
Class 2
Class 2
Sectional centers
Class 3
Class 3
Class 3
Primary centers
Toll (tandem) offices
Class 4
Class 4
Class 4
Class 4
circuits,trunks
Central (end) offices
Class 5
Class 5
Class 5
Class 5
Class 5
last mile
subscriber lines
local loop
Class 5 switch is the sole interface to the
subscriber lines
11
Optimized Telephony Routing
PSTN Review
Circuit switching (route is maintained for
duration of call) Route set-up is an expensive
operation, just as it was for manual
switching Today, complex least cost routing
algorithms are used Call duration consists of
set-up, voice and tear-down phases
12
Signaling
PSTN Review

• PSTN with automatic switching requires signaling
• The present PSTN has thousands of features
• and all require signaling support
• Examples

• On-hook / off-hook
• Pulse / Tone dialing
• Receiver off-hook
• Call waiting
• Caller number identification
• Call forwarding
• Hook-flash

Fax transmission detect Inter-CO messaging Echo
cancellation Voice mail Conference
calls Coin-drop Billing
13
Signaling Methods
PSTN Review

• Signaling can be performed by several methods
• Analog voltage signaling EM, ground-start,
  loop-start
• In-band signaling DTMF, MFR1, MFR2
• Channel associated signaling (CAS) AB bits, ABCD
  bits
• Common channel signaling (CCS) SS7, QSIG
• Trunk Associated CCS
• Separate signaling network CCS

14
The PSTN circa 1960
PSTN Review
trunks circuits
local loop subscriber line
automatic routing through universal telephone
network

• Analog voltages used throughout, but extensive
  Frequency Division Multiplexing
• Voice signal arrives at destination after
  amplification and filtering to 4 KHz
• Automatic routing
• Universal dial-tone
• Voltage and tone signaling
• Circuit switching (route is maintained for
  duration of call)

15
The Digitalization of the PSTN
PSTN Review

• Shannon (Bell Labs) proved
• is better
  than
• and the PSTN became digital
• Better means
• More efficient use of resources (e.g. more
  channels on trunks)
• Higher voice quality (less noise, less
  distortion)
• Added features

• Digital
• Communications

• Analog
• Communications

16
Timing
PSTN Review

• In addition to voice, the digital PSTN transports
  timing
• This timing information is essential because of
• the universal use of TDM
• the requirement of accurate playback (especially
  for fax/modem)
• Receiving switches can recover the clock of the
  transmitting switch
• Every telephony network has an accurate clock
  called stratum 1
• Clocks synchronized to it are called stratum 2
• Clocks synchronized to them are called stratum
  3
• and so on

17
The Present PSTN
PSTN Review
core backbone
PSTN Network
subscriber line

• Analog voltages and copper wire used only in
  last mile,
• but core designed to mimic original situation
• Voice signal filtered to 4 KHz at input to
  digital network
• Time Division Multiplexing of digital signals in
  the network
• Extensive use of fiber optic and wireless
  physical links
• T1/E1, PDH and SONET/SDH synchronous protocols
• Signaling can be channel/trunk associated or via
  separate network (SS7)
• Automatic routing
• Circuit switching (route is maintained for
  duration of call)
• Complex routing optimization algorithms (LP,
  Karmarkar, etc)

18
Nonvoice services
PSTN Review

• The PSTN can even be used to transport non-voice
  signals
• such as FAX
  or DATA
• These services disguise themselves as voice by
  using a modem
• Proper timing is essential
• Special signaling is required
• turn off LEC
• turn off call waiting
• service recognition

PSTN

• capabilities negotiation
• mutual identification
• end of page/document

• modem recognition
• modem training
• data compression

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Digital Loop Carrier
PSTN Review

• Pushes the digital PSTN closer to customer
• ATT SLC-40, SLC-96, Nortel DMS P-phone,
  pair-gain

TR-08 Mode 1 pair-gain Replace 96 pairs with 5
T1s (one spare for span protection)
96 10 86 TR-08 Mode 2 pair-gain Replace 96
pairs with 2 T1s (without span protection)
96 4 92
Access Network
CLASS 5
Street cabinet
UTP/coax/fiber
CPE
FTTB/FTTC
pedestal
UTP
TR-08 multiplex 96 lines on
Mode 1 4 T1s Mode 2 2 T1s (21
concentration) GR303/V5.1/V5.2 multiplex up to
2048 lines
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Packet Networks
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Earliest Data Comm
Packet Networks

• Earliest data communications were serial bit
  streams
• Basic data unit is the character (5, 6, 7, or 8
  bits)
• Start-stop protocol delineates individual
  characters
• Rate limited to thousands of characters per
  second
• Initially range limited to tens of meters
• later modems extended range
• Terminal computer and
• computer computer
• used the same protocol

RS 232
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Data in Packets
Packet Networks

• Problems with serial communications protocols
• Large overhead (encapsulation per character)
• Dedicated resources
• 1961 Kleinrock article on packet switching
  network
• 1962 ARPA computer program begins
• 1967 first use of word packet
• 1969 ARPANET becomes operational (UCLA, SRI,
  UCSB, Utah)
• 1972 first email

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Packet Switched Networks
Packet Networks

• US DOD project to design a data communications
  network
• Design goal was reliability under attack
• Advanced switch technology enabled
  routing-on-the-fly
• Design produced Internet Protocol
• Data stream divided in variable-length packets
• Each packet routed individually (connectionless)
• Perhaps less optimal, but its only for one
  packet!
• Consecutive packets may take different paths
• Best-effort packet delivery
• No inherent timing, QoS or traffic-engineering
  mechanisms
• Packets can
• be corrupted or lost
• arrive out-of-order
• be duplicated

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Different PSNs
Packet Networks

• Many different Packet Switched Networks
• Internet Protocol TCP, UDP, SCTP, RTP
• Frame Relay
• ATM
• MPLS
• Ethernet LAN, GbE, EFM
• DSL HDSL, SHDSL, ADSL, VDSL
• L2TP L2TP/UDP, L2TPv3

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Commonality

• Layered structure not always OSI 7-layer model
• Use of headers, trailers and payload
• Payload may be adapted
• Successive SDU -gt PDU

Headers more prevalent OSI uses only headers
Service Data Unit Protocol Data Unit
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IP
Packet Networks

• designed to robustly interconnect data terminals
• protocol suite for intranets and internets
• defines all layers except physical (layer 1) Eth,
  ATM, SONET
• variable length packets
• best effort packet delivery, no QoS guarantees
• connectionless, virtual connection TCP, SCTP
• unreliable UDP, reliable TCP, highly reliable
  SCTP
• RT support RTP, RTCP, RTSP
• tunneling support PPP, L2TP
• standards body IETF

27
IP
Packet Networks
TCP
UDP
dest port
src port
chksum
length
28
Frame Relay (FR)
Packet Networks

• designed as WAN to connect LANs over low-speed
  link
• low overhead and simple processing
• defines layers 1 (physical) and 2 (data-link)
• variable length packets
• best effort packet delivery, no QoS guarantees
• connection oriented
• unreliable, but committed info rate
• standards bodies ITU-T, FRF

29
Frame Relay (FR)
Packet Networks
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ATM
Packet Networks

• Asynchronous Transfer Mode
• designed as wideband ISDN
• fast switching
• defines layers 1-4 (physical, data-link, network,
  transport)
• small constant length packets (cells) 53548
  cell tax
• multiservice (data, CBR/VBR voice/video)
• QoS levels and guarantees
• connection oriented
• standards bodies ITU-T, ATMF

31
ATM
Packet Networks
AAL1 connection oriented CBR AAL2 connection
oriented VBR AAL5 connectionless data packets
GFC General Flow Control VPI Virtual Path
Indentifier VCI Virtual Channel Indentifier PTI
Payload Type Identifier CLP Cell Loss
Priority HEC Header Error Control
VC
VP
VC
32
DSL

• designed to reuse subscriber lines for broadband
• layer 1 (physical) protocol (modem)
• many varieties HDSL, SHDSL, ADSL, VDSL
• FDM of data with POTS
• synchronous but transports packet data
• cVoDSL for synchronous voice
• standards bodies ITU-T, ETSI TM6, T1E1.4, DSLF

33
TDM / PDH / SDH
Packet Networks
Same data rate even when no data!

• The PSTN is not a PSN !

T1
Frame every 125 msec
STM-1
frame
9 270 B
34
The Casefor VoP
35
Voice over PSN
VoP Case

• We saw that data transported over voice network
• Should we turn the tables and transport voice
  over data networks?
• Economics PSTN keeps circuit open for call
  duration
• packet networks use only resources
  truly needed
• Convergence we need only maintain a single
  network
• Added value enables new applications
• (video,
  white-boards, ftp, presence, voice browsing,
  etc.)

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Voice over PSN
VoP Case

• There are a few problems
• Voice has to be packetized (what size packets?,
  preprocessing?)
• Not a synchronous stream no timing distribution
• Packets arrive after random delays
• Packets may arrive out-of-order
• Packets may be lost
• Reliability

37
PSTN Accessibility
VoP Case

• The PSTN has
• 560 Million subscriber lines worldwide (156 M in
  US)
• Total traffic CAGR 5
• 100 Million fax machines (45 M in US)
• Fax traffic CAGR 12
• gt1.5 Billion people with access to fax
• Is there any business reason
• to transport voice
  otherwise?

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Data Traffic Growth
VoP Case
Data traffic growing much faster than voice
(already more) Internet capacity increasing by
factor of 10 each year
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Revenue Breakdown
VoP Case

• ATT 1998 figures
• 51 switched (long distance) voice (incl. fax)
  service
• 45.3 leased line service
• 1.6 FR
• 1.5 IP
• 0.7 ATM
• So data traffic is increasing fast because its
  cheap!
• The killer-app from revenue point of view is
  voice

40
Typical VoP Applications
VoP Case

• PC PC communications (VoIP,VoDSL)
• Integrated Access Devices (VoATM,VoIP)
• Enterprise/campus convergence (VoFR,VoATM)
• Toll-bypass (VoFR, TDMoIP)
• Access networks (VoDSL, VoATM, VoCATV)

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PSTN Emulation
42
Encapsulation
PSTN Emulation

• We would like to use the standard PSN technique
• but TDM payloads have no natural size packet !
• The header will typically contain
• addresses
• identifiers
• status, alarms
• sequence number
• timestamp
• control information

43
Sequence Numbers
PSTN Emulation

• Packet numbering is needed to
• detect packet loss (mainly for timing - RT
  systems do not retransmit)
• correct for misordering
• supply timing when source is synchronous

135
137
138
136
44
Timing
PSTN Emulation

• PSNs introduce delay variation (jitter)
• How does PSTN emulation replicate timing?
• Station clock
• Clock distribution
• Adaptive clock

45
RTP with IP/UDP
PSTN Emulation
IP header (5 dwords)
UDP header (2 dwords)
RTP header (gt 3 dwords)

PAYLOAD
46
RTP Header (RFC 1889)
PSTN Emulation
0
1
2
3
2
3
4
5
6
1
7
0
8
9
2
3
4
5
6
1
7
0
8
9
1
0
2
3
4
5
6
1
7
0
8
9
V P X CSRC M PAYLD TYPE SEQUENCE
NUMBER
TIMESTAMP
SSRC ID
CSRC contributing source M marker bit SSRC sync
source identifier
V version number P padding indicator X
extension indicator
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Types of PSTN Emulation
PSTN Emulation

• Call (session) emulation
• Emulates single call
• Voice and end-user signaling
• Loop emulation
• Emulates trunk composed of individual loops
• Only transports active loops (timeslots)
• Circuit emulation
• Emulates entire circuit (trunk)
• Does not deal with individual timeslots

48
Payload Types
PSTN Emulation

• Call emulation
• Leased line emulation
• H.225
• CES (AAL1)
• LES (AAL2)

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Extent of Emulation
PSTN Emulation

• End-to-end emulation
• Edge-to-edge emulation
• Link emulation

core switches
edge switch
edge switch
50
Emulation Elements
PSTN Emulation

• PSTN emulations may have the following elements
• End-points phone, user agent (UAC,UAS), terminal
• Gateways IWF, SoftSwitch
• Intermediaries
• Proxies, Redirectors
• Mixers
• Address and location servers gatekeeper,
  registrar

51
Decomposed GWs
PSTN Emulation

• Voice GWs need to handle both voice and signaling
• Scalability increased by separating this
  functions
• Media GW MG handles all voice (DSP) functions
• Media GW controller MGC handles all signaling
  intelligence
• Optionally there may be a signaling GW SG
• MGC (master) can control multiple MGs
• MG (slave) is stateless and is unaware of call
  status
• MGC-MG Megaco/H.248, MGCP
• MGC-SG SCTP
• MGC-MGC SIP, H.323, SDP
• MG-MG RTP, AAL1, AAL2

52
Switching/routing emulation?
PSTN Emulation

• Emulation relies on switching/routing
• of the underlying PSN
• Need to convert PSTN address to PSN address
• Often put PSN address in header
• Need virtual connection for duration of the call
• Call phases
• Setup
• media transfer
• Tear down

53
Signaling emulation?
PSTN Emulation

• Only in-band signaling is automatically
  transported
• For other methods there are two options
• Transparent Signaling
• Carry CAS bits (e.g. TDMoIP)
• Trunk associated SS7
• Signaling Translation
• H.323 translates some PSTN signaling
• to H.225/H.245/H.450
• Problem there are thousands of features!

54
PSTNoPSN
PSTN Emulation
Fax
PSN
G W
PBX
G W
PSTN