Transcript Voice over Packet - Part 1

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O
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Voice
over
Packet
Yaakov (J) Stein
Chief Scientist
RAD Data Communications
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Voice over Packet
What is this course all about?
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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
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TOC
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Course Outline -1-
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Introduction
–
–
–
–
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Digital Voice Processing
–
–
–
–
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PSTN Review history and terminology
Packet networks IP, FR, ATM
The case for VoP
PSTN Emulation
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
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TOC
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Course Outline -2-
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Other features
–
Echo cancellation
–
Modem/fax relay
Qos
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Paying for QoS
–
Speech quality measurement PSQM, PESQ, E-model
VoX
–
–
–
–
–
–
–
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VoIP
VoFR
VoATM
VoDSL
VoCATV
TDMoIP
VoCATV
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PSTN Review
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PSTN
Review
The PSTN circa 1900
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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
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PSTN
Review
Multiplexing
O 1900: 25% of telephony revenues went to copper mines
 standard was 18 gauge, long distance even heavier
P  two wires per loop to combat cross-talk

needed method to place multiple conversations on a single trunk
1918: “Carrier system” (FDM)
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5 conversations on single trunk
later extended to 12 (group)
still later supergroups, master groups, supermaster groups
1963: T-carrier system (TDM)
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channels
f
timeslots
T1 = 24 conversations per trunk
later T3 = 28 T1s
still later SDH rates with 1000s of conversations per trunk
t
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PSTN
Review
PSTN Topology
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local loop
Local
Exchange
subscriber line
Local
Exchange
Long distance
network
Local
Exchange
trunk
circuit
Many local telephone exchanges had sprung up
Bell Telephone acquired them
and interconnected them for long distance
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PSTN
Review
Switching / routing
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
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PSTN
Review
Old US PSTN
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Regional centers
Class 1
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Class 2
Class 3
Class 4
Class 3
Class 4
Sectional centers
Class 2
Class 4
Class 3
Class 4
Primary centers
Toll (tandem) offices
circuits,trunks
Class 5
Class 5
local loop
Class 5
subscriber lines
Class 5
Class 5
Central (end) offices
last mile
Class 5 switch is the sole interface to the subscriber lines
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PSTN
Review
Optimized Telephony Routing
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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
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PSTN
Review
Signaling
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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
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Fax transmission detect
Inter-CO messaging
Echo cancellation
Voice mail
Conference calls
Coin-drop
Billing
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PSTN
Review
Signaling Methods
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Signaling can be performed by several methods
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Analog voltage signaling
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In-band signaling
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Channel associated signaling (CAS)
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Common channel signaling (CCS)
E&M, ground-start, loop-start
DTMF, MFR1, MFR2
AB bits, ABCD bits
SS7, QSIG
– Trunk Associated CCS
– Separate signaling network CCS
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PSTN
Review
The PSTN circa 1960
trunks
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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)
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PSTN
Review
The Digitalization of the PSTN
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Shannon (Bell Labs) proved
Digital
Analog
is better than
Communications
Communications
and the PSTN became digital
Better means
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More efficient use of resources (e.g. more channels on trunks)
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Higher voice quality (less noise, less distortion)
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Added features
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PSTN
Review
Timing
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In addition to voice, the digital PSTN transports timing
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This timing information is essential because of
– the universal use of TDM
– the requirement of accurate playback (especially for fax/modem)
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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
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PSTN
Review
The Present PSTN
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core
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backbone
subscriber line
PSTN Network
• 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)
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PSTN
Review
Nonvoice services
The PSTN can even be used to transport non-voice signals
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such as
FAX
or
DATA
VoP course
PSTN
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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
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– capabilities negotiation
– mutual identification
– end of page/document
– modem recognition
– modem training
– data compression
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PSTN
Review
Digital Loop Carrier
Pushes the digital PSTN closer to customer
“pair-gain”
AT&T SLC-40, SLC-96, Nortel DMS P-phone,
TR-08 Mode 1 pair-gain:
Replace 96 pairs with 5 T1s
Access Network
CLASS
5
(one spare for “span protection”)
UTP/coax/fiber Street
FTTB/FTTC cabinet
CPE
96 – 10 = 86
TR-08 Mode 2 pair-gain:
Replace 96 pairs with 2 T1s
pedestal
(without “span protection”)
UTP
96 – 4 = 92
TR-08 multiplex 96 lines on:
Mode 1: 4 T1s
Mode 2: 2 T1s (2:1 concentration)
GR303/V5.1/V5.2 multiplex up to 2048 lines
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Packet Networks
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Packet
Networks
Earliest Data Comm
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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Packet
Networks
Data in Packets
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
Networks
Packet Switched Networks
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US DOD project to design a data communications network
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Design goal was reliability under attack
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Advanced switch technology enabled routing-on-the-fly
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Design produced Internet Protocol
– Data stream divided in variable-length packets
– Each packet routed individually (connectionless)
• Perhaps less optimal, but it’s 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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Packet
Networks
Different PSNs
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
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Layered structure not always OSI 7-layer model
Headers more prevalent
Use of headers, trailers and payload OSI uses only headers
header
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Payload may be adapted
Successive SDU -> PDU
header
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payload
header
payload
trailer
Service Data Unit
Protocol Data Unit
trailer
trailer
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Packet
Networks
IP
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designed to robustly interconnect data terminals
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protocol suite for intranets and internets
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defines all layers except physical (layer 1) Eth, ATM, SONET
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variable length packets
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best effort packet delivery, no QoS guarantees
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connectionless, virtual connection TCP, SCTP
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unreliable UDP, reliable TCP, highly reliable SCTP
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RT support RTP, RTCP, RTSP
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tunneling support PPP, L2TP
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standards body: IETF
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Packet
Networks
IP
O
ver len
TOS
ID
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TTL
total len
flgs
prot
frag offs
hdr chksum
src IP add
dest IP add
options
padding
TCP
UDP
src port
dest port
src port
seq num
length
dest port
chksum
ack num
offs res
flags
chksum
options
window
urgent ptr
padding
payload
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Packet
Networks
Frame Relay (FR)
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designed as WAN to connect LANs over low-speed link
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low overhead and simple processing
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defines layers 1 (physical) and 2 (data-link)
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variable length packets
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best effort packet delivery, no QoS guarantees
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connection oriented
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unreliable, but committed info rate
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standards bodies: ITU-T, FRF
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Packet
Networks
Frame Relay (FR)
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flag
payload
FR header
8
flag
16
8
8
DLCI C/R EA
6
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FCS
1
1
DLCI
4
F
E
C
N1
B
E
C
N1
DE EA
1
1
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Packet
Networks
ATM
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Asynchronous Transfer Mode
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designed as wideband ISDN
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fast switching
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defines layers 1-4 (physical, data-link, network, transport)
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small constant length packets (cells) 53=5+48 cell tax
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multiservice (data, CBR/VBR voice/video)
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QoS levels and guarantees
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connection oriented
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standards bodies: ITU-T, ATMF
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Packet
Networks
ATM
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5-byte
header
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48 byte payload
AAL1 connection oriented CBR
(GFC) VPI
VCI PTI CLP HEC
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
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VC
VC
VP
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DSL
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designed to reuse subscriber lines for broadband
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layer 1 (physical) protocol (modem)
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many varieties HDSL, SHDSL, ADSL, VDSL
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FDM of data with POTS
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synchronous but transports packet data
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cVoDSL for synchronous voice
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standards bodies : ITU-T, ETSI TM6, T1E1.4, DSLF
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Packet
Networks
TDM / PDH / SDH
The PSTN is not a PSN !
T1
…S 1
2 3 4
Same data rate
even when no data!
…
23 24 S 1 2 3
…
31 32 1 2
…
frame
193b
E1
…
1 2 3 4
32 B
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…
Frame every 125 msec
frame
STM-1
3
…
…
frame
9 * 270 B
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The Case
for VoP
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VoP
Case
Voice over PSN
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We saw that data transported over voice network
Should we “turn the tables” and transport voice over data networks?
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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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VoP
Case
Voice over PSN
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Voice has to be “packetized” (what size packets?, preprocessing?)
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Not a synchronous stream; no timing distribution
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Packets arrive after random delays
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Packets may arrive out-of-order
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Packets may be lost
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Reliability
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VoP
Case
PSTN Accessibility
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The PSTN has
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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%
>1.5 Billion people with access to fax
Is there any business reason
to transport voice otherwise?
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VoP
Case
Data Traffic Growth
Relative Capacity
250
200
150
100
50
0
1996
1997
1998
1999
2000
2001
Data traffic growing much faster than voice (already more)
Internet capacity increasing by factor of 10 each year
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VoP
Case
Revenue Breakdown
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AT&T 1998 figures
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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 it’s cheap!
The killer-app from revenue point of view is voice
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VoP
Case
Typical VoP Applications
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PC – PC communications (VoIP,VoDSL)
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Integrated Access Devices (VoATM,VoIP)
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Enterprise/campus convergence (VoFR,VoATM)
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Toll-bypass (VoFR, TDMoIP)
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Access networks (VoDSL, VoATM, VoCATV)
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PSTN Emulation
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PSTN
Emulation
Encapsulation
We would like to use the standard PSN technique
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header
TDM payload
trailer
but TDM payloads have no natural size packet !
The header will typically contain
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addresses
identifiers
status, alarms
sequence number
timestamp
control information
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PSTN
Emulation
Sequence Numbers
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Packet numbering is needed to
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detect packet loss (mainly for timing - RT systems do not retransmit)
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138
139
correct for misordering
135

136
137
136
138
supply timing when source is synchronous
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PSTN
Emulation
Timing
PSNs introduce delay variation (jitter)
How does PSTN emulation replicate timing?

Station clock

Clock distribution
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Adaptive clock
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PSTN
Emulation
RTP with IP/UDP
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IP header (5 dwords)
UDP header (2 dwords)
RTP header (> 3 dwords)
PAYLOAD
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PSTN
Emulation
RTP Header (RFC 1889)
0
1
2
3
01234567890123456789 012345678901
V P X CSRC M PAYLD TYPE
SEQUENCE NUMBER
TIMESTAMP
SSRC ID
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V version number
CSRC contributing source
P padding indicator
M marker bit
X extension indicator
SSRC sync source identifier
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PSTN
Emulation
Types of 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
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PSTN
Emulation
Payload Types
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Call emulation
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Leased line emulation
header

H.225
… 31 32
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CES (AAL1)
header
N voice samples
P
1 2 3 4

1 2 3 4
… 31 32 1
2
ptr
3 4
… 31 32 1
… 31 32
1 2
2
…
LES (AAL2)
TS1
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header
4
TS2
TS3
…
TSn
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PSTN
Emulation
Extent of Emulation
End-to-end emulation
Edge-to-edge emulation
Link emulation
core
switches
edge
switch
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edge
switch
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PSTN
Emulation
Emulation Elements
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PSTN emulations may have the following elements

End-points phone, user agent (UAC,UAS),

Gateways IWF, SoftSwitch

Intermediaries
– Proxies, Redirectors
– Mixers

Address and location servers gatekeeper, registrar
terminal
PSN
PSTN
GW
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PSTN
Emulation
Decomposed GWs
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
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PSTN
Emulation
Switching/routing emulation?
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Emulation relies on switching/routing
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of the underlying PSN

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
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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
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PSTN
Emulation
Signaling emulation?
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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!
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PSTN
Emulation
PSTNoPSN
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Fax
PSN
GW
PBX
GW
PSTN
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