401
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1
© 1999, Cisco Systems, Inc.
Introduction to Voice and
Telephone Technology
Session 401
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
1
Voice Is Not A Network
• Voice is an Application
• Complete understanding of Voice
Application fundamentals helps us to
design and build better Networks
401
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© 1999, Cisco Systems, Inc.
Objective
To Prepare the Data
Communications Professional
for Voice and Data Network
Integration by Providing Voice
Technology Fundamentals
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
2
Agenda
• Basic Analog Telephony
• Basic Digital Telephony
• Voice Coding and Compression
Techniques
• Voice Transport and Delay
• Supplemental Slides: Digital Voice
Signaling Techniques
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5
© 1999, Cisco Systems, Inc.
Telephony Equipment
• Telephone set
• Key system
Optimizes use of telephone sets to lines
Mechanical to electronic
Two to ten telephone handsets is typical
• PBX (Private Branch Exchange)
Advanced features and call routing
Tens to hundreds of telephone handsets
• Central office switch
401
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
3
Analog Telephony—
Connection Basics
Tip
Ring
Sleeve
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© 1999, Cisco Systems, Inc.
Basic Call Progress: On-Hook
Telephone
Switch
Local
Loop
Local
Loop
DC Voltage
Open Circuit
No Current Flow
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
4
Basic Call Progress: Off-Hook
Off-Hook
Closed
Circuit
DC Current
Dial Tone
Telephone
Switch
Local
Loop
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Local
Loop
9
© 1999, Cisco Systems, Inc.
Basic Call Progress: Dialing
Off-Hook
Closed
Circuit Dialed Digits
Pulses or
Tones
DC Current
Telephone
Switch
Local
Loop
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
5
Basic Call Progress: Switching
Off-Hook
Closed
Circuit
Telephone
Switch
DC Current
Local
Loop
401
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Address
to
Port
Translation
Local
Loop
11
© 1999, Cisco Systems, Inc.
Basic Call Progress: Ringing
Off-Hook
Closed
Circuit
Ring Back
Tone
DC Current
Local
Loop
401
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DC Open Cct.
Ringing Tone
Telephone
Switch
Local
Loop
12
© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
6
Basic Call Progress: Talking
Off-Hook
Closed
Circuit
Voice Energy
DC Current
Telephone
Switch
Voice Energy
DC Current
Local
Loop
401
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Local
Loop
13
© 1999, Cisco Systems, Inc.
Analog Telephony—Signaling
• Supervisory
• Addressing
• Call progress
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
7
Analog Telephony—
Supervisory Signaling
Switch
401
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Switch
• Loop start
• Ground start
Almost all
telephones
Switch Trunk
Lines
Current flow
sensed
Momentary
ground ring lead
15
© 1999, Cisco Systems, Inc.
Loop Start
Station
PBX or Central Office
Loop
(Local or Station)
DC Current
Ringing
401
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AC
Switch
+
–
Switch
+
–
Switch
+
–
16
© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
8
E&M Signaling
• PBXs, switches
Separate signaling leads for each direction
E-Lead (inbound direction)
M-Lead (outbound direction)
Allows independent signaling
401
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State
E-Lead
M-Lead
On-Hook
Open
Ground
Off-Hook
Ground
Battery Voltage
17
© 1999, Cisco Systems, Inc.
Signaling and Addressing
Dial Pulse
DTMF
Analog Transmission
“In-Band” Signaling
0–9, *, # (12 Digits)
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ISDN
Digital Transmission
“Out-of-Band”
Message-Based
Signaling
18
© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
9
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)
401
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© 1999, Cisco Systems, Inc.
Tone Dialing
Dual Tone Multifrequency (DTMF)
401
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1209
1336
1477
1633
697
1
2
3
A
770
4
5
6
B
852
7
8
9
C
941
*
0
#
D
20
© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
10
Voice Channel Bandwidth
Voice Channel
Voice Signal
Output
Voltage
or Energy
.2
1
Tone Dialing
Signals
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2
3
4
Frequency
(K-Hertz)
Systems Control
Signals
21
© 1999, Cisco Systems, Inc.
Switching Systems
Manual Control—Switch/Cord Boards
Off-Hook Indicator
Tip
Ring
Patch Cord
Pairs
Manual Ring
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
11
Local Access Network
Feeder Route Boundary
Central
Office
40,000 to
50,000 Lines
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Serving
Area
Boundary
23
© 1999, Cisco Systems, Inc.
PSTN Network Hierarchy
1
1
1
Class Name
2
4C
4P
3
3
5
4C
4C
4X
4P
5
5
4P
5
5R
5R
5
4X
1
Regional Center
2
Sectional Center
3
Primary Center
4C
Toll Center
4P
Toll Point
4X
Interm. Point
5
End Office
5R
EO w/ RSU
R
Remote Sw. Unit
R
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
12
Types of Voice Circuits
Serving Area
415-NXX-XXX
415-577-3800
Serving Area
510-NXX-XXX
Class 5
Class 5
Switch
Switch
OPX
Off-Premises
Ext.
415-577-3801
510-655-1400
FX
Foreign Exchange
ARD
Auto Ring Down
401
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© 1999, Cisco Systems, Inc.
Echo in Voice Networks
Talker
Listener
Delay
Talker Echo
Listener Echo
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
13
Normal Signal Flow
Two-Wire
Local Loop
Central Office
Receive
Direction
2w-4w
Hybrid
Transmit
Direction
• Two- to four-wire hybrid combines
receive-and transmit-signals over
the same pair
• Two-wire impedance must match
four-wire impedance
401
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© 1999, Cisco Systems, Inc.
How Does Echo Happen?
Echo Is Due to a Reflection
Echo Is Experienced here
Transmit
Direction
Impedance Mismatch is here
2w-4w
Hybrid
2w-4w
Hybrid
Central Office
Central Office
Reflected
Signal
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Receive
Direction
Impedance Mismatch at the 2w-4w Hybrid
Is the Most Common Reason for Echo
28
© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
14
Echo Is Always Present
Echo as a Problem Is a Function
of the Echo Delay, and the
Magnitude of the Echo
Echo Is Unnoticeable
(dB)
Echo Path Loss
Echo Is a Problem
Echo Path Delay (ms)
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© 1999, Cisco Systems, Inc.
Ways to Defeat Echo
• Increase the loss in the echo path
Can often be the solution
Disadvantage: static setting and reduces
the signal strength of the speaker
• Echo suppresser
Acts like a noise gate, effectively
making communications half-duplex
401
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
15
Echo Canceler
Most Effective Means for Removing Echo
Echo “Cancelled”
Here
Voice Endpoint
E/C
Received
Voice
Signal
+
Echo Canceler
Block Diagram
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Adaptive
Filter
31
© 1999, Cisco Systems, Inc.
Summary
• Information exchange based on
voltage, current flow, grounding,
and so on
• Analog voice technology dates
back to the late 1800s
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
16
Agenda
• Basic Analog Telephony
• Basic Digital Telephony
• Voice Coding and Compression
Techniques
• Voice Transport and Delay
• Supplemental Slides: Digital Voice
Signaling Techniques
401
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© 1999, Cisco Systems, Inc.
Digital Telephony
Digital Trunking
Switch
Switch
Analog Loop
Digital Network
Switch
POTS
A to D
Conversion
Digital Loop Digital Network
Switch
ISDN
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
17
Digital Telephony
Pulse Code Modulation—Nyquist Theorem
Voice Bandwidth =
200 Hz to 3400 Hz
Analog Audio Source
Sampling Stage
= Sample
Codec Technique
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8 bits per sample
8 kHz (8,000 Samples/Sec)
35
© 1999, Cisco Systems, Inc.
Pulse Code Modulation—
Analog to Digital Conversion
Quantizing Noise
A—Law (Europe)
10010011011
Stage 1
µ—Law (USA)
Quantizing Stage
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
18
Time Division Multiplexer
Example: T1 Channel Bank
INPUTS
OUTPUT
8,000 Frames per Second
(1 Frame per 125 µs)
Analog or Digital
Interface Cards
Ch. 1
Ch. 2
Ch. 3
Ch. 4
Ch. 5
Ch. 6
Framing
Bit
(8000 per Second)
Ch 1 Ch 2 Ch 3
Ch 4 Ch 5
Ch 6
Next Frame
Framing
Bit
Ch 24
Chs 7-23
Ch 1, etc
T1
Multiplexer
Chs.
7-23
64 kbps x 24 = 1.536 Mbps
Add Framing Bits = 8 Kbps
Total Bit Rate: 1.544 Mbps
Ch. 24
Each Input
Represents
64 kbps
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Eight Bits from
Each Channel Input
In Sequential Order
37
© 1999, Cisco Systems, Inc.
DS1 Superframe (D4) Format
• 193rd bit of each
frame used for
frame synchronization
• D4 framing is 12
frames
• D4 framing pattern is:
100011011100
• Channel Associated
Signaling (CAS) robs
the LSB of every byte
in frames 6 and 12 for
AB bits
Framing
Bits
Frame
Number
1
2
3
4
5
6
7
8
9
10
11
12
Framing Bit
Value
1
0
0
0
1
1
0
1
1
1
0
0
Bit Use in Each Channel
Time Slot
Signaling—Bit
Use Options
Traffic
Signaling
T
2
4
Bits 1–7
Bit 8
*
A
A
Bits 1–7
Bit 8
*
A
B
• Common Channel
Signaling (ISDN)
uses TS 24
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
19
Extended Superframe (ESF)
S Bits
Frame
Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
401
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Bit Use in Each Channel
Time Slot
Fe
DL
BC
–
–
–
0
–
–
–
0
–
–
–
1
–
–
–
0
–
–
–
1
–
–
–
1
m
–
m
–
m
–
m
–
m
–
m
–
m
–
m
–
m
–
m
–
m
–
m
–
–
C1
–
–
–
C2
–
–
–
C3
–
–
–
C4
–
–
–
C5
–
–
–
C6
–
–
Signaling—Bit
Use Options
Traffic
Signaling
T
2
4
16
Bits 1–7
Bit 8
*
A
A
A
Bits 1–7
Bit 8
*
A
B
B
Bits 1–7
Bit 8
*
A
A
C
Bits 1–7
Bit 8
*
A
B
D
39
© 1999, Cisco Systems, Inc.
Digital Signaling Schemes
Channel Associated Signaling
Extended Superframe
“In-Band” Audio
Address Signaling
(DTMF)
401
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Bit
A
B
C
D
Supervision
On/Off Hook
Frame
6th
12th
18th
24th
Address Signaling
(Dial Pulse)
40
© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
20
Digital Signaling Schemes
Common Channel Signaling
Extended Super Frame
64 Kbps Signaling
Channel in TS24
of Each Frame
(e.g. ISDN D Channel
Q.931 Messages)
“In-Band” Audio
Address Signaling
(DTMF)
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© 1999, Cisco Systems, Inc.
Digital Telephony—
Synchronization
• Bit synchronization
Primary reference source
Ones density
• Time-slot synchronization
Bits/bytes/channels
• Frame alignment
193rd Bit Pattern
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
21
Digital Telephony—
Synchronization
One Multiframe (ESF)
3 ms
12
1
24
1 Frame,
125µs, 193bits 24 Time Slots
1
12
24
1 Channel Time
Slot, 5.18µs
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© 1999, Cisco Systems, Inc.
Synchronization—Traditional
Network Clocking Strata
Master Clock
Stratum
PRS
1
Timing
Toll Office
Timing
2
Timing
Timing
End Office
End Office
DCS
3
PBX
PBX
4
401
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
22
Agenda
• Basic Analog Telephony
• Basic Digital Telephony
• Voice Coding and Compression
Techniques
• Voice Transport and Delay
• Supplemental Slides: Digital Voice
Signaling Techniques
401
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© 1999, Cisco Systems, Inc.
Voice Coding and Compression
• Speech-coding schemes
• Subjective impairment analysis:
mean opinion scores
• Digitizing voice
• Voice compression
ADPCM
CELP (LD-CELP and CSA-CELP)
Silence removal techniques (DSI using VAD)
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
23
Voice Compression Technologies
Unacceptable
Business
Quality
Toll
Quality
*
PCM (G.711)
64
PCM (G.711)
(Cellular)
Bandwidth
(Kbps)
32
*
* (G.726)
ADPCM 32
*
24
16
ADPCM 24 (G.726)
*
*
*
ADPCM 16 (G.726) LDCELP 16 (G.728)
8
0
*
CS-ACELP* 8 (G.729)
* LPC 4.8
Quality
401
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© 1999, Cisco Systems, Inc.
Speech-Coding Schemes
• Waveform coders
Non-linear approximation
of the actual waveform
Examples: PCM, ADPCM
• Vocoders
Synthesized voice
Example: LPC
• Hybrid coders
Linear waveform approximation
with synthesized voice
Example: CELP
401
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
24
Subjective Impairment Analysis:
Mean Opinion Scores
5
Hybrid Coders
4
Waveform Coders
Subjective
Quality 3
(MOS)
2
Vocoders
1
2
Score
5
4
3
2
1
401
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4
8
16
Kbps
32
Quality
Description of Impairment
Excellent
Good
Fair
Poor
Bad
Imperceptible
Just Perceptible, not Annoying
Perceptible and Slightly Annoying
Annoying but not Objectionable
Very Annoying and Objectionable
64
©
© 1998,
1999, Cisco
Cisco Systems,
Systems, Inc.
Inc.
49
Measuring Mean Opinion Scores:
ITU P.800 Series
Source
Channel Simulation
Impairment
Codec ‘X’
1
2
3
4
5
1
2
3
4
5
“Nowadays, a chicken leg is a rare dish”
Rating
Rating
Level
Level of
of Speech
Speech Quality
Quality
Distortion
Distortion
5
Excellent
Imperceptible
4
Good
Just perceptible but not annoying
3
Fair
Perceptible and slightly annoying
2
Poor
Annoying but not objectionable
1
Unsatisfactory
Very annoying and objectionable
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
25
Digitizing Voice: PCM
Waveform Encoding Review
• Nyquist Theorem: sample at twice the
highest frequency
Voice frequency range: 200-3400 Hz
Sampling frequency = 8000/sec (every 125µs)
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
= DS-0
64 Kbps
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© 1999, Cisco Systems, Inc.
Nonlinear vs. Linear Encoding
Output
Output
Input
Input
Nonlinear Encoding
Linear Encoding
Closely Follows Human Voice Characteristics.
High Amplitude Signals have
More Quantization Distortion.
Relatively Easy to Analyze, Synthesize and
Regenerate. All Amplitudes Have Roughly
Equal Quantization Distortion.
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
26
Encoding
Quantizing
Filtering
Sampling
Voice CODECs: Waveform Coders
1110010010010110
Waveform
ENCODER
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Waveform
DECODER
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© 1999, Cisco Systems, Inc.
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)
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
27
Voice Compression—ADPCM
• Adaptive Differential Pulse
Code Modulation
Waveform coding scheme
Adaptive: automatic companding
Differential: encode the changes
between samples only
Rates and bits per sample:
32 Kbps = 8 Kbps x 4 bits/sample
24 Kbps = 8 Kbps x 3 bits/sample
16 Kbps = 8 Kbps x 2 bits/sample
401
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© 1999, Cisco Systems, Inc.
Voice Compression—CELP
• Code excited linear predictive
• Very high voice quality at low-bit rates,
processor intensive, use of DSPs
• G.728: LD-CELP—16 Kbps
• G.729: CSA-CELP—8 Kbps
G.729a variant— “stripped down” 8 kbps
(with a noticeable quality difference)
to reduce processing load, allows two
voice channels encoded per DSP
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
28
Voice CODECs: Hybrid Coders
PCM Encoder
PCM
Decoder
Filtering
11100100100101
Sampling
1
Quantizing
Sample
Encoding
Frames
VocalCords
Throat
Nose
Mouth
Human
Speech
Model
401
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Model
Parameters
Model
Parameters
10110010
Parameters
Analysis
Synthesis
57
© 1999, Cisco Systems, Inc.
G.729
Cake
A/D
Cake
Code DSP Packet Recipe
10.1.1.1
16-Bit Linear PCM
Code
Look-up
IngredientsDirections
A-sound Play K, A,
K-sound and K
Recipe or Code Book
401
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
29
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
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© 1999, Cisco Systems, Inc.
Voice Activity Detection
- 31 dbm
B/W Saved
Voice
Activity
(Power
Level)
Hang Timer
No Voice
Traffic Sent
SID
SID Buffer
- 54 dbm
Pink Noise
Voice “Spurt”
Silence
Voice “Spurt”
Time
401
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
30
Bandwidth Requirements
Voice Band Traffic
Encoding/
Compression
G.711 PCM
A-Law/µ
A-Law/
µ-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
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Result
Bit Rate
6.3/5.3 kbps
Variable
61
© 1999, Cisco Systems, Inc.
Agenda
• Basic Analog Telephony
• Basic Digital Telephony
• Voice Coding and Compression
Techniques
• Voice Transport and Delay
• Supplemental Slides: Digital Voice
Signaling Techniques
401
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
31
Voice Network Transport
• Voice Network Transport is
typically TDM circuit-based:
T1/E1
DS3/E3
SONET (OC-3, OC-12, etc.)
• But can also be packet-based:
ATM
Frame Relay
IP
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© 1999, Cisco Systems, Inc.
Data Is Overtaking Voice
Evolution from TDM-based
transport to packets/cells
or a combination
Relative
Load
Data Is 23x
Voice
Traffic
30
25
20
Data
15
10
Data Is 5x
Voice Traffic
5
0
1990
Voice
1995
2000
2005
Year
Source: Electronicast
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
32
The Tyranny of the DS0
• Switching and transport based on circuits
• Rigid structure yields high cost for packet
Customer
Premise
Local
CO
Class-5 DS0
Switch
DS1
DS0
Class-4
Switch
DS3
DS0
Switching
Class-4
Switch
DS1
DS3
DS0
DS3
DS3
401
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Customer
Premise
DS0
DS1
DS0
3/1 DACS
DS3
DS3
DS1
SONET
ADM
SONET
ADM
OC-3/12
Class-5
Switch
DS1
Transport
3/1 DACS
DS1
Local
CO
Interexchange
OC-48
OC-48
OC-48
OC-3/12
65
© 1999, Cisco Systems, Inc.
TDM Transport Efficiency
Types of Traffic
Voice
Utilization
PBX
Wasted Bandwidth
Legacy
50–60%
LAN
Video
Single WAN Link
Time Slot Assignments
• Wasted bandwidth
• No congestion
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
33
Packet Transport Efficiency
Types of Traffic
Voice PBX
Utilization
Q
U
E
U
E
Legacy
90–95%
LAN
Cells/Frames/Packets
Video
Individual Packets
• High bandwidth efficiency
• Congestion management
401
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© 1999, Cisco Systems, Inc.
Delay
Sender
PBX
Receiver
Network
PBX
First Bit
Transmitted
Last Bit
Received
A
Processing
Delay
A
Network
Transit
Delay
t
Processing
Delay
End-to-End Delay
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© 1999, Cisco Systems, Inc.
Cisco Systems Confidential
34
Delay Variation—“Jitter”
Sender
Receiver
Network
A
B
C
Sender Transmits
t
A
B
D1
401
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D2 = D1
C
Sink Receives
D3 = D2
t
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Voice Delay Guidelines
One Way Delay
(msec)
Description
0–150
Acceptable for Most User Applications
150–400
Acceptable Provided That
Administrations Are Aware
of the Transmission Time Impact
on the Transmission Quality
of User Applications
400+
Unacceptable for General Network
Planning Purposes; However, It Is
Recognized That in Some Exceptional
Cases This Limit Will Be Exceeded
ITU’s G.114 Recommendation
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Delay in Perspective
Cumulative Transmission Path Delay
CB Zone
Satellite Quality
Fax Relay, Broadcast
High Quality
0
100
200
300
400
500
600
700
800
Time (msec)
Delay Target
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Fixed Delay Components
Propagation Delay
Serialization Delay—
Buffer to Serial Link
Processing Delay
• Propagation—Six microseconds per kilometer
• Serialization
• Processing
Coding/compression/decompression/decoding
Packetization
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Variable Delay Components
Queuing
Delay
Queuing
Delay
Queuing
Delay
Dejitter
Buffer
• Queuing delay
• Dejitter buffers
• Variable packet sizes
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An Example
• Assumptions:
We have eight trunks
We are going to use CS-ACELP that uses
8 Kbps per voice channel
Our uplink is 64 Kbps
Voice is using a high priority queue and
no other traffic is being used
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Delay Calculation
Los
Coder Delay Queuing Delay
Angeles
25 ms
6 ms
Propagation
Delay—32 ms
Dejitter Buffer
50 ms
Munich
(Private Line Network)
Serialization Delay
3 ms
Fixed
Delay
Coder Delay G.729 (5 msec Look Ahead)
Coder Delay G.729 (10 msec per Frame)
Packetization Delay—Included in Coder Delay
Variable
Delay
5 msec
20 msec
21 msec
Max Queuing Delay 64 kbps Trunk
Serialization Delay 64 kbps Trunk
3 msec
Propagation Delay (Private Lines)
32 msec
Variable
Delay
Component
Network Delay (e.g., Public Frame Relay Svc)
Dejitter Buffer
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Total
50 msec
110 msec
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Variable Delay Calculation
• We have eight trunks, so in the worst case we will
have to wait for seven voice calls prior to ours
• To put one voice frame out on a 64Kbps link
takes 3msec
• 1 byte over a 64Kbps link takes 125 microseconds.
We have a 20 byte frame relay frame with 4 bytes of
overhead. 125 * 24 = 3000 usecs or 3 msec
• Does not factor in waiting for a possible data
packet or the impact of variable sized frames
• Assumes voice prioritization of frames
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Delay Calculation
Site B
Fixed
Delay
Variable
Delay
Site A
Delay #1
DELAY #1
Coder Delay G.729
Packetization Delay
25 msec
(Included in Coder Delay)
Max Queuing Delay 64 kbps Trunk
Serialization Delay 64 kbps Trunk
3 msec
Propagation Delay (Private Lines)
Dejitter Buffer
32 msec
50 msec
Private Line
Network
21msec
Delay #2
Tandem Switch
Delay #1 Total
—
110 msec
Site C
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Delay Calculation
Site B
Fixed
Delay
DELAY #1 Total
Variable
Delay
Site A
Delay #1
110 msec
DELAY #2
Coder Delay G.729
Packetization Delay
25 msec
Private Line
Network
(Included in Coder Delay)
Max Queuing Delay 2 Mbps Trunk
Serialization Delay 2 Mbps Trunk
0.1 msec
.7 msec
Propagation Delay (Private Lines)
Dejitter Buffer
5 msec
50 msec
Delay #2 Total
80 msec
Total Delay
190 msec
Delay #2
Site C
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Other Useful Voice QoS
Schemes in IP
• Custom Queuing, Priority Queuing
and Weighted Fair Queuing (WFQ)
• Resource Reservation Protocol
(RSVP)
• IP Precedence Bit setting in the ToS
Field of the IP Header
• Compressed Real Time Protocol
(CRTP)
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Summary
• Voice traffic engineering principles
still apply
• Packet-based voice trunks can
provide efficiency with high quality if
properly engineered
• The biggest impact on voice quality
over a data network will be as a result
of the delay and delay variation
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Repeat: Voice Is Not A Network
• Voice is an Application
• Complete understanding of Voice
Application fundamentals helps us to
design and build better Networks
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© 1999, Cisco Systems, Inc.
Agenda
• Basic Analog Telephony
• Basic Digital Telephony
• Voice Coding and Compression
Techniques
• Voice Transport and Delay
• Supplemental Slides: Digital Voice
Signaling Techniques
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Digital Voice Signaling
Techniques
• ISDN
• Q.930/Q.931
• Signaling System 7
• Voice addressing
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ISDN
• Integrated Services Digital Network
Part of a network architecture
Definition for the access to the network
Allows access to multiple services
through a single access
• Standards-based
ITU recommendations
Proprietary implementations
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Network Access
Traditional Access
Public Packet-Switched Network
PSTN (CO Lines)
800
Tie Trunks
FX
Private Lne Data
Customer
Equipment
(PBX)
ISDN Access
Customer
Equipment
(PBX)
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Telephone
Switch
Public Packet-Switched Network
PSTN (CO Lines)
800
Tie Trunks
FX
Private Line Data
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Terminology
• B channel “bearer channel”
64 kbps
Carries information (voice, data,
video, etc.)
DS-0
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Terminology (Cont.)
• D channel “signaling channel”
16 Kbps or 64 Kbps
Carries instructions between customer
equipment and network
Carries information
Can also carry packet switch data (X.25)
for the public packet switched network
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Terminology (Cont.)
• BRA/BRI (Basic Rate Access/
Basic Rate Interface)
2B+D
2 x 64 Kbps + 16 Kbps = 144 Kbps
(not including overhead)
Designed to operate using the average
local copper pair
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Terminology (Cont.)
• PRA/PRI (Primary Rate
Access/Primary Rate Interface)
23 B + D
23 x 64 Kbps + 64 Kbps (D Channel) + 8
Kbps (Frame Alignment bit) = 1.544 Mbps
Designed to operate using T1/E1
In E1 environments: 30 B + D
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ISDN Reference Points
TE1
.
TE1
NT1
S/T
.
TE2
.
BRA
U
TA
R
TE1
Carrier
.
.
R
S
TE2
TE2
.
TA
R
NT2
(PBX)
.T
.
U
PRA
.
S
Customer Premises
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NT1
Local Loop
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ISDN Reference Points
• NT1
Terminates local loop
Coding and transmission conversion
Maintenance and performance
monitoring
Functions as a CSU
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ISDN Reference Points (Cont.)
• TE1
ISDN compatible equipment
• TE2
Non-ISDN compatible equipment
Requires TA
• TA
Interfaces available for different TE2
E.g. RS-232, X.21, V.35, PC-Bus, video, etc.
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ISDN Reference Points (Cont.)
• NT2
Typically a PBX
Provides switching functions
Handles Layer 2 and Layer 3 protocols
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Access to ISDN
• At the S-reference point:
RJ-45 (receive and transmit pair)
Optional power can be provided
for TE devices
Distance:
1 Km (1 x TE only),
200 m (8 x TE), 500 m (4 x TE)
When more than one TE, wires
act as a bus
CSMA/CD
Limitation: cannot have an
extension phone
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Access to ISDN
• At the U-Reference point (BRA)
Standards differ NA, France,
UK vs. Germany vs. Japan
In North America, designed to use as
much of existing copper plant available
Two wire, unloaded local loops are
99% of total
Up to 5.5 Km loop length
• At the U-Reference point (PRA)
T1/E1 standard
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D Channel
• ISDN Access Protocols are carried in the D channel
• Layer 2 and Layer 3 protocol specifications
Protocol specifications are identical for BRA and PRA
• Layer 2, Q.920/921, LAP-D
Supports the communications for Layer 3
Maintains the connections between devices
• Layer 3, Q.930/931
Call setup, call supervision, call tear down, and
supplementary services
Uses standard set of messages to communicate
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D-Channel Encapsulation
Layer 3
Layer 2
Protocol
Length of
Discriminator Call Reference
Flag
Address Control
Layer 1
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Call
Reference
Message
Type
Information
CRC
Information
Elements
Flag
D Channel
(16 Kbps or 64 Kbps)
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ISDN CCS (Q.930/931) Messages
Call Establishment
Call Information
Call Clearing
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Alerting
Call proceeding
Connect
Connect ack
Progress
Setup
Setup ack
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Hold
Hold ack
Hold reject
Resume
Resume ack
Resume reject
Retrieve
Retrieve ack
Retrieve reject
Suspend
Suspend ack
Suspend reject
User information
Disconnect
Release
Release complete
Restart
Restart ack
Miscellaneous
•
•
•
•
•
•
•
Congestion control
Facility
Information
Notify
Register
Status
Status inquiry
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Public ISDN and
Signaling System 7
Signaling
Network
BRI
PBX1
BRI
Switch
PRI
DSS1
Transmission
Network
Switch
PRI
Signaling System 7
PBX2
DSS1
DSS1 Is a Public ISDN Protocol
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ISDN and SS7 “The Bridge
Between the Islands”
Voice Transmission
STP
Switch
SSP
Switch
SSP
SS7
SCP
Signaling Network
Voice
Transmission
Switch
STP
STP
STP
STP
PBX1
SSP
STP
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Voice
and
ISDN—
Signaling PRI
SCP
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SS7 Components
SCP
Network 1
SCP
SSP
Network 2
STP
STP
STP
STP
SCP
SCP
SSP
SSP
SSP
Voice Trunk
Signaling Link
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SSP: Signal Switching Point
STP: Signal Transfer Point
SCP: Signal Control Point
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Network Addressing
LEC 1-609-555-1234
IXC
555-1234
LEC
PSTN
E.164 Addressing
1-609-5551234 Dials:
9+1-609-555-1234
555-1234
PBX
PBX
Dials:
8+555-1234
555-1234
555-1234
VCI/VPI
1234
1234
VCI/VPI
WAN
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Agenda
• Basic Analog Telephony
• Basic Digital Telephony
• Voice Coding and
Compression Techniques
• Voice Transport and Delay
• Supplemental Slides: Digital Voice
Signaling Techniques
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© 1999, Cisco Systems, Inc.
Thank You!
•Q & A
• Please Fill Out
Evaluation Forms
• THANK YOU!
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Please Complete Your
Evaluation Form
Session 401
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105
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