Introduction to Packet Voice
Technologies
Cisco Networking Academy
Program
Traditional Telephony
Basic Components of a
Telephony Network
Central Office Switches
What Is a PBX?
Basic Call Setup
Supervisory Signaling
Address Signaling
–Tone telephone
•DTMF dialing
• Rotary telephone
– Pulse dialing
Informational Signaling
Digital vs. Analog Connections
Time-Division Multiplexing
Frequency-Division Multiplexing
Packetized Telephony
Networks
Packet Telephony vs.
Circuit-Switched Telephony
• More efficient use of bandwidth and
equipment
• Lower transmission costs
• Consolidated network expenses
• Increased revenue from new services
• Service innovation
• Access to new communications devices
• Flexible new pricing structures
Call Control
Distributed Call Control
Centralized Call Control
Packet Telephony Components
Real-Time vs. Best-Effort Traffic
• Real-time traffic needs guaranteed delay
and timing.
• IP networks are best-effort with no
guarantees of delivery, delay, or timing.
• Solution is quality of service end-to-end.
Foreign Exchange Station
Interface
Foreign Exchange Office
Interface
E&M Interface
T1 Interface
E1 Interface
BRI
Physical Connectivity Options
Cisco IP Phone
Analog Voice Basics
Local Loops
Types of Local-Loop Signaling
• Supervisory signaling
• Address signaling
• Informational Signaling
On Hook
Off Hook
Ringing
Ringing (Cont.)
Pulse Dialing
Dual Tone Multifrequency
Informational Signaling
with
Call-Progress Indicators
Trunks
Foreign Exchange Trunks
• Foreign Exchange Office
–Connects directly to office equipment
–Used to extend connections to another
location
• Foreign Exchange Station
–Connects directly to station equipment
–Used to provision local service
Types of Trunk Signaling
• Loop start
• Ground start
• E&M Wink Start
• E&M immediate start
• E&M delay start
Loop-Start Signaling
Ground-Start Signaling
E&M Signaling
• Separate
signaling leads
for each direction
• E-lead
(inbound
direction)
• M-lead
(outbound
direction)
• Allows
E&M Type I
E&M Type V
E&M Type II
E&M Type III
E&M Type IV
Trunk Supervisory Signaling—
Wink Start
Trunk Supervisory Signaling—
Immediate Start
Trunk Supervisory Signaling—
Delay Start
2-Wire to 4-Wire Conversion
and Echo
• Echo is due
to a
reflection.
• Impedance
mismatch at
the 2-wire to
4-wire hybrid
is the most
common
reason for
Echo Is Always Present
• Echo as a
problem is a
function of
the echo
delay and
the
loudness of
the echo.
Echo Suppression
Echo Cancellation
Analog-to-Digital Voice
Encoding
Digitizing Analog Signals
1. Sample the analog signal regularly.
2. Quantize the sample.
3. Encode the value into a binary
expression.
4. Compress the samples to reduce
bandwidth, optional step.
Basic Voice Encoding:
Converting Digital to Analog
1. Decompress the samples, if compressed.
2. Decode the samples into voltage
amplitudes, rebuilding the PAM signal.
3. Filter the signal to remove any noise.
Nyquist Theorem
Voice Compression Techniques
• Waveform algorithms
–PCM
–ADPCM
• Source algorithms
–LDCELP
–CS-ACELP
Example: Waveform
Compression
• PCM
–Waveform coding scheme
• ADPCM
–Waveform coding scheme
–Adaptive: automatic companding
–Differential: encode changes between
samples only
• ITU standards:
–G.711 rate: 64 kbps = (2 * 4 kHz) * 8
bits/sample
–G.726 rate: 32 kbps = (2 * 4 kHz) * 4
Compression Bandwidth
Requirements
Perceptual Speech Quality
Measurement
Signaling Systems
T1 Digital Signal Format
Robbed-Bit Signaling
Channel Associated Signaling—
T1
E1 Framing and Signaling
Channel Associated Signaling—
E1
Common Channel Signaling
ISDN
• ISDN
–Part of network architecture
–Definition for access to the network
–Allows access to multiple services through
a single access
–Used for data, voice, or video
• Standards-based
–ITU recommendations
–Proprietary implementations
ISDN Network Architecture
Layer 3 (Q.930/931) Messages