voip 101: the fundamentals of ip telephony william simmelink, general manager voip business unit...
TRANSCRIPT
VOIP 101: The Fundamentals of IP Telephony
William Simmelink, General ManagerVoIP Business UnitTexas Instruments
February 2003
Page 2
Agenda
Internet Telephony Call Basics
Fundamental Components of VoIP
Gateways
VoIP Applications
Page 3
Voice Over Internet Protocol (IP)
There are three styles of Voice over IP calls:
Phone to Phone
PC to Phone
PC to PC
InternetIntranet
Gateways adapt traditional telephony to the Internet.
Page 4
Telephony Signaling
CentralOfficeSwitch
Idle
First Digit is Dialed
DTMF Detector Activated in the CO
Dial-Tone On
Remaining Digits Dialed
Dial Tone Off
Ring Back
Voice-mode
Connected
On-Hook
Off-hook
Signals are exchanged between a telephoneand the switch at the Central Office. Thesesignals connect and disconnect calls as wellas inform the caller of the progress of the call.
Signals are exchanged between a telephoneand the switch at the Central Office. Thesesignals connect and disconnect calls as wellas inform the caller of the progress of the call.
Page 5
InternetIntranet
Packet Signaling
All three VoIP calls can use H.323, or SGCP/MGCP to set up the Internet portion of the call.
Calls involving gateways must also perform telephony signaling.
Page 6
Voice over Internet Signaling
Sending voice over a data network requires advanced signaling techniques in the gateways.
InternetIntranet
CentralOfficeSwitch
The gateway connected to the central office must emulate the telephone.
The gateway connected to the phone must emulate the signaling functions of the central office.
Page 7
Voice over Internet Signaling
Telephone numbers are translated to data network addresses (Internet addresses).
InternetIntranet
CentralOfficeSwitch
Telephony signals are interpreted by the gateway and mapped to the appropriate network protocol (H.323/SGCP/MGCP for IP) set-up, maintenance, billing and tear-down messages.
Page 8
PBXTelephone
DSP MICRO MICRO DSP
Off-hook
Voice-modevoice mode
On-hook
Idle-mode
Off-hook
DTMF Modefirst digits
Dial-tone
digits
digitsdigits
idle mode
Dial-tone off
Dial-tone
Switched CAS (FXS-FXO)
Network
connectconnect_ack
release
setup
setupcall_proceeding
call_proceeding
connectconnect_ack
Call Progress In Band
release
H.323SGCP/MGCP
Page 9
Fundamental Components of VoIP Gateways
Page 10
Micro Ethernet(Internet)
Micro Processor(s) Telephony Protocols Network Protocols Management Routing Billing
How is it all Done?
Within the Gateway a series of processors perform the adaptation from Traditional to Internet Telephony.
DSP
DSP
DSP
DSP
Telephones(Circuits)
Digital Signal Processor(s) (DSP) Voice Compression Tone Detection/Generation Echo Cancellation Silence Suppression
Page 11
Analog Voice to PCM
An analog voice signal is received.
The Signal is converted to a Pulse Code Modulation(PCM) digital stream.
10110101 11010011 11001001 00100100 00111100 10010011 11100001 00100100 00111100 10010011 10110101 11010011 11001001 00100100 00111100 10010011 11100001 001
DSP
Page 12
10110101 11010011 11001001 00100100 00111100 10010011 11100001 00100100 00111100 10010011 10110101 11010011 11001001 00100100 00111100 10010011 11100001 00100100
PCM Processing
The PCM stream is analyzed.
DSP
Detected signaling tones are routed around the CODEC. (needed, since most CODECs garble signaling tones to the point that they are unrecognizable)
Tone Detection is performed:
Echo is removed.
The Voice Activity Detector (VAD) removes silence.
Remaining stream is passed to CODEC.
Page 13
PCM to Frames
11010011 11001001 00100100 00111100 10010011 11100001 00100100 00111100
. . . and voice frames are created
10110101 11010011 11001001 00100100 00111100 10010011 11100001 00100100 00111100
Most CODECs also compress the PCM stream: PCM G.711 generates 64,000 bits per second G.729a compression generates 8,000 bits per second
DSP
The PCM stream is fed into the CODEC . . .
10110101
Each Frame is 10 ms long (G.729a) and contains 10 bytes of “speech.”
Page 14
Frames to Packets
DSP
10110101
Packet Assembler Software within the DSP takes frames from the CODEC and creates packets.
The packet is forwarded to the gateway’s host processor.
Several frames may be combined in a single packet
10110101 10110101 10110101RTP
A 12 byte Real Time Protocol (RTP) Header is added: Provides sequence number Time stamp
Page 15
IP
A 20 byte IP header is added to the packet containing: The IP address of this gateway (the source address) The IP address of the destination gateway
An 8 byte UDP header containing source and destinationsockets is also added.
UDP
Addressing
Dialed digits identified by the tone detection performed in the DSP are used to determine the destination number.
1011010110110101 1011010110110101RTP
301-999-1212
This number is mapped to an IP Address.
= 192.128.100.2
Micro
Page 16
In the Internet
Routers and Switches in the Internet examine the addressesin the IP address in order to identify the route to the destination.
Several routers and or switches may be in the path thatthe packets take to their destination.
Page 17
IP
Upon Arrival at the Destination
The IP and UDP headers are removed from the packet in the Microprocessor.
UDP
Micro
RTP
The Packet is forwarded to the DSP where theRTP Header is removed.
Finally, the packet is disassembled leaving thevoice frames.
1011010110110101 10110101 10110101
Page 18
Various Network Problems are Dealt With
Voice Packets are generated at a constant rate while someoneis speaking; there is essentially no gap between packets.
These gaps, known as jitter, must be removed by the receivinggateway in order to accurately reproduce the original speech
Devices in the network cause an unpredictable amount ofdelay to occur between packets.
Page 19
Jitter Removal
An adaptive jitter buffer in the receiving DSP is used to smooth the playout of packets arriving from a “jittery” network.
DSP
DSP
This eliminates the jitter induced distortion that would have been heard by the listener.
Page 20
Lost Packets
Congestion in the network may cause some packets to be dropped.
1
2
4
6
5
3
Left untreated, the listener hears annoying pops & clicks.
Page 21
Lost Packets
An algorithm in the DSP detects missing packets.
1
2
4
6
And replays the last successfully received packet at a decreased volume in order to fill the gaps.
1
2
4
6
4
2
DSP
35
Page 22
Turning “Hello”…..
Into “oHell”
Out of Order Packets
Out of order packets are not played in the order they arrive…..
Packets may take diverse routes through a network and may arrive out of order.
1
2
4
5
3
DSP
Page 23
Out of Order Packets
1
2
4
5
DSP
When an out of order condition is detected the missing packet is replaced by its predecessor as if it is lost.
2
When the late packet finally arrives it is discarded.
3
Page 24
PCM Back to Analog
A Comfort Noise Generator fills in the gaps that were created during silence detection and suppression.
The PCM Stream is reconstituted as an analog signal and is played out to the listener.
10110101 11010011 11001001 00100100 00111100 10010011 11100001 00100100 00111100 10010011 10110101 11010011 11001001 00100100 00111100 10010011 11100001 00100100
Page 25
VoIP Applications
Page 26
Central Office/Infrastructure
CentralOffice Gateway
PacketNetwork
Traditional carriers migrate to packet core for lower network costs.
Gradual capping of Class 4 tandem switches drives CO/Infrastructure VoIP ports.
Carriers proposing new packet architectures with dramatically lower cost structures.
Page 27
Enterprise
SME Gateway
IP Phone
PBX
PacketNetwork
Enterprises deploying to avoid access charges and settlement fees.
Businesses take advantage of existing data networks.
Reduced operating costs by managing one network.
Page 28
IP Phones and PBX Trunking
Office 2Office 2
Office 1Office 1
T1
PacketNetwork
Gateway
Router
IP Phone
PBX
LAN-based PBX for cost reduction, flexibility, and new applications: Integrated voice/data LAN infrastructure Integrated voice/data applications Open hardware platform
Page 29
Residential Broadband
Residential voice alternatives, leveraging broadband connections
VoCable solutions in trials in US, and deployments in Europe
VoDSL deployments in Asia and Europe
Fiber to the Home potential in China
CMTS DSLAM
CPE Gateway Cable
Modem
Packet Network
Cable or DSL Modem Based IAD
VoiceGateway
Page 30
Summary
VoIP solutions require well integrated, robust set of functional components for toll quality operation.
VoIP implementations are in current systems deployed worldwide.
VoIP value proposition exists in different vertical markets.