Transcript 1 - TMCnet
VOIP 101: The Fundamentals of IP Telephony William Simmelink, General Manager VoIP Business Unit Texas Instruments February 2003 Agenda Internet Telephony Call Basics Fundamental Components of VoIP Gateways VoIP Applications Page 2 Voice Over Internet Protocol (IP) There are three styles of Voice over IP calls: PC to PC PC to Phone Phone to Phone Internet Intranet Gateways adapt traditional telephony to the Internet. Page 3 Telephony Signaling Off-hook Signals are exchanged between a telephone Dial-Tone On and the switchDTMF at Detector the Central Office. These Activated in the CO signals connect and disconnect calls as well First Digit is Dialed as inform the caller of the progress of the call. Dial Tone Off Central Office Switch Remaining Digits Dialed Ring Back Connected Voice-mode On-Hook Idle Page 4 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. Internet Intranet Page 5 Voice over Internet Signaling Sending voice over a data network requires advanced signaling techniques in the gateways. Internet Intranet Central Office Switch The gateway connected to the phone must emulate the signaling functions of the central office. The gateway connected to the central office must emulate the telephone. Page 6 Voice over Internet Signaling 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. Internet Intranet Central Office Switch Telephone numbers are translated to data network addresses (Internet addresses). Page 7 Telephone Switched CAS (FXS-FXO) DSP MICRO Off-hook Dial-tone DTMF Mode first digits Dial-tone off digits Network setup call_proceeding MICRO PBX setup call_proceeding H.323 SGCP/MGCP connect connect_ack Voice-mode On-hook Idle-mode connect connect_ack Call Progress In Band release DSP release Off-hook Dial-tone digits digits voice mode idle mode Page 8 Fundamental Components of VoIP Gateways Page 9 How is it all Done? Within the Gateway a series of processors perform the adaptation from Traditional to Internet Telephony. Digital Signal Processor(s) (DSP) Voice Compression Tone Detection/Generation Echo Cancellation Silence Suppression Telephones DSP DSP Micro (Circuits) Micro Processor(s) Telephony Protocols Network Protocols Management Routing Billing Ethernet (Internet) DSP DSP Page 10 Analog Voice to PCM DSP 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 Page 11 PCM Processing The PCM stream is analyzed. DSP Echo is removed. The Voice Activity Detector (VAD) removes silence. Tone Detection is performed: Detected signaling tones are routed around the CODEC. (needed, since most CODECs garble signaling tones to the point that they are unrecognizable) 10110101 11010011 11001001 00100100 00111100 10010011 11100001 00100100 00111100 10010011 10110101 11010011 11001001 00100100 00111100 10010011 11100001 00100100 Remaining stream is passed to CODEC. Page 12 PCM to Frames DSP The PCM stream is fed into the CODEC . . . 10110101 11010011 11001001 00100100 00111100 10010011 11100001 00100100 00111100 11010011 11001001 00100100 00111100 10010011 11100001 00100100 00111100 . . . and voice frames are created 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 Each Frame is 10 ms long (G.729a) and contains 10 bytes of “speech.” 10110101 Page 13 Frames to Packets DSP Packet Assembler Software within the DSP takes frames from the CODEC and creates packets. Several frames may be combined in a single packet RTP 10110101 10110101 10110101 10110101 A 12 byte Real Time Protocol (RTP) Header is added: Provides sequence number Time stamp The packet is forwarded to the gateway’s host processor. Page 14 Addressing Micro Dialed digits identified by the tone detection performed in the DSP are used to determine the destination number. 301-999-1212 = 192.128.100.2 This number is mapped to an IP Address. 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 destination sockets is also added. IP UDP RTP 10110101 10110101 10110101 10110101 Page 15 In the Internet Routers and Switches in the Internet examine the addresses in the IP address in order to identify the route to the destination. Several routers and or switches may be in the path that the packets take to their destination. Page 16 Upon Arrival at the Destination Micro The IP and UDP headers are removed from the packet in the Microprocessor. IP UDP RTP 10110101 10110101 10110101 10110101 The Packet is forwarded to the DSP where the RTP Header is removed. Finally, the packet is disassembled leaving the voice frames. Page 17 Various Network Problems are Dealt With Voice Packets are generated at a constant rate while someone is speaking; there is essentially no gap between packets. Devices in the network cause an unpredictable amount of delay to occur between packets. These gaps, known as jitter, must be removed by the receiving gateway in order to accurately reproduce the original speech Page 18 Jitter Removal DSP An adaptive jitter buffer in the receiving DSP is used to smooth the playout of packets arriving from a “jittery” network. DSP This eliminates the jitter induced distortion that would have been heard by the listener. Page 19 Lost Packets Congestion in the network may cause some packets to be dropped. 6 5 4 3 2 1 Left untreated, the listener hears annoying pops & clicks. Page 20 Lost Packets DSP An algorithm in the DSP detects missing packets. 6 5 4 3 2 1 And replays the last successfully received packet at a decreased volume in order to fill the gaps. 6 4 4 2 2 1 Page 21 Out of Order Packets DSP Packets may take diverse routes through a network and may arrive out of order. 3 5 4 2 1 Out of order packets are not played in the order they arrive….. Turning “Hello”….. Into “oHell” Page 22 Out of Order Packets DSP When an out of order condition is detected the missing packet is replaced by its predecessor as if it is lost. 3 5 4 2 2 1 When the late packet finally arrives it is discarded. Page 23 PCM Back to Analog A Comfort Noise Generator fills in the gaps that were created during silence detection and suppression. 10110101 11010011 11001001 00100100 00111100 10010011 11100001 00100100 00111100 10010011 10110101 11010011 11001001 00100100 00111100 10010011 11100001 00100100 The PCM Stream is reconstituted as an analog signal and is played out to the listener. Page 24 VoIP Applications Page 25 Central Office/Infrastructure Packet Network Central Office Gateway 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 26 Enterprise IP Phone SME Gateway Packet Network PBX Enterprises deploying to avoid access charges and settlement fees. Businesses take advantage of existing data networks. Reduced operating costs by managing one network. Page 27 IP Phones and PBX Trunking Office 2 IP Phone Router Packet Network Gateway T1 PBX Office 1 LAN-based PBX for cost reduction, flexibility, and new applications: Integrated voice/data LAN infrastructure Integrated voice/data applications Open hardware platform Page 28 Residential Broadband Cable or DSL Modem Based IAD CMTS CPE Gateway DSLAM 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 Cable Modem Voice Gateway Packet Network Page 29 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. Page 30