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H.323 Liane Tarouco Leandro Bertholdo RNP  POP/RS
Videoconference solutions <ul><li>CuSeeMe </li></ul><ul><li>Mbone </li></ul><ul><li>H.323 </li></ul><ul><li>Proprietary so...
<ul><li>Cornel University 1992 </li></ul><ul><ul><li>CuSeeMe  </li></ul></ul><ul><ul><li>White Pine  </li></ul></ul><ul><u...
Using CuSeeMe
MBone <ul><li>1992  </li></ul><ul><li>Session reservation </li></ul><ul><li>SDR protocol  </li></ul><ul><li>IP  multicast ...
Mbone tools <ul><li>VIC ( Video Conference ) </li></ul><ul><li>RAT ( Robust Audio Tool ) </li></ul><ul><li>WB ( White Boar...
Standard entities <ul><li>Telecommunications standards are set by the United Nations agency, International Telecommunicati...
<ul><li>Microsoft  Neteeting </li></ul><ul><li>Application sharing </li></ul><ul><li>File Transfer </li></ul><ul><li>White...
Videoconference topology <ul><li>Point to point </li></ul><ul><li>Multippoint </li></ul>
Point to point
Group to group
Multipoint
H.323 Architecture V.70 Terminal H.324 Terminal Speech Terminal H.322 Terminal Speech Terminal H.320 Terminal H.321 Termin...
Standards for  videoconference <ul><li>H.320 ISDN Videoconferencing </li></ul><ul><ul><li>groupvideoconferencing  </li></u...
Available Transport Media <ul><li>ISDN, LAN, WAN, Internet, ADSL (Asynchronous Digital Subscriber Lines) and VPN, (Virtual...
Available Transport Media <ul><li>The worldwide availability of the Internet has virtually stopped the use of POTS (Plain ...
Available Transport Media. <ul><li>However, the forthcoming media-enabled 3G mobile phone has caused the creation of a der...
ISDN <ul><li>There are two available ISDN connections </li></ul><ul><ul><li>Basic Rate Interface (BRI) </li></ul></ul><ul>...
ISDN <ul><li>ISDN connections usually aggregate the BRI and share the same number for both B channels.  </li></ul><ul><ul>...
ISDN for multipoint conference <ul><li>To hold a multipoint conference over ISDN, participants use a Multipoint Control Un...
H.320 <ul><li>H.320 - ITU standard for ISDN conferencing </li></ul><ul><ul><li>Audio: G.711, G.722, G.722.1, G.728 </li></...
Lan & WAN <ul><li>H.323 is the ITU standard for LAN conferencing and includes: </li></ul><ul><ul><li>Audio: G.711, G.722, ...
Multipoint conference <ul><li>To hold a multipoint conference over IP, H.323 systems require some form of Multipoint Confe...
Cellular Networks.  <ul><li>The cellular phone network is a readily available form of wireless multimedia delivery and wit...
Cellular networks <ul><li>3G-324M is an extension by the 3rd Generation Partner Project (3GPP) and 3rd Generation Partner ...
Internet, VPN & ADSL.  <ul><li>Internet, VPNs and ADSL are other forms of TCP/IP networks and hence can be used as a trans...
Directory services <ul><li>Alternatively, users can register their presence using LDAP with a Directory Service </li></ul>
Videoconference over Internet <ul><li>H.323 is the ITU standard used for Internet conferencing and includes: </li></ul><ul...
Codex de Vídeo H.261 - video codec for >= 64kbps  H.263 - video codec for < 64kbps  Frames no formato H.261/H.263 não comp...
Video standards <ul><li>H.261 - video codec for audiovisual services at speed > 64Kbps. </li></ul><ul><li>H.263 - video co...
Image size <ul><li>QCIF - Quarter Common Intermediate Format </li></ul><ul><ul><li>176x144 pixel image.  </li></ul></ul><u...
Video Sizes <ul><li>NTSC - National Television Standards Committee, used in USA, Canada & Japan. 640 x 480 pixels. </li></...
Video Sizes <ul><li>CIF - Common Intermediate Format </li></ul><ul><ul><li>optional for both H.261 & H.263 </li></ul></ul>...
PC Window Sizes <ul><li>SXGA - 1280 x 1024 pixels - used by high end graphics workstations. </li></ul><ul><li>XGA - 1024 x...
H.264 <ul><li>Ratified in late 2003, this new codec standard was a development between the ITU and ISO/IEC Joint Video Tea...
Audio standards <ul><li>G.711 - Pulse Code Modulation of voice frequencies (PCM), were 3.1 kHz analogue audio is encoded i...
Audio standards <ul><li>G.722.2 - Coding of speech at around 16 kbps using Adaptive Multi-Rate Wideband, AMR-WB.  </li></u...
Audio standards <ul><li>G.728 - 3.4 kHz Low Delay Code Excited Linear Prediction (LD-CELP) were 3.4 kHz analogue audio is ...
Control standards <ul><li>H.221 - defines the transmission frame structure for audovisual teleservices in channels of 64 t...
Control standards <ul><li>H.231 - specifies multipoint control units used to bridge three or more H.320 systems together i...
Control standards <ul><li>H.239 - defines role management and additional media channels for H.300-Series multimedia termin...
Control standards <ul><li>H.241 - defines extended video procedures and control signals for H.300-Series multimedia termin...
Control standards <ul><li>H.243 - defines the control procedures and protocol for establishing communications between thre...
Control standards <ul><li>H.282 - Remote device control protocol for multimedia applications. </li></ul><ul><li>H.283 - Re...
Numbers & names <ul><li>E.164 Number - (User Number).   </li></ul><ul><ul><li>A numeric string given to an H.323 endpoint....
H.323 Alias <ul><li>H.323 Alias </li></ul><ul><ul><li>A logical name given to an H.323 endpoint. </li></ul></ul><ul><ul><l...
Q.931 <ul><li>Signalling protocol for establishing and terminating calls. </li></ul>
What is H.323 <ul><li>H.323 *  is a multimedia  conferencing protocol,  which includes  </li></ul><ul><ul><li>voice,  </li...
H.323 umbrella <ul><li>H.323 was first approved in February 1996 </li></ul><ul><li>Designed to operate over IP networks  <...
Elements of an H.323 System <ul><li>• Terminals  </li></ul><ul><li>• Multipoint Control Units (MCUs)  </li></ul><ul><li>• ...
Terminals <ul><li>Videoconference dedicated units </li></ul><ul><li>“ Soft phones” (e.g., NetMeeting®) </li></ul>
Terminals <ul><li>H.323 Terminals are the endpoints on the LAN that provide real-time two way communications.  </li></ul><...
Terminals <ul><li>The H.323 standard specifies what modes must be supported so that all these endpoints can work together....
Terminal identification <ul><li>When initiating an H.323 Video Conference, we need some means of identifying the User or H...
H.323 User Number <ul><li>This number is referred to as the H.323 User Number and when registered with a Gatekeeper, we ha...
Multipoint Control Units (MCUs): <ul><li>To allow three or more participants into a conference, most H.323 systems usually...
MCUs <ul><li>Responsible for managing multipoint conferences </li></ul>
MCU <ul><li>The H.323 MCU's basic function is to maintain all the audio, video, data and control streams between all the p...
MCU <ul><li>The main components of an H.323 MCU are </li></ul><ul><ul><li>MC - multipoint controller </li></ul></ul><ul><u...
MCU: MC & MP <ul><li>The MC also controls conference resources such as multicasting.  </li></ul><ul><ul><li>Most H.323 sys...
Switching/mixing
MCU: MC & MP <ul><li>Both the MC and MP functions can exist in one unit or as part of other H.323 components.  </li></ul><...
MCU - H.320 <ul><li>H.320 conferences are essentially a point-to-point connection and need to use an H.320 MCU to link and...
Endpoint with Embedded MCU <ul><li>An alternative to using a dedicated MCU for small conferences involving 3 or 4 particip...
Terminal & MCU <ul><li>VSX 7000s has both BRI or PRI ISDN options that when used in conjunction with the multipoint capabi...
Gatekeeper <ul><li>The Gatekeeper is an  optional  component in  the H.323 </li></ul><ul><li>Primarily used for  </li></ul...
Gatekeeper <ul><li>Although the H.323 standard describes the Gatekeeper, as an optional component, it is in practice an es...
Gatekeeper <ul><li>Responsible for  </li></ul><ul><ul><li>providing address translation between an endpoints current IP ad...
Gatekeeper <ul><li>Registered endpoints can be H.323 Terminals, Gateways or MCU's. </li></ul>
Gatekeeper <ul><li>Gatekeeper allow network administrators to </li></ul><ul><ul><li>configure,  </li></ul></ul><ul><ul><li...
Gateway and zone <ul><li>Only one Gatekeeper can manage a H.323 zone, but this zone could include several Gateways and MCU...
Gatekeeper and zone <ul><li>Since a zone is defined and managed by only one Gatekeeper, endpoints such as Gateways and MCU...
Example of gatekeepers <ul><li>The INVISION 100 from RADVISION combines Gateway and MCU functionality in one box and has a...
Example of gatekeeper <ul><li>VCON  - Media XchangeManager™, MXM.  </li></ul>
Media Xchange Manager™ - VCON <ul><li>From a remote console, the administrator can now perform centralised management func...
Interconnected Gatekeeper Zones: <ul><li>Gatekeeper defines the zone and manages the registered endpoints within.  </li></ul>
Calling an endpoint <ul><li>To call an endpoint within the same zone, we simply dial that endpoints H.323 User Number. </l...
Calling na endpoint <ul><li>But what happens when we want to call an endpoint that is located in another zone?  </li></ul>...
Interconnected Gatekeeper Zones:
Neighbour Gatekeepers <ul><li>When Gatekeepers are arranged in a single tier 'Peer-to-Peer' manner with no particular hier...
Directory Gatekeepers <ul><li>When the Gatekeepers are arranged in a multi-tier manner with a hierarchical structure, they...
Directory Gatekeeper <ul><li>A Directory Gatekeeper only knows  its Parent and Child Gatekeepers.  </li></ul><ul><li>If th...
Gateways  <ul><li>Gateways interface H.323 to other networks, including the PSTN, H.320 systems, and other H.323 networks ...
Gateways  <ul><li>The Gateway is composed of a “Media Gateway Controller” (MGC) and a “Media Gateway” (MG), which may co-e...
Gateway
Gateways: <ul><li>H.320 and H.323 systems can interoperate with the use of a Gateway.  </li></ul><ul><li>Essentially, the ...
Gateway <ul><li>Most Gateways work in conjunction with, or include a Gatekeeper functionality </li></ul>
H.323 Stack
The Protocols  <ul><li>H.323 is a “framework” document that describes how the various pieces fit together  </li></ul><ul><...
Protocols <ul><li>H.450.x is a series of supplementary service protocols  </li></ul><ul><li>H.460.x is a series of version...
QoS <ul><li>H.460.9 allows an endpoint to report Quality of Service information to the Gatekeeper, aiding in determine how...
Data Collaboration Using T.120
Data collaboration applications <ul><li>Data collaboration applications offer very useful services like application/deskto...
Tools for collaboration  <ul><li>Whiteboard </li></ul><ul><li>Application sharing </li></ul><ul><li>Remote control  </li><...
Standards  for Collaboration Services <ul><li>Standards  </li></ul><ul><li>T.120 </li></ul><ul><ul><li>Layered, Network In...
ITU-T  T.120 series Whiteboard Overhead Proj Photos Documents File Transfer App Sharing Reservations A/V Control Applicati...
H.323 and T.120 components MCU - multipoint control unit contains MC and MP optionally a T.120 MCU Terminals may contain T...
H.323 e T.120
ITU-T  T.120 series <ul><li>Kinds of data and standards </li></ul><ul><ul><li>Photos and Documents (T.126) </li></ul></ul>...
ITU-T  T.120 series <ul><li>Kinds of control & standadrds </li></ul><ul><ul><li>Conference setup, entry, modify, exit (T.1...
Protocols T.120  <ul><li>T.123 - OSI transport protocol </li></ul><ul><li>T.122, T.125  - Multipoint Communication Service...
T.124 - Generic Conference Control (GCC):   <ul><li>T.124 - Generic Conference Control  </li></ul><ul><ul><li>Provides set...
Conformance for T.120 service <ul><li>Transport protocol profile (T.123); </li></ul><ul><li>MCS Multipoint Communication S...
Additional standards <ul><li>T.130 series, High level audio/video control </li></ul><ul><ul><li>Remote cameras & VCRs, vid...
data,  video  audio  Applications Broadcast application Application Sharing  Required services in H.323 & T.120 Teaching a...
Lecture delivery performance <ul><li>T.120 protocols uses a lot of performance management and when the data channel or the...
Some monitoring results   <ul><li>T.120 communication between terminals (Netmeeting), start before logical channels open  ...
QoS <ul><li>Quality of Service - needed quality to attend specific application user request </li></ul><ul><ul><li>telephon...
QoS <ul><li>Usual specifications </li></ul><ul><ul><li>Bandwidth  </li></ul></ul><ul><ul><li>Delay  </li></ul></ul><ul><ul...
MOS - Mean Opinion Scores  <ul><li>Bad </li></ul><ul><ul><li>unintelligible, user do not understand decoded message. Inter...
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H.323 Liane Tarouco Leandro Bertholdo

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  • T.120 Does file transfer and still image well Fairly widespread Inexpensive or Free Implementation Compliance Lack of industry interest in further development No support for Video or Animation Scalability Many standards in use (RTP, SMTP, etc. but not as a package)
  • Transcript of "H.323 Liane Tarouco Leandro Bertholdo"

    1. 1. H.323 Liane Tarouco Leandro Bertholdo RNP POP/RS
    2. 2. Videoconference solutions <ul><li>CuSeeMe </li></ul><ul><li>Mbone </li></ul><ul><li>H.323 </li></ul><ul><li>Proprietary solutions </li></ul>
    3. 3. <ul><li>Cornel University 1992 </li></ul><ul><ul><li>CuSeeMe </li></ul></ul><ul><ul><li>White Pine </li></ul></ul><ul><ul><li>First Virtual Communications </li></ul></ul><ul><li>Reflector - proprietary protocol </li></ul><ul><li>Audio, vídeo, chat </li></ul>Videoconference with CuSeeme
    4. 4. Using CuSeeMe
    5. 5. MBone <ul><li>1992 </li></ul><ul><li>Session reservation </li></ul><ul><li>SDR protocol </li></ul><ul><li>IP multicast </li></ul>
    6. 6. Mbone tools <ul><li>VIC ( Video Conference ) </li></ul><ul><li>RAT ( Robust Audio Tool ) </li></ul><ul><li>WB ( White Board ) </li></ul><ul><li>NTE ( Network Text Editor </li></ul>
    7. 7. Standard entities <ul><li>Telecommunications standards are set by the United Nations agency, International Telecommunications Union (ITU) </li></ul><ul><ul><li>The ITU has developed the H, G and T Series of standards </li></ul></ul><ul><li>Internet Engineering Task Force (IETF) defines stadards for the Internet </li></ul><ul><ul><li>IETF has developed Real-Time Protocol (RTP), Real-Time Control Protocol (RTCP) & Resource Reservation Protocol (RSVP). </li></ul></ul><ul><li>Products that adhere to these standards allow users to participate in a conference, regardless of their platform. </li></ul>
    8. 8. <ul><li>Microsoft Neteeting </li></ul><ul><li>Application sharing </li></ul><ul><li>File Transfer </li></ul><ul><li>Whiteboard </li></ul><ul><li>Chat </li></ul><ul><li>ILS server </li></ul><ul><li>H.323 </li></ul>H.323 became popular
    9. 9. Videoconference topology <ul><li>Point to point </li></ul><ul><li>Multippoint </li></ul>
    10. 10. Point to point
    11. 11. Group to group
    12. 12. Multipoint
    13. 13. H.323 Architecture V.70 Terminal H.324 Terminal Speech Terminal H.322 Terminal Speech Terminal H.320 Terminal H.321 Terminal GSTN GQOS LAN N-ISDN B-ISDN H.323 MCU H.323 Terminal H.323 Gatekeeper H.323 Gateway H.323 Terminal H.323 Terminal
    14. 14. Standards for videoconference <ul><li>H.320 ISDN Videoconferencing </li></ul><ul><ul><li>groupvideoconferencing </li></ul></ul><ul><li>H.323 Audiovisual communication </li></ul><ul><ul><li>desktop videoconferencing </li></ul></ul><ul><li>H.324 High Quality Video and Audio Compression over POTS </li></ul>
    15. 15. Available Transport Media <ul><li>ISDN, LAN, WAN, Internet, ADSL (Asynchronous Digital Subscriber Lines) and VPN, (Virtual Private Networks) are the popular transport media used in desktop video conferencing. </li></ul>
    16. 16. Available Transport Media <ul><li>The worldwide availability of the Internet has virtually stopped the use of POTS (Plain Old Telephone Service) as a direct means of connecting video conferencing systems. </li></ul>
    17. 17. Available Transport Media. <ul><li>However, the forthcoming media-enabled 3G mobile phone has caused the creation of a derivative of the H.324 POTS standard in the form of 3G-324M as well as next generation Gateways to transcode the new protocols. </li></ul>
    18. 18. ISDN <ul><li>There are two available ISDN connections </li></ul><ul><ul><li>Basic Rate Interface (BRI) </li></ul></ul><ul><ul><ul><li>two 64kbps B-channels and </li></ul></ul></ul><ul><ul><ul><li>one 16kbps D-channel </li></ul></ul></ul><ul><ul><li>Primary Rate Interface (PRI). </li></ul></ul><ul><ul><ul><li>PRI in Europe provides 30 x 64kbps B-channels and one 64kbps D-channel. </li></ul></ul></ul>USA PRI
    19. 19. ISDN <ul><li>ISDN connections usually aggregate the BRI and share the same number for both B channels. </li></ul><ul><ul><li>Known as ISDN-2, this provides a line speed of 128kbps is typically used in a desktop conference over ISDN. </li></ul></ul><ul><ul><li>For increased bandwidth, ISDN-6 provides a line speed of 384kbps and is typically used in room-based conferences over ISDN. </li></ul></ul>
    20. 20. ISDN for multipoint conference <ul><li>To hold a multipoint conference over ISDN, participants use a Multipoint Control Unit (MCU), that connects and manages all the ISDN lines. </li></ul><ul><ul><li>separate MCU or </li></ul></ul><ul><ul><li>endpoint with an embedded H.320 multipoint capability </li></ul></ul>
    21. 21. H.320 <ul><li>H.320 - ITU standard for ISDN conferencing </li></ul><ul><ul><li>Audio: G.711, G.722, G.722.1, G.728 </li></ul></ul><ul><ul><li>Video: H.264, H.263, H.261 </li></ul></ul><ul><ul><li>Data: H.239, T.120 </li></ul></ul><ul><ul><li>Control: H.221, H.231, H.242, H.243 </li></ul></ul>
    22. 22. Lan & WAN <ul><li>H.323 is the ITU standard for LAN conferencing and includes: </li></ul><ul><ul><li>Audio: G.711, G.722, G.722.1, G.723.1, G.728, G.729 </li></ul></ul><ul><ul><li>Video: H.264, H.263, H.261 </li></ul></ul><ul><ul><li>Data: H.239, T.120 </li></ul></ul><ul><ul><li>Control: H.225, H.245 </li></ul></ul>
    23. 23. Multipoint conference <ul><li>To hold a multipoint conference over IP, H.323 systems require some form of Multipoint Conference Server (MCS). </li></ul><ul><ul><li>This is also referred to as an H.323 Multipoint Control Unit (H.323 MCU), which is not the same as an H.320 MCU; </li></ul></ul>
    24. 24. Cellular Networks. <ul><li>The cellular phone network is a readily available form of wireless multimedia delivery and with the forthcoming media-enabled 3G mobile phone or Personal Digital Assistants, PDAs, that support the CDMA2000 or WCDMA Air Interface, there is sufficient bandwidth to enable IP-based multipoint audio and video conferencing to existing desktop video conferencing systems when used in-conjunction with next generation Gateways and MCU's that also support these new protocols. </li></ul>
    25. 25. Cellular networks <ul><li>3G-324M is an extension by the 3rd Generation Partner Project (3GPP) and 3rd Generation Partner Project2 (3GPP2) to the ITU H.324M standard for 3G mobile phone conferencing and includes: </li></ul><ul><ul><li>Audio: G.722.2 (AMR-WB), G.723.1 </li></ul></ul><ul><ul><li>Video: MPEG-4, but not H.264 </li></ul></ul><ul><ul><li>Control: H.223 A/B, H.245 </li></ul></ul>
    26. 26. Internet, VPN & ADSL. <ul><li>Internet, VPNs and ADSL are other forms of TCP/IP networks and hence can be used as a transport media in desktop conferencing systems. </li></ul><ul><li>Users must get a fixed IP address </li></ul>
    27. 27. Directory services <ul><li>Alternatively, users can register their presence using LDAP with a Directory Service </li></ul>
    28. 28. Videoconference over Internet <ul><li>H.323 is the ITU standard used for Internet conferencing and includes: </li></ul><ul><ul><li>Audio: G.723.1, G.722.1, G.728 </li></ul></ul><ul><ul><li>Video: H.264, H.263, H.261 </li></ul></ul><ul><ul><li>Data: H.239, T.120 </li></ul></ul><ul><ul><li>Control: H.225, H.245 </li></ul></ul>
    29. 29. Codex de Vídeo H.261 - video codec for >= 64kbps H.263 - video codec for < 64kbps Frames no formato H.261/H.263 não comprimidos Frames comprimidos Frames comprimidos Rede Digital Vídeo analógico Frames no formato H.261/H.263 não comprimidos Sistema de Captura Amostras digitais Conversor A/D Sistema de transformação de cores Sistema de redimensionamento de frames Frames modificados para exibição Sistema de Exibição Sistema de transformação de cores Sistema de redimensionamento de frames Codificador Decodificador
    30. 30. Video standards <ul><li>H.261 - video codec for audiovisual services at speed > 64Kbps. </li></ul><ul><li>H.263 - video codec for narrow telecommunications channels at < 64 Kbps. </li></ul><ul><li>H.264/AVC - a new video codec standard offering major improvements image quality. </li></ul>
    31. 31. Image size <ul><li>QCIF - Quarter Common Intermediate Format </li></ul><ul><ul><li>176x144 pixel image. </li></ul></ul><ul><ul><li>minimum size in H.320 </li></ul></ul><ul><li>CIF - optional H.320 full- screen 352x288 pixels </li></ul><ul><ul><li>requires more computing capability. </li></ul></ul>
    32. 32. Video Sizes <ul><li>NTSC - National Television Standards Committee, used in USA, Canada & Japan. 640 x 480 pixels. </li></ul><ul><li>PAL - Phase Alternation by Line, used in Europe (except France), Africa & Middle East. 768 x 576 pixels. </li></ul>
    33. 33. Video Sizes <ul><li>CIF - Common Intermediate Format </li></ul><ul><ul><li>optional for both H.261 & H.263 </li></ul></ul><ul><ul><li>352 x 288 pixels. </li></ul></ul><ul><li>QCIF - Quarter Common Intermediate Format </li></ul><ul><ul><li>required by both H.261 & H.263 </li></ul></ul><ul><ul><li>176 x 144 pixels. </li></ul></ul><ul><li>SQCIF - Sub Quarter Common Intermediate Format </li></ul><ul><ul><li>used by 3G mobiles MPEG4 video and H.263, </li></ul></ul><ul><ul><li>88 x 72 pixels. </li></ul></ul>
    34. 34. PC Window Sizes <ul><li>SXGA - 1280 x 1024 pixels - used by high end graphics workstations. </li></ul><ul><li>XGA - 1024 x 768 pixels - typical PC or laptop resolution. </li></ul><ul><li>SVGA - 800 x 600 pixels. </li></ul><ul><li>VGA - 640 x 480 pixels. </li></ul>
    35. 35. H.264 <ul><li>Ratified in late 2003, this new codec standard was a development between the ITU and ISO/IEC Joint Video Team, (JVT) and is known as H.264 (ITU name) or ISO/IEC 14496-10/MPEG-4 AVC (ISO/IEC name). </li></ul><ul><li>This new standard surpasses H.261 and H.263 in terms of video quality, effective compression and resilience to transmission losses </li></ul><ul><li>Potential to halve the required bandwidth for digital video services over the Internet or 3G Wireless networks. </li></ul><ul><li>H.264 is likely to be used in applications such as Video Conferencing, Video Streaming, Mobile devices, Tele-Medicine etc. Current 3G mobiles use a derivate of MPEG-4, but not H.264. </li></ul>
    36. 36. Audio standards <ul><li>G.711 - Pulse Code Modulation of voice frequencies (PCM), were 3.1 kHz analogue audio is encoded into a 48, 56 or 64 kbps stream. </li></ul><ul><ul><li>Used when no other standard is equally supported. </li></ul></ul><ul><li>G.722 - 7 kHz audio encoded into a 48, 56 or 64 kbps stream. </li></ul><ul><ul><li>Provides high quality, but takes bandwidth. </li></ul></ul><ul><li>G.722.1 - 7 kHz audio encoded at 24 and 32 kbps for hands-free operation in systems with low frame loss. </li></ul>
    37. 37. Audio standards <ul><li>G.722.2 - Coding of speech at around 16 kbps using Adaptive Multi-Rate Wideband, AMR-WB. </li></ul><ul><ul><li>Five mandatory modes, 6.60, 8.85, 12.65, 15.85 and 23.85 kbps. </li></ul></ul><ul><li>G.723.1 - 3.4 kHz dual rate speech codec for telecommunications at 5.3 kbps & 6.4 kbps. </li></ul>
    38. 38. Audio standards <ul><li>G.728 - 3.4 kHz Low Delay Code Excited Linear Prediction (LD-CELP) were 3.4 kHz analogue audio is encoded into a 16 kbps stream. This standard provides good quality results at low bitrates. </li></ul><ul><li>G.729 A/B - 3.4 kHz speech codec that provides near toll quality audio encoded into an 8 kbps stream using the AS-CELP method. </li></ul>
    39. 39. Control standards <ul><li>H.221 - defines the transmission frame structure for audovisual teleservices in channels of 64 to 1920 Kbps; used in H.320 </li></ul><ul><li>H.223 - specifies a packet-orientated multiplexing protocol for low bit rate multimedia communications; Annex A & B handles light and medium error prone channels of the mobile extension as used in 3G-324M. </li></ul><ul><li>H.224 - defines real-time control protocol for simplex applications using the H.221 LSD, HSD and HLP channels. </li></ul><ul><li>H.225 - defines the multiplexing transmission formats for media stream packetisation & synchronisation on a non-guaranteed QoS LAN. </li></ul>
    40. 40. Control standards <ul><li>H.231 - specifies multipoint control units used to bridge three or more H.320 systems together in a conference. </li></ul><ul><li>H.233 - Confidentiality systems for audiovisual services, used by H.320 devices. </li></ul><ul><li>H.234 - Encryption key management and authentication system for audiovisual services, used by H.320 devices. </li></ul><ul><li>H.235 - Security and encryption for H.323 and other H.245 based multimedia terminals. </li></ul>
    41. 41. Control standards <ul><li>H.239 - defines role management and additional media channels for H.300-Series multimedia terminals. </li></ul><ul><ul><li>How data and web-enabled collaboration work in parallel with video in a conference, allowing endpoints that support H.239 to receive and transit multiple, separate media streams - typically voice, video and data collaboration. </li></ul></ul>
    42. 42. Control standards <ul><li>H.241 - defines extended video procedures and control signals for H.300-Series multimedia terminal. </li></ul><ul><li>H.242 - defines the control procedures and protocol for establishing communications between audiovisual terminals on digital channels up to 2 Mbps </li></ul><ul><ul><li>used by H.320. </li></ul></ul>
    43. 43. Control standards <ul><li>H.243 - defines the control procedures and protocol for establishing communications between three or more audiovisual terminals - H.320 multipoint conferences. </li></ul><ul><li>H.245 - defines the control procedures and protocol for H.323 & H.324 multimedia communications. </li></ul><ul><li>H.246 - Interworking of H-Series multimedia terminal. </li></ul><ul><li>H.248 - Gateway Control Protocol. </li></ul><ul><li>H.281 - defines the procedures and protocol for far end camera control (FECC) in H.320 calls. </li></ul>
    44. 44. Control standards <ul><li>H.282 - Remote device control protocol for multimedia applications. </li></ul><ul><li>H.283 - Remote device control logical channel transport. </li></ul><ul><li>H.350 - Storing and retrieving video and voice over IP information from enterprise directories. </li></ul>
    45. 45. Numbers & names <ul><li>E.164 Number - (User Number). </li></ul><ul><ul><li>A numeric string given to an H.323 endpoint. </li></ul></ul><ul><ul><li>If this endpoint registers with a Gatekeeper, then the Gatekeeper can translate the E.164 Number into the endpoints IP address. </li></ul></ul>
    46. 46. H.323 Alias <ul><li>H.323 Alias </li></ul><ul><ul><li>A logical name given to an H.323 endpoint. </li></ul></ul><ul><ul><li>If this endpoint registers with a Gatekeeper, then the Gatekeeper can translate the H.323 Alias into the endpoints IP address. </li></ul></ul>
    47. 47. Q.931 <ul><li>Signalling protocol for establishing and terminating calls. </li></ul>
    48. 48. What is H.323 <ul><li>H.323 * is a multimedia conferencing protocol, which includes </li></ul><ul><ul><li>voice, </li></ul></ul><ul><ul><li>video, </li></ul></ul><ul><ul><li>data conferencing </li></ul></ul><ul><li>for use over packet-switched networks </li></ul>
    49. 49. H.323 umbrella <ul><li>H.323 was first approved in February 1996 </li></ul><ul><li>Designed to operate over IP networks </li></ul>
    50. 50. Elements of an H.323 System <ul><li>• Terminals </li></ul><ul><li>• Multipoint Control Units (MCUs) </li></ul><ul><li>• Gateways </li></ul><ul><li>• Gatekeeper </li></ul>
    51. 51. Terminals <ul><li>Videoconference dedicated units </li></ul><ul><li>“ Soft phones” (e.g., NetMeeting®) </li></ul>
    52. 52. Terminals <ul><li>H.323 Terminals are the endpoints on the LAN that provide real-time two way communications. </li></ul><ul><li>The H.323 standard states that all H.323 Terminals must support voice , </li></ul><ul><ul><li>video and data are optional. </li></ul></ul><ul><ul><li>Hence the basic form of an H.323 Terminal is the IP Phone; </li></ul></ul><ul><ul><li>However most H.323 Terminals are Video Conferencing Systems. </li></ul></ul>
    53. 53. Terminals <ul><li>The H.323 standard specifies what modes must be supported so that all these endpoints can work together. </li></ul><ul><li>H.323 Terminals must support </li></ul><ul><ul><li>H.245 protocol to control channel usage and capabilities; </li></ul></ul><ul><ul><li>Q.931 protocol for call setup and signalling; </li></ul></ul><ul><ul><li>RAS (Registration/Admission/Status) protocol to communicate with the Gatekeeper and </li></ul></ul><ul><ul><li>RTP/RTCP protocol to sequence audio and video packets. </li></ul></ul>
    54. 54. Terminal identification <ul><li>When initiating an H.323 Video Conference, we need some means of identifying the User or H.323 Endpoint that we wish to conference with. </li></ul><ul><li>The thought of having to remember IP addresses is daunting enough; but the use of DHCP to dynamically allocate the IP address of an endpoint means that this method is impractical. </li></ul><ul><li>Hence the concept of a Dial Plan and the use of an H.323 User Number registered to a Gatekeeper. </li></ul><ul><ul><li>A Dial Plan is simply a method of allocating a unique number to an H.323 Endpoint. </li></ul></ul>
    55. 55. H.323 User Number <ul><li>This number is referred to as the H.323 User Number and when registered with a Gatekeeper, we have a means of translating this User Number into an IP address. </li></ul><ul><li>The H.323 User Number is often loosely referred to as the E.164 Number. </li></ul>
    56. 56. Multipoint Control Units (MCUs): <ul><li>To allow three or more participants into a conference, most H.323 systems usually require a Multipoint Conference Server (MCS). </li></ul><ul><li>This is also referred to as an H.323 Multipoint Control Unit (H.323 MCU). </li></ul>
    57. 57. MCUs <ul><li>Responsible for managing multipoint conferences </li></ul>
    58. 58. MCU <ul><li>The H.323 MCU's basic function is to maintain all the audio, video, data and control streams between all the participants in the conference. </li></ul><ul><ul><li>hardware based </li></ul></ul><ul><ul><li>software based </li></ul></ul>
    59. 59. MCU <ul><li>The main components of an H.323 MCU are </li></ul><ul><ul><li>MC - multipoint controller </li></ul></ul><ul><ul><ul><li>The MC is the conference controller and handles H.245 negotiations between all terminals to determine common capabilities for audio and video processing. </li></ul></ul></ul><ul><ul><li>MP - multipoint processor (optional) </li></ul></ul>MC MP
    60. 60. MCU: MC & MP <ul><li>The MC also controls conference resources such as multicasting. </li></ul><ul><ul><li>Most H.323 systems support IP multicast and use this to send just one audio and one video stream to the other participants. </li></ul></ul><ul><ul><li>The MC does not actually deal directly with any of the audio, video and data streams. </li></ul></ul><ul><li>This is left to the MP, which does all the audio mixing, data distribution and video switching/mixing of the bits. </li></ul><ul><li>It also provides the conversion between different codecs and bit rates. </li></ul>
    61. 61. Switching/mixing
    62. 62. MCU: MC & MP <ul><li>Both the MC and MP functions can exist in one unit or as part of other H.323 components. </li></ul><ul><li>Most H.323 MCU's work in conjunction with, or include a Gatekeeper functionality. </li></ul>
    63. 63. MCU - H.320 <ul><li>H.320 conferences are essentially a point-to-point connection and need to use an H.320 MCU to link and manage all the ISDN lines in order to hold a conference with three or more participants. </li></ul>
    64. 64. Endpoint with Embedded MCU <ul><li>An alternative to using a dedicated MCU for small conferences involving 3 or 4 participants is to equip one of the endpoints with an embedded multipoint capability. </li></ul><ul><ul><li>The Polycom VSX 7000s has an embedded multipoint options that supports itself and up to 3 other sites in a Voice-Activated or Continuous Presence session. </li></ul></ul>
    65. 65. Terminal & MCU <ul><li>VSX 7000s has both BRI or PRI ISDN options that when used in conjunction with the multipoint capability, allows mixed-mode operation between both ISDN and IP networks. </li></ul><ul><li>In a simplistic manner, it also acts like a Gateway, bridging between the other 2 or 3 ISDN and IP endpoints. </li></ul>
    66. 66. Gatekeeper <ul><li>The Gatekeeper is an optional component in the H.323 </li></ul><ul><li>Primarily used for </li></ul><ul><ul><li>admission control and </li></ul></ul><ul><ul><li>address resolution </li></ul></ul><ul><li>The gatekeeper may allow calls to be placed directly between endpoints or it may route the call signaling through itself to perform functions such as follow-me/find-me and forward on busy </li></ul>
    67. 67. Gatekeeper <ul><li>Although the H.323 standard describes the Gatekeeper, as an optional component, it is in practice an essential tool for defining and controlling how voice and video communications are managed over the IP network. </li></ul>
    68. 68. Gatekeeper <ul><li>Responsible for </li></ul><ul><ul><li>providing address translation between an endpoints current IP address and its various H.323 aliases, </li></ul></ul><ul><ul><li>call control and routing services to H.323 endpoints, </li></ul></ul><ul><ul><li>system management and security policies. </li></ul></ul>
    69. 69. Gatekeeper <ul><li>Registered endpoints can be H.323 Terminals, Gateways or MCU's. </li></ul>
    70. 70. Gatekeeper <ul><li>Gatekeeper allow network administrators to </li></ul><ul><ul><li>configure, </li></ul></ul><ul><ul><li>monitor and manage the activities of registered endpoints, </li></ul></ul><ul><ul><li>set policies and control network resources such as bandwidth usage within their H.323 zone. </li></ul></ul><ul><li>These services provided by the Gatekeeper in communicating between H.323 endpoints are defined in RAS. </li></ul>
    71. 71. Gateway and zone <ul><li>Only one Gatekeeper can manage a H.323 zone, but this zone could include several Gateways and MCU's. </li></ul>
    72. 72. Gatekeeper and zone <ul><li>Since a zone is defined and managed by only one Gatekeeper, endpoints such as Gateways and MCU's that also have a built-in Gatekeeper must provide a means for disabling this functionality. </li></ul><ul><li>This ensures that multiple H.323 endpoints that contain a Gatekeeper can all be configured into the same zone. </li></ul>
    73. 73. Example of gatekeepers <ul><li>The INVISION 100 from RADVISION combines Gateway and MCU functionality in one box and has an embedded Gatekeeper that can be disabled; </li></ul><ul><ul><li>this allows the zone to be controlled by a more powerful Gatekeeper </li></ul></ul>
    74. 74. Example of gatekeeper <ul><li>VCON - Media XchangeManager™, MXM. </li></ul>
    75. 75. Media Xchange Manager™ - VCON <ul><li>From a remote console, the administrator can now perform centralised management functions such as configure endpoints, monitor the status and availability of endpoints, control and limit bandwidth usage and more. </li></ul><ul><li>MXM automatically generates Call Detail Reports, CDR; which can be used for network planning or billing purposes. </li></ul><ul><li>With video telephony services such as Call Forward, Call Transfer and Call Pickup, MXM provides the functions that make Video Conferencing as simple as making a telephone call. </li></ul><ul><li>Furthermore, MXM includes an H.323 Gatekeeper. </li></ul>
    76. 76. Interconnected Gatekeeper Zones: <ul><li>Gatekeeper defines the zone and manages the registered endpoints within. </li></ul>
    77. 77. Calling an endpoint <ul><li>To call an endpoint within the same zone, we simply dial that endpoints H.323 User Number. </li></ul>
    78. 78. Calling na endpoint <ul><li>But what happens when we want to call an endpoint that is located in another zone? </li></ul><ul><li>Well, we then also need to know the zone where that endpoint is registered. </li></ul><ul><li>Each Gatekeeper on the same network is identified by a unique number, its Zone Number. </li></ul><ul><li>All the Gatekeepers on the network must know how they are related to eachother. </li></ul>
    79. 79. Interconnected Gatekeeper Zones:
    80. 80. Neighbour Gatekeepers <ul><li>When Gatekeepers are arranged in a single tier 'Peer-to-Peer' manner with no particular hierarchical structure, they are termed as being Neighbour Gatekeepers. </li></ul><ul><li>This would typically be on a corporate network within a multi-site company who has a Gatekeeper at each site. </li></ul><ul><li>Each Gatekeeper manages its own site (Zone), with inter-zone communications routed directly between zones and controlled on an individual basis specifically defined by the direct relationship between each Gatekeeper </li></ul>
    81. 81. Directory Gatekeepers <ul><li>When the Gatekeepers are arranged in a multi-tier manner with a hierarchical structure, they are termed as being Directory Gatekeepers (DGK). </li></ul><ul><li>This would typically be within a large scale deployment such as the national schools network. Whilst each Gatekeeper still manages its own zone, inter-zone communications are routed indirectly on a Parent-Child basis between zones. </li></ul>
    82. 82. Directory Gatekeeper <ul><li>A Directory Gatekeeper only knows its Parent and Child Gatekeepers. </li></ul><ul><li>If the Gatekeeper does not know the Zone of the dialled number, it routes the call to its Parent DGK, which then searches its database to see if the Zone known. </li></ul><ul><li>If not known, this Parent routes the call to its Parent and so on until it eventually reaches a Parent DGK that has a Child DGK that matches the Zone. </li></ul><ul><li>The call is then routed down through each Child DGK tier until it reaches the specific endpoint. </li></ul>
    83. 83. Gateways <ul><li>Gateways interface H.323 to other networks, including the PSTN, H.320 systems, and other H.323 networks (proxy) </li></ul>
    84. 84. Gateways <ul><li>The Gateway is composed of a “Media Gateway Controller” (MGC) and a “Media Gateway” (MG), which may co-exist or exist separately </li></ul><ul><ul><li>The MGC handles call signaling and other non-media-related functions </li></ul></ul><ul><ul><li>The MG handles the media </li></ul></ul>
    85. 85. Gateway
    86. 86. Gateways: <ul><li>H.320 and H.323 systems can interoperate with the use of a Gateway. </li></ul><ul><li>Essentially, the Gateway provides translation between circuit-switched networks ISDN and packet-based networks LAN, enabling the endpoints to communicate. </li></ul><ul><li>To do this, it must translate between the H.225 to H.221 transmission formats and between the H.245 to H.242 communications control protocols. </li></ul><ul><li>The Gateway also has to transcode between the various audio and video codecs used between the LAN and ISDN devices. </li></ul>
    87. 87. Gateway <ul><li>Most Gateways work in conjunction with, or include a Gatekeeper functionality </li></ul>
    88. 88. H.323 Stack
    89. 89. The Protocols <ul><li>H.323 is a “framework” document that describes how the various pieces fit together </li></ul><ul><li>H.225.0 defines the call signaling between endpoints and the Gatekeeper </li></ul><ul><li>RTP/RTCP (RFC 3550) is used to transmit media such as audio and video over IP networks </li></ul><ul><li>H.225.0 Annex G and H.501 define the procedures and protocol for communication within and between Peer Elements </li></ul><ul><li>H.245 is the protocol used to control establishment and closure of media channels within the context of a call and to perform conference control </li></ul>
    90. 90. Protocols <ul><li>H.450.x is a series of supplementary service protocols </li></ul><ul><li>H.460.x is a series of version-independent extensions to the base H.323 protocol </li></ul><ul><li>T.120 specifies how to do data conferencing </li></ul><ul><li>T.38 defines how to relay fax signals </li></ul><ul><li>V.150.1 defines how to relay modem signals </li></ul><ul><li>H.235 defines security within H.323 systems </li></ul><ul><li>X.680 defines the ASN.1 syntax used by the Recommendations </li></ul><ul><li>X.691 defines the Packed Encoding Rules (PER) used to encode messages for transmission on the network </li></ul>
    91. 91. QoS <ul><li>H.460.9 allows an endpoint to report Quality of Service information to the Gatekeeper, aiding in determine how to route calls </li></ul><ul><li>H.323 devices may utilize IETF standards for providing quality of service, including DiffServ and RSVP </li></ul>
    92. 92. Data Collaboration Using T.120
    93. 93. Data collaboration applications <ul><li>Data collaboration applications offer very useful services like application/desktop sharing to provide real remote collaboration specially for use in conjunction with videoconferencing. </li></ul><ul><li>T.120 based systems are the most used approach for this kind of applications providing interoperability between different vendors </li></ul>
    94. 94. Tools for collaboration <ul><li>Whiteboard </li></ul><ul><li>Application sharing </li></ul><ul><li>Remote control </li></ul><ul><li>Chat </li></ul><ul><li>File transfer </li></ul><ul><li>Printed documents sharing (document camera) </li></ul>Videoconference needs complement for supporting collaboration:
    95. 95. Standards for Collaboration Services <ul><li>Standards </li></ul><ul><li>T.120 </li></ul><ul><ul><li>Layered, Network Independent </li></ul></ul><ul><ul><li>P2P Communications </li></ul></ul><ul><ul><li>8K channel </li></ul></ul><ul><ul><li>NetMeeting (now dead product) </li></ul></ul><ul><ul><li>No development of standard </li></ul></ul><ul><li>ITU-T H.239 (July 2003) </li></ul><ul><ul><li>supports one or more parallel streams for sharing data and presentations </li></ul></ul><ul><ul><li>Defines multiple channels (video, audio, or data.) and their “role” (Live, Presentation). </li></ul></ul><ul><ul><li>Limited Implementations </li></ul></ul>
    96. 96. ITU-T T.120 series Whiteboard Overhead Proj Photos Documents File Transfer App Sharing Reservations A/V Control Application Protocols T.126 - Still Image, T.127 - File Transfer T.130 - A/V Control, T.SHARE, T.RES T.124 - Generic Conference Control T.123 - Transport Stacks MCU T.122 / T.125 - Multipoint Comm. Service T.126 T.127 TERMINAL Switching T.130 ISDN POTS Voice/ Data LAN ATM
    97. 97. H.323 and T.120 components MCU - multipoint control unit contains MC and MP optionally a T.120 MCU Terminals may contain T.120 capabilities
    98. 98. H.323 e T.120
    99. 99. ITU-T T.120 series <ul><li>Kinds of data and standards </li></ul><ul><ul><li>Photos and Documents (T.126) </li></ul></ul><ul><ul><li>Pointing and Annotating (T.126) </li></ul></ul><ul><ul><li>File transfer (T.127) </li></ul></ul><ul><ul><li>PC Application Sharing </li></ul></ul><ul><ul><li>Virtually any multipoint flow of data </li></ul></ul>
    100. 100. ITU-T T.120 series <ul><li>Kinds of control & standadrds </li></ul><ul><ul><li>Conference setup, entry, modify, exit (T.124) </li></ul></ul><ul><ul><li>Camera, mic, peripheral control (T.130) </li></ul></ul><ul><ul><li>Who sees whom (T.130) </li></ul></ul><ul><ul><li>Director control, browsing (T.130) </li></ul></ul><ul><ul><li>Reservations </li></ul></ul><ul><ul><li>Add site, extend conference time </li></ul></ul>
    101. 101. Protocols T.120 <ul><li>T.123 - OSI transport protocol </li></ul><ul><li>T.122, T.125 - Multipoint Communication Service (MCS): </li></ul><ul><li>T.124 -Generic Conference Control (GCC) </li></ul>
    102. 102. T.124 - Generic Conference Control (GCC): <ul><li>T.124 - Generic Conference Control </li></ul><ul><ul><li>Provides set of facilities to establish and manage multipoint . </li></ul></ul><ul><ul><li>Centralize an information base (state of serving conferences) </li></ul></ul>
    103. 103. Conformance for T.120 service <ul><li>Transport protocol profile (T.123); </li></ul><ul><li>MCS Multipoint Communication Service (T.125); </li></ul><ul><li>Required components of Generic Conference Control (T.124); </li></ul>
    104. 104. Additional standards <ul><li>T.130 series, High level audio/video control </li></ul><ul><ul><li>Remote cameras & VCRs, video routing, continuos presence </li></ul></ul><ul><ul><ul><li>How a student request the floor to ask a question </li></ul></ul></ul><ul><ul><li>Automatic video switching </li></ul></ul><ul><ul><ul><li>Social rules e.g. hand raising are used </li></ul></ul></ul><ul><ul><ul><li>Submit and cancel floor requests </li></ul></ul></ul>
    105. 105. data, video audio Applications Broadcast application Application Sharing Required services in H.323 & T.120 Teaching and learning environment
    106. 106. Lecture delivery performance <ul><li>T.120 protocols uses a lot of performance management and when the data channel or the end station do not perform according established thresholds unexpected decision may occur disconnecting one or more users from the conference or disabling some data collaboration function. </li></ul>
    107. 107. Some monitoring results <ul><li>T.120 communication between terminals (Netmeeting), start before logical channels open </li></ul><ul><li>Centralized conference model (tightly coupled) use intensively MCU resources </li></ul>Event> Mon Nov 26 17:15:54 2001 Pkts in 25655 Pkts Event> client Leandro Bertholdo - T.120 session closed Event> Mon Nov 26 17:16:54 2001 Pkts in 27438 Pkts Event> Mon Nov 26 17:17:55 2001 Pkts in 1695 Pkts Event> client Alexei Korb timeout -- holding down Event> Mon Nov 26 17:18:55 2001 Pkts in 3324 Pkts Event> client Alexei Korb - T.120 session closed due to insufficient bandwidth Event> Mon Nov 26 17:19:56 2001 Pkts in 4708 Event> Mon Nov 26 17:20:56 2001 Pkts in 5850 Event> client Liane Tarouco - T.120 session closed due to insufficient bandwidth Event> Mon Nov 26 17:21:57 2001 Pkts in 7114 Event> Mon Nov 26 17:22:58 2001 Pkts in 8182
    108. 108. QoS <ul><li>Quality of Service - needed quality to attend specific application user request </li></ul><ul><ul><li>telephony </li></ul></ul><ul><ul><li>videoconference </li></ul></ul><ul><ul><li>file download </li></ul></ul><ul><ul><li>TV </li></ul></ul>
    109. 109. QoS <ul><li>Usual specifications </li></ul><ul><ul><li>Bandwidth </li></ul></ul><ul><ul><li>Delay </li></ul></ul><ul><ul><li>Jitter </li></ul></ul><ul><li>QoS from user point of view ? </li></ul><ul><ul><li>ITU P800 Perceptual Speech Quality Measurement (PSQM) </li></ul></ul><ul><ul><li>Mean Opinion Scores (MOS) </li></ul></ul>
    110. 110. MOS - Mean Opinion Scores <ul><li>Bad </li></ul><ul><ul><li>unintelligible, user do not understand decoded message. Interruptions due degradation </li></ul></ul><ul><li>Poor </li></ul><ul><ul><li>signal present interruption due degradations; user needs considerable effort to understand some segments </li></ul></ul><ul><li>Moderate </li></ul><ul><ul><li>voice quality is bad; user fell annoyed with degradations but there are no interruption and still can understand the message (requires moderated effort) </li></ul></ul><ul><li>Good </li></ul><ul><ul><li>voice is good to listen, user perceive degradations but do not bother because are minimal (no big effort is needed) </li></ul></ul><ul><li>Excellent </li></ul><ul><ul><li>user can not differentiate original message from corrupted, that means, do not perceive signal degradation (no effort is required) </li></ul></ul>
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