H.323

1
H.323

• Liane Tarouco
• Leandro Bertholdo

2
Standard entities

• Telecommunications standards are set by the
  United Nations agency, International
  Telecommunications Union (ITU)
• The ITU has developed the H, G and T Series of
  standards
• Internet Engineering Task Force (IETF) defines
  stadards for the Internet
• IETF has developed Real-Time Protocol (RTP),
  Real-Time Control Protocol (RTCP) Resource
  Reservation Protocol (RSVP).
• Products that adhere to these standards allow
  users to participate in a conference, regardless
  of their platform.

3
Available Transport Media.

• ISDN, LAN, WAN, Internet, ADSL (Asynchronous
  Digital Subscriber Lines) and VPN, (Virtual
  Private Networks) are the popular transport media
  used in desktop video conferencing.
• 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.
• 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.

4
ISDN

• There are two available ISDN connections
• Basic Rate Interface (BRI)
• Primary Rate Interface (PRI).
• Essentially, a BRI provides two 64kbps B-channels
  and one 16kbps D-channel whilst a PRI in Europe
  provides 30 x 64kbps B-channels and one 64kbps
  D-channel.

5
ISDN

• ISDN connections usually aggregate the BRI and
  share the same number for both B channels. Known
  as ISDN-2, this provides a line speed of 128kbps
  is typically used in a desktop conference over
  ISDN.
• For increased bandwidth, ISDN-6 provides a line
  speed of 384kbps and is typically used in
  room-based conferences over ISDN.
• With ISDN-6, the sequence in which the lines are
  aggregated must be known and adhered too!
  Furthermore, if the connection is going to use
  some form of 'switch', this must be configured to
  pass both voice and data!

6
ISDN for multipoint conference

• To hold a multipoint conference over ISDN,
  participants use a Multipoint Control Unit (MCU),
  that connects and manages all the ISDN lines.
• This can be either a separate MCU or an endpoint
  with an embedded H.320 multipoint capability

7
H.320

• H.320 is the ITU standard for ISDN conferencing
  and includes
• Audio G.711, G.722, G.722.1, G.728
• Video H.264, H.263, H.261
• Data H.239, T.120
• Control H.221, H.231, H.242, H.243

8
Local Area Network (LAN) or Intranet and Wide
Area Network (WAN).

• LANs and WANs use TCP/IP protocol and the H.323
  standard defines how to assemble the audio,
  video, data and control (AVDC) information into
  an IP packet.

9
DHCP

• Most companies use DHCP and allocate dynamic IP
  addresses to PC's.
• Therefore, in order to correctly identify a user,
  the H.323 endpoints are usually registered with a
  Gatekeeper and 'called' into a conference by
  their H.323 alias.
• The Gatekeeper translates the alias into the
  corresponding IP address.
• Another method of identifying H.323 users is for
  them to register their presence using Light
  Directory Access Protocol (LDAP) with a Directory
  Service such as Microsoft's Site Server ILS or
  Windows 2003 Active Directory.

10
Multipoint conference

• To hold a multipoint conference over IP, H.323
  systems require some form of Multipoint
  Conference Server (MCS).
• 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

11
Lan WAN

• H.323 is the ITU standard for LAN conferencing
  and includes
• Audio G.711, G.722, G.722.1, G.723.1, G.728,
  G.729
• Video H.264, H.263, H.261
• Data H.239, T.120
• Control H.225, H.245

12
Cellular Networks.

• 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.

13
Cellular networks

• 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
• Audio G.722.2 (AMR-WB), G.723.1
• Video MPEG-4, but not H.264
• Control H.223 A/B, H.245

14
Internet, VPN ADSL.

• Like LANs, the Internet, VPNs and ADSL are other
  forms of TCP/IP networks and hence can be used as
  a transport media in desktop conferencing
  systems.
• What the users must do is to get their Internet
  Service Provider (ISP) to provide them with a
  fixed IP address

15
Directory services

• Alternatively, users can register their presence
  using LDAP with a Directory Service such as
  Microsoft's Site Server ILS or Windows 2003
  Active Directory. This is how you determine the
  address of the machine that you want to
  conference with. Obviously, speed is limited to
  that of the slowest link, but most ISPs now
  support ISDN Dial-up at 128kbps or V.92 modems at
  56kbps

16
Internet

• H.323 is the ITU standard used for Internet
  conferencing and includes
• Audio G.723.1, G.722.1, G.728
• Video H.264, H.263, H.261
• Data H.239, T.120
• Control H.225, H.245

17
Video standards

• H.261 - video codec for audiovisual services at p
  x 64Kbps.
• H.263 - video codec for narrow telecommunications
  channels at lt 64 Kbps.
• H.264/AVC - a new video codec standard offering
  major improvements image quality.

18
Image size

• QCIF is Quarter Common Intermediate Format and
  represents a 176x144 pixel image.
• This is the minimum size that must be supported
  to be H.320 compliant.
• CIF is the optional full- screen H.320 video
  image of 352x288 pixels and requires considerably
  more computing capability.
• Note whilst this is termed full-screen, it is
  nowhere near the size of a typical PC screen
  (1024x768) pixels or that of a UNIX workstation
  (1280x1024) pixels.

19
Video and PC Window Sizes

• NTSC - National Television Standards Committee,
  used in USA, Canada Japan. 640 x 480 pixels.
• PAL - Phase Alternation by Line, used in Europe
  (except France), Africa Middle East. 768 x 576
  pixels.

20
Video and PC Window Sizes

• CIF - Common Intermediate Format optional for
  both H.261 H.263, 352 x 288 pixels.
• QCIF - Quarter Common Intermediate Format
  required by both H.261 H.263, 176 x 144 pixels.
• SQCIF - Sub Quarter Common Intermediate Format
  used by 3G mobiles MPEG4 video and H.263, 88 x 72
  pixels.

21
Video and PC Window Sizes

• SXGA - 1280 x 1024 pixels - used by high end
  graphics workstations.
• XGA - 1024 x 768 pixels - typical PC or laptop
  resolution.
• SVGA - 800 x 600 pixels.
• VGA - 640 x 480 pixels.

22
H.264

• 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).
• This new standard surpasses H.261 and H.263 in
  terms of video quality, effective compression and
  resilience to transmission losses, giving it the
  potential to halve the required bandwidth for
  digital video services over the Internet or 3G
  Wireless networks. 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.

23
Audio standards

• 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. Used
  when no other standard is equally supported.
• G.722 - 7 kHz audio encoded into a 48, 56 or 64
  kbps stream. Provides high quality, but takes
  bandwidth.
• G.722.1 - 7 kHz audio encoded at 24 and 32 kbps
  for hands-free operation in systems with low
  frame loss.
• G.722.2 - Coding of speech at around 16 kbps
  using Adaptive Multi-Rate Wideband, AMR-WB. Five
  mandatory modes, 6.60, 8.85, 12.65, 15.85 and
  23.85 kbps.
• G.723.1 - 3.4 kHz dual rate speech codec for
  telecommunications at 5.3 kbps 6.4 kbps.

24
Audio standards

• 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.
• 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. Annex A is a
  reduced complexity codec and Annex B supports
  silence suppression and comfort-noise generation.

25
Data standards

• T.120 - defines protocols and services for
  multimedia conferencing.
• T.121 - Generic Application Template (GAT).
  Defines a template as a guide for developers in
  managing T.120 resources.
• T.122 - defines Multipoint Communication Services
  (MCS) available to developers.
• T.123 - defines Network Specific Data protocol
  for multimedia conferencing.
• T.124 - defines Generic Conference Control (GCC),
  mandatory for 'group' conferences.
• T.125 - defines MCS data transmission protocol.

26
Data standards

• T.126 - Multipoint Still Image and Annotation
  protocol. Defines the protocol used to provide
  interoperability with graphics data in
  applications such as whiteboarding, annotated
  image exchange, screen sharing and remote apps
  control.
• T.127 - Multipoint Binary File Transfer protocol.
  Defines the protocol used to support binary file
  transfer within a conference.
• T.128 - defines Multipoint Application Sharing
  protocol (also known as T.SHARE)
• T.134 - defines Multimedia Application Text
  Conversation protocol (also known as T.CHAT).

27
Data standards

• T.135 - User-to-reservation system transactions
  within T.120 conferencing.
• T.136 - Remote device control application
  protocol.
• T.137 - Virtual meeting room management -
  services protocol.
• T.140 - Protocol for multimedia application text
  conversation.

28
Control standards

• H.221 - defines the transmission frame structure
  for audovisual teleservices in channels of 64 to
  1920 Kbps used in H.320
• 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.
• H.224 - defines real-time control protocol for
  simplex applications using the H.221 LSD, HSD and
  HLP channels.
• H.225 - defines the multiplexing transmission
  formats for media stream packetisation
  synchronisation on a non-guaranteed QoS LAN.

29
Control standards

• H.231 - specifies multipoint control units used
  to bridge three or more H.320 systems together in
  a conference.
• H.233 - Confidentiality systems for audiovisual
  services, used by H.320 devices.
• H.234 - Encryption key management and
  authentication system for audiovisual services,
  used by H.320 devices.
• H.235 - Security and encryption for H.323 and
  other H.245 based multimedia terminals.

30
Control standards

• H.239 - defines role management and additional
  media channels for H.300-Series multimedia
  terminals. 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.
• H.241 - defines extended video procedures and
  control signals for H.300-Series multimedia
  terminal.
• H.242 - defines the control procedures and
  protocol for establishing communications between
  audiovisual terminals on digital channels up to 2
  Mbps used by H.320.

31
Control standards

• H.243 - defines the control procedures and
  protocol for establishing communications between
  three or more audiovisual terminals - H.320
  multipoint conferences.
• H.245 - defines the control procedures and
  protocol for H.323 H.324 multimedia
  communications.
• H.246 - Interworking of H-Series multimedia
  terminal.
• H.248 - Gateway Control Protocol.
• H.281 - defines the procedures and protocol for
  far end camera control (FECC) in H.320 calls.

32
Control standards

• H.282 - Remote device control protocol for
  multimedia applications.
• H.283 - Remote device control logical channel
  transport.
• H.350 - Storing and retrieving video and voice
  over IP information from enterprise directories.

33
Numbers names

• E.164 Number - (User Number). A numeric string
  given to an H.323 endpoint. If this endpoint
  registers with a Gatekeeper, then the Gatekeeper
  can translate the E.164 Number into the endpoints
  IP address.
• H.323 Alias - A logical name given to an H.323
  endpoint. If this endpoint registers with a
  Gatekeeper, then the Gatekeeper can translate the
  H.323 Alias into the endpoints IP address.

34
Q.931

• Q.931 - Signalling protocol for establishing and
  terminating calls.

35
Evolution

• H.323 was first approved in February 1996, the
  same month that the first SIP draft was published
  
• Designed to operate over IP networks
• Today, H.323 is the most widely deployed
  standards-based voice and videoconferencing
  standard for packet-switched networks, with
  literally billions of minutes of billable traffic
  every month
• ITU-T now considering work on H.323v6

36
What is H.323

• H.323 is a multimedia conferencing protocol,
  which includes voice, video, and data
  conferencing, for use over packet-switched
  networks

H.323 is ITU-T Recommendation H.323
Packet-based multimedia communications systems
37
Elements of an H.323 System

• Terminals
• Multipoint Control Units (MCUs)
• Gateways
• Gatekeeper
• Border Elements

Endpoints
38
Terminals

• Video phones
• IVR devices
• Voicemail Systems
• Soft phones (e.g., NetMeeting)

39
Terminals

• H.323 Terminals are the endpoints on the LAN that
  provide real-time two way communications.
• The H.323 standard states that all H.323
  Terminals must support voice, with video and data
  being optional.
• Hence the basic form of an H.323 Terminal is the
  IP Phone however most H.323 Terminals are Video
  Conferencing Systems.

40
Terminals

• The H.323 standard specifies what modes must be
  supported so that all these endpoints can work
  together.
• H.323 Terminals must support
• H.245 protocol to control channel usage and
  capabilities
• Q.931 protocol for call setup and signalling
• RAS (Registration/Admission/Status) protocol to
  communicate with the Gatekeeper and
• RTP/RTCP protocol to sequence audio and video
  packets.

41
Terminal identification

• 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.
• 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.
• Hence the concept of a Dial Plan and the use of
  an H.323 User Number registered to a Gatekeeper.
• A Dial Plan is simply a method of allocating a
  unique number to an H.323 Endpoint.

42
H.323 User Number

• 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.
• The H.323 User Number is often loosely referred
  to as the E.164 Number.

43
MCUs

• Responsible for managing multipoint conferences
  (two or more endpoints engaged in a conference)
• The MCU contains a Multipoint Controller (MC)
  that manages the call signaling and may
  optionally have Multipoint Processors (MPs) to
  handle media mixing, switching, or other media
  processing

44
Multipoint Control Units (MCUs)

• To allow three or more participants into a
  conference, most H.323 systems usually require a
  Multipoint Conference Server (MCS).
• This is also referred to as an H.323 Multipoint
  Control Unit (H.323 MCU).
• This is not the same as an H.320 MCU hence it is
  important to be clear about what you mean when
  using the term MCU

45
MCU

• 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.
• Whilst most H.323 MCU's, such as the mcu-15v or
  mcu-xx cards with the viaIP are hardware based,
  VCON have introduced the VCON Conference Bridge,
  VCB that provides a basic software MCU capable of
  allowing Ad-Hoc Conferencing in both Continuous
  Presence or Voice-Activated Switching modes.

46
MCU

• The main components of an H.323 MCU are
• MC - multipoint controller
• MP - multipoint processor (optional)
• The MC is the conference controller and handles
  H.245 negotiations between all terminals to
  determine common capabilities for audio and video
  processing.

47
MCU MC MP

• The MC also controls conference resources such as
  multicasting.
• Most H.323 systems support IP multicast and use
  this to send just one audio and one video stream
  to the other participants.
• The MC does not actually deal directly with any
  of the audio, video and data streams.
• This is left to the MP, which does all the audio
  mixing, data distribution and video
  switching/mixing of the bits. It also provides
  the conversion between different codecs and bit
  rates.

48
MCU MC MP

• Both the MC and MP functions can exist in one
  unit or as part of other H.323 components.
• Most H.323 MCU's work in conjunction with, or
  include a Gatekeeper functionality.

49
MCU - H.320

• 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.

50
Endpoint with Embedded MCU

• 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.
• 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.
• Furthermore, the 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. In a
  simplistic manner, it also acts like a Gateway,
  bridging between the other 2 or 3 ISDN and IP
  endpoints.

51
Gatekeeper

• The Gatekeeper is an optional component in the
  H.323 system which is primarily used for
  admission control and address resolution
• 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

52
Gatekeeper

• 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.
• Gatekeepers are responsible for providing address
  translation between an endpoints current IP
  address and its various H.323 aliases, call
  control and routing services to H.323 endpoints,
  system management and security policies.
• These services provided by the Gatekeeper in
  communicating between H.323 endpoints are defined
  in RAS.

53
Gatekeeper

• Gatekeepers provide the intelligence for
  delivering new IP services and applications.
• They allow network administrators to configure,
  monitor and manage the activities of registered
  endpoints, set policies and control network
  resources such as bandwidth usage within their
  H.323 zone.
• Registered endpoints can be H.323 Terminals,
  Gateways or MCU's.

54
Gateway and zone

• Only one Gatekeeper can manage a H.323 zone, but
  this zone could include several Gateways and
  MCU's.
• 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.
• This ensures that multiple H.323 endpoints that
  contain a Gatekeeper can all be configured into
  the same zone.

55
Examples of gatekeepers

• The INVISION 100 from RADVISION combines Gateway
  and MCU functionality in one box and has an
  embedded Gatekeeper that can be disabled
• this allows the zone to be controlled by a more
  powerful Gatekeeper

56
Example of gatekeeper

• With media networks becoming more and more
  complex, the ability for the administrator to
  effectively manage and control their usage
  becomes crucial.
• To address these issues, VCON have introduced
  Media XchangeManager, MXM.
• 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.
• MXM automatically generates Call Detail Reports,
  CDR which can be used for network planning or
  billing purposes.
• 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. Furthermore, MXM includes an H.323
  Gatekeeper.

57
Interconnected Gatekeeper Zones

• As stated earlier, the Gatekeeper defines the
  zone and manages the registered endpoints within.
  To call an endpoint within the same zone, we
  simply dial that endpoints H.323 User Number. But
  what happens when we want to call an endpoint
  that is located in another zone? Well, we then
  also need to know the zone where that endpoint is
  registered. Each Gatekeeper on the same network
  is identified by a unique number, its Zone
  Number. To call an endpoint in a different zone,
  we prefix that endpoints H.323 User Number with
  its Zone Number and dial this extended number.

58
Interconnected Gatekeeper Zones

• The telephone analogy to the Gatekeeper Zone
  Number is the STD code for the local exchange. If
  we want to telephone a person locally, we just
  dial their local number, but if we want to
  telephone somebody further afield, we need to
  prefix their local number with their STD code.
• Behind the scenes, all the Gatekeepers on the
  network must know how they are related to
  eachother.

59
Interconnected Gatekeeper Zones
60
Neighbour Gatekeepers

• When Gatekeepers are arranged in a single tier
  'Peer-to-Peer' manner with no particular
  hierarchical structure, they are termed as being
  Neighbour Gatekeepers.
• This would typically be on a corporate network
  within a multi-site company who has a Gatekeeper
  at each site.
• 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

61
Directory Gatekeepers

• When the Gatekeepers are arranged in a multi-tier
  manner with a hierarchical structure, they are
  termed as being Directory Gatekeepers (DGK).
• 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.

62
Directory Gatekeeper

• A Directory Gatekeeper only knows its Parent and
  Child Gatekeepers.
• 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.
• 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.
• The call is then routed down through each Child
  DGK tier until it reaches the specific endpoint.

63
Gateways

• The Gateway is composed of a Media Gateway
  Controller (MGC) and a Media Gateway (MG),
  which may co-exist or exist separately
• The MGC handles call signaling and other
  non-media-related functions
• The MG handles the media
• Gateways interface H.323 to other networks,
  including the PSTN, H.320 systems, and other
  H.323 networks (proxy)

64
Gateways

• H.320 and H.323 systems can interoperate with the
  use of a Gateway.
• Essentially, the Gateway provides translation
  between circuit-switched networks ISDN and
  packet-based networks LAN, enabling the endpoints
  to communicate.
• 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.
• The Gateway also has to transcode between the
  various audio and video codecs used between the
  LAN and ISDN devices.

65
Gateway

• Most Gateways work in conjunction with, or
  include a Gatekeeper functionality

66
Border Elementsand Peer Elements

• Peer Elements, which are often co-located with a
  Gatekeeper, exchange addressing information and
  participate in call authorization within and
  between administrative domains
• Peer Elements may aggregate address information
  to reduce the volume of routing information
  passed through the network
• Border Elements are a special type of Peer
  Element that exists between two administrative
  domains
• Border Elements may assist in call
  authorization/authentication directly between two
  administrative domains or via a clearinghouse

67
The Protocols

• H.323 is a framework document that describes
  how the various pieces fit together
• H.225.0 defines the call signaling between
  endpoints and the Gatekeeper
• RTP/RTCP (RFC 3550) is used to transmit media
  such as audio and video over IP networks
• H.225.0 Annex G and H.501 define the procedures
  and protocol for communication within and between
  Peer Elements
• 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

68
Protocols

• H.450.x is a series of supplementary service
  protocols
• H.460.x is a series of version-independent
  extensions to the base H.323 protocol
• T.120 specifies how to do data conferencing
• T.38 defines how to relay fax signals
• V.150.1 defines how to relay modem signals
• H.235 defines security within H.323 systems
• X.680 defines the ASN.1 syntax used by the
  Recommendations
• X.691 defines the Packed Encoding Rules (PER)
  used to encode messages for transmission on the
  network

69
Registration, Admission, and Status - RAS

• Defined in H.225.0
• Allows an endpoint to request authorization to
  place or accept a call
• Allows a Gatekeeper to control access to and from
  devices under its control
• Allows a Gatekeeper to communicate the address of
  other endpoints
• Allows two Gatekeepers to easily exchange
  addressing information

70
Registration, Admission, and Status RAS
71
H.225.0 Call Signaling

• Allows an endpoint to initiate and terminate a
  call with another endpoint

72
H.245 Signaling

• H.245 is used to negotiate capabilities and to
  control aspects of the conference between two or
  more endpoints

73
Fast Connect and H.245

• Some H.323 calls do not utilize the rich
  capabilities offered by H.245 and simply media
  channels using the Fast Connect procedures
• In this mode, a call may be established with as
  few as two messages (Setup / Connect)

74
H.323 Stack
75
Resolving Addresses

• A Gatekeeper may resolve addresses in a number of
  ways
• Sending a Location Request (LRQ) message to
  another Gatekeeper
• Accessing a Peer Element
• Accessing a back-end database (e.g., LDAP)
• Gatekeepers and Peer Elements may query other
  Gatekeepers and Peer Elements and may exchange
  address information outside the context of a call

76
Resolving Addresses

• Since a Gatekeeper is not required, endpoints may
  resolve addresses themselves using, for example,
  DNS, LDAP, or a local phonebook containing
  static IP addresses

77
Using LRQs

• A Gatekeeper may send an LRQ to one ore more
  Gatekeepers
• It may accept any LCF response and utilize that
  information to satisfy the original ARQ

78
Using LRQs with Hierarchical Gatekeepers

• A Gatekeeper may forward an LRQ received on to
  another Gatekeeper in order to resolve the
  address
• The response may be directed back to the
  originating Gatekeeper or the intermediate
  Gatekeeper

79
Advanced features

• Advanced Videoconferencing
• Supports advanced videoconferencing features,
  including
• Cascading MCUs
• MCU control over audio and video mixing
• Chair control
• Far-end camera control

80
Supplementary Services

• Standard mechanisms to provide a variety of
  services, including
• Call transfer
• Call forward
• Call park/pick-up
• Call Hold
• Call Waiting
• Message Waiting Indication
• Call Completion on Busy / No-Answer
• Call Intrusion

81
QoS

• H.460.9 allows an endpoint to report Quality of
  Service information to the Gatekeeper, aiding in
  determine how to route calls
• H.323 devices may utilize IETF standards for
  providing quality of service, including DiffServ
  and RSVP

82
Scalability

• H.323 allows calls to be routed directly between
  endpoints without the need for an intermediate
  entity that maintains call state
• Ability to utilize network services for address
  resolution, including ENUM, LDAP, and DNS

83
Flexibility

• Voice over IP (or any packet-based network)
• Videoconferencing
• Support for T.120 data conferencing
• Support for real-time text communication