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H.323. Liane Tarouco Leandro Bertholdo. 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

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h 323


Liane Tarouco

Leandro Bertholdo

standard entities
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.
available transport media
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.
  • 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.
  • 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!
isdn for multipoint conference
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
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
local area network lan or intranet and wide area network wan
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.
  • 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.
multipoint conference
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;
lan wan
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
cellular networks
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.
cellular networks13
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
internet vpn adsl
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
directory services
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
  • 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
video standards
Video standards
  • H.261 - video codec for audiovisual services at p x 64Kbps.
  • H.263 - video codec for narrow telecommunications channels at < 64 Kbps.
  • H.264/AVC - a new video codec standard offering major improvements image quality.
image size
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.
video and pc window sizes
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.
video and pc window sizes20
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.
video and pc window sizes21
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.
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.
audio standards
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.
audio standards24
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.
data standards
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.
data standards26
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).
data standards27
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.
control standards
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.
control standards29
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.
control standards30
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.
control standards31
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.
control standards32
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.
numbers names
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.
q 931
  • Q.931 - Signalling protocol for establishing and terminating calls.
  • 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
what is h 323
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”

elements of an h 323 system
Elements of an H.323 System
  • •Terminals
  • •Multipoint Control Units (MCUs)
  • •Gateways
  • •Gatekeeper
  • •Border Elements


  • Video phones
  • IVR devices
  • Voicemail Systems
  • “Soft phones” (e.g., NetMeeting®)
  • 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.
  • 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.
terminal identification
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.
h 323 user number
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.
  • 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
multipoint control units mcus
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
  • 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.
  • 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.
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.
mcu mc mp48
  • 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.
mcu h 320
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.
endpoint with embedded mcu
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.
  • •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
  • 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.
  • 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.
gateway and zone
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.
examples of gatekeepers
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
example of gatekeeper
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.
interconnected gatekeeper zones
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.
interconnected gatekeeper zones58
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.
neighbour gatekeepers
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
directory gatekeepers
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.
directory gatekeeper
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.
  • 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)
  • 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.
  • Most Gateways work in conjunction with, or include a Gatekeeper functionality
border elements and peer elements
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
the protocols
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
  • 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
registration admission and status ras
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
h 225 0 call signaling
H.225.0 Call Signaling
  • Allows an endpoint to initiate and terminate a call with another endpoint
h 245 signaling
H.245 Signaling
  • H.245 is used to negotiate capabilities and to control aspects of the conference between two or more endpoints
fast connect and h 245
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)
resolving addresses
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
resolving addresses76
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
using lrqs
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
using lrqs with hierarchical gatekeepers
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
advanced features
Advanced features
  • Advanced Videoconferencing
  • Supports advanced videoconferencing features, including
    • Cascading MCUs
    • MCU control over audio and video mixing
    • Chair control
    • Far-end camera control
supplementary services
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
  • 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
  • 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
  • Voice over IP (or any packet-based network)
  • Videoconferencing
  • Support for T.120 data conferencing
  • Support for real-time text communication