Video Conferencing - Understanding the Terminology


Third Generation Mobile System – The generic term for the next generation of mobile wireless communications networks.


Third Generation Partnership Project – A body comprising several Organizational partners working to produce technical specifications for a third generation mobile system based on GSM core networks and the radio access technologies they support.


Admissions Confirm message – A RAS message that the Gatekeeper sends to the calling endpoint, accepting the ARQ.

Address Resolution

A mechanism for identifying the address of a called endpoint in terms of the network, such as an IP address.

Address Translation

The ability of a Gatekeeper to translate an alias address, such as a name or e-mail address, to a transport address. One method of translation uses a Translation Table, which is updated by the Registration messages on the RAS channel.


An alternative identification string for an IP address. An alias can be a name, a URL address, an e-mail address, a transport address in the form of ‘IP address: port number, or a Party Number.


Any step-by-step problem-solving procedure. Transmission of compressed video over a communications network requires sophisticated compression algorithms. Some video conferencing systems offer both proprietary and standard compression algorithms.

Alternate Gatekeeper

Support for an Alternate Gatekeeper enables you to make Gatekeeper failures transparent to the endpoints that are registered to the Gatekeeper. In Radvision implementations, a backup Gatekeeper (the ‘Secondary’ Gatekeeper) runs in parallel to each online Gatekeeper (the ‘Primary Gatekeeper’).

Application Level Gateway

Application Level Gateways (ALGs) serve as communicators between two networks. ALGs are protocol-aware entities that examine application protocol flows and only allow messages that conform to security policies to pass. See also proxy server.


Admissions Reject message – A RAS message that the Gatekeeper sends to the calling endpoint, rejecting the ARQ.


Admissions ReQuest message – A RAS message sent by an endpoint placing a call or an endpoint receiving a call asking for bandwith allowance and permission to continue the Call Setup.


Asynchronous Transfer Mode – A high bandwith, high speed (up to 155Mbps), controlled-delay packet switching and transmission system integrating multiple data types (voice, video and data). Uses fixed size packets also known as ‘cells’ ATM is often referred to as ‘cell relay’.


The process of verifying the identity of a user trying to log on to a system, or the sender of a message.

B Channel

Bearer channel. In ISDN communications a B channel transmits data or voice at 64 or 56 Kbps.


The transmission capability of a communications channel. In analog communications bandwith is measured in Hertz (cycles per second). In digital communications it is measured in bits per second (bps). Ethernet bandwith is typically 10 or 100 Mbps.


Bandwidth ON Demand. Increasing the bandwidth of a call by linking two or more B channels of an ISDN line.


Bits per second – A unit of measurement of the speed of data transmission and thus of bandwidth (lower case is significant).

Bps (or BPS)

Bytes (8-bit) bytes per second. A unit of measurement of the speed of data transmission and thus of bandwidth (upper case is significant).


Basic Rate Interface. An ISDN subscriber interface supporting two bearer B channels at 64 Kbps and one data D channel at 16 Kbps.


A connection between three or more conference sites, enabling data, voice or video traffic to pass simultaneously between the sites. Video conferencing bridges are often called MCUs (Multipoint Conferencing Units).


A method of transmitting larger amounts of data, voice and video than telephony networks allow. In ISDN, broadband channels support rates above the primary E1 (2.048 Mbps) rate.


Transmission of data to everybody on the network or network segment.


See jitter buffer.

Call Authorization

Acceptance or rejection of calls from an H.323 terminal. The Gatekeeper may reject calls from a terminal because of restricted access to or from particular terminals or Gateways, or restricted access during certain periods of time. Call authorization is an optional Gatekeeper service.

Call Control

Also called Call Processing. Refers to the signaling involved in setting up, monitoring, transferring and disconnecting (tearing down) a call.

Call Fallback

Support for Call Fallback enables you to configure a Gatekeeper Forwarding policy to deal with cases such as Gatekeeper failure to resolve a destination address in the IP network, lack of Gatekeeper bandwidth resources, or unsuccessful Call Setup to the destination endpoint due to network failure.

Call Setup Routing

Two alternative modes for routing the Call Setup (Q.931) and Control (H.245) channels. Routed Mode routes the Call Setup and Control channel through the Gatekeeper. See also Routed Mode, Direct Mode, Q.931 + H.245 Routed Mode.

Cascaded MCU

The MCU allows you to combine two or more conferences resulting in a larger conference with many more participants. This is called Cascading. Cascading creates a distributed environment that helps to reduce the drain on network resources.


Call Detail Record – Information in a simple text format that can be used as input to third party billing programs or other software for billing purposes.

Centralized MCU Conferences

In a centralized topology, the Multipoint Controller and Media Processor unit components of the MCU work together to manage conference signaling and to perform media processing for all connected terminals. The MCU can handle multipoint conferences simultaneously. See also MCU, cascaded MCUs, clustered MCUs.

Chat Room

A virtual room where a chat session takes place. Technically, a chat room is really a channel, but the term room is used to promote the chat metaphor.


Common Intermediate Format – A standard video format used in video conferencing. CIF requires four times the bandwidth of QCIF.

Circuit Switching

A networking technology that provides a temporary but dedicated connection between two stations regardless of the number of switching devices through which data is routed. Analog circuit switching (FDM) has been replaced by digital circuit switching (TDM). The digital technology still maintains the connection until one speaker hangs up.

Cisco Proxy

The Cisco H.323 Proxy is a device that acts like a Gateway and relays H.323 data between H.323 zones.

Clustered MCUS

The Multipoint Controller (MC) and Media Processor (MP) unit components of the MCU operate independently. The MCU can be set up in a clustered layout to use a single MCU to control several units configured to operate only as MP units performing media processing. MCUs configured as MP Only units have their MC component disabled. The controlling MCU unit also makes use of the local MP component.


COder / DECoder. Hardware or software that converts sound, speech or video signals from analog to digital code and vice versa.


A technique for reducing the bandwidth or bit rate required to encode a block of information so that it occupies less space on a transmission channel or storage device.

Conference Hunting

The purpose of Conference Hunting in Radvision implementations is to maintain conferences and ignore Line Hunting where necessary.

Conferencing Service

The conferencing service is a mechanism supported on the Radvision MCU which defines the qualities, capabilities and management policies of a conference.

Continuous Presence

Continuous Presence (CP) allows you to view multiple participants in one screen at the same time. Incoming participant images are combined into a video image layout set according to the policies of the conferencing service. The range of video layouts available depends upon the type of media processing supported.

D Channel

The data signaling channel of an ISDN line. The channel is used to carry call control messages between the ISDN terminal and public switch.

Default Extension

Any endpoint on the network that has been defined an alternative destination for incoming calls routed by a Gateway.


The time taken for a signal to pass through a video conference from the sending station to the receiving station.


Data Encryption Standard – A cryptography method that makes it impossible for anyone without the decryption key to restore the data ti its unscrambled form.

Desktop Video Conferencing

Video conferencing on a personal computer. This is most appropriate for small groups or individuals. Many desktop video conferencing systems support document sharing.


Dynamic Host Configuration Protocol. In a DHCP environment IP policy is dynamic. This means that a terminal does not have a constant IP address. Management keys for identifying endpoints in a DHCP environment are the alias name or phone number of an endpoint.

Dial Plan

In traditional telephony systems, a dial plan is a front end system that allows users to call each other by dialing a number on a telephone. In voice and video conferencing over IP, a dial plan is a system that allows participants in point-to-point or multipoint conferences to call each other or join in conferences. For example, the Radvision ECS Dial Plan provides ‘configuration tools’ which allow network administrators to build an IP dial plan that suits the requirements of their organization and network.


Direct Inward Dialing. A method in which incoming calls are routed directly to endpoints on the LAN, without operator intervention.

Direct Mode

Routing of the Call Setup channel (Q.931) and the Control channel (H.245) to form a direct connection between two endpoints without Gatekeeper intervention. See also Call Setup routing.

Distance Learning

Incorporation of video and audio technologies so that students can ‘attend’ classes and training sessions presented at a remote location.


Domain Name Server. On TCP/IP networks, DNS converts the domain name (URL) of a host computer into a numeric IP address using the following format:

Document Sharing

A video conferencing feature that enables multiple participants to view and edit the same computer document.


The ability of a Gateway to maintain an ISDN video call by reconnecting the call at a lower rate when one of the B channels is lost. Downspeeding contributes to a higher percentage of call completion on the network.


Dual Tone Multi-Frequency signals, also called touch-tone dialing. The tones that are heard when you press the buttons on a touch-tone telephone.


The European interface for PRI ISDN lines. An E1 connection has up to 30 B channels and 1 D channel, which transmit at rates of up to 2.048 Mbps.


The term ‘E.164 number’ differentiates an ‘absolute’ telephone number from the number you must dial to reach an endpoint from a specific location. E.164 numbers include country codes, national destination codes and subscriber numbers.


A process which attenuates or eliminates the acoustic echo effect on video conference calls. Echo cancellation is largely replacing obsolete suppression.

Echo Effect

A time-delayed electronic reflection of a speaker’s voice. This is largely eliminated by modern digital echo cancellation.

Echo Supression

A technique for reducing annoying echoes in the audio portion of a video conference by temporarily deadening the communication link in one direction.


A network element at the end of the network such as an H.323 terminal, a Gateway, a Multipoint Controller Unit (MCU), a PC terminal, IP or ISDN phone, or video camera.


A LAN physical and data link protocol running over the lowest two layers of the OSI Reference Model at speeds of up to 10 or 100Mbps.


European Telecommunications Standards Institute. A France-based non-profit making organization that produces telecommunications standards to be used throughout Europe and beyond.

Exit Zone

When you define a prefix for the Exit Zone Service, you need to dial the prefix to reach an endpoint in another zone. This can be useful for restricting unauthorized users from making calls to other zones. The Exit Zone prefix affects the way in which the Gatekeeper tries to complete calls to other zones.


See Call Fallback.

Fast Connect

Also called Fast Start and H.323 Fast Start. A procedure for shortening the time it takes to start a call by skipping the H.245 phase and transferring channel parameters in the Call Setup messages.

Fast Start

See Fast Connect.


Federal Communications Commission. An independent US government agency charged with regulating interstate and international communications by radio, television, wire, satellite and cable.


A barrier device placed between two separate networks. A firewall can be implemented in a single router that filters out unwanted packets or it can use a variety of technologies in a combination of routers and hosts. Today many firewalls combine filtering functionality with Network Address Translation (NAT) functions.

Flow Control

Comprised of the hardware, software and procedure for controlling the transfer of IP packets between two points on a network.

Frame Rate

The rate at which individual video pictures (frames) in a sequence are displayed in a video conference. Frame rate is measured in frames per second (fps).


An ITU-T standard for speech codecs that provides toll quality audio at 64 Kbps using the PCM method and either u-law or A-law.


An ITU-T standard for speech codecs that provides toll quality audio at 64 Kbps.


An ITU-T standard for speech codecs that compresses 50Hz-7KHz audio signals into one of two bit rates, 24 or 32Kbps.


An ITU-T standard for speech codecs that provides good quality audio at 5.3 or 63 Kbps.


An ITU-T standard for speech codecs that provides near-toll quality audio at 16 Kbps

G.729 A/B.

An ITU-T standard for speech codecs that provides near-toll quality audio at 8 Kbps. G.729 Annex A is a reduced complexity codec and G.729 Annex B supports silence suppression and comfort noise generation.


In H.323 telephony, a Gatekeeper is an optional server that is responsible for network-based services including registration, admission and status, for which it uses a special protocol called RAS. Gatekeeper functions include address translation, call authorization and bandwidth management, as well as providing accounting information.

Gatekeeper Zone

A group of endpoints together with their Gatekeeper constitute a zone. You can configure a zone by predefining endpoints that entitled to register with the Gatekeeper. See also predefined endpoint, registered endpoint, zone.


A Gateway is a network entity, which provides a bridge between networks. An H.323 Gateway provides real-time, two-way communication between H.320 terminals on the packet-based network and other ITU terminals on a circuit switching network, or to other H.320 Gateways. A video Gateway provides real-time, two-way multimedia communication between video terminals on different networks. For example, between H.323 terminals on a packet-based network and H.320 terminals on an ISDN network, or between H.323 terminals on a packet-based network and H.324/M terminals on a 3G mobile network.

Gateway Redundancy

The Gateway Redundancy feature enables the Gatekeeper to request that other Gatekeepers search their zones to locate a Gateway when Line Hunting fails to find a suitable service provider. If no service provider is found, the Gateway Redundancy policy enables the Gatekeeper to complete the call by referring the call to other Gatekeepers.

Gateway Supported Prefixes

H.323 version 2 enables a Gateway to specify prefixes that the user should dial before the WAN number in order to make a call using a certain media.


Gatekeeper ConFirm Message – A RAS message that the Gatekeeper sends to the requesting endpoint, accepting the GRQ.


Gatekeeper ReJect Message – A RAS message that the Gatekeeper sends to the requesting endpoint, rejecting the GRQ.

Group Hunting

Group Hunting enables a Gatekeeper to perform load balancing for a group of H.323 endpoints defined with the same alias.


Gatekeeper ReQuest Message – A RAS message that an endpoint sends to locate a Gatekeeper to which the endpoint can register.


Global System for Mobile Communications – The standard digital cellular phone service of Europe , Japan , Australia and elsewhere.


H.225.0 specifies the procedures and messages applicable to Gatekeepers, including the RAS protocol for Registration, Admission and Status.

H.235 Security

H.235 Security ensures the authentication of each endpoint and the integrity of messages. The basis of the security process is the shared secret between the endpoint and the Gatekeeper.


ITU-T standard for role management and additional media channels for H.300 – series terminals.


The ITU-T standard used for the Control Protocol for Multimedia Communication. H.245 is included in the H.225.0 Recommendation. H.245 provides signaling for the proper operation of the H.323 terminal, including capabilities exchange, opening and closing of logical channels together with a full description of these channels, mode preference requests, flow control messages and general commands and indications.


The H.245 Proxy enables routing H.245 channels in a point-to-point H.323 call. The H.245 Proxy is a natural extension of a Gatekeeper.

H.245 Tunneling

H.245 tunneling decreases the time between the point at which an endpoint initiates a call and the point at which the call participants are ready to open multimedia channels. The endpoint must also support H.245 tunneling.


The ITU-T standard for signaling and control between circuit-switched PSTNs and VoIP networks.


The ITU-T standard for compression that allows higher quality calls to pass over a lower bandwidth for advanced video coding in generic audiovisual services.


An ITU-T standard for compressing an H.320 video conferencing transmission. H.261 supports CIF and QCIF resolutions.


An ITU-T standard video codec based on and compatible with H.261. H.263 offers improved compression over H.261 and transmits video with QCIF support.


The ITU-T standard for video conferencing over digital networks such as ISDN.


The ITU-T standard for video conferencing over packet-switched networks such as LANs and the Internet.

H.323 Fast Start

See Fast Connect


The ITU-T standard for video conferencing over analog telephone lines (POTS) using modems.


ITU-T standard for directory services architecture for multimedia conferencing.


The Third Generation Partnership Project (3GPP) has adopted the ITU-T Recommendation H.324 Annex C, also referred to as H.324/M ( Mobile ) or H.324M (with very few changes), on their 3G-324M as the standard for multimedia telephony over 3G circuit switched cellular networks.


See Supplementary Services.


Internet Engineering Task Force. Formed in 1986, the IETF sets the technical standards that run the Internet. IETF working groups seek the advice of the Internet community through RFCs (Request for Comment), and then submit recommendations to the IETF for final approval.

Instant Messaging (IM)

A communications service that enables you to create a private chat room with another individual in order to communicate in real time over the Internet.

Incoming Call Routing

See default extension, DID, MSN, sub-addressing, TCS4.

IP Address

The unique address of a computer attached to a TCP/IP network. IP addresses are 32 bits long. Each octet is represented in decimal and is separated by dots.

IP Multicast

A means of simultaneous transmission of data from a server to a group of selected users on a TCP/IP network, (internal, intranet or Internet). IP multicast is used for streaming audio and video over the network.

IP Network

A network that uses the TCP/IP protocol.

IP Telephony

A set of technologies that enables voice, data and video collaboration over existing IP-based LANs, WANs and the Internet. IP telephony uses open IETF and ITU standards to move multimedia traffic over any network that uses IP.


Information ReQuest Message – A RAS message in which the Gatekeeper asks the endpoint for its current status.


Information Request Response Message – A RAS message that the Gatekeeper sends to the calling endpoint, rejecting the IRQ.


Integrated Services Digital Network – ISDN is an entirely digital telephone network that allows both data and voice communications over the same line. ISDN replaces the old analog local loop and operates at significantly faster speeds than the traditional telephone service.

ISDN Rollover

In Radvision implementations, Gateway support for the ISDN Rollover feature ensures that a call is completed even when call volume is high. ISDN Rollover requires support by the PSTN switch application and presumes the availability of a pool of stacked Gateways across the managed network.


International Telecommunications Union – The most important telecom standards setting organization in the world. With headquarters in Geneva , the ITU is an international organization founded in 1865, now part of the United Nations System that sets communications standards for global telecom networks.


The Telecommunication Standardization Sector of the ITU, developing standards for interconnecting telecommunication equipment across networks.


Interactive Voice Response is a two-stage incoming call routing method supported by the Gateway. It enables DID to a LAN terminal, even when the ISDN lines do not have multiple numbers allowing direct dialing to an endpoint.


The result of a change in latency or the tendency towards lack of synchronization caused by mechanical or electrical changes. Technically, jitter is the phase shift of digital pulses over a transmission medium.

Jitter Buffer

A portion of memory specifically allocated to storing IP packets awaiting transmission, or to storing received IP packets. The buffer facilitates flow control by capturing IP packets and then transmitting packets as ‘playback’ using speeds and rates of delay that the destination device can handle without causing packet loss through overloading.

Jitter Buffer Management

Jitter buffer management represents the trade-off between a larger buffer and increased rates of jitter.


Kilobits per second. One thousand bits per second. Used to define data transfer bit rates.


Local Area Network. A communications network internal to an organization.

LAN/WAN Connectivity

The practical set of tools, from operating system layer protocols to support services that make a remote access device an effective link between LANs and WANs.


A measure of accumulated waiting time or delay, representing the length of time required for information to pass through a network.


Location ConFirm Message – A RAS message that a Gatekeeper sends to the requesting Gatekeeper/endpoint accepting the LRQ.


Lightweight Directory Access Protocol – A protocol for accessing online directory services. LDAP is both an information model and a protocol for querying and manipulating the model.

Leased Line

A dedicated connection providing a telecommunications link for voice, data and Internet traffic. Leased lines are delivered along predetermined routers and enable control over them. For example, QoS, bandwith allocation and connection speed.


Light Emitting Diode. A display technology that uses a semiconductor diode that emits light when charged. LEDs usually indicate both correct and problematic operation.

Line Hunting

A Gateway supports a list of prefixes (services). Gateway unavailability to receive a call means that the Gateway cannot accept calls with the particular prefix in question. The Line Hunting function of a Gatekeeper searches for a Gateway which is free to accept calls with this prefix.

Load Balancing



Location ReQuest Message – A RAS message that a Gatekeeper / endpoint sends to another Gatekeeper requesting the location of an endpoint.


Location ReJect Message – A RAS message that a Gatekeeper / endpoint sends to the requesting Gatekeeper rejecting the LRQ.


Media Access Control – A system of rules used to move data from one physical medium to another.


The side in communications which initiates and controls the session. The ‘slave’ is the other side that responds to the master’s commands.


Megabits per second. One million bits per second. Used to define data transfer bit rates.


Multipoint Conferencing Unit. A device that manages a multipoint conference by connecting the multiple sites and stations in the same video conference. The current version of the Radvision MCU can connect video conference endpoints supporting different protocols into the same conference including H.323 and SIP. The MCU can be used in conjunction with a Gateway to connect H.320 and H.324 video conference endpoints. The MCU combines video, audio and data streams from multiple conference endpoints into one multi-location, interactive session.


Media Gateway Control Protocol / Media Gateway Controller – An IP telephony signaling protocol from the IETF. MGCP was the original protocol, which evolved into MEGACO. Both protocols are designed for implementation in IP phones that are cheaper than SIP or H.323 phones.


Management Information Database – An SNMP structure that describes the particular device being monitored.


Moving Pictures Experts Group. MPEG is a series of standards designed to reduce the storage requirements of digital video. MPEG-4 provides the standardized technological elements for the integration of interactive graphics applications and interactive multimedia.


Multiple Subscriber Number – A method of incoming call routing in which a group of telephone numbers is assigned to a particular ISDN line by the telephone company.


Communication configuration in which several terminals or stations are connected. Contrast with point-to-point.

Multipoint Video Conference

Video conference with more than two sites. The sites must connect via a video bridge.


Transmission of duplicate data streams, one to each user. In multi-unicast, multiple users request the same data from the same server at the same time. Contrast with IP multicast, unicast.


Network Address Translation – NAT devices translate IP addresses so that users on a private network can see the public network, but public network users cannot see the private network users.

Neighbour Gatekeeper

A mechanism by which the Radvision H.323 Gatekeeper optimizes inter-zone communication. A list of Neighbour Gatekeepers and their IP addresses allows the Gatekeeper to resolve destination IP addresses when the source endpoint is not in the same zone as the destination endpoint.


A group of stations (computers, telephones, or other devices) connected by communications facilities for exchanging information. Connection can be permanent, via cable, or temporary, through telephone or other communication links. The transmission medium can be physical (fibre optic cable) or wireless (satellite).

Network Load Balancing

See RAI / RAC.


Network Specific Facility – The Network Specific Facility Information Element (NSFIE) feature enables system administrators to co-ordinate their network and service requirements with Service Providers.

Online Endpoint

When an endpoint registers with a Gatekeeper, the endpoint is active and ready to receive calls. By registering, the endpoint informs the Gatekeeper that it is online.


A block of data used for transmission in packet-switched systems.

Packet Loss

The discarding of data packets in a network when a device is overloaded and cannot accept incoming data at a given moment.

Packet Re-Ordering

Packet re-ordering ensures that all packets reach their destination in the correct sequential order.

Packet Switching

A networking technology that breaks up a message into smaller packets for transmission and switches them to their required destination.

Parent Filters

When the Radvision Gatekeeper fails to resolve a destination address, the Gatekeeper searches for the destination first among its Children, then among its neighbours and then via its parent. Parent filters enable the Gatekeeper to avoid unnecessary searches among its Children and Neighbour Gatekeepers.

Party Number

The dialing number of an endpoint. This number can be a telephone number or a number used by other mechanisms on various networks, such as telex and ISDN.


Private Branch Exchange. A small, company-owned version of the telephone company central switching office. Used to enable direct dialing between IP phones on the corporate LAN, or for long-distance direct dialing between IP phones and H.323 endpoints.


Communication configuration in which communication is between two stations only. Contrast with multipoint.


Point of Presence – The point at which a line from a long distance carrier connects to the line of the local telephone company, or to the user if the local company is not involved. For online services and Internet providers, the POP is the local into which exchange users dial via a modem.


A pathway into and out of a computer or a network device, such as a switch or router.


Plain Old Telephone Service.

Predefined Endpoint

An endpoint entitled to register with a specified Gatekeeper.


A prefix is a part of the dialing sequence used to access a service or conference type. See also Gateway supported prefixes and conferencing service.


Primary Rate Interface. An ISDN subscriber interface supporting 23 bearer channels at 64 Kbps and two data D channels at 16 Kbps in North America, or 30 bearer B channels at 64 KBPS and two data D channels at 16 Kbps in Europe. A PRI connection runs at 1.544 Mbps over a T1 connection and at 2.048 Mbps over an E1 connection.

Proxy Server

An application that breaks the connection between sender and receiver. All input is forwarded out on a different port, closing a straight path between two networks and preventing a cracker from obtaining internal addresses and details of a private network.


Public Switched Telephone Network – The worldwide voice telephone network. Once only an analog system, the heart of most telephone networks today is all digital. In the US, most of the remaining analog lines are the ones from your house or office to the telephone company’s central office.


A protocol for Call Signaling, consisting of Setup, Teardown and Disengage. Q.931 is included in the H.225.0 Recommendation.

Q.931 + H.245 Routed Mode

The routing of the Call Setup channel (Q.931) and the Control channel (H.245) through the Gatekeeper. See also Call Setup routing, Routed Mode.


Quarter Common Intermediate Format – A standard video format used in video conferencing (176 x 44 pixels). QCIF requires a quarter of the bandwidth of CIF.


Quality of Service – The ability to define a level of performance in a data communications system. For example, ATM networks specify modes of service that ensure optimum performance for traffic such as real-time voice and video.


Remote Access Dial-In User Service – A server for authentication, authorization and accounting of endpoints and endpoint aliases.


Resource Available Indication / Resource available Confirmation – The RAI / RAC function automatically manages load balancing on the network. RAI / RAC messages are exchanged between a Gatekeeper and a Gateway to determine whether the Gateway is available to receive calls.


A protocol for Registration, Admission and Status. In an H.323 audio or video system, the RAS is a control channel over which H.225.0 signaling messages are sent.


Registration ConFirm message. A RAS message that a Gatekeeper sends to the calling endpoint accepting the RRQ.


The processing of information that returns a result so rapidly that the interaction appears to be instantaneous. Video conferencing is an example of a real-time application.

Real-Time Srreaming

Delivery of a real-time stream of a live video conference while the conference is in progress.


See Gateway Redundancy.

Registered Endpoint

A Registered Endpoint is an endpoint that has informed the Gatekeeper that it is online, active and ready to receive calls and has received confirmation from the Gatekeeper of its registration request.

Routed Mode

The routing of the Call Setup channel (Q.931) and the Control channel (H.245) through the Gatekeeper. See also Call Setup routing, Q.931 + H.245 Routed Mode.


A device or setup that finds the best route between any two networks, even if there are several networks to transverse. Like bridges, remote sites can be connected using routers over dedicated or switched lines to create WANS.


Registration ReJect message. A RAS message that a Gatekeeper sends to the requesting endpoint rejecting the RRQ.


Registration ReQuest. A RAS message in which an endpoint identifies itself to a specific Gatekeeper and asks for service. The RRQ message binds the endpoint aliases – names or phone numbers – to the IP addresses of the endpoint.


Real Time Transport Protocol / Real Time Control Protocol – RTP is an IP protocol that supports real-time transmission of voice and video. It is widely used for IP telephony. RTCP is a companion protocol that is used to maintain QoS.

RTP Redundancy

A method of overcoming packet loss by doubling packet payload without increasing the number of packets sent.

Serial Interface

A channel that transfers digital data in a serial fashion, one bit after the other over one wire or fibre. The serial port on a PC is a serial interface that is used to attach modems and scanners. Serial interfaces may have multiple lines, but only one is used for data. See also V.35 and X.21.

Service Prefix

The prefix identifies the service and can usually be a numeric code, an alphanumeric string, a name or a phone number.


A service is a function that is supported by a subset of endpoints in a zone. You access a service by dialing a prefix attached to the name or phone number. Services allow you to dynamically add more resources, such as a Gateway, into the system. In Radvision implementations you can define access privileges per endpoint for each service.

Silence Suppression

Silence information within the audio stream can consume LAN bandwidth and burden MCU voice processing. Using compression techniques, Silence Suppression can greatly reduce the wasted bandwidth in a multipoint conference and on congested networks.

Single Station MCU

In Radvision implementations, a solitary Radvision MCU that is facilitating a conference.


Session Initiation Protocol – An IP telephony signaling protocol developed by the IETF. SIP is a text-based protocol that is suitable for integrated voice-data applications. SIP is designed for voice transmission and uses fewer resources and is considerably less complex than H.323.


The side in communications which responds to session commands. The ‘master’ is the other side that initiates and controls the session.


Simple Network Management Protocol – A widely used network monitoring and control protocol.


Sub Quarter Common Intermediate Format. A video format usually used to refer to video with dimensions of 88 x 72 pixels.


Sub-addressing is a one-stage DID dialing mechanism in which a phone sends two numbers. One number is for routing on the circuit-switched network. The other number is forwarded to the Gateway inside a Q.931 Sub-addressing Information Element for IP address resolution by the Gatekeeper.


The current version of the Radvision MCU allows the creation of audio sub-conferences to which selected participants in an existing conference can be temporarily diverted. When the sub-conference is over, the participants return to the main conference.


A subnet is a portion of an IP network defined by a subnet mask. Devices on the same subnet have the same subnet mask.

Supplementary Services

The ITU-T H.450 Recommendation comprises a number of separate Recommendations for implementing Supplementary Services of the H.450.x series. For full details of the H.450.x Recommendations please refer to the appropriate ITU documents.

T.120 Data Standard

Data sharing protocol for multipoint data communication in a multimedia conferencing environment. T.120 enables white board collaborations, file transfers, graphic presentations and application sharing between participants in a conference.


The interface for PRI ISDN lines used in the USA and some Asia Pacific countries. A T1 connection has up to 24 B channels and transmits at rates of up to 1.544 Mbps.


Transmission Control Protocol. A reliable transport layer on top of IP. TCP provides transport functions which ensure that the total amount of bytes sent is received correctly by the called party.


TCS4 is a special routing method for incoming H.320 video calls. TCS4 allows direct inward dialing to an endpoint on the IP network via the Gateway when DID is not available.

Topology Islands

IP subnets, characterized by homogeneous and fast LAN connectivity. Dividing the network into islands enables a Gatekeeper to direct calls through the most optimal routes, therefore avoiding slow connections or bottlenecks as much as possible.


A standard way of defining blocks of data in a TCP stream, since TCP does not have delimiters. During configuration you can define the maximum number of TPKT channels allowed.


Auto transcoding is the conversion of one audio transmission format into another using various algorithms to achieve different audio quality levels at reduced bandwidth levels.


Time To Live. A maximum amount of time a packet is allowed to propagate through the network before it is discarded. TTL is a time, typically in seconds, after which the fragment can be deleted by any device on the network.


Unregistration ConFirm Message – A RAS message that a Gatekeeper or an endpoint sends accepting the URQ.


User Data Protocol – A transport protocol within the TCP / IP protocol suite that is used in place of TCP when a reliable delivery is not required.


A means of transmitting a message from one station to another. Contrast with IP.

Unrecognized Alias

An alias that is not in the registration database of the Gatekeeper.

Unregistered Endpoint

An endpoint that is no longer online and registered with a Gatekeeper.


Unregistration ReJect message. A RAS message that a Gatekeeper or an endpoint sends rejecting the URQ.


Uniform Resource Locator – An Internet address. The address that defines the route to a file on a computer connected to the Internet.


Unregistration ReQuest Message – A RAS message sent when an endpoint wishes to terminate its session with a Gatekeeper.


An ITU standard for group band modems that achieve higher data rates by combining the bandwidth of several telephone circuits. Typically used for modem-to-multiplexer communication. See also serial interface and X.21.

Video Bit Rate

Bit rate is the speed at which bits are transmitted, in bits per second.

Video Bridge

Computerized switching system which allows multipoint video conferencing.

Video Conferencing

Communication across long distances with video and audio contact, that may also include graphics and data exchange. Digital video transmission systems typically consist of a camera, codec (coder-decoder), network access equipment, network and audio system.

Video On Demand Streaming

Delivery of a Video on Demand stream to a viewer upon request at any given time. Contrast this to a real-time stream that is delivered when the conference is in progress.

Voice-Activated Video Switching

Automatic switching of a video image viewed at each conference terminal according to the voice level of each participant.


Voice over IP – See IP telephony.


Virtual Private Network – VPN modules create closed secure tunnels for communication between two firewalled LANs. VPN technology is one of the approaches being used today for providing secure communications over IP networks.


Wide Area Network – A communication network spread over a wide geographical area.

Web Conferencing

Enables two or more logged in users to set up a typed, real-time, online conversion across the World Wide Web.


A term used to describe the placement of shared documents on an on-screen ‘shared notebook’. See also document sharing.

Wildcard Digit Manipulation

A mechanism which enables a Gatekeeper to manipulate an incoming call source number, before searching for the destination endpoint.


An ITU standard protocol for a circuit switched network. See also serial interface and V.35.


An H.323 zone is a logical collection of terminals, Gateways and Multipoint Control Units (MCUs) managed by a single Gatekeeper. A zone must include at least one terminal and may include several LAN segments connected by routers. See also Gatekeeper Zone.

Zone Prefix

Zone prefixes are similar to telephone area codes. If an endpoint in a zone dials a zone prefix before its number, and the Gatekeeper cannot resolve it in its zone, the Gatekeeper attempts to locate and route the call to the appropriate zone of the Neighbour Gatekeeper.

LifeSize Range
Aethra Range
Polycom Range
Desktop Range
Desktop Video Conferencing
Second User
Video conferencing systems and equipment
Copyright © JKC - All rights reserved