© copyright ezenia inc, 2000 1 multimedia conferencing standards there are two standards...

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© Copyright Ezenia Inc, 2000

Multimedia conferencing standards

There are two standards associated with Multimedia Conferencing.

H.320 applies to connections across Wide Area Networks. H.323 applies to connections across Local Area Networks.

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H.320

H.323Standards

Review Questions

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H.320 conferences

H.320 is an umbrella standard linking into other standards where necessary. Many standards exist that are relevant to a multimedia conference.

The H.320 standard defines how a multimedia conference running over Wide Area (specifically ISDN) should operate. Many of the individual standards are common to both the WAN and LAN environments. The major difference is in the area of signaling and control. WAN environments are connection based; those in the LAN are connectionless.

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• What other standards are under the H.320 Umbrella?

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H.320 conferences

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VideoH.261H.263

DataT.120

AudioG.711G.722G.728

MultiplexingH.221

Controlsignaling

H.242H.243

NetworkInterfaces

V.xxX.xxI.xx

LineProtocols

I.361I.363I.400

H.320

Click on the box to find out more about these standards

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H.323 standards

H.323 acts as the umbrella standard linking into other standards where necessary.

Release 1 - 1996; Release 2 – 1998 ; Release 3 - 1999 Multimedia conferencing over IP LANs uses standards

from many organisations, some within ISO and some not, the specific standard H.323 is predominantly used to identify those standards that are relevant. Those listed on the next page are not a complete set, but the ones most likely to be of immediate relevance in configuring and using the Encounter products.

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• What other standards are under the H.323 Umbrella?

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H.323 standards

VideoH.261H.263

DataT.120

AudioG.711G.722G.723G.728

ControlsignalingH.225H.244H.450.x

NetworkInterfaces8802V.xxX.xxI.xx

NetworkProtocolsTCPUDPIP

H.323

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SecurityH.235

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Video Standards

There are 2 standards for video encoding used for Multimedia Conferencing. The standards control such factors as how the signal is compressed and digitised for transmission, the quality of the picture and how frequently the screen will be refreshed. But remember, the higher the picture quality, and the more frequent the refresh rate, the higher the bandwidth requirement.

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H.261 H.263

Multimedia Standards

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H.261

This is the older video encoding standard, but the most widely supported. The choice of refresh rates and image format allows choices to be made balancing bandwidth and picture quality. There are two defined image formats:

Common Intermediate Format (CIF),

which is 352 pixels by 288 lines

Quarter CIF (QCIF), which is 176 pixels by144 lines

H.261 supports the following refresh rates:

30, 15, 10 and 7.5 times per second

This standard is the default for all H.323 traffic and all endpoints must be able to support at least this video mode

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H.263

This is the newer video standard. It offers improved video coding and decoding, with higher quality options (4CIF has 4 times the picture element count of CIF), and greater refresh options. There are five defined image formats: CIF and QCIF (as for H.261) 16CIF - 1408 pixels by 1152 lines 4CIF - 702 pixels by 576 lines SQCIF - 128 pixels by 96 lines

H.263 supports the following refresh rates: 30, 15, 10, 7.5, 6, 5, 4, 3, 2, 1 times per second

For some applications a high quality image with very slow refresh will be desirable, while others would prefer rapid refresh at the expense of quality.

H.263 is not directly compatible with H.261.

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Audio standards

There are more variations of audio standards than there are of video, but there is only limited flexibility within each standard. It is possible to inter-work these standards providing a device on the network translates the codes.

Each encoding standard relies on an analogue to digital technique called Pulse Coded Modulation (PCM), of which there are two standard methods of operating - A-law is used mainly in North America, and m-law is used mainly in Europe. Some encoding standards support both techniques, some only one.

Where both are supported within the standard, the endpoints must be compatible with the chosen technique.


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G.722

G.723

G.728

G.711

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G.711

This is the most common audio standard used within both H.320 and H.323.

G.711 encodes 3.1 KHz audio range using pulse code modulation. G.711 audio is transmitted at 48kbps or 56kbps in H.320, and 64 kbps in H.323.

This is the default standard applied to H.323 endpoints, regardless of what other standards they can support, they must support G.711. Previous

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G.722

G.722 encodes 7.0 KHz audio range based on sub-bands (it divides the total frequency band into higher and lower bands called sub-bands). It only transmits the change in an input signal, instead of the whole signal.

G.722 audio is transmitted at 48kbps or 56kbps

in H.320, and 64 kbps in H.323.

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G.723

G723 is also known as IP Audio and is often used in H.323 networks. It provides audio quality like the Mobile Phone standard, and not normal telephone standard sound quality.

G.723 encodes 3.0 KHz audio range and is transmitted at 5.3kbps or 6.3kbps.

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G.728

G.728 is an audio coding algorithm that takes 128K pulse code modulated data and transmits it at 16kbps.

It is frequently used in low-speed conferences since it makes more of the transfer rate bandwidth available for video.

It is used widely within H.320 networks for this reason.

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Control standards

There are two significant control protocols used within H.323, H225 and H.245.

H.225 is used for control signalling during the initial call set up between Gatekeeper and endpoint.

H.245 is then used for control signalling during the conference itself.

Another set of standards were introduced with H.323 Release 2. These are the H.450 family of protocols which add a range of additional services to H.323.

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H.225 H.245H.450.x

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H.225 RAS

H.225

H.225 is the principle “language”of the Gatekeeper. A Gatekeeper has a range of special functions to fulfill, and so requires a special language of it’s own. H.225 also uses another standard protocol, Q.931.

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H.225/Q.931/RAS H.225/Q.931

Q.931

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H.225 RAS

The three prime functions of H.225 are those of endpoint Registration, Admission and Status. Each of these functions is performed as a separate step within H.225. The Gatekeeper sends “keep alive” messages to endpoints while it is processing RAS requests.

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Admission StatusRegistration

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Q.931

Q.931 is one of the common call signaling protocols used in Wide Area Networks – it is used extensively in ISDN.

In H.323, Q.931 is used to establish a connection between an endpoint and the Gatekeeper, and between one endpoint and another.

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H.225 Registration

Any endpoint that wishes to take part in a conference must Register with a Gatekeeper. When an endpoint attached to a LAN is started it will automatically send a message looking for a Gatekeeper to register with. There can only be one Gatekeeper that is “allowed” to accept that registration. Once an endpoint is registered it will periodically send an update message to the Gatekeeper to maintain it’s registration.

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• What does the Gatekeeper do with the registration information?

“Hello, my name is ‘Mary’”

Register ‘Mary’ as available for conferences

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H.225 Registration

The Gatekeeper will use the information gained during registration to build a “translation” table. This will include learning the name allocated to each endpoint as well as it’s network (IP) address.

A Gatekeeper can be set to accept any address, or set to only accept specific addresses. An endpoint could be set to register with any Gatekeeper, or a specific Gatekeeper. To build a zone (in a multi-gatekeeper system), requires that one or other device is correctly configured. In a large network it will be easiest to configure the Gatekeepers.

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H.225 Admission

Before a conference can commence the Gatekeeper will have to give permission. The originating endpoint will request a connection by specifying with whom it wishes to communicate and the amount of bandwidth it requires.

The Gatekeeper will permit the connection providing 1. Security conditions are satisfied (e.g. valid user, valid

destination, etc.). 2. Sufficient bandwidth exists.

If sufficient bandwidth does not exist, then the Gatekeeper can negotiate with the endpoint to attempt to agree a lower value providing the endpoint supports such an option.

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• How does admission happen?

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H.225 Admission

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I would like to talk to Mary @ 400kbps

Is he allowed to?Is there enough capacity?Can we agree on capacity?

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H.225 Status

The Gatekeeper will monitor the status of all registered endpoints and connections, by sending and receiving “Keep Alive” messages. The prime purpose of this is to ensure that registered endpoints remain available. An endpoint should de-register when finished, but may not. To allow for this the Gatekeeper regularly contacts each endpoint to see that they are still operational. An endpoint that fails to respond is removed from the registered list.

Similarly the endpoints are required to report periodically to the Gatekeeper for the duration of a call. If an endpoint fails to report for four consecutive times the call is assumed to have ended and the Gatekeeper will send Disconnect messages to both endpoints and release the network resources.

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• How does this happen?

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H.225 Status

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No reply from Harry, so I’ll de-register

him.

“Are you still there ‘Mary’”

“Are you still there ‘Harry’”

“I’m still here Gatekeeper”

Mary’s still active, so she

can stay registered

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H.245

Connections need to be set-up for a specific conference, monitored to ensure they are functional, and then cleared at the end of the conference. This is the function of the H.245 standard.

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H.245 Control

Audio

Video

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Security standards

H.235 is a security protocol which was introduced into H.323 at release 2. It provides the following facilities to ensure that conferences can be more tightly controlled. By authenticating users, it can control who can join the

conference. H.235 can enforce endpoint integrity which forces endpoints

to be associated with Participants. It can ensure data stream privacy by using a pseudo

encryption technique. Using a technique, known as non-repudiation, which checks

that the caller is who they say they are, it can ensure that callers are billed correctly.

Many multimedia endpoints do not currently support H.235.

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H.450.x

Introduced into H.323 at release 2, H450.x protocols facilitate the use of several supplementary services in the H.323 environment.

H.450.1 is the signalling protocol which controls the operation of the other protocols.

H.450.2 provides call transfer facilities, which allow callers to be answered by one endpoint and then transferred to another.

H.450.3 allows calls to be diverted to other endpoints for answering.

H.450 is intended primarily for telephony applications (VoIP) and so not all multimedia endpoints support it.

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The H.323 standard is used over Wide Area Networks only.

True

False

Multimedia Conferencing Standards - Review Question 1

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H.320 is the Multimedia conferencing standard for WANs. H.323 is used over any network that uses IP as it’s

transport protocol. This could be a LAN or a WAN network.

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Correct Answer

Well done H.323 is used over any network that uses IP as it’s

transport protocol. This could be a LAN or a WAN network.

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Which of the following standards are used for conference control, between endpoints,once they have registered with the Gatekeeper.

H.225

H.245

Q.931

RAS


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H.225 is the protocol used by the Gatekeeper to communicate with endpoints.

H.245 is used to control the conference traffic between endpoints.

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Q.931 is part of the H.225 protocol, used by the Gatekeeper to communicate with endpoints.


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RAS is part of the H.225 protocol, used by the Gatekeeper to communicate with endpoints.


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Correct Answer


Well done H.245 is used to control the conference traffic between

endpoints.

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As a minimum, an H.323 endpoint must support the H.263 video standard.

True

False


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H.263 is the newer video standard, offering more efficient compression, or higher image quality.

An H.323 endpoint must support H.261, as a minimum.

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Correct Answer


Well done An H.323 endpoint must support H.261, as a minimum.

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Which of the following audio standards are most widely used in H.323?

G.711 and G.722

G.711 and G.723

G.711 and G.728

G.723 and G.728


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G.711 is the most commonly used audio standard in both H.320 and H.323.

G.722 is not widely used in H.323. G.711 and G.723 are the most commonly used audio

standards in H.323.

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G.711 and G.728 are the most commonly used audio standards in both H.320.

G.711 and G.723 are the most commonly used audio standards in H.323.

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G.723 is widely used in H.323. G.728 is widely used in H.320. G.711 and G.723 are the most commonly used audio

standards in H.323.

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Correct Answer


Well done G.711 and G.723 are the most commonly used audio

standards in H.323.

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Which of the following standards was introduced in Release 2 of the H.323 Standard?

H.225

H.235

H.245


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H.225 is the protocol used by the Gatekeeper, and has been part of H.323 since release 1 of the standard.

H.235 was introduced to provide enhanced security in H.323, release 2.

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H.245 is the protocol used by endpoints for conference control, once they have registered with the Gatekeeper and has been part of H.323 since release 1 of the standard.

H.235 was introduced to provide enhanced security in H.323, release 2.

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Correct Answer


Well done H.235 was introduced to provide enhanced security in

H.323, release 2.

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