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. Review Questions. Standards. H.323. H.320. Beginning. Help. - PowerPoint PPT Presentation
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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?
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?
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.
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
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.
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.
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.
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.
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.
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.
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.
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?
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.
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
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.
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.
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.
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.
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.