Application in the video meeting system of H.248 agreement
Commercial large-scale video meeting system of network connection based on the intersection of H.323 and agreement basically at present. Though the voice of SIP is very high, should reach the level employed on a large scale, SIP also needs to do a lot of improvements. But the reason why have new agreement that attempts to substitute H.323 ceaselessly, first, come from the autogenous complexity of H.323 agreement, second, the restriction on network connection of H.323 system equipment element.
After H.323 agreement is put out, master application is divided for two directions: First, VoIP, second, video-information meeting.
Relied on the advantage of the expenses, the development of VoIP is very quick, even proposed " everythingonIP " that year Slogan. But development is set up and begun the limitation of embodying it in the network of H.323 early edition so far, limitation display at the intersection of VoIP and gateway device of system mainly, one concentrate the intersection of signalling and deal with and the intersection of media and " loud " of handling capacity on The gateway begins to make inadaptation network more large-scale development. Then IETF has put out MGCP agreement timely, the full name is " control protocol of the media gateway " ,Intend to enable " It is big " Deal with and deal with separating with the media in signalling of the gateway, but the limit of MGCP makes IETF put out MEGAC O quickly, in order to solve the intersection of MGCP and flexibility and augmentability, support the intersection of multimedia and scheduling problem, MEGACO to there was H.248 agreement that ITU-T was deliver formally later on.
H.248 and MGCP have much similarity at the intersection of agreement and concept and structure, main thought of them " large " in the so-called H.323 Gateways are resolved into two parts: MG (media gateway) And MGC (media gateway controller) . The function of MGC is: (l)Deal with H with gate keeper. 225RAS news; (2)It is (available) to deal with No.7 signalling ; (3)It is (available) to deal with H.323 signalling . The function of MG is: (l) Termination point interface of I P network; (2)Termination point interface of the circuit switching network; (3)Deal with H.323 signalling (in a kind of resolving) ; (4)Dealing with has RAS (registerationadmissionstatus) Circuit switching signalling of the function (in a kind of resolving) ; (5)Handle the media flow. Define from MGC and function of MG we can find out traditional the intersection of video-information and MCU and hinder " loud " that VoIP develop from for a time in the system The gateway is no matter just in position or network protocol of support in the network, it is very similar. And already put forward MCU in H.323 actually and is divided into two partial concepts, but the concept. How divide MCU, what kind of structure is network separate, how go on, control and manage, most important, through what kind of agreement Communication comes two parts of MCU after separating, these questions will turn into and hinder the expanded bottleneck of the network size as large gateway at that time too after the network size of the video-information meeting is expanded.
The technical rapid development of Communication does not allow such a situation to exist. Because H.248/MeGaCo agreement is simple, powerful, and augmentability is very good, have been widely paid close attention to after putting out, and used widely rapidly. One that is about H.248 supplementary to propose and expand, make, put out constantly even. ITU-T propose Ericsson one about solve the intersection of MCU and draft T01-SG16-021 of separation problem at the meeting of 16th will it be October 200 2 year 015-D-0281-" H.248.MCUMultipointControl Unit" ,Regarded as H formally after this draft did more detailed supplement the meeting in March of 2003. 248.19 drafts are proposed, T01-SG16-030520-TD-WP2 -0017-" DecoMPosedMultipoint ControlUnit, Audio, VideoandD ataConferencingPackages" ," the multi-point control unit is separated, the sound, picture and data meeting are chartered " .
The purport that H.248.19 puts forward is: MCU can be resolved into MC (the multi-point control device) And MP (multi-point processor) . Generally speaking, MC and MP Apparatus structures are different, MC can form with computer and protocol software, it has no relations with handling the media, lax too with the number of users. MP correlates with media, it is formed by specialized hardware, closely related to number of users. The separation of the two will have a very great advantage no matter from network design and network optimization, and can give play to the specialty by the division among specialized departments from the angle of Manufacturing industry. Adopt H.248 agreement to manage and control between MC and MP. And VoIP system architecture after the network architecture after separating can consult the gateway to separate, can allow to set up a plurality of subregions MP (as shown) on the call control layer . This kind of common control, decentralized processing mode, can give full play to the advantage of the packet-switched network, make the network possess the good expansibility at the same time.
H.248.19 has put forward the way that MC separates with MP, has supplemented the news parameter about conference control lacked in H.248 agreement, have defined media's synchronous method and compounded the media to spread and give the way. Though the bag that H.248.19 defined has been already more detailed and more feasible for convening, to whole network connection of the video-information meeting, only define parameters about carrying on in the course at the meeting are not obviously enough. Because of the complexity of the conference system, generally speaking video-information meetings run try hard to use one MCU at present, but when the meeting is large in scale, do not have
No cascade. After MCU separates, the advantage brought at first can try hard to reduce MC cascade, a meeting only uses one MC, but MP can consult the present media gateway, or the so-called IAD (IAD) ,Users adopt the way of inserting nearby. In this way, topological diagram convene meeting, by star shaped instead of complicated the intersection of number and the intersection of picture and form, no matter conference control, it had simplification to a great extent that conference management or media dealt with.
But these advantages are not absolute, MCU has separated and brought a lot of questions, for instance the revising of news parameter, change of the Communication procedure,etc., but there are two master problems: First, the problem of management of MP, second, the safety problem of MP.
Problem of management of MP
The problem of management of MP comes from the autogenous characteristic of the conference system. The meeting is the multipoint communmication, there is the difference in this point and VoIP, at the time of point-to-point communication, the media gateway is only responsible for inserting users nearby, receive the control of corresponding MGC; But multipoint communmication, one MP may need simultaneous processing to be in a plurality of users of a plurality of meetings, these a plurality of meetings do not probably go on on the same MC, in this way MP needs to accept the control of a plurality of MC. It seems that the question seems to be a little great like this, because accept that it usually means the confusion to control in many ways, but the fact is not like this. When one MP needs to support the meeting that a plurality of different MC convenes at the same time, MP must can support to accept a plurality of MC controls with virtual MP, but the top management right of MP is still in its acquiescence MC. In fact H.248 has already considered this question while making, has just never provided the concrete application mode. In the real application, MP starting up register, remain to default MC its registering, by MC offer corresponding GK at message of MP; Need MP participate in meeting at other MC, notify until what kind of by default MC its ability and form register to other MC; After the meeting, and then require MP to withdraw from other MC by its default MC. MG in H.248 accepts the control of MGC, uses mID (mean the news promoter) Such a parameter, fictitious MP need offer designated MC to be right away getting all right at the fictitious mID of it only.
The safety problem of MP is actually a public problem of all telecommunication equipment, but the reason why put forward here alone, because MP puts forward MCU and separates a piece of development after the concept. But since position and ability and MG of MP are so alike, the safety problem can be drawn lessons from and solved naturally. In fact generally speaking the safety problems should all mention to use H.235, but before H.235 can really be used, a lot of ciphering algorithms are used as media's gateway device safety measure, these measures are used to the video-information system to hardly need changing.
The management of concrete MP and settlement of safety problem reflect in the course of registering and authorizing, only need to expand the parameter of ServiceChange correspondingly.
MP sends ServiceChange to register to MC, if is the registration of starting up, TerminationId in ServcieChange sets up as Root, , in order to be must be selected, set up as Restart, the item ServiceChang eReason, in order to be must be selected, the item version is available in item Method. There should also be four X word separators used for authorizing in the order: An numeric string MGAUTH, algorithm ID and random number Rand used for digital signature; Also used in A that DH exchanged; If fictitious MP registers to other MC, then mID numbers for fictitious MP, there should also be three X word separators that is used in registering in the order fictitiously: Point out the address of MP controlled MC, the meeting number joined, the number of required termination point of meeting joined, ServiceChangeMethod increases by one, is used for pointing out to the other land MC registrations or cancelling.
According to technological angle, MCU can be realized while separating and the trend of the times. Because the video-information business certainly will carry out the transition to the network of future generation, the separation of MCU accords with thoughts separated with the media to the network signalling of future generation put forward now very much. But there is not a producer realized yet at present, there are a lot of reasons, but the major cause is now the market of the video-information meeting is not very big. Though 911 of U.S.A. and SARS of 2003 make the voice of Communication of video-information higher and higher, but there are many genuine popularization of video-information business hindered. This awaits Apparatus manufacturers, telecommunication operator, joint efforts in many ways such as the content provider,etc.. But the advantage that MCU separates is obvious, believe with the constant enlargement of the meeting business scope of video-information, superiority will show obviously. And with the constant completion and expanding of H.248 agreement, it can really be applied to the video-information meeting apparatus, will generate an important impact on popularization of the meeting business of video-information.
