Video Conference Agreement Research Report
Summary
This article introduces the currently popular H.323 and SIP protocol, analyzes the advantages and disadvantages of the two, using the reference to the E-CP selection agreement
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At present, the main standards of IP network communications have H.323 and SIP. Both have a complete solution to IP telephone system signaling, and H.323 uses traditional telephone signaling mode, including a series of protocols. And the SIP draws on other Internet agreements, using the text-based agreement, and the two agreements are elaborated and compared.
H.323 agreement
I. System composition of the H.323 architecture
As shown in Figure 1, the video conference network using the H.323 architecture is mainly composed of terminal, gatekeerage (GK), gateway (GW), and multipoint control unit (MCU):
Figure 1 Structure of video conferencing system based on H.323 protocol
1. Terminal: The terminal is a node device that provides an immediate, two-way communication function. The main function of the terminal is to collect video / audio signals. After processing, the MCU or other terminal is processed, and the video / audio signal is received, and the processing is sent to the corresponding output device.
2. Neighborhood: The gatekeepers are managers in a domain, play an important role in the system, and its main features are:
1) Certification accounting: Collect authentication billing information, and give the authentication request, billing request to the AAA server (authentication, authorization, and billing) with the RADIUS message
2) Address resolution: An alias address that will be given to the gatekeeper is parsed as an IP address
3) Domain management: The collection of all terminals, gateways and multi-point control units under single gatekeeper management, called us. The gatekeeper is responsible for the management of terminals, MCU, GW and other devices in the domain;
4) Bandwidth Management: The gatekeeper can set the user bandwidth within a viable range of the total network total bandwidth.
3. DVD: Take the H.323 agreement and other non-H.323 agreements, so that H.323 terminals and other non-H.323 terminals can communicate. The H.323 system can be compatible with a variety of terminals through GW to protect existing investments.
4. MCU: MCU is a device-specific device, consisting of two parts, part of the MC, mainly responsible for processing control information in the conference; another part is MP, mainly used to process audio, video, information. MC and MP can be physically a device, or a separate device.
Second, H.323 Agreement Standard
H.323 is an agreement, including the following criteria:
Audio compression encoding: H.323 The terminal must support G.711 voice standards. Support other ITU standards, such as G.722, G.723.1, G.728, G.729 is optional
Video compression encoding: H.323 The terminal must support H.261 codec standards; support H.263 standard is an optional feature. H.263 is primarily used for low-rate video encoding at the beginning, and later H.263 is recommended based on the H.261 proposal, the code rate and improves coding quality, and the current application of H.263 becomes more widely.
Media Transfer: H.225.0 The standard describes the packaging packet and synchronous transport mechanism of the media stream on the LAN without QoS guarantee. H.225.0 Formatting the transmitted video, audio, data and control flows, outputs to the network interface, and compensates for the received video, audio, data and control flow from the network interface input packet. In addition, it also completed logically frame, sequential number, error correction and error detection. At the same time, H.225.0 also includes two parts: Q.931 / H.225.0 and Ras / H.225.0. Q.931 / H.225.0 is responsible for initiating calling and establishing media streams; RAS / H.225.0 is responsible for device authentication and collection of billing information.
Multimedia Communication Control: H.245 Agree Defines four information, responsible for communication capabilities, open / close logical channels, and control in the meeting. Multi-point controllers should follow the H.245 control agreement to manage the meeting.
Flow delivery: RTP / RTCP. RTP Agreements are used to instantly transmit media stream information, instant Agreements (RTP), and Instant Control Agreements (RTCP) collaborative work, RTCP is responsible for monitoring RTP. These protocols (and H.245) are also working with IP multicast to ensure UDP The accurate error of the group timing. RTP processing timing problem is to make time tag and sorting each UDP packet transmitted, and timely, the transmitter is plus audio and video stream synchronization information, desire data rate, desired packet rate And the distance and other information. SIP protocol
I. Introduction to the SIP Agreement
Session Initiation Agreement (SIP) is a signaling protocol for initial, managed, and terminating voice and video sessions in grouping networks. Specifically, it is used to generate, modify and end a session between one or more participants. . The so-called session means the information exchange between users. In an application based on the SIP agreement, each session can be a variety of different information, which may be ordinary text data, or a digitized audio, video data, can also be a data such as a game, etc. Huge flexibility.
II. Basic composition of the SIP system
The SIP session uses four main components: SIP user agent, SIP registration server, SIP proxy server, and SIP redirection server. These systems complete the SIP session by transmitting messages including SDP Agreements (for defining the content and features of messages). The following summary introduces each SIP component and its role in this process.
SIP User Agent (UA) is a terminal user equipment, such as mobile phones, multimedia handheld devices, PCs, PDAs, etc. used to create and manage SIP sessions. The user agent client issued a message. The user agent server responds to the message.
The SIP registration server is a database that contains all the user agents in the domain. In SIP communication, these servers retrieve the IP address of the participant and other related information, and send it to the SIP proxy server.
The SIP proxy server accepts the session request of the SIP UA and queries the SIP registration server to obtain the address information of the recipient UA. It then forwards session invitation information directly to the recipient UA (if it is in the same domain) or proxy server (if UA is in another domain).
The SIP redirect server allows the SIP proxy server to direct SIP session invitation information to an external domain. The SIP redirection server can be in a hard body with the SIP registration server and the SIP proxy server.
Example Description:
When the user A invitation is using the multimedia handheld device to perform a SIP session, the SIP proxy server in domain A distinguish the user B is not in the same domain. The SIP proxy server then queries the IP address of User B on the SIP redirect server. The SIP redirective server can be used in domain A, or in domain B, or both in domain A in the domain B. The SIP redirective server feeds the contact information of User B to the SIP proxy server, which forwards the SIP session invitation information to the SIP proxy server in Domain B. The SIP proxy server in domain B sends the invitation information of User A to the user B. User B is then forwarded from the same path via the same path to accept the invitation.
Call user B
2. Inquiry "How do I pick up the user B in the field B?"
3. Respond to the "Domain Orientation of the Agent Controller"
4. "Agent" call domain B's SIP agent
5. Query "Where is the user b?"
6. User B's address
7. Agent call
8. Respond
9. Respond
10. Response
11. Multimedia channel has been established
Three. SIP message structure
SIP draws on all other widely existing Internet protocols, such as HTTP, SMTP, etc., like these agreements, SIP is also used for text-based encoding methods, there are two types of messages:
(1) Request: From the client to the server message. (2) Response: Send the message from the server to the client.
Where request messages include:
· Invite: Initial call, change the parameter (Re-invite).
· ACK: Confirm the end response of the INVITE. · BYE: Terminate the call.
· Cancel: revoke the search and Zhen Ling.
· Options: Query the other capability.
· Register: Register the location service.
· INFO: Send information in the session without changing the session state.
• PRACK: The same is the same as the ACK, but it is used for temporary response.
Subscribe: This method is used to book a notification of its status change to the remote endpoint.
· Notify: This method sends a message to notify the subscriber's change in the state it is.
• Update: Allows customers to update the parameters of a session without affecting the current state of the session.
· Message: Instant messages are implemented by carrying instant messaging content in its requestor.
· REFER: The function is to indicate that the acceptor contacts a third party by using contact address information provided in the request.
The response message contains a number of pixel response code. The SIP response code set part is based on the HTTP response code. There are two types of responses, they are:
· Temporary response (1xx): Temporary response is used by the server to indicate the process, but does not end the SIP.
· Final response (2xx, 3xx, 4xx, 5xx, 6xx): eventually responded to termination of SIP.
Each SIP message consists of three parts:
(1) Start line: Each SIP message starts by the starting line. The starting line conveys the message type (the method is the method type in the request, and the response is the response code) and the protocol version. The starting line can be a request line (request) or a status line (response).
(2) SIP header: Used to deliver message attributes and modify the message meaning. They are the same as the HTTP header field in syntax and semantics (in fact, some heads are borrowed from HTTP), and always keep formats:
(3) Message: Used to describe the initial session (eg, in the multimedia session including audio and video encoding type, sampling rate, etc.). The message can be displayed in the request and response. SIP clearly distinguishes signaling information transmitted in SIP starting lines and headers with session description information outside of the SIP range. Possible body types include the SDP session description agreement to which this article will be described.
SDP is used to describe an agreement to describe multimedia session notices, multimedia session invitations, and other forms of multimedia session initialization. SDP packs typically include the following information:
(1) Session information
· Session name and purpose.
· Session activity time.
Since the resources involved in the session are limited, it includes the following additional information.
· Bandwidth information used by sessions.
· Contact Information of the session of the session.
(2) Media information
· Media type, such as video and audio.
· Transfer protocol, such as RTP / UDP / IP and H.320.
· Media format, such as H.261 video and MPEG video.
· Multi-broadcast sites and media transfer (IP multicast sessions).
· The remote order (IP unicast session) for the contact address of the contact address. SDP description consists of many text lines, the format of the text line is
Use H.323 advantages:
1. H.323 Agreement is earlier, the main agreement is more stable and mature, software products and related support hardware equipment, so it can be very good for software and hardware, and there are many hardware that supports H.323. But the software is relatively small.
Use H.323 Disadvantages:
1. H.323 Agreement includes many agreements more complicated, although OpenH323 open source project reference, due to its
The structure is huge, and the light is studying it takes a long time, and it is lacking RFC-based development.
Inspection, therefore the development is difficult, the development is high, the development cycle is long, and the company has more investment.
2. If H.323 is used as a standard, the communication of the entire product must be done according to the standard, so that a part is meaningless, so because the code of the electronic whiteboard and the screen shared does not do it according to the agreement, the front needs to overthrow, follow the agreement do.
3. Nowadays, there are many hardware that support H.323, software is also, but not a lot, they also claim to implement and hardware interconnects, but we can't confirm that it can be realistic.
4. Since Microsoft and IBM began to pay attention to SIP, if the future h.323 is eliminated or unknown, if we
After H.323, the overall frame is set, due to the difference between the H.323 and SIPs, the structure is very different.
If you want to turn in the direction, it is almost impossible, and you need to overthrow.
Use SIP advantages:
1. Migrant manufacturers have begun to pay attention to and support SIP, and they will be able to achieve interconnection in the future.
Microsoft has selected SIP as its real-time communication strategy and in Microsoft XP, Pocket PC, and MSN
Deployment in Messenger. Microsoft also announces the next version of CE.NET will use the SIP-based VoIP application interface layer and promise to provide SIP-based voice and video calls to the user PC. That is, through SIP, Microsoft's software can implement voice, data, and text exchange between different communication devices in a single network environment based on IP agreement, including computers, mobile devices, handheld devices, and more. Common features related to the phone, such as voice calls, voicemails, voice transfer, etc., will be the function of enterprise networks based on IP protocol, 3GPP selection SIP as the control of the multimedia subsystem of the 3G full IP network Agreement has proved that SIP will have extensive development prospects, which will achieve interconnection of all devices in the future, which is also trend.
2. Because a large manufacturer began to pay attention to the SIP protocol, if the future SIP really becomes a standard, and our property
Hold, then our products will not be able to implement the interconnects of any other products including hardware and software,
Our products may lose the market.
3. The SIP protocol is low relative to H.323 complexity, and the development difficulty should be lower than H.323, and the relative development risk is low.
One, Microsoft now provides SIP client SDK (XP and 2003),
The SDK of the server will be released later, and if it can be used, it is difficult to develop a lot. Use SIP Disadvantages:
1. Agreement is relatively new relative to H.323, the stability of the agreement is not enough, there may be an imperfect place to need to be modified.
In this way, we must also modify the code with the modification of the agreement, and the stability of the product has a big question.
question.
2. Since SIP is a lightweight row agreement, you need to use other agreements, such as SDP protocols, so development efforts increase, increased complexity, although the SIP protocol is not as complex as H.323, but if it is like H.323 According to RFC development, development difficulty and development risks are still very large.
3. SIP has no conference control capabilities, and does not propose MCU concept as H.323, so for point-to-point multimedia instant messaging, it is very suitable, how to apply to multi-person instant messaging is suitable for instant messaging
Existing product use agreements (only from the official website, this is unable to determine its reliability):
H.323 agreement
V2 Conference 4.0
2. NetMeeting
3. AVCON Video Conference System 4.0
4. Net Multimedia Video Conference System Active Meeting 2004
5. WebEx video conference
6. VCON video conferencing system
SIP protocol
1. MSN Messager (will be launched)
2. IBM Sametime
3. Microsoft Live Communications Server 2003
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