Protocol learning: H.323

Author: gnuhpc
Source: http://www.cnblogs.com/gnuhpc/

1. h323 is a VoIP Signaling Control technology that was first adopted by the VoIP standard. It defines the system logic components, message definitions, and communication processes for two-point/multi-point real-time media communication based on the sub-network.

2. basic organization unit "Domain": In the H.323 system, the domain refers to a gateway, multi-point control unit (MCU), Multi-Point Controller (MC) managed by the Customs) A collection of multi-point processor (MP) and all terminals. A domain contains at least one terminal, and there must be only one terminal. Similar to a gateway in ancient China.

2. Architecture and logical components:

Including the gateway controller (Gateway Controller), MCU, gateway, and proxy ). The gateway serves as a bridge between the PSTN and the IP network. The terminal, gateway, and multi-point control unit (MCU) are terminal devices in H.323 and logical units in the network. Terminal devices are callable and called.

3. Communication Mechanism with the PSTN network:

Calls from the PSTN are accepted by the H.323 gateway. The audio streams are converted into IP data packets and transmitted to the specified telephone terminal under the destination gateway through the IP network; at the same time, the call from the IP network (from the computer terminal or another PTT Network) can be transferred to the PSTN destination terminal.

 

4. Protocol features:

Based on the concept of network guard, the H.323 network is centrally controlled to facilitate unified maintenance and management. However, an obvious drawback is that the call processing latency is large and the network size is greatly limited.

5. Component details:

1) Terminal)

A node device that provides real-time and bidirectional communication in a group network. It is also an end user device that communicates with gateways and multi-point access control units and must support voice communication. video and data communication are optional.

2) gateway (gageway)

Convert media information and messaging information. That is to say, if one of the communication terminals is not an H.323 terminal, the gateway will convert the transmission formats between terminals.

3) multi-point control unit (MCU)

It includes command Multi-Point Controller (MC) and optional multi-point processor (MP). The former processes H.245 control information between terminals, thus determining its processing capabilities for videos and audios. When necessary, you can determine which video streams and audio streams need to be multicast to control the resources used by the conference system.
MC does not directly process any media information, but leaves it to MP for processing. MP performs mixing, switching, and other processing of audio, video, or data information. Multi-point processor and Multi-Point Controller may exist in a dedicated device or as part of another H.323 component, but MCU is generally an independent unit device and has its own transport layer address.

4) gagekeeper)

A network guard manages a group of terminals, gateways, and MCU. This group is called a region. A region is a logical connection composed of these elements, which may physically span multiple LANs. In addition, it manages H.323 endpoints and calls, such:

Address Translation

Call Admission Control Service

Bandwidth control and management

Call control signaling

Call permission

Call Management

Network Management

6. protocol stack structure

H.323 includes different standards and references other standards.

For audio applications, g.711 is required. You can also select other G series suggestions.

For video applications, H.261 and H.263 are used.

Provides data support through the T series.

H.225.0 and H.245 are the core protocols of the H.323 system. The former is mainly used for call control, and the latter is used for media channel control.

Various control, signaling, and maintenance operations are provided by H.245, q.931, and network guard specifications.

Audio and video groups must be encapsulated in Real-Time Protocol (RTP) and transmitted through a UDP socket pair between the sender and the receiver.

The Real-time Control Protocol (RTCP) is used to evaluate the quality of sessions and connections and provide feedback between communication parties.

7. RAS (registration, admission and status) Protocol

1) Definition: The Protocol executed between the endpoint and the network guard. It provides the network guard with functions such as determining the endpoint address and status, and implementing call acceptance control. These functions use H.225 messages to provide a series of supported operations.

2) functions:

A. network Guard Search: the endpoint searches for its own network guard, and the multicast mechanism is used to complete the search. After that, all Ras messages are sent between the endpoint and its own network guard, which is divided into manual and automatic modes, manually configure the gateway and preset the transport layer address of the gateway to the configuration file or initialization file. The automatic mode allows the relationship between the endpoint and its home location to change over time. When the original network guard fails, it can automatically switch to the replaced network guard.

B. endpoint registration: Used for the endpoint to register its own information with the network guard, mainly the alias and the call control channel transport layer address. This includes removing the registration process. The endpoint must be registered on the network guard identified during the search process. It must be registered before it can initiate and receive calls. Registration indicates that the endpoint has been added to the management area.

C. endpoint positioning: used by the endpoint or the network guard to ask the transport layer address of the call control channel at a certain end point to the corresponding network guard.

D. Call acceptance: when initiating a call, you must first ask the network guard whether the call can be initiated.

E. Call Exit: The network guard is notified after the call ends, and the endpoint has exited the call.

F. bandwidth management: the endpoint puts forward the bandwidth change request to the network guard during the call process.

G. status query: The network guard queries the terminal's startup/shutdown status

H. Gateway status resource indication: notifies the network guard of available resources of the gateway.

8. q.931 Protocol
1) Definition: as the network layer protocol of the telecommunications system, it mainly provides the establishment, maintenance and termination of calls between two devices for logical network connections.

2) function: Provides end-to-end signaling to ensure normal communication between H.323 terminals.

9. RTP protocol:

1) Definition: A transmission protocol for multimedia data streams over the Internet. Generally, UDP is used to transmit data. If the underlying network provides multicast allocation, then, RTP can use this multicast allocation to support data transmission of multiple destination files.

2) Functions

Provides the Net Load type indication (that is, the Data Type and encoding method)

Data Group No.

Data sending Timestamp

Data source ID

10. RTCP protocol --- RTP Control Protocol

1) Definition: uses the same distribution mechanism as Data grouping to periodically send control groups to all participants in the RTP session, so as to provide QoS detection methods for data transmission and obtain information about participants.

2) function: it provides feedback on the quality of data transmission. The feedback information is used to control adaptive coding and diagnose data allocation faults. This is closely related to the traffic control and congestion control functions of other transport layer protocols.

A. Transmit the permanent identification of the RTP source transport layer, and implement synchronization between multiple media sets.

B. determine the RTCP group sending rate. The RTCP group needs to be sent regularly. In the case of large conferences, the network will generate considerable traffic control, therefore, the sending rate of the RTCP group must be determined based on the available bandwidth and meeting Scale.

C. Transmit a small amount of session control information.

11. H.245 Protocol

1) Definition: A general multimedia communication control protocol used to control the establishment, maintenance, and release of communication channels.

2) functions:

A. Determine the master-slave relationship: Determine the master-slave relationship of the call to avoid conflicts during the signaling process.

B. capability exchange: exchange the ability to receive and encode media data before transmission, and confirm that the sender knows the receiving capability of the local end.

C. Logical Channel Signaling: The validation protocol is used to enable and disable the Logical Channel, ensuring that media signals and data are transmitted over the logical channel.

D. Receiver's request to disable the Logical Channel: this allows the receiver to initiate a request to disable the logical channel. The sending terminal can accept the request or reject the request.

E. Round-trip latency determination: provides a mechanism to measure the latency and checks whether the remote terminal is working.

F. Loop maintenance: A required function of the gateway for loop testing.

G. Other commands or instructions: such as traffic control commands, multi-point mode commands, user input instructions, etc.

Appendix: Glossary

SCN: Circuit Switching Network

PBN: Group Network

Cname: Also known as canonical name. This record allows you to map multiple names to the same computer. It is usually used on computers that provide both WWW and mail services. For example, there is a computer named "host.mydomain.com" (a record ). It provides WWW and mail services at the same time, in order to facilitate user access to the service. You can set two aliases for this computer (Cname): WWW and mail. The full names of these two aliases are www.mydomain.com and mail.mydomain.com ". In fact, they all point to "host.mydomain.com ".

Author: gnuhpc
Source: http://www.cnblogs.com/gnuhpc/