introduction
In recent years, with the rapid development of data communications and IP services, IP networks based on packet switching have become more and more widely used because of their simplicity and openness.
Experts have predicted that future telecommunications services will be unified on the IP network. The traditional telephone network will inevitably transition to a converged NGN (Next Generation Network) centered on data services, especially IP services. NGN will take the IP network as the core and carry all communication services including voice through the packet-switched network based on TCP / IP.
1 NGN network system and softswitch technology
NGN takes the packet switching network as the core and aims at transmitting voice, data, and multimedia integrated services, and can complete real-time applications or non-real-time applications. It interoperates with various existing networks, and gradually moves toward integration and unification, is compatible with existing telecommunications services and Internet services, and creates a favorable environment for quickly providing new services.
At present, the industry divides the NGN architecture into four layers: the edge layer, the core layer, the control layer, and the service layer from the vertical direction. Each layer is interconnected through a standard open interface, and is implemented through standard interfaces and protocols. For the interconnection and interworking of existing communication networks, see Figure 1.
In these 4 levels, the control layer can be regarded as the core, which is currently mainly implemented using softswitch. Interconnection with existing wired or wireless networks depends on various MGs (Media Gateways).
The specific functions of these 4 levels are as follows:
1) Edge layer
The main function of this layer is to connect various traditional networks (PSTN, ISDN, IN, H.323, Internet, private network, etc.) and various user terminals to the core packet transmission network to centralize, aggregate and transmit user services. At the same time, the interconnection between NGN and the existing circuit-switched network is realized through various media gateways. Provide a variety of broadband, narrowband, mobile, fixed user access. The main network components are TG (relay gateway), SG (signaling gateway), AG (access gateway) and IAD (integrated access component).
2) Core layer
This layer is a data bearing network that can provide QoS (Quality of Service) guarantee. Its main function is to complete the high-speed exchange and transmission of business information. The main network components at this layer are data exchange and transmission equipment such as broadband switches, high-speed routers, and high-speed optical transmission networks.
3) Control layer
This layer is the intelligent heart of the entire network and a centralized control platform. Its main function is to provide end-user end-to-end call / session control, access protocol adaptation, interconnection and interworking, and resource management to achieve network service control and integration. The main network functions of this layer are soft switch, MGC (Media Gateway Controller), call agent, call controller, call server, etc.
4) Application layer
This layer is the supporting environment for NGN services and services. In addition to providing traditional smart services, it can also provide open and powerful APIs (application programming interfaces) for third-party service developers to call in order to quickly develop new services . This layer consists of two parts, the application and the middleware in the vertical direction. Among them, the main network components of the application part are various AS (application servers), such as AAA (authentication, authentication, accounting) server, PS (policy server) and OSS (operation support system), etc., providing control of various services Logic, complete value-added business and corresponding service processing. Middleware includes software components such as authentication, billing, directory, security, browsing, searching, navigation, and format conversion.
The idea of ​​softswitch technology is to separate business, call control, and media control. The softswitch equipment is located in the layered call control layer, interacts with the gateway of the media layer, receives the relevant information of the terminal, and instructs the gateway to complete the connection control. The main function of MG is to convert the media in one network into the media format required by another network. For example: MG can convert between the bearer channel of the circuit-switched network and the media stream of the packet network. TG is the gateway between the circuit-switched network and the packet network, used to terminate a large number of digital circuits. AG is the gateway that connects the analog line to the packet network.
Figure 2 is a schematic diagram of the system structure of the soft switch equipment. The dotted frame in the figure represents the soft switching equipment entity.
As can be seen from Figure 2, the internal softswitch equipment is mainly divided into functional modules such as resource management function, MG access function, call control function, interconnection and interworking function, and service providing function. Standard protocols are used for all external interfaces. For example, Sigtran (SS7 / IP) protocol is used for interface with SG; Radius protocol is used for interface with AAA server; SIP protocol is used for interface with application server; SNMP protocol is used for interface with network management server; The intercommunication with H.523 network adopts H.323 protocol family; the interface with MG (TG, AG) adopts H.248 protocol; the call between softswitch or the call between softswitch equipment and SIP terminal adopts SIP protocol.
Through this separation, the softswitch network system has many advantages. One softswitch device can control multiple MGs at the same time, and the scalability of the system is improved. Second, the conversion of specific media streams is completed by the corresponding MG, which is beneficial to The unity and reliability of the equipment; when a soft switch equipment fails, it can be replaced by other soft switch equipment to complete the control of the MG, which improves the redundancy of the system; finally, a standard protocol is adopted between the soft switch equipment and the MG Control is conducive to competition and cooperation between manufacturers of different equipment.
2 Application of H.248 protocol in NGN
2.1 H.248 protocol structure
H.248 / Megaco protocol (MG control protocol), referred to as H.248 protocol, is a non-peer-to-peer protocol established by IETF and ITU-T, and is used for communication between MGC and MG. The main function is to establish a good service bearer connection model, separate the call and bearer connection, and realize the packet network and PSTN (public switched telephone) through the management of various service gateways (TG, AG, RG (registered gateway)), etc. Network) business interoperability.
An H.248 message can be divided into several layers. Layer 1 can be regarded as a message header and several transactions. The transaction can be a transaction request (TransacTIon Req) or a transaction response (TransacTIon Reply). Each transaction can be seen as a transaction header and a number of actions, each action is related to a context. An action (AcTIon) includes a context header and several commands. Each command (Command) contains the command header and several descriptors. This structure is shown in Figure 3.
Transactions guarantee the execution of sequential commands, that is, in a transaction, the commands are executed in order. When all commands are successfully executed, the transaction is successfully executed. When one of the commands fails, the entire transaction fails.
Action is closely related to the context. It consists of a series of commands limited to one context. Within an action, commands need to be executed sequentially.
The command is the main content of the H.248 message, which realizes the control of the context and terminal attributes, including specifying what signals and actions the terminal reports events can be applied to the terminal, and specifying the topology of the context.
The signal means that something will happen to the terminal, such as sending a sound or displaying a text message. The signal is specified by the softswitch through the signal descriptor, and its duration can also be specified. Under normal circumstances, when the terminal detects an event, it will automatically stop the playback of the signal.
2.2 Call flow description
Due to space limitations, this article only describes the typical analog user-initiated calls in flowchart form. The flow chart of call establishment for simulated users is shown in Figure 4. In the figure, it is assumed that MG1 and MG2 belong to the same softswitch.
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