introduction
In recent years, with the rapid development of data communication and IP business, the IP network based on group exchange is simple and open, and it has been made in more and more extensive applications.
Experts predict that future telecom services will be unified on the IP network. Traditional telephone networks will inevitably transition to NGN (next-generation networks) centered on data services, especially IP services. NGN will use the IP network as the core, and carry all the communication types including the voice in a TCP / IP-based packet switching network.
1 NGN network system and softswitch technology
NGNs can complete real-time applications or non-real-time applications in packet switched networks as the core to transmit voice, data, multimedia integrated services. It communicates with existing networks and gradually integrates and uniform, compatible with existing telecommunications services and Internet services, and creates a favorable environment for fast providing new business.
At present, the industry is divided into an edge layer, a core layer, a control layer, and a business layer 4 floors in the vertical direction, and through the standard open interface between the various layers, and implements the standard interface and protocol. Interconnect and interconnection of existing communication networks, see Figure 1.
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In these 4 levels, the control layer can be considered as the core, and is mainly to be implemented in a softswitch. Interconnection interconnect with existing wired or wireless networks relies on various MG (Media Gateways).
The specific features of these four levels are as follows:
1) Edge layer
The main function of this layer is to connect various traditional networks (PSTN, ISDN, ISDN, IN, H.323, Internet, private networks, etc.) and various user terminals to the core packet transfer network, and collect, aggregate, and transfer for user services. At the same time, the interconnection interconnection between NGN and existing circuit switched networks is achieved through various media gateways. Provides access to various broadband, narrowband, movement, and fixed users. The main network components are TG (relay gateways), SG (signaling gateways), AG (access gateways), and IAD (integrated access components), and the like.
2) Core layer
This layer is a data bearer network that provides QoS (Quality) guaranteed, the main function is to complete the high-speed exchange and transmission of business information. The main network components of this layer are exchange and transmission equipment such as broadband switches, high-speed routers, high-speed optical transmission networks.
3) Control layer
This layer is the smart heart of the entire network, which is a centralized control platform. Its main function is to provide a network service control and fusion of the end user-ended call / session control, access protocol adaptation, interconnection interconnection and resource management. The main network function of this layer is a soft exchange, MGC (Media Gateway Controller), Call Agent, Call Controller, Call Server, and the like.
4) Application layer
This layer is the support environment of NGN business and services. In addition to providing traditional intelligent business, it can also be called by providing open, powerful APIs (Application Programming Interfaces) for rapid development of new services. . This layer consists of two parts of the application and the middleware in the vertical direction. Among them, the main network components of the application part provide various services to all kinds of AS (application servers), such as AAA (authentication, authentication, billing) servers, PS (policy servers), and OSS (operating support systems), etc. Logic, complete value-added services and corresponding service processing. Middleware includes software components such as authentication, billing, directory, security, browsing, lookup, navigation, format conversion.
The idea of soft exchange technology is to separate business, call control, and media control. The soft switching device is located in the layered call control layer, interacting with the gateway of the media layer, receives information about the terminal, indicating the gateway to complete the connection control. The main function of MG is to convert media in a network into the media format required by another network. For example, the MG can be converted between the carrier of the circuit switched network and the media stream of the packet network. The TG is a gateway between the circuit switched network and the packet network to terminate a large number of digital circuits. AG is a gateway that connects the analog line to the packet network.
Fig. 2 is a schematic diagram of the structure of the soft switching device. The dashed frame in the figure indicates a softswitch device entity.
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As can be seen from Figure 2, the soft switching device is mainly divided into resource management functions, MG pick-up functions, call control functions, interconnection, and other functional modules such as service providing functions. All standard protocols are used in the external interface, for example, the interface with the SG uses the SigTRAN (SS7 / IP) protocol; using the RADIUS protocol with the AAA server; uses a SIP protocol with the interface of the application server; the interface with the network management server adopts the SNMP protocol; Interoperability with the H.523 network adopts the H.248 protocol; the interface between the MG (TG, AG) is used; the call between the call or softswitch and the SIP terminal is used by the SIP protocol.
Through this separation, the softswitch network system has many advantages. A soft switching device can control multiple MG at the same time, and the expandability of the system is improved; secondly, the conversion of the specific media stream is completed by the corresponding MG, facilitating Singleness and reliability of the device; when a softswitch is faulty, it can be replaced by other softswitch devices to complete the control of the MG, improve the redundancy of the system; finally, the standard protocol is used between softswitch devices and MG Control, manufacturers who are conducive to different equipment to conduct competition and cooperation.
Application of 2 H.248 protocol in NGN
2.1 H.248 protocol structure
H.248 / Megaco protocol (MG Control Protocol), referred to as the H.248 protocol, is an unsuccessful protocol that IETF, ITU-T, is used in communication between MGC and MG. The main function is to establish a good service carrying model, separating the calls and bearer connections, achieve packet network and PSTN by management of various business gateways (TG, AG, RG (registration gateway)), etc. (public exchange call The business interoperability made.
A H.248 message can be divided into several layers, and the 1st layer can be seen as a message head and several transactions, and the transaction can be a transaction request (Transaction Rece). Each business can be seen as a transaction head and several actions, each action is related to a context. A action (Action) includes a context header and a number of commands. Each command (Command) contains the command head and several descriptors. This structure is shown in Figure 3.
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The transaction guarantees the execution of the order command, that is, in a transaction, the command is executed in order. The entire transaction fails when all commands successfully executed when the commands are successfully executed. When one of the commands fails, the entire transaction fails.
Action is closely related to the context, which consists of a series of commands that are limited to a context. During an action, the command needs to be executed.
The command is the main content of the H.248 message, implements control of context and terminal attributes, including what signals and motions that specify terminal report events can be applied to terminals, and specify topology of the context.
The signal means that some things happen, such as sending or displaying text messages. The signal is specified by a soft exchange through a signal descriptor, and it can be specified for its duration. In general, the playback of the signal is automatically stopped when the terminal detects an event.
2.2 Call Process Description
Due to the limited space limit, this article only initiates a call to the typical simulated user in the form of flowchart. The simulated user call setup flow diagram is shown in Figure 4. In the figure, it is assumed that MG1 and MG2 are affiliated with the same softswitch. Technology area
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