Evolution of Circuit switches to media gateway controllers

Softswitch is a key technology for implementing next-generation networks. Media Gateway controller is a device that uses Softswitch technology to implement call control. This article mainly introduces the function and system structure of the Media Gateway controller, and discusses a solution for evolution from a circuit switch to a media gateway controller.

I. SoftSwitch and media gateway Controller　

Softswitch is the core of the Next Generation Network NGN) to implement call control. It provides Call Control and Connection Control for businesses with real-time requirements of the Next Generation Network. According to the next generation network hierarchy in China, Softswitch is positioned on the control layer. In the latest group communication structure reference of the International Group Communication Alliance (IPCC), Softswitch is located at the business layer of the access plane. Its main task in the next generation network is to establish relationships between different terminals. These relationships can be simple calls or complex processing relationships. The business to be processed includes voice, data, video, and multimedia services. The core idea of Softswitch is to break down a traditional circuit switch into different layers by function, and connect different layers through open interfaces. The call control function is equivalent to the call control function of a traditional switch. However, the call control function implemented by Softswitch is unrelated to the service. That is to say, Softswitch provides basic call control that can process various services, including call routing, management control, Connection Control, establish session and remove Session), signaling intercommunication, such as from SS7 to IP) and so on. The Call Control Function of a traditional switch is combined with the service. Different services require different call control functions.

The reason why Softswitch is different from the hard exchange between the traditional telephone network and the ATM network is that the softswitch only exchanges the IP addresses of the caller and the called party, including IP addresses and UDP ports, the end-to-end connection does not require any circuit switching unit. At the same time, the user's voice stream is implemented by the lower-layer bearer network IP network, while the IP network is a non-Connection Network Based on Packet Switching and supports end-to-end transparent access. Because the Softswitch technology separates business implementation functions and uses open application interface APIs, the service provider can provide more services based on unified and open interfaces, it also facilitates the flexible introduction of new services in vswitches. Softswitch will facilitate the integration of traditional PSTN networks and IP networks and facilitate the smooth evolution to the next generation network.

Soft switches are a specific implementation of Softswitch technology. Media Gateway Controller MGC: Media Gateway Controller) is a type of soft switch. It is a concept proposed after the IP Telephone Gateway is decomposed. In the IP telephone system, the IP Telephone Gateway is used to adapt to the PSTN network and the IP network. With the development of the IP telephone system, it is found that the IP phone user voice stream IP phone user plane) and the IP phone connection control plane) there is no physical connection between the two. They are only associated with the IP address at the transport layer, so they do not need to be physically put together, resulting in the decomposition of the IP Phone gateway and the generation of H.248/Megaco suggestions. The IP Telephone Gateway can be divided into three parts: the media gateway MG), the Media Gateway controller MGC), and the signaling gateway SG, the media gateway is responsible for media transformation and connection between the PSTN and the IP addresses. The signaling gateway is responsible for signaling conversion, and the Media Gateway Controller is responsible for controlling the establishment and release of the Media Gateway connection based on the received signaling. H.248/Megaco recommends that the gateway controller control the gateway. The first advantage of this architecture is the separation of call and bearer control, which is also the core of Softswitch technology. Media Gateway controller MGC) is the source and end point of signaling messages. It controls the entire network through multiple protocols: monitors various resources and controls all connections, and is also responsible for user authentication and network security. The media gateway controller also controls the media gateway and the signaling gateway, and converts the corresponding signaling to communicate with different networks.

Ii. Functions of the Media Gateway Controller

1. function structure of the Media Gateway Controller　

The media gateway controller is a collection of multiple logical functions and provides comprehensive call control, connection, and business functions.

2. Main Functions of the Media Gateway Controller

The main functions of media gateway controller include call control, service provision, service exchange, protocol, intercommunication, resource management, billing, authentication and authorization, and address resolution. voice processing control, control and management of various terminals, and signaling 7. Some of the functions are briefly described below.

1) Call Control Function　

Provides control functions for the establishment, maintenance, and release of basic calls, including call processing, Connection Control, Intelligent Call trigger detection, and resource control. It can receive monitoring requests from the service exchange function and process the call-related events. Receives call control information from the service exchange function and supports call establishment and monitoring. Supports basic two-party call control and multiple-party call control functions. Provides multiple-party call control functions, including the special logical relationship of multiple-party calls, call Member addition, exit, isolation, listening, and sound mixing process control. The Softswitch device should be able to identify events such as user disconnection, dial-up, and dismounting reported by the media gateway, control the media gateway to send various audio signals to users, and provide numbering solutions meeting the needs of operators. It should also have the function of supporting various terminals to connect to the signaling gateway.

2) business functions　

In the evolution from circuit switching to group switching, end users should have full inheritance of services. Therefore, MGC must be able to implement all services provided by the PSTN/ISDN switch, including basic services and supplementary services, as well as to provide intelligent service in combination with the existing intelligent network. In addition, MGC should be able to provide new services such as mobile and multimedia services. At the same time, MGC should provide programmable and open api interfaces to achieve interconnection with external application platforms, so as to facilitate the introduction and development of new businesses.

3) Business Exchange Function

The service exchange function is combined with the call control function to provide a series of functions required for communication between the call control function and the Service Control Function SCF. The business exchange functions mainly include:

◆ Recognition triggered by business control and communications with SCF

◆ Manage the signaling between the call control function and SCF

◆ Modify the call/connection processing function as required to process IN business requests under the control of SCF

◆ Business interaction management

4) protocol functions　

As an open, multi-protocol entity, standard protocols must be used to communicate with various media gateways, terminals, and networks. These protocols include: h.248, SCTP, ISUP, TUP, INAP, H.323, RADIUS, SNMP, SIP, M3UA, MGCP, BICC, PRO, BRI, etc.

5) interconnection functions　

The signaling gateway can be used to achieve intercommunication between the branch network and the existing Signaling Network No. 7. The signaling gateway can be used to communicate with the existing intelligent network to provide users with a variety of intelligent services. The SCF can be used to control VoIP calls, and call information operations, such as number display, can be achieved through the intercommunication module, the use of H.323 protocol to achieve intercommunication with the existing H.323 system IP telephone network, the SIP protocol is used to achieve intercommunication with the future SIP network system. It can be interconnected with other Softswitch devices, and the protocol between them can be SIP or BICC; provides interconnection between H.248 terminals, SIP terminals, and MGCP terminals in the IP network.

4) Resource management　

Centrally manage various resources in the system, such as resource allocation, release and control, resource status detection, and resource usage statistics.

5) billing　

Collect detailed information about call records, and send tickets to the billing center through the RADIUS protocol or other methods. It also supports real-time billing, and determines the call duration based on your account balance to achieve real-time disconnection.

6) authentication and authorization functions

It can communicate with the authentication center through the RADIUS protocol and authenticate the information of end users and media gateways under its jurisdiction to avoid illegal access.

7) Address Resolution　

To communicate with various networks such as PSTN, H.323, and SIP, you need to support multiple address formats and complete mutual conversion between them. The address resolution function is completed by the internal address resolution module of the Media Gateway controller.

8) management and maintenance functions　

The media gateway Controller provides three network management interfaces: CLI, Web, and SNMP. Maintenance personnel can manage them through serial ports, Web browsers, and SNMP management systems. Management functions include configuration management, performance management, fault management, and security management.

9) voice processing control function

It can control whether the media gateway adopts voice compression, and provide the audio compression algorithm that can be selected. It can also control whether the media gateway adopts the echo cancellation technology. It provides the media gateway with the voice packet cache area, to reduce the impact of jitter on speech quality.

3. Media Gateway Controller Interface Protocol　

As the control center of the NGN network, MGC must communicate with devices of different levels through various open interface protocols. There are many interface protocols, which are briefly described below:

1) interface protocol between MGC and media gateway MG: MGC controls and manages the bearer control resources of media gateway, including MGCP and H.248/MEGACO.

2) interface protocol between MGC and the signaling gateway (SG): used to transmit the signaling information between MGC and the signaling gateway. This interface protocol can use the SIGTRAN protocol. The lower layer of SIGTRAN uses the SCTP protocol to provide reliable connections for TCP/IP network transmission of signaling 7. The higher layers are classified into M2PA, M2UA, and M3UA.

3) interface protocol between MGC and MGC: used for interaction between different SoftSwitch. There are BICC protocols and SIP-T protocols.

4) interface protocol between MGC and the application/service layer: Provides interfaces for accessing various databases, third-party application platforms, and various functional servers, supports various value-added services, management services, and third-party applications. For example, the interface protocol between MGC and the application server is SIP or API, which provides support for third-party applications and various value-added services. MGC implements the network management interface protocol as SNMP protocol.

Iii. Evolution from circuit switches to MGC　

The Next Generation Network (NGN) will be an IP-centric full-service network that supports the integration of voice, data, and multimedia services. The process from the current circuit switching network to the next generation network will be a long-term gradual process. There are two main transitional modes:

1) Business node replacement mode: this means to gradually replace the Business node devices in the existing network to complete the transformation to the next generation network.

2) overlapping network mode: it refers to the construction of an overlapping network of IP addresses that coexist with the existing telecommunication network. The two networks are interconnected first, and then users in the current network are gradually transitioned to the next generation network.

1. Softswitch-an important foundation for media gateway and network evolution