Softswitch technology and its application

Abstract: Through careful analysis of the basic principles of softswitch, the main features and functions of softswitch are discussed, the application of softswitch in VoIP is briefly introduced, and the problems that need to be studied in the development of softswitch are discussed.

Keywords: Softswitch Protocol Network Principle Application

With the rapid development of communication network technology, people's requirements for broadband and services are also growing rapidly. In order to provide users with more flexible and diverse existing services and new services, and provide users with more personalized services, the following proposals The concept of a first-generation network, and currently major telecommunications operators have begun to experiment with next-generation communication networks. Softswitch technology is also one of the focuses in next-generation communication network solutions, and has become a hot topic in the industry in recent years. The China Network and Switching Standards Research Group has completed the overall technical requirements framework for the softswitch system, and the 863 project has also initiated research projects related to the softswitch system in multimedia and mobile communication systems.

1 The concept and definition of softswitch

The concept of softswitch originated in the United States. At that time, in the enterprise network environment, users used Ethernet-based telephones to realize the PBX function (IP PBX) through a set of PC server-based call control software (Call Manager, Call Server). For such a set of equipment, the system does not need to lay the network separately, but can achieve the unification of management and maintenance only by sharing with the local area network, the overall cost is much lower than the traditional PBX. Because the enterprise network environment does not have high requirements for device reliability, billing, and management, it is mainly used to meet communication needs, and the device threshold is low. Many equipment vendors can provide such solutions, so the IP PBX application has achieved great success. Inspired by the success of IP PBX, in order to improve the comprehensive operating efficiency of the network, the development of the network is more reasonable and open, and better serves users. The industry has proposed such an idea: the traditional switching equipment is divided into call control and media processing, the standard protocol (MGCP, H248) is used between the two, and the main software is used for processing. ) Technology came into being.

Once the concept of softswitch was put forward, it was quickly recognized and valued by the industry. The establishment of ISC (Interactive Security Switch) has accelerated the development of softswitch technology, and the relevant standards and protocols of softswitch have received IETF, ITTU-T, etc. The importance of the International Organization for Standardization.

According to the definition of the International Security Forum ISC, Security Switch is a device and system based on a packet network that uses program-controlled software to provide separate call control functions and media processing. Therefore, the basic meaning of softswitching is to separate the call control function from the media gateway (transport layer), and realize the basic call control function through software, so as to realize the separation of call transmission and call control, for control, switching and software programmable functions Create a separate plane. Softswitch mainly provides functions such as connection control, translation and routing, gateway management, call control, bandwidth management, signaling, security, and call detail recording. At the same time, Softswitch also encapsulates network resources and network capabilities, and connects to the business application layer through standard open service interfaces, which can easily provide new services on the network quickly.

2 Network structure based on softswitch technology

Softswitch is one of the core equipment of the next generation network. When constructing a network structure based on softswitch technology, each operator must consider the interoperability with other various networks. In the next generation network, there should be a more unified network system structure. The network structure based on softswitch technology is shown in Figure 1.

It can be seen from the figure that the softswitch is located at the network control layer, and the function of separating the call control function and the media processing by using the program control software based on the packet network is well realized.

The interface between the softswitch and the application / business layer provides access to various databases, third-party application platforms, functional servers, and other interfaces to support value-added services, management services, and third-party applications. Among them: the interface between the softswitch and the application server can use SIP, API, such as Parlay, to provide support for three-party applications and value-added services; the interface between the softswitch and the policy server to dynamically intervene in the work of network equipment, can use the COPS protocol; The interface between the softswitch and the gateway center implements network management and uses SNMP; the interface between the softswitch and the intelligent network SCP implements support for existing intelligent network services and uses the INAP protocol.

Through the interaction between the core packet network and the media layer gateway, receive call-related information in process and instruct the gateway to complete the call. Its main task is to establish relationships between points. These relationships can be simple calls or a more complex process. Softswitch technology is mainly used to handle real-time services, such as voice services, video services, and multimedia services.

The interface between softswitches realizes the interaction between different and softswitches. SIP-T and H. 323 or BICC agreement.

3 Design principles of softswitch technology and its realization goals

Softswitch technology is a distributed software system that can provide seamless interoperability between networks based on various technologies, protocols, and devices. Its basic design principle is to try to create a highly scalable, interface The distributed software system with the characteristics of standardization and business openness is independent of the specific underlying hardware / operating system and can handle the synchronous communication protocol required by various businesses well. Push towards the Moore curve track. And it should be able to support the following basic requirements:

(1) Calls independent of protocol and equipment? Call processing and / or simultaneous meeting management application development for smoke equipment.

(2) It can safely execute multiple third-party applications in its softswitch network without any harmful effects caused by malicious or erroneous applications.

(3) Third-party hardware vendors can increase their ability to support new devices and protocols.

(4) Service and application providers can increase the ability to support system-wide strategies without jeopardizing their performance and security.

(5) Ability to perform synchronous communication control to support a variety of after-sales room systems including billing, network management, and other operational support systems.

(6) Support the dynamic topology of the synchronous communication control network that is bound by runtime or contributes to structural improvement.

(7) Network scalability from small to large and supports complete fault recovery capability.

The realization goal of softswitch is to realize the protocol conversion of various media streams by means of computer software programming with the cooperation of media equipment and media gateway, and realize the IP network and ATM network based on the architecture of packet network (IP / ATM) , PSTN network, etc., to provide the same functions as circuit switches and facilitate business value-added and flexible scaling.

The reference model of the softswitch system is shown in Figure 2.

4 Main protocols used by softswitch

There are many protocols involved in the softswitch system, including H. 248, SCTP, ISUP, TUP, INAP, H. 323, Radius, SNMP, SIP, M3UA, MGCP, BIC, PRI, BRI, etc. Internationally, organizations such as IETF, ITU-T, and Switchorg have always played an active and leading role in the research of softswitches and protocols. Many key agreements have been formulated or tend to be completed. These agreements will standardize the research and development of the entire softswitch, and make the product from the stage of using the private agreement of each manufacturer to the stage of using the industry's common standard agreement. It is possible for the products to communicate with each other. Open system structure, each network component can be developed independently. In terms of research progress in softswitching, China is at the same level as the world. The "Network and Switching Standards Research Group" of the Ministry of Information Industry started the research on the softswitch project in the second half of 1999, and has now completed the "Overall Technical Requirements for Softswitch Equipment". In addition, the "IP Standards Research Group" is developing technical specifications for equipment such as relay media gateway (TG), signaling gateway? Point SG? Snow, access gateway (AG), integrated access equipment? Point IAD? Snow, etc.

The following is a brief introduction to several major agreements.

1. H. 248 / MEGACO

H. Both 248 and MEGACO protocols are called media gateway control protocols and are used in media gateways and H. Between 248 / MEGACO and the softswitch equipment. The contents of the two agreements are basically the same, only H. 248 was proposed by ITU, and MEGACO was proposed by IETF, and it was an agreement recommended by both parties. They introduce the two abstract concepts of Terminion (terminal) and Contex (association). In Termination (terminal), the media stream parameters, MODEM, and carrying capacity parameters are encapsulated, and Content (association) indicates the interconnection relationship between some Termination (terminal). H. 248 / MEGACO completes the operation between Termination (terminal) and Connext (association) through eight commands such as Add, Mode, Subtract, and Move, thus completing the call establishment and release.

2. Media Gateway Control Protocol (MGCP)

Media Gateway Control Protocol (MGCP) was proposed by IEFT and is the product of the combination of Simple Gateway Control Protocol (SGCP) and IP Device Control Protocol (IPDC). The MEGACO protocol is a further improvement, improvement and improvement of the MGCP protocol. The MGCP protocol can be said to be a relatively mature protocol. The content of the protocol is similar to the MEGACO protocol. At present, most of the softswitch system equipment supports this protocol, and its shortcomings are gradually manifested. Replaced by the 248 / MEGACO agreement.

In the softswitch system, MGCP protocol and H. The 248 / MEGACO protocol is applied between the media gateway and the MGCP terminal and the soft switch equipment. Through this protocol, the connection, establishment and release of the media / control flow on the media gateway and the MGCP terminal are controlled.

3. Session Initiation Protocol (SIP)

Session Initiation Protocol (SIP) is an application layer control protocol for multimedia communication on IP networks proposed by IETF. Based on the Internet Protocol (HTTP), it follows the Internet's design principles and is based on a peer-to-peer work model. Using SIP can realize the connection, establishment and release of the session, and support unicast, multicast and mobility. In addition, if SIP is used in conjunction with SDP, it can dynamically adjust and modify session attributes, such as call bandwidth, transmitted media type, and codec format. The specific content can be found in IETF RFC 2543bis.

In the softswitch system, SIP protocol is mainly used between softswitch and SIP terminal, and some manufacturers apply SIP protocol between softswitch and application server to provide value-added services based on SIP protocol. In general, SIP protocol is mainly used in the combination of voice and data services, and the establishment and release of calls between multimedia services. In particular, the SIP protocol, with its simple and flexible characteristics, has made 3GPP, which is a mobile communication standardization organization, decided to build a third-generation mobile communication full IP network on its basis, and requires that future 3G terminals must support SIP protocol.

4. SCTP protocol

SCTP protocol is a flow control transmission protocol proposed by IETF. It mainly transmits PSTN signaling information on a connectionless network. This protocol can provide reliable data transmission on the IP network. SCTP can transmit user data without errors and without duplication in the confirmation mode, and segment user data according to the limitations of the MTU of the channel; ensure the order of user messages on multiple streams and deliver messages from multiple users Copy to the data block of SCTP. The mechanism of SCTP coupling is used to ensure the self-processing of some faults at the network level. SCTP also has the characteristics of avoiding congestion and being protected from any flooding and anonymous attacks.

The SCTP protocol plays the role of the main bearer of the control protocol in the softswitch.

5 Main features and functions of softswitch technology

1. The main characteristics of softswitch technology are shown in the following aspects:

(1) Programmable call processing system supporting various networks such as various PSTN, ATM and IP protocols.

(2) It can be easily run on various commercial computers and operating systems.

(3) High efficiency and flexibility. E.g:

· Softswitch plus a trunk gateway is an alternative to a long-distance / tandem switch (C4 switch), which has VoIP or VTOA functions in the backbone network.

· Softswitch plus an access gateway is a substitute for a voice virtual private network (VPN) / private branch exchange (PBX) trunk line, and has VoIP function in the backbone network.

Softswitch plus a RAS can use public bearer relay to provide managed MODEM services.

· Softswitch plus a relay gateway and a local performance server is a substitute for a local switch (C5 switch), with VoIP or VTOA functions in the backbone network.

(4) Openness The IN (Intelligent Network) service can be reused through an open and flexible directory interface. For example, it provides a directory embedding mechanism with access to a relational database management system, a lightweight directory access protocol, and transaction capability application partial directory.

(5) Provide an open application programming interface (API) for third-party developers to create next-generation services.

(6) It has programmable after-sales room features. For example: Programmable event detailed records and detailed call events are written to a service provider's event collection device.

(7) It has the characteristics of managing all software components based on advanced policy server. It includes a simple network management protocol interface exposed to all components, a policy description language, and a system for writing and executing customer policies.

2. Softswitch is a collection of multiple logical function entities. It provides integrated service call control, connection and part of service functions. It is the core equipment for the next-generation telecom network voice / data / video service call, control and service provision. The main functions are shown in the following aspects:

(1) Call control and processing provide control functions for the establishment, maintenance, and release of basic calls.

(2) The protocol function supports the corresponding standard protocol, including H. 248, SCTP, H. 323, SNMP, SIP, etc.

(3) The service providing function can provide various general or personalized services.

(4) Business exchange function

(5) The interworking function can realize the interworking with the response equipment through various gateways.

(6) The resource management function centrally manages various resources in the system, such as resource allocation, release, and control.

(7) The billing function transmits the bill to the billing center according to the operational requirements.

(8) The authentication / authorization function can perform authentication and authorization to prevent illegal users or devices from accessing.

(9) Address resolution function and voice processing function.

6 Application based on softswitch

With the continuous development of communication network technology and the formulation and supplementation of various standards for softswitching, many manufacturers have introduced solutions for softswitching, and various operators are actively conducting related experiments. At present, many domestic and foreign telecommunications equipment manufacturers, such as Siemens, Alcatel, Ericsson, Nortel, ZTE, etc., are actively developing new exchange transition platforms, and have proposed solutions for softswitches in next-generation networks. Here is a brief introduction to the application of softswitch in VoIP. Figure 3 shows the VoIP network structure based on softswitch technology.

It can be seen from the figure that its function is very similar to the exchange / long-distance network between current circuit-switched transmission systems. The C4 switch is replaced by a combination of a soft-switch system and a group of relay gateways. The relay gateway itself is controlled by the softswitch technology using the calling / called protocol. This protocol is the MGCP / IPDC protocol associated with a specified time slot from a circuit switch with a specified source / destination RTP / UDP / IP flow. .

As part of call processing, softswitch technology identifies the most likely egress gateway to be used to terminate the call, and uses this information to command the relay gateway to perform the specified function, that is, softswitch technology can A minimum cost routing is used to complete each call, so that the selected egress gateway is closest to the destination phone. Thus, the original call operation function performed through the circuit-switched network is completed.

The access gateway can either terminate the PRI of the ISDN or the CAS signaling from the enterprise PBX. This access gateway can be controlled by the softswitch in various ways based on the packet telephony protocol; 323 protocol gateway, softswitch can be like an H. The 323 gateway acts like this; if the access gateway tunnel PRI (Q.931) or CAS signaling returns to the softswitch, then the softswitch can also use protocols like MGCP / IPDC to control the access gateway in a better way. This also reflects the ability of the softswitch technology to handle the access gateway.

Softswitch technology is connected to the cable network through SIP protocol to support the enterprise's IP PBX and IP phones. It can also access SCP through TCP / IP protocol, and can also be connected to SS7 network through TCP / SCCP protocol, making seamless interconnection possible.

7 Main issues that need continued attention in the next generation network based on softswitch

Although the next-generation network based on softswitch is a relatively complete network solution that can be applied in various communication fields, due to its new technology, most of the current solutions are in the experimental stage, and large-scale applications have not yet been formed. Continue to pay attention to issues such as QOS, gateways, security, service delivery methods, and organic integration with existing networks.

1. QOS problem

For any network, QOS guarantee is a very important issue. Fundamentally speaking, the softswitch itself cannot solve the QOS problem, but depends on its bearer network to ensure the quality of service. There are currently two ways to carry the network: ATM and IP. For ATM's bearer network, it has a strong QOS mechanism itself. However, for the IP bearer network, how to solve the QOS problem. It is a very critical issue in the next-generation network based on softswitch, because from the current equipment development situation of manufacturers and the general trend of network development, it should be the general trend to use IP as the bearer network.

2. Softswitch network management

Judging from the current implementation of softswitches, most of them use the SNMP protocol as the network management protocol of the softswitch system. However, the SNMP management system has certain limitations. The SNMP management is mainly based on static management methods and cannot be used for various services Comprehensive management of changes in demand. Because SNMP uses a UDP-based bearer, it cannot guarantee the reliable transmission of network management information. At the same time, the next-generation network based on softswitch provides real-time services, and the network management system must have certain QoS management capabilities. However, the current capability of the network management system based on softswitch is relatively poor, and further improvement and improvement are needed to meet the user's requirements for service quality.

3. The protocol involved in the softswitch needs to be improved

Each network interface of the softswitch network uses an open protocol for communication. However, at present, no matter from the formulation of the protocol or the development of various manufacturers, the standardization of the interface is not perfect, and most protocols are still in the stage of expansion and improvement. Therefore, it will take some time before the final open network.

In summary, although the softswitch has great development potential, it is still in the initial stage of development. The communication system with softswitch as the core will provide business opening capabilities, in line with the development trend of triple play, providing voice, data, video services and multimedia integration services to meet the development of personalized, mobile and access to information anytime, anywhere aims.