Soft switch
Concept
Origin
In the traditional PSTN switching network, service control, call control, and transmission bearer are all concentrated in the switch, and all PSTN services must Realized through TDM switch. In order to meet the constant demands of users for new services, a public service generation platform, namely the intelligent network, has appeared in the PSTN network. The intelligent network of PSTN proposed and realized for the first time that some network-wide service generation and call control were separated, and thus improved the development of PSTN network-wide services, improved the ability of the network to provide services centrally, and shortened the cost of new services provided by the whole network. Cycle. The IP telephone technology system not only further strengthens the idea of separating service generation and call control, but also further expects to realize the separation of call control and media transmission bearer. The goal of this separation is to make the control and generation of IP telephony services truly independent of the media transmission network facilities, so as to realize the integration of IP telephony and its related services and multimedia applications more flexibly and effectively. More importantly, this separation also helps telecom operators to implement centralized operation, management, maintenance, and configuration on packet networks. Telecom operators and users can define, configure and manage their own business characteristics in the central office system through IP connection, without worrying about the type and configuration of user terminals, so that the provision of services and applications has greater possibilities, flexibility and Operable and manageable.
Definition
According to the definition of the International Softswitch Forum ISC, Softswitch is a device and system that uses program-controlled software to provide call control functions based on a packet network. According to this definition, Softswitch is a typical technical practice designed for voice purposes in packet networks. This definition also shows that Softswitch borrows the concept of "hard" switch in the PSTN network in the traditional telecommunications field. The difference is that it emphasizes its meaning based on the separation of the packet network and call control from the media transmission bearer. Softswitch is an important part of NGN, but it is more of a device and system that focuses on call control functions, and it does not constitute a special overall networking technology mechanism and network architecture by itself.
Technical basis
The technical basis of the soft switch is the MGCP/H.248 media control protocol. The media control protocol, also known as the device control protocol, is a so-called master-slave relationship control protocol. The accused party generally does not contain a simple device or device in an intelligent state, and all occurrences and changes of state events must be reported to the master control device. Therefore, the media control protocol should generally not be regarded as call signaling, and can only be used for endpoint (including user endpoints and relay endpoints) control. The interoffice call signaling protocol of the soft switch cannot use MGCP/H.248, but must use the existing multimedia international standards, that is, the ITU H.323 protocol and/or the IETF SIP protocol.
The soft switch can realize the idea of separating call control and media transmission through media control protocol technology. Therefore, some foreign scholars call the soft switch as a call agent (CA) or a media gateway controller (MGC). The proposal of the soft switch concept not only enables the voice service functions of NGN and the switch function of the traditional PSTN network to be completely and transparently compatible? It fundamentally ensures that the IP telephone technology can completely replace the PSTN network.
Advantages
As a commercial hardware platform based on a technical representative of open, standard, multi-protocol and operational management, soft switches will become important and important in the next generation communication network (NGN). One of the key systems and equipment technologies. However, it must be noted that since domestic softswitch networking and practice of a certain scale are still rare, the discussion and practice of softswitches still only remain on the conceptual level of partial and individual technologies.
Functions
Soft switches mainly provide the following functions:
Call control function (CCF);
Address resolution/routing function;
Service provision function;
Network management/billing function;
Service switching function (SSF);
SIP (Session Initiation Protocol) interworking function;
li>H.323 interworking function;
Application server connection function;
Signaling gateway Connection function;
Relay gateway control function.
In addition, the system is required to have flexible scalability, carrier-class high availability, and strong software fault tolerance and hardware fault tolerance. Therefore, the target system is a multi-machine, real-time telecommunication system.
Technical Features
People's views on the technical features of soft switches are basically the same. According to the concept of the soft switch, it can be seen that the technical feature of the soft switch is based on the MGCP/H.248 media control protocol, which realizes the idea of separating call control and media transmission.
Although NGN’s goal is to gradually integrate voice, data, and video services into multiple services on IP networks, the fundamental challenge it faces is to seek mature, large-scale operation and management. IP telephony technology. Because voice is the most popular, basic and most important service for operators, and its market covers the widest range, and its commercial income has a far greater impact than other services. After nearly a hundred years of development, traditional PSTN technology and TDM network have provided operators with mature business models and quality standards for their voice services. The data IP network was originally designed for data services? Voice is a time-sensitive service. The IP telephony technology that provides large-scale voice services in the data IP network is the primary challenge faced by any operator during the transition to NGN. .
Soft switches are basically specially designed to realize the functions of PSTN switches on IP networks. The switch of the traditional PSTN network is a vertical, closed and private system structure, and the soft switch is a standard-based, open system structure that separates call control and media. In addition, soft switches and PSTN "hard" switches have many common attributes. For example, they are all relatively centralized control and management system devices, and all user-oriented data and call status information are stored in the system. This makes the soft switch pay special attention to the telecom operators? Because only in this way can the telecom operation requirements be met to implement effective call service control and operation and maintenance management for the endpoint access of the IP packet network and the interconnection with the PSTN. Therefore, the soft switch is an important means of realizing IP telephony technology in NGN.
It is particularly worth mentioning that in order to support the access of smart endpoints, video endpoints and multimedia endpoints, general soft switches may also support H. 323 protocol and SIP protocol to control and manage user endpoints of H.323 and SIP. As a result, the soft switch itself cannot completely separate call control and media transmission for all endpoints it controls and manages. In addition, as mentioned earlier, the soft switch must also support the H.323 and SIP protocols in the "interoffice" to interconnect with the H.323 network and SIP network and other soft switches. However, MGCP/H.248 media control protocol technology is always the basic feature of soft switches. If a certain call control equipment system does not support MGCP or H.248 media control protocol, it generally cannot be called a soft switch. The industry will generally support H. The call control device with the prominent feature of the 323 protocol is called the gatekeeper, and the call control device with the salient feature of supporting the SIP protocol is called the proxy server (ProxyServer).
In addition, although the soft switch supports the access of intelligent endpoints, video endpoints, and multimedia endpoints, it does not mean that all voice, data, and video services must be implemented on the soft switch. In theory, even if there is the possibility of such a device, it is completely unnecessary to do so, because this system does not have any advantages but has a series of defects in the overall architecture. In fact, the core design of the soft switch focuses on the call control functions of various services and the management of endpoint capabilities to a certain extent. For example, the soft switch does not directly provide any form of IP data services; the soft switch can support video endpoint services, but the control function of the video conference is not directly provided by the soft switch, but is implemented by interconnecting to other application servers.
Through a unified, IP-based transmission infrastructure, call control equipment (mainly soft switches) can be combined with other IP technologies and data application technologies to construct a system to support multimedia and other intelligent services. We can call such a system a soft switch system. Broadly speaking, the soft switch system is a collection of multiple logical functional entities for call, media, and service control. It provides call control, connection and some other business functions including telephone terminals, smart terminals, and multimedia terminal services. Such a soft switch system can be used for NGN control, management service access and central office equipment at the edge of the PSTN network. In this way, a variety of services related to call control can be integrated and managed and operated in this system. However, such a soft switch system will still adopt multiple distributed subsystem modes in order to flexibly provide applications of intelligent and multimedia services including voice in the integrated system.
So from the point of view of the telephone service provided by the soft switch and its technical characteristics, the soft switch is an important part of the NGN system, but it does not provide any fundamental innovation of the entire network architecture.
Key technologies
Key subsystems
Figure 1 shows the four key subsystems of the soft switch: protocol stack subsystem, protocol conversion subsystem , General Protocol Subsystem and General Model Subsystem. The universal model subsystem communicates with 5 call-related universal protocol subsystems, and the 5 universal protocol subsystems correspond to 5 protocol groups respectively. These five subsystems are the General Call Control Protocol Subsystem, the General Service Control Protocol Subsystem, the General Call Bearer Control Protocol Subsystem, the General Mobility Management Protocol Subsystem and the General AAA (Authentication, Authorization, Accounting) protocol subsystem. The protocol conversion subsystem converts each specific protocol into a corresponding general protocol. Protocol stack subsystems can be further divided into specific protocol stack subsystems according to different communication protocols, and these specific protocol stack subsystems can also be roughly divided into five categories.
General Protocol
Figure 2 shows the PDU (Protocol DataUnit) format of the general protocol to achieve uniformity and is suitable for various general protocols. The idea of combining translation and encapsulation draws on SIP-T (Session Initiation Protocol for Telephony) protocol. The design of the protocol data unit is mainly based on translation, supplemented by encapsulation. Translation ensures that most useful parameters that need to be converted can be easily obtained to achieve high efficiency, while encapsulation ensures the integrity of information to meet the needs of special parameters and achieve scalability.
The main benefits of designing a general protocol are as follows:
It can realize the protocol independence of the core of the soft switch, and the general model subsystem of the soft switch only needs to be You can handle five general protocols, and you don’t need to care about which protocol is actually used with the external functional entity;
The protocol component of the soft switch can be realized, if you need to add Or to delete an external protocol, you only need to add or remove the corresponding protocol stack subsystem and protocol conversion subsystem through the configuration of the system;
Reduce the general model of softswitch The difficulty of system design;
Protocol conversion, protocol collaboration and protocol cooperation are key technologies in soft switches. In the case of a large number of protocols, the use of general protocols as an intermediary can greatly Reduce the number of protocol conversions, protocol collaborations, and protocol collaborations.
General model
In the intelligent network system, IN-SSM (Intelligent Network Service Switching Model) in SSF implements a business control model, The general business control model in the softswitch can be implemented with reference to this model. The general call control model can be carried out on the basis of the state machine of the outgoing procedure and the incoming procedure on the network side in the Q.931 protocol and the INAP (Intelligent Network Application Part) CS-2 (Capability Set 2) BCSM (Basic Call State Model) Design. The general call bearer control model can be designed according to the currently only two call bearer control protocols H.248 and MGCP (Media Gateway Control Protocol).
Protocol relationship processing in the protocol group
According to the protocol relationship theorem in the softswitch, there is a protocol conversion relationship in the call control protocol group, and there is protocol cooperation in the call bearer control protocol group. Relationship, and there is a protocol independent relationship in the service control protocol group, the mobility management protocol group, and the AAA protocol group. The processing of the protocol relationship in the protocol group is embodied in the general protocol subsystem, that is, the translation information unit part of the general protocol PDU. For the protocol independent relationship, no processing is required, that is, the translation information unit in the corresponding general protocol PDU is empty. For the protocol cooperation relationship, the information that needs to be transferred between the two protocols should be translated into the translation information unit in the general call bearer control protocol PDU in the general call bearer control protocol subsystem. For the protocol conversion relationship, the necessary information should be translated into the translation information unit in the general call control protocol PDU according to the atomic protocol function. It is worth noting that the above discussion of agreement relations are all from the perspective of the outside of the system, and from the perspective of the inside of the system, since the general agreement is generally used as an intermediary in the target system, each specific agreement is It is necessary to perform protocol conversion with the corresponding general protocol.
Protocol relationship processing between protocol groups
According to the protocol relationship theorem in softswitch, the three protocol groups of service control protocol group, mobility management protocol group, and AAA protocol group are There is a protocol cooperation relationship between the call control protocol groups, and there is also a protocol cooperation relationship between the call bearer control protocol group and the call control protocol group. The three protocol groups of the service control protocol group, the mobility management protocol group, and the AAA protocol group are related to the call. There is a protocol independence relationship between the bearer control protocol groups, and there is a protocol cooperation relationship among the three protocol groups of the service control protocol group, the mobility management protocol group, and the AAA protocol group. The processing of the protocol independent relationship and protocol cooperation relationship between these protocol groups is embodied in the general model subsystem, that is, in the message interaction between the general models.
Scalability
Good scalability is the key to the success of a soft switch. The following points can ensure that the target system has good protocol scalability.
The use of indirect protocol conversion instead of direct protocol conversion in the call control protocol suite facilitates the addition of new protocols in the future;
In the same way, indirect protocol cooperation or indirect protocol cooperation is also used in other protocol groups to facilitate the addition of new protocols in the future;
When deleting a specific protocol in the protocol stack subsystem , Only need to delete the corresponding protocol conversion subsystem, and will not affect the general protocol subsystem and general model subsystem. But if the specific protocol to be deleted is the only protocol in this protocol group, the corresponding general protocol subsystem and general model subsystem need to be deleted;
When a specific protocol is added to the protocol stack subsystem, if the protocol can belong to one of the existing five types of protocol groups, then only the corresponding protocol conversion subsystem needs to be added at the same time without the general protocol subsystem. And general model subsystem, otherwise the corresponding protocol conversion subsystem, general protocol subsystem and general model subsystem need to be added;
The common information unit in the general PDU is partially realized The scalability of the general protocol;
The encapsulated information unit part of the general PDU contains the original PDU of the encapsulated protocol. When the general model subsystem needs to process the information unit that is not in the translation When the content of the agreement appears in the PDU, it can be extracted from the original PDU, ensuring that the system is changed as little as possible and new functions are provided as soon as possible.
Availability
High availability is a basic requirement for telecommunications systems. The following points ensure that the target system can achieve a satisfactory availability .
Using a multi-process method, the processes of the same function adopt a load sharing method to work;
Using a multi-machine system, the same Yes, the physical devices also work in a load sharing manner;
Using an overload control mechanism to prevent the system from crashing under the impact of large traffic;
li>The guardian subsystem is used to guard the other subsystems to realize software fault tolerance.
Role in NGN
Soft switches are important equipment and systems that provide call control in various services of next-generation networks. However, the soft switch is not all of the next generation network. As an important part of next-generation network call control, soft switches still need to complement other key technologies and equipment to develop richer video and multimedia services in collaboration.
The system program and office data of the soft switch are the operating basis of the soft switch, and also the most important and non-fault-tolerant link during fault recovery. Therefore, timely backup of system data and office data will be truly important. At the same time, it also provides a guarantee for failure recovery.
Thinking about the formulation
Conceptually, the formulation "softswitch is the core of the next-generation network" will bring some confusion and logic to the basic propositions Confusion on the. The core technology of the next generation network should fundamentally include support for various network technology systems such as voice, data, and video services. The soft switch itself is only a partial equipment system and does not have the characteristics of a comprehensive network system architecture. The soft switch itself does not directly provide IP data services. Soft switches can support video and multimedia endpoint services through the expansion of call control modes and functions, but soft switches still need to collaborate with other key technologies and equipment to develop richer video and multimedia services. Soft switch technology is mainly designed for centralized call control functions. In addition to soft switch technology, the next-generation network will also rely on a series of existing IP technologies and other application technologies to support multiple services such as voice, data, and video.
In terms of strategy, the term "softswitch is the core of the next-generation network" often leads people to think that softswitch technology is equivalent to the next-generation network technology, and thus will discuss and practice The focus of attention is only on the single soft switch technology and its surrounding range. Put the consideration of future investment only on the equipment and systems related to the soft switch. This leads to a short-sighted strategic thinking that only looks at the trees but not the forest.
Technically, the term "softswitch is the core of the next-generation network" often leads people to place all expectations of the next-generation network on the soft switch equipment, and then use the soft switch Think about the construction of the so-called next-generation network as the core, making the discussion and practice of the next-generation network only stay at the conceptual level of primary, partial, small-scale and individual equipment system technology for a long time, thus falling into the partial structure of the soft switch. There is no solution to the overall network architecture of the first generation network.
In practice, the experiment and discussion of networking methods around the soft switch is very easy to cause people to ignore the practice and discussion of other next-generation network technologies, thus limiting the next-generation The net can only be the so-called "no net mode" of the so-called full-planar network where the soft switches are interconnected. Obviously, the "no-network mode" next-generation network architecture can only support one or a few soft switches, which is not suitable for telecom operators to operate on a large scale. The author refers to this phenomenon as the "islanding phenomenon" in the practice of soft switch technology.
On the network structure, "softswitch is the core of the next-generation network" cannot answer questions such as whether the next-generation network needs core network technology. If not, how to avoid the soft switch "Islanding phenomenon", if necessary, what is the core network technology of the next-generation network, how to solve the coordination and interconnection between soft switches, how to develop the network-wide business of the next-generation network and other basic issues.
Strategy analysis
In the discussion of the network architecture of the next generation network, there have always been two very different strategic ideas: one is to emphasize Intelligent endpoints and edges, simple network facilities, because the driving force of end-to-end multimedia convergence services comes from endpoints and edges, and the vigorous innovation and prosperity of next-generation network services depend on this. The success of the Internet also proves the importance of this approach to multimedia services. Another way of thinking advocates simple endpoints and edges, and intelligent network facilities, because only a purely unified endpoint and edge devices are conducive to large-scale management and control. The voice service provided by the PSTN network has verified the reliability of the business practice of this idea. ITU's H.323 protocol and IETF's SIP protocol are just technical systems that reflect the first strategic thinking to some extent. The soft switch characterized by the MGCP/H.248 protocol of IETF/ITU is the realizing means that reflects the second strategy. As mentioned above, in order to achieve the goal of end-to-end IP phone telecom operations to a greater extent, people have designed soft switch technology mechanisms for telecom operators. The IP telephone voice service is the most important and fundamental service function in the soft switch.
It is obvious that the true meaning of the next-generation network requires not only the innovation and prosperity of intelligent endpoint services, but also the large-scale operation and management of its systems. The difficulty of discussing the network architecture system of the next generation network is how to comprehensively consider multiple technologies. The author believes that, in terms of technology maturity and development, three technologies such as the H.323 protocol, SIP protocol and soft switch featuring the MGCP/H.248 protocol are all implementation means for IP telephony and next-generation networks. These three technologies have their own differences and specialties and need to be complementary. Only the use of a network architecture system and implementation methods that integrate the three technologies is the best policy.
In fact, the next-generation network system should be composed of the core network and the end office soft switches located at the access and edge of the next-generation network. Among them, the core network of the next-generation network is the backbone communication network covering the service range of operators. It is used to realize the interconnection and intercommunication between soft switches, multiple operators, and different networks, and realize the traffic control and communication of the entire network. Scheduling; sharing the entire network business of the nature of centralized database retrieval, sharing the business creation environment, and providing network management across the entire network, etc. The NGN core network mainly provides functions such as networking, routing strategies, traffic control, creation and management of the entire network, and maintenance and management of the entire network. Obviously, among the three main technologies implemented by the next-generation network IP phone, the H.323 protocol network technology and the SIP protocol network technology will play a major role in the core network. Many telecom operators in the world have used H.323 gatekeepers or SIP proxy servers and other network application servers to realize networking, networking, and development of network-wide services and operation and maintenance.
This does not mean denying the importance and key position of soft switch technology in the next generation network. The next-generation network architecture system is determined by the H.323 and SIP network systems, but the access and convergence of the most popular and basic IP telephone services in the next-generation network mainly rely on the soft switch technology characterized by the MGCP/H.248 protocol . As many scholars have already pointed out, the soft switch can also be regarded as the result of the decomposition of the intelligent gateway under the H.323 and SIP system. In this way, the network architecture system of the integration of the three technologies may be more clear. In fact, the true meaning of the soft switch should be a general-purpose classless soft switch? It is responsible for the regional or end office system platform in the next generation network, and is responsible for simultaneously controlling network access traffic and controlling PSTN network edge interconnection. The function of the service. Only when a regional general-purpose soft switch and a national core backbone network are jointly established can a complete next-generation network communication system be constructed.
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