With the widespread concern of the communication industry in recent years, the Internet community is constantly developing and improving the application of SIP protocol. The telecommunications industry has also begun to be optimistic about this agreement. 3GPP, an internationally renowned wireless communication organization, announced that SIP is the core protocol in IMS(IP Multimedia Subsystem) of Release5, and ETSI adopted IMS in its NGN system. SIP protocol may become the core protocol of the next generation network. As the core entity of SIP system, SIP server has certain differences in functions and performance indicators in different application scenarios. However, in the initial stage of its application, the * * * part in various scenarios should be uniformly stipulated to provide necessary technical reference for telecom operators and technical basis for equipment manufacturers to develop equipment. This is beneficial to the benign development of the transition from telecommunication network to the next generation network.

The technical requirements of SIP server equipment stipulate the general technical requirements of SIP server equipment, including functional requirements, business requirements, interface requirements, protocol requirements, communication flow, operation and maintenance and network management requirements, performance and reliability indicators, interoperability requirements, power supply and grounding requirements, and environmental requirements. The specific requirements of SIP server in some specific scenarios will be studied separately in combination with the actual development.

2. Introduction to 2.SIP

The structure of the SIP network and the location of the SIP server in the SIP network are shown in figure 1, which are used to provide voice and video services to users in the SIP network. The main functional entities in SIP network include: SIP server, user terminal, AAA server, network management server, application server, domain name server and media server. Network operators can choose appropriate functional entities to provide users with voice and video services according to actual operational requirements.

SIP network can communicate with DN/PLMN network, H.323 network or soft switch network through interworking unit (IWF). IWF is a logical functional entity, and its function can be embedded in the SIP server, or realized by other functional entities such as soft switching devices or SIP gateways. When IWF is not embedded in SIP server, the interface protocol between SIP server and IWF is SIP protocol. The interconnection between SIP networks of different operators can be realized by IWF. The technical requirements of SIP server equipment do not specify the specific implementation of IWF.

The definitions of several terms in the figure are given below, and descriptions of other devices can be found in the technical specifications of softswitch series.

SIP server: A network device that provides call control, call routing, user management and other functions in a SIP network.

User agent: It includes two parts: SIP user agent client (UAC) and SIP user agent server (UAS). UAC is used to initiate requests, while UAS is used to respond to requests. User agents can be implemented on entities such as SIP voice terminals, SIP video terminals and SIPIAD.

Softswitch network: a network in which softswitch devices complete the functions of user registration, call control, routing, authentication, billing, etc.

3. Basic functional requirements of 3.SIP

SIP server adopts IP bearer mode, and its functions include: call control and processing function, service provision/support function, user management function, protocol processing function, routing processing, access authentication, billing information collection, operation and maintenance/network management function, interworking function, security function (optional), black and white list function and congestion control function (optional).

The functional structure diagram of SIP server is shown in Figure 2.

Fig. 2 Functional structure diagram of SIP server

3. 1 Call control and processing function

SIP server can complete control functions such as call establishment, maintenance and release of basic services/multimedia services, including call processing, connection control, intelligent call trigger detection and resource control. SIP server should support basic two-party call control function and multi-party call control function. The conference bridge resource function of multi-party call should be completed through the media server.

3.2 Service Provision/Support Function

SIP server should support basic voice services and multimedia services, as well as traditional PSTN supplementary services and data and multimedia related services. SIP server provides traditional intelligent network services to users in the domain, and needs to communicate with traditional intelligent network through soft switch equipment. Therefore, the traditional intelligent network services available in SIP domain are related to the intelligent network services provided by softswitch devices, and there is no specific provision in the specification. In addition, the SIP protocol is not clearly defined in supporting user agents to cross the spring and report * and # signals. Therefore, it is not required in the specification to realize the operation mode of supplementary services through SIP protocol. SIP server can pass through application server or PARLAY gateway. Provide a variety of value-added services and intelligent services.

3.3 Protocol processing function

In terms of protocol processing capability, the SIP server should support SIP, SDP, SNMP, Telnet and other protocols, and optionally support RADIUS and Diameter protocols. The SIP protocol adopted by the SIP server must support UDP and TCP bearer. The default port number is 5060, regardless of whether the SIP protocol uses TCP or UDP.

3.4 Routing processing function

(1) The SIP server should be able to convert domain names to IP addresses. However, the SIP server should realize the redirection function of the request message.

(2)SIP server should support the function of querying domain names from domain name servers.

(3) Number analysis and storage function, which requires the SIP server to support the storage of 20-bit calling numbers and 24-bit called numbers, and expand them to 28 bits. After analyzing the 10-bit number, select the route, and add or delete numbers at any position.

3.5 Operation and Maintenance/Network Management Function

Operation and maintenance system is the part of SIP server responsible for system management and operation and maintenance, and it is a collection of tools for users to use, configure, manage and monitor SIP server. SIP server should not only support local maintenance and remote maintenance and management, but also communicate with the network management center supporting SNMP protocol through the internal SNMP proxy module. The specification stipulates the operation and maintenance system from the aspects of configuration management, fault management, traffic control function, safety management, statistical management and man-machine system.

3.6 Access authentication function

(1) The SIP server should be able to authenticate each call request of users, and reject and discard the call requests initiated by users who fail authentication. SIP server can send the registration and call information of user terminals within its jurisdiction to the authentication center for user authentication and authorization.

(2) The 2)SIP server can also authenticate and authorize the users of prepaid cards through AAA servers.

(3)SIP server should be able to support authentication and authorization of user agents with dynamic IP address configuration.

3.7 Billing information collection

Requirements for billing. In order to improve the compatibility between SIP server and existing network. For example, it can be used in the existing telephone network billing system and the existing IP telephone network system. SIP server should be able to support transmission protocols of billing information, such as CMIS/FTAM protocol or FTAM protocol, RADIUS protocol or Diameter protocol, FTP, etc. The use of specific transport protocols is determined by each operator according to the characteristics of its own network. As for the charging method, the SIP server should at least have the function of collecting charging information according to the charging object, including time, traffic, media format, coding mode, etc., and be responsible for sending the collected information to the charging center. SIP server should at least support detailed billing mode, and optionally support multiple billing, instant billing and other functions. SIP server should at least provide calling number billing and account billing.

3.8 communication flow supported by SIP server

The technical requirements of SIP server equipment also describe the relevant communication processes, including basic registration process, successful call process, reliable temporary response confirmation process, abnormal call process, call release process and several typical supplementary business processes. At the same time, the technical requirements also stipulate that when the user's subscription information is included in the authentication/billing center in the background, it is necessary to complete the identity authentication and billing of the user in the call process through the SIP server. The flow described in the technical requirements of SIP server equipment does not include the interactive flow between SIP server and authentication/billing center.

4.SIP performance and reliability requirements

4. 1 performance requirements

Call connection rate. Call connection rate refers to the ratio of the number of call connections to the total number of call attempts during an observation period.

Call processing capability. The call processing capacity of SIP server should be measured by two indicators: the number of call attempts per hour (BHCA) and the maximum concurrent processing capacity.

Time delay. Delay means that the SIP server completes the message processing after receiving the SIP message. And realize the time interval of successful message forwarding.

4.2 Reliability index

Once an operator adopts a network system based on SIP protocol, since the SIP server is the key equipment in its system, it is necessary to require the reliability of the SIP server according to the carrier-class system equipment. The technical requirements of SIP server equipment are as follows:

SIP server must be designed with fault-tolerant technology, and the system must reach or exceed 99.999% availability. The annual power outage time of the whole system is less than 3min.

SIP server is required to have high reliability and stability (such as dual-machine backup).

5, power supply and environmental requirements

Considering that SIP server may be used in different scenarios, the requirements for its equipment power supply are explained from two aspects: DC power supply and AC power supply, in which DC power supply specifies the rated voltage, voltage fluctuation range and noise voltage. In addition, the grounding requirements of SIP server are specified, including grounding mode, grounding requirements, cross-sectional area of grounding wire, grounding resistance and power redundancy.

The environmental requirements are described in detail from the aspects of temperature and humidity requirements, dust prevention and harmful gas concentration in the computer room, electromagnetic interference resistance, electromagnetic interference generated by SIP server itself, seismic requirements, transportation and storage requirements, etc.