SIP and ISUP Protocol Interoperability
Ling Ying, buttons, Bin, Li Qing
(China Telecom Group Shanghai Research and Development Center, Shanghai 200122)
Abstract: This article is proposed by the development of SoftSwitch technology. From the perspective of network fusion, the structural model of SIP and ISUP intercommunication units (IWU, interworking unit) is analyzed, and the IWU is introduced to implement SIP and ISUP interworking. The principles and mechanisms provide a feasible solution for the smooth transition of PSTN to softswitch networks. Key words: soft exchange; SIP; isup; IWU
Research on Interworking Between Sip and Isup Protocol
Ling Ying, Niu Ying-bin, li qing
Shanghai Research And Development Center of China Telecom Group, Shanghai 200122, China)
Abstract: Based on the rapid development of technology on softswitch, the interworking unit architecture model about SIP and ISUP protocol are analysed from the point of view of the integrating network The principle of realization Interworking Unit (IWU) and its mechanism are explained which provides. AN Effective Solution for the PSTN Network to Evolve Into Softswitch Network Smoothly. Keywords: Soft Switch; Sip; Isup; IWU
I. Foreword As the advancement of technology and the continuous improvement of the user requirements, the softswitch network architecture of multimedia business such as high quality voice, data and video is gradually become a hot spot for telecommunications network development. However, from the current perspective, especially for the Chinese market, there is also a great potential for the development of the original telephone network. There are many users who can excavate, so circuit exchange technology will continue to be applied and developed, and telecom operators will also protect The investment of existing telephone networks, and it is desirable that the technology can be smooth and transitioned, which will present a long-term coexistence of different technologies, so the research on new and old two network fusion interoperability is actual and urgent. The SIP protocol is one of the mainstream control protocols in the VoIP field. It shows powerful vitality in its simple, efficient, easy-to-expand, etc.; Isup is one of the user part of the No.7 signaling system, which is supported ISDN The basic bearing business and supplementary service providing the non-voice use of the non-voice use provides the signal functionality required, which is expanded based on the phone user portion (TUP), which belongs to the control signaling between the switches. Our research on ISUP and SIP protocols is important for the practical application of softswitch technology.
Second, IWU's position in the network This section defines the structural model of the ISUP protocol and the intercommunication unit (IWU, ie Interworking unit) between the IWU, Interworking Unit, and detailing the implementation mechanism of IWU. Figure 1 shows the specific location of the IWU in the network.
The IWU is a functional module that is capable of fusion between two protocols that can be set independently or in combination with ISUP switches. To describe convenience, we divide the IWU from the external interface into two parts of the I-IWU (Incoming Interworking Unit) and O-IWU (Outgoing Interworking Unit). The I-IWU is responsible for transmitting calls from the SIP network to the iSUP domain, and the O-IWU is responsible for passing a call from the ISUP domain to the SIP domain. The IWU and the interface of the SIP domain belong to the NNI (Network-To-Network Interface) interface. Adjacent SIP nodes (ASN, T, Adjacnt Sip Node) connected to IWU are reliable network entities, not user entities. If the NNI interface supports SIP-I, the remote SIP user agent must be able to process the ISUP message. Structural models of IWU IWU architecture model As shown in Figure 2, mainly include 3 modules: call service function module (CSF, Call Service function), bearer control function module (BCF, bearer control function) And media mapping / exchange function modules (MMSF, MAPPING / SWITCHING FUNCTION). The CSF main function is to send call control signaling to peer entities, managing call status, access service control entities, and initiation resource requests; the function of BCF is related to the processing and mapping of SDP bearer control information, mainly including access control and media resources. The function of the acquired; the MMSF function provides the controllable interconnect function between the two bearers, and can freely convert the carrier from one process and adaptation / encoding technology to another, such as implementation of ISUP (Q.761-764) Mutual conversion between media formats between SDP / SIP.
Fourth, IWU implementation of the mechanism of the agreement 1. Basic principles On the SIP interface, IWU is used as a user agent (UA) support IETF's definition in the SIP and SDP protocol standards; on the ISUP interface, IWU supports ISUP Definitions related to standards q.761 to q.764. IWU must be able to identify calls, dialogues, and related call controls, so it must establish a one-to-one relationship between SIP dialog and iSUP call / bearer control instance, so that the interoperability between the relevant signaling of the same call can be guaranteed. Pacup and map of ISUP messages (1) Introduction to SIP Message Structure SIP message has two types of requests and responses, each message contains 3 elements: request row / status line, head domain, and message body. The SIP message headfield defined in RFC 3261 includes 44 VIA, FROM, TO, CALL-ID, CSEQ, Contact, Content-Type, Content-Length, Max-Forwards, Proxy-Authenticate, etc., and these headers The number of domains is scalable. The Content-Type domain of the SIP message indicates the media type of the message carrier, and the Content-Length domain indicates the number of bytes of the message body. The SIP message can carry a variety of types of messages, such as the SDP message body, or the binary format ISUP message body, etc. (2) ISUP messages In order to maintain the end-to-end business, transparently transmitting the iSUP message in the SIP network is considered to be important. The IWU provides a feature that encapsulates the ISUP message in a MIME (Multi-Purpose Internet Mail Extensions, Multi-Function Internet Mail Extension) format in the SIP message body. For example, you can package the ISUP message in the SIP message body below, transparent forwarding in the SIP network: 1) Invite message; 2) 183 session programs temporary response (if the response is the first rearward message of SIP); 3) 200 OK message; 4) Info messages in any case. However, any message body cannot be carried in the CANCEL message body, and some ISUP messages cannot be encapsulated in the SIP message body, because the programs involved in these messages are only related to the ISUP side of the call, and the typical situation is related to ISUP line maintenance. news. If the IWU receives these messages encapsulated in the SIP message body, discard. In addition, the ISUP message of the segment itself cannot be encapsulated in the SIP message body, but is re-equipped by the I-IWU (ISUP side interface) to the original ISUP message and then package. (3) Mapping of iSUP messages For the map of the ISUP and SIP messages, the following 2 cases include: 1) SIP to ISUP message mapping: If the IWU receives the SIP message body carrying the MIME format package ISUP Message, the IWU extracts the ISUP message. Since business calls in the SIP network may cause conflicts between SIP headers and ISUP parameters, IWUs should maintain consistency between ISUP parameters and SIP headers (About the ISUP parameter or ISUP parameter over the SIP header The principle of merger of the SIP header will depend on the relevant application / business situation). If there is no package ISUP message in the SIP message body, the IWU calls the associated ISUP program, using the SIP header field and the SDP message to synthesize the parameters of the appropriate ISUP message.
In order to keep the network integrity and fraudulent behavior of different networks, SIP status codes, and ISUP release reasons to correct the following principles: Mapping only uses the reason why the value described in the Q.850, this reason value is in ISUP / DSS1 applications are also defined. The location of the subdomain should include a network that exceeds the interfacial node value, and the international switch uses the Q.767 encoding structure. 2) The case where ISUP to SIP message mapping: After the IWU receives the ISUP message, according to the received message type, in accordance with the local policy to form the parameters in the ISUP message to generate the corresponding headband value in the SIP message. For example, Request-Uri, from, TO, etc., and encapsulate the iSUP message in the proper SIP message in the message body in the MIME format. 3. Message mapping example below We use the IAM message of the ISUP in the O-IWU (ISUP side) Take the detailed process of generating the INVITE message generated by the SIP. O-IWU receives the IAM message from the ISUP network to perform the ISUP program to process the IAM (initial address message) message, map it to the INVITE message, and select the call routing in the SIP domain. Thereafter, the O-IWU determines from the local configuration whether or not the "group send" address mechanism is used on the SIP side. 1) If the "set" address mechanism is used, the O-IWU determines the address end signal according to the standard below, and calls the appropriate SIP signaling program. Address end signaling is determined by the following criteria: 1 Receive ST signals, or 2 The number of numbers received reached the maximum number in the national dialing scheme, or 3 analyze the called party number, pointing to the call route to be The number of numbers required, or 4 After receiving the least number of numbers required for the route, 4 to 6 s after receiving the latest address messages. 2) If the "overlapping" address mechanism is used, the O-IWU will: 1 Start the timer toiw3 (4 ~ 6 s) 2 call the appropriate go to the SIP signaling program, and 3 According to the specified SAM (Sub-address message) message. In the SIP network that meets the pretreatment conditions, the O-IWU sends an Invite message after confirming the completion of the prerequisites. If it is set to the "Connection Nature Representation" parameter in the "Connection Nature Representation" parameter of the IAM message to "turn on on this circuit" or "turn on the on-circuit", O-IWU The on-the-conduct circuit will be completed in accordance with the leading circuit. O-IWU maps the two main URIs of the invite from the upper and lower relationship established, a representative session, a destination representing it. The former is used to generate the FROM domain, which is used to generate a TO domain and the REQUEST-URI domain. O-IWU reads the CPN (called user number) parameter in the IAM message, "Tel URL" to generate an INVITE message from the ISUP format Request-URI, or first The number mapping (ENUM) is parsed into a URI. iSUP format The translation process of "TEL URL" is as follows: When the NPI of the IAM message indicates that the telephone number is formatted with E.164, the specific translation process should tend to NOA of the CPN parameter (address nature means language The value of the field is set. If the NOA indicates that the number is an "international number", it will be attached to the "Tel: " string directly after the conversion; if the value of NOA is "domestic number", it must be submitted to "Tel URL" The phone number plus the prefix of the country code.
If the O-IWU that performs the conversion is connected to several switches, these switches belong to different countries, and should select the appropriate national code according to the originating switches and trunk groups. If the value of NOA is "user number", in order to use an international telephone number representation, there is a need to add other necessary parts in the national code and number plan (such as regional code and city code). Only " " symbols are not included in the "Tel URL" in the "national or network-specific" value in NOA. At this time, the O-IWU is only copied by the phone number. If you use the SIP URI address format, you need to add the "user = phone" parameter. ISUP has some additional parameters that need to be added to "Tel Uri" after the translation is completed. If the IWU supports the carrier-based route, you must check if there is a TNS (transfer network selection) parameter in the IAM message. If these values are valid, the O-IWU extracts the CIC (bearer network identification code) value from the given parameter and The analysis, and finally the "CIC =" parameter carrying the CIC value is added to the destination URL. If the CIC does not specify the network to which the IWU belongs, in order to maintain the consistency with the local policy, it should also add "CIC =" parameters. When you create a "CIC =" parameter, you must be prefixed as a national code that has been used or hidden in the country code in the CPN. Therefore, if CIC is "5062", it is written into " 86-5062" in China. If the O-IWU supports the number-carrying route, some other domains need to be viewed. In order to correctly map the FCI (forward call representation), if the "NP" bit indicates that the operation of the local number has been carried in the PSTN, an "NPDI = YES" parameter is attached after "Tel URL". If there is a GDP (General Number Parameter) parameter in the IAM, the content of the CPN should be translated from the ISUP format and copy to the "RN =" domain, and the domain is attached to "Tel" URL later. The GDP parameter itself is also translated from the ISUP format to generate the "Tel Url" primary phone number. In some national-level number schemes, the LRN and the dialed numbers are stored in the CPN parameters. In this case, they should be separated and stored in different domains of "Tel URL". Note: The LRN must be a national level, so there is no preamble symbol " " before the "rn =" domain. In most cases, the "Tel URL" of the finally generated destination can be used simultaneously for the TO and REQUEST-URI domain. However, if an OCN (original called number) parameter appears in the IAM, the OCN parameter should be translated from the ISUP format used to generate a TO domain, so the request-uri and the totrium are different. The structure of the FROM header is depends on the CIN (call user number) parameter. If the CIN parameter is invalid, the O-IWU generates a virtual SIP URI containing only hostnames (such as "Sip: GW.SIPCarrier.com") to construct the FROM header. When converting CIN parameters, two additional information fields must be considered: Restriction addresses provide a representation (PRI) and authentication representation (SCI).
If the PRI is set to "limit", the O-IWU creates a special SIP URI (the identity of the calling user) is transmitted to the far end, where "Display-name" and "UserName" are "anonymous", for example:: From: anonymous
6. Conclusion Smooth transition to the traditional telecommunications network to Softswitch Network is a hot spot in the development of telecommunications technology. This article provides a feasible solution for interconnecting interconnections of SIP networks and PSTN networks by detailing the structure of IWU and its implementation principles and mechanisms. Of course, there are still many problems in reality to consider, and complete intervenes to implement two protocols will also involve multiple factors, such as security, billing and other aspects. We believe that in the near future, research on this SIP and ISUP interoperability will be more mature and perfect, which will greatly accelerate the pace of soft exchange technology to the practical direction.
references
[1] RFC3261, SIP: Session Initiation Protocol [S] [2] RFC3312, Integration of Resource Management and Session Initiation Protocol (SIP) [S]. [3] Itu-T Recommendation Q.761, Signalling System No ISDN User Part, Functional Description [S]. [4] Itu-T Recommendation Q.763, Signalling System No. 7 - ISDN User Part Formats and Codes [S]. [5] ITU-T Recommendation Q .764, Signalling System No. 7 - ISDN User Part Signalling Procedures [S].
