ISO/IEC 21990:2002规定了在专用综合业务网络（PISN）内连接在一起的专用综合业务网络交换机（PINX）之间的Q参考点处支持短消息服务（SMS）的信令协议。 该服务基于GSM 03.40。ISO/IEC 21990:2002中描述的服务中心功能等同于GSM 03.40中服务中心的功能。因此，为了与GSM网络的互操作性，只需要实现QSIG接口。 注1:不排除与其他空气接口的互通，但不在ISO/IEC 21990的范围内: 2002 注2:短消息服务是一种特殊类型的基本服务，但在本文件中描述为补充服务。 短消息服务是允许被服务用户向同一PISN或另一网络中的另一用户发送有限大小的消息的服务。 ISO/IEC 11579-1中定义了Q参考点。 按照ETS 300 387规定的方法，分三个阶段制定服务规范。ISO/IEC 21990:2002包含Q参考点的第3阶段规范，并满足ISO/IEC 21989中第1阶段和第2阶段规范确定的要求。 SMS的信令协议在面向连接的呼叫独立APDU传输机制的信令协议之上运行，并使用ISO/IEC 11582中规定的补充业务控制通用程序的某些其他方面。 ISO/IEC 21990:2002还规定了额外的信令协议要求，以支持SMS和补充业务与ANF之间Q参考点的交互。 ISO/IEC 21990:2002适用于可相互连接以形成PISN的PINX。

ISO/IEC 21990:2002 specifies the signalling protocol for the support of the Short Message Service (SMS) at the Q reference point between Private Integrated services Network eXchanges (PINXs) connected together within a Private Integrated Services Network (PISN). This service is based on GSM 03.40. The Service Centre functionality described in ISO/IEC 21990:2002 is equal to the functionality of a Service Centre in GSM 03.40. Thus, for interoperability with a GSM network, it is only necessary to implement a QSIG interface. NOTE 1 The interworking with other air interfaces is not precluded, but is outside the scope of ISO/IEC 21990:2002. NOTE 2 The Short Message Service is a special type of basic service but is described in the present document as a supplementary service. The Short Message Service is a service which permits a served user to send a message of limited size to another user in the same PISN or another network. The Q reference point is defined in ISO/IEC 11579-1. Service specifications are produced in three stages and according to the method specified in ETS 300 387. ISO/IEC 21990:2002 contains the stage 3 specification for the Q reference point and satisfies the requirements identified by the stage 1 and stage 2 specifications in ISO/IEC 21989. The signalling protocol for SMS operates on top of the signalling protocol for the connection oriented call independent APDU transport mechanism and uses certain further aspects of the generic procedures for the control of supplementary services specified in ISO/IEC 11582. ISO/IEC 21990:2002 also specifies additional signalling protocol requirements for the support of interactions at the Q reference point between SMS and supplementary services and ANFs. ISO/IEC 21990:2002 is applicable to PINXs which can be interconnected to form a PISN.