智能运输系统.陆地移动装置用通信信道(CALM).2G单元系统

[中文适用范围]： 本国际标准确定了第二代（2G）蜂窝网络和2G系统（例如使用WAP和I-Mode类型协议）与CALM兼容的空气接口，即在将2G系统纳入CALM系统之前必须满足的要求。特别是，本国际标准规定了2G系统必须包含的协议和参数，以支持在ITS环境中进行长时间、长距离、高数据速率的无线通信链路，其中异构切换或媒体独立切换（MIH）对于维持链路是必要的，或者根据媒体选择策略是理想的。本国际标准提供了在ITS领域使用第二代蜂窝通信进行长距离、中速无线通信的协议和参数。在可行的情况下，本国际标准是通过参考适用的现有标准并加以选择而制定的。提供了所需的区域变体。具体而言，对于本国际标准，通过引用采用了由各种国际和国家标准定义的现有2G系统。本国际标准不包括特定应用的上层，但将由应用标准（可能不针对特定技术）驱动。 [外文原描述]： ISO 21212:2008 determines the air interface for second generation (2G) cellular networks and 2G systems (e.g. using WAP and I-Mode type protocols) to be compliant to CALM, i.e., requirements that must be met before a 2G system can be incorporated into a CALM system. In particular, it specifies protocols and parameters that 2G systems shall include to support prolonged, long-range, high data rate wireless communication links in ITS environments where heterogeneous handovers or media independent handovers (MIH) are either necessary to maintain the link, or desirable as determined by media selection policies. ISO 21212:2008 provides protocols and parameters for long range, medium speed wireless communications in the ITS sector using second generation cellular communications. Wherever practicable, ISO 21212:2008 has been developed by reference to suitable extant standards, adopted by selection. Required regional variations are provided. Specifically, for ISO 21212:2008, extant 2G systems, as defined by various international and national standards, are adopted by reference. Application-specific upper layers are not included in ISO 21212:2008, but will be driven by application standards (which may not be technology specific).