本ETSI标准（ES）规定了终端设备（TE）将提供的技术特性（电气接口要求），终端设备（TE）将连接到PSTN的2线模拟接口，在该接口处，网络寻址将通过环路断开信号进行，只要这些特别于环路断开信号功能。参考文件:prTBR21

This ETSI Standard (ES) specifies the technical characteristics (electrical interface requirements) to be provided by Terminal Equipment (TE) to be connected to a 2-wire analogue interface of the PSTN at which network addressing is intended to be performed by means of loop disconnect signalling, insofar as they are particular to the loop disconnect signalling function. Reference document: prTBR21