Digital Subscriber System No. 1

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Digital Subscriber Signalling System No. 1 (DSS1), also known as Euro-ISDN or E-DSS1 (European DSS1), is a digital signalling protocol (D channel protocol) used for the ISDN. The interface is also called NET3 for BRI and NET5 for PRI lines. Can also be called CTR4. It is defined by ITU-T I.411 (ETS 300 102). It supports Bearer Capability, Low Level Compatibility and High Level Compatibility, ANI, DNIS and redirected number signaling in both directions.

DSS1 is a pan-European standard developed by ETSI. In 1989, 26 network operators from 20 European countries decided to develop this standard to replace earlier national protocols (such as FTZ 1 TR 6 or VN3). DSS1 has been one of the keys to the success of the ISDN within European countries (as compared to, for example, the U.S.).

Non-European countries using DSS1 include Australia, Brazil, India, Israel, New Zealand, Pakistan, Peru, Singapore, South Africa, Sri Lanka, Taiwan and the United Arab Emirates.

The DSS1 protocol knows four different codesets for information elements. Codeset 0 regards to Q.931, Codeset 5 to the ETSI standard, Codeset 6 for national applications and Codeset 7 for PBX applications. The most common Codeset is 0/Q.931.