c telephone. combinations giving 15 discrete combinations in each direction. The system is further extended by reserving some of the combi- 34.4.6 Digital private network signalling nations as shift functions SO that the basic combinations can have up to 3 meanings. The later is used to add features between PABXs In the early 1980s 2Mbit/s private circuits became available for use such as three party conference, executive intrusion, call offering etc. in private networks and this led the need for an inter-PABX signall- The main use within PABX private network operation is to pro- ing system that would maximise the benefits of the all digital vide fast call set up and the basic set of signals is defined for the network. In the absence of any international standards develop- sending of routing digits between registers. ments in this area UK manufacturers and BT formed a working party to create an open standard SO that PABXs from different 34.4.5 Dual Tone Multi Frequency manufacturers could interwork freely within a network for at least the simple call set up process, but also capable of implementation As PABXs were modernised and took advantage of DTMF tele- of advanced features as the need arose in the customer's network. phones on the extensions, the need for fast call set up over private Not all PABXs in the network needed to be capable of implementing circuits became very apparent and the post dialling delays on tie the advanced features such was the flexibility built into the specifi- lines using 10pps E&M or AC15 signals was not adequate for the cation. modern business needs. R2 signalling was used to overcome this Digital Private Network Signalling System No 1 was first publish- problem but only the larger PABX systems could afford the cost of ed by the working party in May 1983 and has since been revised and implementing the full R2 system with its forward and backward extended as customers' needs grew. DPNSS is a common channel signalling. The smaller PABX manufacturer found that he had to signalling syst