ocessing system 37/7 mal or Centrex type PABX ports equipped with the recall and dial 37.2.3 Signalling information out feature. Using the voice processing system in this way may extraction/insertion make it "Call Routing Apparatus" in regulatory terms, adding signi- ficantly to the regulatory approval issues and to installation and on Usually in voice processing systems signalling information sent going support standards. Most voice processing equipment manu- from the telephone exchange is extracted to select the opening facturers have therefore not taken the step of becoming call routing service. Information can be sent by decadic dial pulsing loop dis- apparatus. connect signalling or DTMF tone and/or digital signalling in E1 or T1 connections. In more advanced E1 and T1 systems, signalling 37.2.2 Digital network connection information can be sent as HDLC framed data such as in ISDN (CCITT), DMI(AT&T), DASS2 and DPNSS signalling schemes. There are two main standards for digital connections: T1 (24 chan- These allow the calling party to be identified if this information is nels) in the US and Japan and E1 (30 channels) in the rest of the provided by the PTT. In the USA this feature, known as Automatic world. T1 standards are set by ANSI; E1 standards are set by CEPT. Number Identification (ANI) is widely supported. Most current US T1 has a clock rate of 1.544Mbit/s and E1 2.048Mbit/s. T1 signall- systems currently use DTMF techniques to receive this data from ing is carried within each telephone channel; E1 signalling is carried the network. In the UK regulatory and legislative issues may pre- in time slot 16. vent its widespread availability. The feature of providing this infor- mation within the UK is known as Calling Line Identity (CLI). Outbound signalling can be performed using a new circuit or the 37.2.2.1 Channel Associated Signalling (CAS) same circuit used for the incoming call once the exchange has been notified by a recall signal, by any of the above methods. In the E1 version of CAS the E1 framing s