 The next frame is not transmitted until the within the local call charging area. It will then 'break out' into the last is correctly acknowledged. public network via a normal PSTN trunk circuit. Within DASS2 there are three types of call identified: These types of facilities require intelligent tie line signalling systems and like the PSTN signalling, tie line signalling comes in 1. Category 1 calls, which are calls that require end-to-end digital varying degrees of complexity depending on the application within paths e.g. a data call. In this type of call no in band tones or the private network. announcements are given by the network. 2. Category 2 calls, which are calls for which an end-to-end digital 34.4.1 E&M signalling path is not essential and if a non-digital path is encountered then a 'not suitable for data' message is sent back to the PABX. E&M signalling is the simplest form of tie line signalling and has 3. Telephony, which cannot be used for data and the swap facility been borrowed for PABX usage from the public network where it in the system cannot be used, nor can the user-to-user sup- has been used to interface public switching systems with the long plementary data services. distance transmission line signalling systems. The signalling scheme can be applied to both 2-wire transmission circuits and The public network offers 5 services to the DASS2 PABX user as 4-wire transmission circuits as the signalling is carried on an addi- follows: tional two wires between the PABX and the distant end, or the intermediate transmission or multiplexing equipment. 1. User to user signalling, which allows several blocks of 32 bytes The E&M signalling system derives its name from the historical of data to be transmitted between the calling and the called designations of the signalling leads between the PSTN trunk circuits PABXs. There is also the option to configure one or more and the transmission equipment which tended to be housed in DASS2 speech paths as tie lines, to permit transmission of large different pa