"This ETS aims to meet the requirements of network operators and equipment manufacturers who are designing equipment to operate with an increased speed on Videotex services over the Public Switched Telephone Network (PSTN). This ETS has a close relationship to the set of ETSs 300 072 [1] to 300 076 [5] describing the Videotex data syntax. This ETS specifies how to use the end-to-end protocols defined by ETS 300 223 [6] in the case of syntaxbased Videotex lower layer protocols using packet mode access over the PSTN. It also specifies how to use the lower layers protocols defined by ETS 300 080 [7]. This ETS: - identifies the parts of ETS 300 223 [6] which apply; - defines the applicable protocol pillar; - distinguishes the differences which apply to some parameters; - distinguishes the lower layers elements which are applicable. It is applicable to terminals supporting syntax-based Videotex lower layer protocols using packet mode access over the PSTN to be attached to a modem. In this context@ a terminal is either a Videotex Terminal (VT)@ a Videotex Service Centre (VSC)@ a Videotex Access Point (VAP) or a Videotex Host (VH) whilst a modem is a device providing an interface according to one of the CCITT V. series of Recommendations contained in the Normative references of this ETS (see Clause 2). This ETS applies only for modems using@ in both directions@ a synchronous interface to the attached terminal. For Videotex services using modems operating in ""asynchronous interface""@ the protocols to be used are those already defined by the national Videotex services. In the case of syntax-based Videotex lower layer protocols using packet mode access over the PSTN@ the following principles apply when using ETS 300 223 [6]: - the term ""S/T reference point"" should read ""modem interface""; - the PSTN network should be used instead of the Integrated Services Digital Network (ISDN); - in the case of syntax-based Videotex lower layer protocols using packet mode access over the PSTN there is neither D-channel nor B-channel; the communication and signalling data are exchanged using the protocols described in Clause 6 of this ETS; - all parts of ETS 300 223 [6] related to the use of ""ISDN supplementary services"" do not apply for syntax-based Videotex lower layer protocols using packet mode access over the PSTN""; - a communication channel is equivalent to a Virtual Circuit (VC)."