 44.1 and described in detail in switching type. Trunk routes then interconnected these main net- Recommendation I.110. It comprises six main parts as follows work switching centres and it is on these high capacity routes that we first saw the introduction of Frequency Division Multiplexing 1. Part I - General Structure; 1.100 series. (FDM) techniques which required separate go and return channels. 2. Part II - Service Capabilities; 1.200 series. During the late 60's and early 70's digital techniques began to 3. Part III - Overall Network Aspects and Functions; 1.300 series. emerge to replace the analogue techniques. First we saw the intro- 4. Part IV - ISDN User/Network Interfaces; 1.400 series. duction of digital transmission to carry the analogue telephony 5. Part V - Internetwork Interfaces; 1.500 series. signal. The 4kHz analogue signal is sampled at 8kHz and the 6. Part VI - Maintenance Principles; 1.600 series. resulting samples quantised into 256 levels according to the A law logarithmic code and encoded into 8 bit bytes to produce a rate of The recommendations state that ISDNs will be based on the 64kbit/s. Thirty of these channels are then assembled, along with a concepts developed for telephony IDNs and may evolve by pro- 64kbit/s signalling channel and a 64kbit/s framing channel, into a gressively incorporating additional functions and network features, 2048kbit/s system. Digital transmission has gradually become more including those of any other dedicated networks such as circuit cost effective to such an extent that a higher order TDM structure switching and packet switching for data, SO as to provide for existing has been established at rates up to and above 565 Mbit/s and it is now and new services. As far as practicable, new services introduced into the normal method of provision. ISDN should be arranged to be compatible with the 64kbit/s In the same way that technology had made the use of digital switched digital connections, although later enhancements may techniques in the transmission field m