erlay switch to For 14.4kbit/s operation, the speech data would need to be en- allow direct 384kbit/s access and switching (with guaranteed time coded to about 4.8 kbit/s and the picture data encoded to about 9.6 slot integrity) within primary rate ISDN access (CCITT, 1984b). kbit/s. To achieve the same picture quality as a 1B ISDN video- For premium service at rates greater than 384kbit/s, switched phone, the video coding algorithm needs to be at least 5 times more access at multiples of 384kbit/s and at the two primary rates of the powerful, but there is little international activity in standardising existing digital transmission hierarchies (2048kbit/s and such an algorithm at present. 1544kbit/s) are likely to be available in the future. A second option is that PSTN videophone pictures could be Although the ISDN is considered the most suitable available designed to have lower spatial and/or temporal resolution while still network to support videophone and videoconferencing services using the algorithm of Rec. H.261. Yet a third option is that PSTN over the next 5 years, it is worth considering the ability of other videophones with lower picture (and speech) quality and proprie- existing and future networks to support such services, and the tary algorithms will be launched by different manufacturers, leaving consequential effects on the services. market forces to determine an eventual 'de facto' standard. How- ever, if incompatible videophones are available at the same time, this can give rise to user uncertainty and dissatisfaction until the de 57.5.3.3 Broadband networks facto standard emerges. The marketing of PSTN videophones would ideally extend be- If customer switched digital broadband networks were widely avail- yond the business community into the residential market, but in the able, and call charges were acceptably low, the main advantages for latter case, the terminal costs need to be about 3 to 5 times lower videophone and videoconferencing services would be that the pic- than the ISDN terminal cos