nger distances without ISDN has a very important roll to play in the local loop. Most of the amplification; optoelectronic integrated circuits; optical ampli- network, apart from the subscriber loop to the local exchange, is fiers; and optical switches. now digital in the developed countries. ISDN provides the means 4. Wireless, which covers a wide range of technologies for the for obtaining digital channels over these analogue lines. Basic efficient transmission of radio signals, including by satellite. access ISDN provides two digital channel (plus a signalling chan- Services and technology 11/15 nel) whilst primary access ISDN provides 30 digital channels (23 2. The PTOs want to move users away from leased private cir- North America) plus signalling. Broadband ISDN gives much cuits, partly to avoid them reselling unused capacity to the higher speeds. There are many applications for ISDN (Wood, 1991) PTO's other customers. The preference is to move customers such as: onto the PSTN which is featured to meet their diverse needs. 1. The connection of a PABX to the public network. The advantage of virtual private networks to the customer are: 2. The connection of small branches to the headquarters of an organisation. 1. They are cheaper to set up since no expensive transmission 3. As a back up to leased lines use within an organisation, either equipment is needed. as an added security measure, or to carry overflow traffic. 2. The services provided over a VPN can be rapidly changed to 4. To provide enhanced features, such as Calling Line Identifica- meet changing customer needs. tion (CLI). 3. They are based on the PSTN, which is designed to high stand- ards to meet public network needs. 4. Bandwidth can be provided on demand or on an agreed basis, 11.5.2.4 Multimedia e.g. at certain times of the day or night to cater for transmission of large data files. Figure 11.10 provides a simplified definition of multimedia service- 5. The VPN in effect provides a managed service of the user's s, as the convergence of voice o