The idea owes much to statistical multiplexing, in which many over 25% per annum, largely in the private sector, while most switched circuits can be active on a single physical link - an efficient use of a circuit's available bandwidth. Extending the idea industrialised countries offer at least one public data network based beyond statistical multiplexing, however, packet switching offers on the technology. the following advantages: The driving force behind the development of packet switching was the need in the late 1960s for asynchronous terminals to access 1. Each terminal in a group sharing the same physical circuit may numerous remote hosts. Its rapid acceptance owes much to the wide be connected to a totally different destination. This versatility implementation of the X.25 international standard interface to is one of the major strengths of packet switching. (Figure 54.2). packet switched networks (PSNs). While private leased lines offer fixed bandwidth, security and 2. Unlike message switching, packet switching has a block size protocol transparency, data transfer speed is limited by the capa- limit, usually 128 bytes or 256 bytes, but up to 4096 bytes on bility of the attached modems, re-routeing is impossible, unused some networks, thereby reducing storage demands or buffering bandwidth is unavailable to other users and a charge is levied for the in the nodes, as well as delay at each node. No single user or large data block can tie up circuit or node line irrespective of use. resources indefinitely, making it well suited for interactive Circuit switching, as found in a conventional telephone network, traffic. provides a dedicated circuit for the duration of the call and charges 4. Data protection against corruption or loss; errors are corrected according to time and distance. However, public voice networks are by retransmission. optimised for voice, prone to data-corrupting noise and are normally 5. Users can select different destinations for each virtual call, without security, such as call screening,