Access to remote databases online (especially via remote database hosts), are usually via several different global computer data communication protocols and networks, i.e. Wide Areas Networks or WANs. In Europe, some widely used switched WAN services are based on: a) public X.25, European EuropaNet networks, such as the (IXI International X.25 Interconnect), e.g. EARN nodes such as JANET in the UK, DFN in Germany, GARR in Italy and SWITCH in Switzerland, b) TCP/IP, c) ISDN, d) frame relay, e) ATM and f) SMDS. Numerous private (WAN) data networks such as DIALOG's DIALNET also exists as an alternative.

X.25 PDNs were for many years, (before the availability of web or telnet access to databases and databanks via TCP/IP), the most popular packet switched public WANs used for accessing databases. Using such networks, one can access very remote databases overseas, for the cost-effective price of a local modem call (e.g. via a Point of Presence or POP, i.e. public-dial node), instead of calling the remote database server directly, via a costly international modem call. E.g. librarians in the US can access BLAISE-LINE in the UK via a local modem number using the American Sprintnet (Telenet) service, likewise UK medical information scientists can access DIALOG and NLM databases in the US using British Telecom's GNS Dialplus service.

Since national/government telecommunications (PTT) monopolies like British Telecom, were the first to offer packet switching data services, most major PDNs worldwide as such are owned by them, although following deregulations in the telecommunications industry, some telecom operators such as British Telecom have been privatised and the telecommunications market open to foreign competitors. Using PDNs such as X.25 services, is quite simple, once you have a "Network User Identification" (NUI) from the PDN carrier, and also the Network User Address (NUA) of the hosts you want to access. X.25 database access may be via dial-up access (cheaper, but temporary), or dedicated access (expensive but permanent). Most PDNs end with "pac" which means Packet Switching!

Starting from about middle 1991, X.25 PDNs has been overshadowed (and in many developed nations almost replaced) by the more advanced and faster frame relay PDN and ATM (a.k.a. cell relay), alongside the popular TCP/IP. But X.25 database access remains popular in developing nations.

The following are the official websites of some major X.25/frame relay PDNs services. Telecom operators or PDN operators are in brackets, when you access thier web site, look under products and services etc, then under data or data communications etc. For more telecom operators, see world telecom operating companies.