Editing X.25 (section)

===User device support===
[[Image:Televideo925Terminal.jpg|thumb|A [[Televideo]] terminal model 925 made around 1982]]
X.25 was developed in the era of [[computer terminal]]s connecting to host computers, although it also can be used for communications between computers. Instead of dialing directly “into” the host computer{{snd}} which would require the host to have its own pool of modems and phone lines, and require non-local callers to make long-distance calls{{snd}} the host could have an X.25 connection to a network service provider. Now dumb-terminal users could dial into the network's local “PAD” ([[packet assembler/disassembler|packet assembly/disassembly]] facility), a gateway device connecting modems and serial lines to the X.25 link as defined by the [[X.29]] and [[X.3]] standards.

Having connected to the PAD, the dumb-terminal user tells the PAD which host to connect to, by giving a phone-number-like address in the [[X.121]] address format (or by giving a host name, if the service provider allows for names that map to [[X.121]] addresses). The PAD then places an X.25 call to the host, establishing a [[virtual call capability|virtual call]]. Note that X.25 provides for virtual calls, so ''appears'' to be a [[circuit switched]] network, even though in fact the data itself is [[packet switching|packet switched]] internally, similar to the way TCP provides connections even though the underlying data is packet switched. Two X.25 hosts could, of course, call one another directly; no PAD is involved in this case. In theory, it doesn't matter whether the X.25 caller and X.25 destination are both connected to the same carrier, but in practice it was not always possible to make calls from one carrier to another.

For the purpose of flow-control, a [[sliding window]] protocol is used with the default window size of 2. The acknowledgements may have either local or end to end significance. A D bit (Data Delivery bit) in each data packet indicates if the sender requires end to end acknowledgement. When D=1, it means that the acknowledgement has end to end significance and must take place only after the remote DTE has acknowledged receipt of the data. When D=0, the network is permitted (but not required) to acknowledge before the remote DTE has acknowledged or even received the data.

While the PAD function defined by [[X.28]] and [[X.29]] specifically supported asynchronous character terminals, PAD equivalents were developed to support a wide range of proprietary intelligent communications devices, such as those for IBM [[System Network Architecture]] (SNA).