Since "thin clients" are not text terminals, this HOWTO only provides a brief overview of them. There are other HOWTOs that cover them in more detail but (as of 2010) they are unfortunately not up to date. See Related HOWTOs, etc.. Thin clients are thin (minimal) client computers that behave something like terminals. Since text terminals (except for very old ones) run an embedded operating system, they are also like a computer. In contrast to text-terminals, thin clients all display a modern high-speed GUI. They are dependent on more powerful computers (servers) for their operation.
For a true thin client terminal, the computing work and disk storage will all be done on the server. At the other extreme, most of this work and storage is done at the thin client but some things such as administration, still depend on the server. Since such a client is not really "thin" it may more correctly be called a "fat client".
Instead of "thin client" they could have been simply called "graphic terminals" but promoters of this technology wanted a catchy name to get customers to believe that it was perhaps a totally different technology. This trend of renaming what is basically just graphic terminals (with new features) continues unabated. Some of these names are "Virtual Desktop" and "Cloud Computing".
Thin clients may be created from an ordinary PC by using software or may be a stand-alone piece of hardware. But such stand-alone hardware may often be just a conventional PC monitor plus a small box for the computer part of the hardware. Linux seems to favor the use of PCs as clients.
Some claim that text-terminals are also a type of thin client but they are not really since they don't conform to the client-server model. However, connecting a terminal via telnet does invoke the client-server model in the use of telnet as a means of transport of data. But the relation of the text-terminal to it's host is not one of client-server. The text-terminal is just another means of access to the computer just like the monitor and its keyboard is to access a PC. One could apply this same reasoning to a thin client and say that the client-server relationship is only for the transport of data.
Thus a thin client is like a terminal. It has a GUI with a mouse that makes it seem like you are using a computer. You are, but that computer may be far away and have many other people using it at the same time you are. Communication is over a high speed network (wireless and/or cable) or even over the Internet. Some thin clients can, in addition, emulate a text terminal and have a serial port connector for that purpose. Recent ones also have USB interfaces as well as ethernet.
There are various types of thin clients. One type is the "Window Terminal" which runs under MS servers (and software). Another type is the "network computer" which is supposed to be platform neutral. This implies they should work with both MS Windows and Linux but early models may not be easy to use with Linux. For Linux, the X Window protocol is used. See Thin clients and NCs under Linux
These are true terminals since all the computing work is done by a server running Windows. They are also called "Window-based Terminals" (WBT). These terminals (clients) are something like computers since they often run an embedded operating system such as Linux or Microsoft's CE, NT, or XP. It's often stored in flash memory so that it may be updated. Also, ordinary PCs can be used as clients (including, in some cases, Linux PCs) with the appropriate software, Some clients can support X Window (from a Linux server) and some can emulate text-terminals. Many so called "network computers" can also run X Window. This will be discussed in the next section.
The server for these clients usually runs MS's Terminal Services (for Windows 2000 servers). Prior to this there was Windows NT Terminal Server Edition (starting mid 1998 with codename "Hydra"). MS uses RDP (Remote Desktop Protocol) which is based on the ITU T.120 protocol. In addition, there is an optional ICA protocol (with added features) which can inter-operate with RDP.
Prior to this there was a modified Windows NT 3.51 (1995) called "WinFrame" by Citrix using the proprietary ICA protocol (Independent Computing Architecture). After MS came out with its own terminal server, Citrix still remained on the scene. It created MetaFrame software (formerly pICAsso) as an add-on to MS's Terminal Server (or Services) so that it could support ICA-based terminals and provide other additional features. Before MS got into the act, there were other proprietary systems for terminals that could display the MS Windows GUI but later on they all switched to support Microsoft's system.
PCs running Linux can be turned into ICA based client terminals using "free" (in price only) proprietary ICA client software from Citrix: Installing the Linux Client. Unfortunately, MS requires that you purchase a license to cover the clients, even if the clients all run Linux. So if you want to save money on software costs by using Linux, you'll have to go all-Linux and use both Linux servers and clients using the free X-Window protocol.
The above is sometimes called "network computing" since the terminals and servers connect to each other over a network (such as the common TCP/IP based network used by both Linux and MS). Network computers may be somewhat different as described below.
See Network Computer - Wikipedia which claims that NC's are defunct as a type of hardware, but that NC is used as a synonym for "thin client".
These are neither true computers nor true terminals but are something in-between. One type of network computer (NC's) is a computer with a CPU but no hard Disk. The OS it needs to run is sent to it over a network. NCs are full-graphics and use the services of a server computer. They are a little different from terminals since some (or most) of the programs they run may execute on their own CPU chip. Running a browser was supposed to be one of their primary functions and thus Java code applets may be sent to them for execution. Many NCs support X Window so that one may use a Linux server to support it. Such a server may be called a "Linux Terminal Server". IBM called their NC a "NetStation" but now calls it "NetVista". They should work on Intranet type networks and NetVista can run the Linux OS.
Wintel came out with a "NetPC" which, unlike the above, is almost a PC computer. However, it has no removable disks so users can't install their own software or obtain copies of anything.
There is a "Linux Terminal Server Project" (LTSP or ltsp) to use Linux as a server for diskless thin clients. They use X Window and by default, applications run on the server. But with additional effort, one can set it up so that some or all applications run on the "terminal". See http://www.ltsp.org/.
"Terminal" in LTSP is actually a thin (or fat) client. This project's client can also run a telnet session and thus behave like a text-terminal. A software package named "lts" for the LTSP is available in the major Linux distributions.
It's claimed that if one has only a few "terminals", they will work without the ltsp software. But if one has many "terminals", ltsp software is needed. So use ltsp if what you want to do is to use old PCs, etc. as diskless thin clients. It works OK on systems with over 100 thin-client workstations.
Linux provides NFS (Network File System) so that if ordinary computers are connected to each other via a network, then a person on one computer can run programs on another computer. Such a program sends messages over the network so that it appears just like a program was being run by your local computer. But such a program is actually being run on another computer on the network. It works also with X Window so that one may see GUI images generated on another computer.
Linux also allows a computer to be diskless and boot over a network. See the "Terminal Server Project" above which has special software for this purpose. Network-boot-HOWTO gives an overview. Older documents are Diskless-HOWTO and Diskless-root-NFS-HOWTO. Thus using a diskless computer which runs NFS enables you to run programs on another computer (the server). This is just like using a NC (Network Computer). It's not really a NC but it's emulating a type of NC. It's also often called a "terminal" and in some sense it is.
Thus if you have an old PC with an ethernet card (NIC) you may be able to use it as a NC. One source of info on this is Thinclient-HOWTO. Even if your old PC doesn't have a NIC, you could still use it to emulate a text-terminal. See Terminal Emulation.
There are also a number of genuine Network Computers (NC) that will work with a Linux server. Today some NCs run the Linux OS inside the NC. Before Linux became popular, NCs didn't run the Linux OS but required some other OS. But even if the NC uses a non-linux OS, it's often possible to make it work with a Linux Server. The non-linux OS is simply stored as files on the Linux Server. Then when the NC starts up it sends a message to the Linux Server asking for the non-linux OS files. This non-linux OS is thus sent to the NC over the network and the NC boots.
The Linux Server runs the NFS and X Window both of which must be supported by the NC. This enables one to use the NC as if it were an X Window terminal.
There are some Linux HOWTOs for certain brands of NCs:
There are 3 different types of hardware arrangements for thin clients. The first type just uses a PC computer as a thin client by emulating a thin client. It really isn't a thin client but it behaves like one. The second type (no longer made ?) looks just like a text-terminal. It just looks like a monitor, with a connector for a keyboard and another connector for a network cable. It's a dedicated thin client and can't be used for anything else. The third type looks like a tiny computer. It uses a standard PC monitor and keyboard both of which plug into a small box which is a "thin" computer. This box provides an interface between the monitor/keyboard and the network.
Promoters of NCs and related Window-Terminals projected that they would soon replace millions of PCs. In 1998 about .7 million thin clients were sold worldwide with (about 27% of them being NCs). In 1999 it dropped to .6 million but went up to .9 million in 2000 (vs. 1.3 million predicted). In 2001 it reached 1.09 million with 1.4 million predicted for 2002.
Microsoft servers (as of 2003) still dominate the market, but the clients may run Linux for which users still have to pay license fee for each Linux client to Microsoft. Thus free all-linux systems are gaining ground.
A major reason why growth was not as rapid as predicted is that PCs have come down in price in recent years so that they are often not much more expensive than a thin client. However, it's argued that even though thin clients may cost the same as PCs, the maintenance and administration costs are less. Note that thin clients sometimes replace text terminals instead of PCs.