|
Types of Terminals
Dumb Terminals
There are various conflicting definitions of "dumb terminal" but as
time goes by, more and more terminals are called dumb. This document
mainly covers text terminals which display only text on the screen.
It could have been titled "Dumb-Terminal-HOWTO". But in some
magazines articles, any terminal, no matter how smart, including ones
which present a full graphical user interface (GUI), are called dumb.
If all terminals are "dumb" then there is no point of prefixing the
word "dumb" to terminal (except as a sales pitch to sell computers or
the like in place of "smart" terminals). Due to the ambiguous meaning
of "dumb terminal" it is not classified here as a type of terminal.
Text Terminals
For a text terminal, a 2-way flow of information between the computer
and the terminal takes place over the cable that connects them
together. This flow is in bytes (such as ASCII) where each byte
usually represents a printable character. Bytes typed at the keyboard
go to the computer and most bytes from the computer are displayed on
the terminal screen. Special control bytes (or sequences of bytes)
from the computer tell the terminal where to move the cursor to, what
to erase, where to begin and end underlining and/or blinking and/or
bold, etc. There are often hundreds of such special commands and most
terminals can even change fonts.
The communication uses characters (letters) encoded using a code chart
for the character set being used. Usually, the first 128 bytes out of
256 possible bytes use ASCII codes. Terminals for Unix-like systems,
normally connect to computers via a cable running between the
asynchronous serial ports (RS-232-C = EIA-232-D) of the host computer
and terminal. Sometimes the connection is via modem or terminal
server, etc.
Other names for text terminals are "dumb terminal", "serial terminal",
"character-cell terminal", "character terminal", "ASCII/ANSI
terminal", "asynchronous terminal", "data terminal", "video terminal",
"video display terminal" (VDT), and "green terminal" (since many used
green displays). In olden days "video display unit" (VDU) was used
for terminals but strictly speaking, it excludes the keyboard.
"Block mode" was used exclusively by old IBM mainframe terminals but
many modern terminals also have this capability (which is not used
much). In block mode, the characters you type are temporarily
retained in the terminal memory (and may possibly be edited by a
built-in editor at the terminal). Then when the send key (or the
like) is pressed, a block of characters (sometimes just a line of
characters) is sent to the computer all at once. Block mode (as of
late 1998) is not supported by Linux. See section ``Block Mode''.
Graphic GUI Capabilities of Text Terminals
Many text terminals can display bit-mapped images, but not in color.
Unfortunately, the popular image formats used on the Internet are not
supported. The protocols for terminal graphics include: Tektronix
Vector Graphics, ReGIS (DEC), Sixel (DEC), and NAPLPS (North American
Presentation Level Protocol Syntax).
Even without bit-mapped images, ordinary text terminals can sort of
display images. One may form arrows <--- and draw boxes with _ and
|. With special graphic character sets, even more is possible. By
using all the letters, one may produce "ascii graphics" art. The term
"graphics terminal" usually means a terminal that can display bit
mapped images. However, this term is sometimes applied also to textonly
terminals since text is a limited form of graphics.
Graphics GUI displays
There are two basic types of graphics displays: raster and vector
(rarely used). Raster graphics (bit-mapped) puts dots on the screen
by horizontal scan lines drawn by an electron beam (or by activating
pixels or dots on a flat screen). Vector graphic displays were
intended to be used for monochrome screens that don't have any dots.
They use smart electronics to draw lines and curves with an electron
beam that can move in any direction (at any angle and location). True
vector graphics draws high quality lines without zig-zags but is both
rare and expensive. For more details see
<http://www.cca.org/vector/>. Raster graphics is almost universally
used today for both PCs and text terminals. For PCs, images encoded
in vector graphic format are sometimes used but they are translated to
raster graphics format for display (with a resulting drop in image
quality).
Thin Clients (Terminals ?)
Introduction
Since "thin clients" are not text terminals, this HOWTO only provides
a brief overview of them. 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. Thin-clients need more computing power. In
contrast to text-terminals thin clients all display a modern highspeed
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".
Such clients are either sold as a stand-alone piece of hardware or
created from an ordinary PC by using software. Linux seems to favor
the use of PCs for this purpose.
Some claim that text-terminals are also thin clients 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 clientserver.
The text-terminal is just another means of access to the
computer just like the monitor and its keyboard is. 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 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. One even has a USB interface.
There are various types of thin clients. One type is the "Window
Terminal" which usually runs under MS software and servers. 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.
Wndow terminals
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 1988 with codename "Hydra"). MS uses RDP
(Remote Desktop Protocol) which is based on the ITU T.120 protocol.
Prior to this (late 1995 on) there was a modified Windows NT 3.51
called "WinFrame" by Citrix using the proprietary ICA protocol (not
RDP). 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:
Citrix Systems, Inc. <http://www.citrix.com/download/unixdownloads.
asp>. Unfortunately, MS requires that you purchase a
license to cover the clients, even if they all run Linux.
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.
Network computers (NCs)
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 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.
Thin clients and NCs under Linux
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 this name is actually a thin (or fat) client. This
project's client can also run a telnet session and thus behave like a
text-terminal. They have created a software package named "lts" which
is available in the major Linux distribution.
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 nonlinux
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:
· JavaStation-HOWTO (by Sun)
· NC-HOWTO (IBM NetStation)
· NCD mini-HOWTO (NCD-ThinSTAR)
· NCD-X-Terminal mini-HOWTO
· XDM-and-X-Terminal mini-HOWTO
Hardware hookups
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 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.
History and the future
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). Someone predicted 8.7 million in 2005. I
wouldn't bet on it.
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.
Emulation on a PC
Since a PC has a screen and keyboard (as does a terminal) but also has
much more computing power, it's easy to use some of this computing
power to make the PC computer behave like a text terminal. This is
called "terminal emulation". They usually emulate text-terminals.
See ``Terminal Emulation''
|
