A thin client (sometimes also called a lean or slim client) is a client computer or client software in client-server architecture networks which depends primarily on the central server for processing activities, and mainly focuses on conveying input and output between the user and the remote server. In contrast, a thick or fat client does as much processing as possible and passes only data for communications and storage to the server.
Many thin client devices run only web browsers or remote desktop software, meaning that all significant processing occurs on the server. However, recent devices marketed as thin clients can run complete operating systems such as Debian Linux, qualifying them as diskless nodes or hybrid clients. Some thin clients are also called "access terminals."
As a consequence, the term "thin client", in terms of hardware, has come to encompass any device marketed as, or used as, a thin client in the original definition – even if its actual capabilities are much greater. The term is also sometimes used in an even broader sense which includes diskless nodes.
In a thin client/server system, the software for the user interface is installed on the thin client, some frequently/heavily used applications, and a networked operating system and nothing else. This software can be loaded from a local drive, the server at boot, or as needed. By simplifying the load on the thin client, it can be a very small, low-powered device giving lower costs to purchase and to operate per seat. The server, or a cluster of servers has the full weight of all the applications, services, and data. By keeping a few servers busy and many thin clients lightly loaded, we can expect easier system management and lower costs, as well as all the advantages of networked computing: central storage/backup and easier security.
Because the thin client is relatively passive and low-maintenance, but numerous, the entire system is simpler and easier to install and to operate. As the cost of hardware plunges and the cost of employing a technician, buying energy, and disposing of waste rises, the advantages of thin clients grow. From the user's perspective, the interaction with monitor, keyboard, and cursor changes little from using a thick client.
A single PC can usually power five or more thin clients. A more powerful PC or server can support up to a hundred thin clients at a time. A high-end server can power over 700 clients.
Thin clients are a great investment for schools and businesses who want to maximize the number of workstations they can purchase on a budget. A simple $70 unit could replace a computer in a school or business. It would also save a lot of power in the long run, due to low power consumption.
What are now called thin clients were originally called "graphical terminals" when they first appeared, because they were a natural development of the text terminals that had gone before them. (Text terminals are essentially the ultimate thin client, but are generally not classified as such because they come from an earlier computing era.)
X terminals were a relatively popular form of graphical terminal in the 1990s.
Late in the Windows NT 3.51 life cycle, Citrix Systems approached Microsoft with an idea for a multi-user version of Windows similar to what had been done with Unix. Microsoft agreed to license the Windows NT 3.51 source code which Citrix then turned into a product called WinFrame; a version of NT 3.51 that allowed multiple users to run on the same server. Microsoft later licensed the technology back from Citrix and incorporated it into a special version of NT 4.0 (known as NT 4.0 TSE, or Terminal Server Edition) and then into all subsequent version of their server operating systems. The code name for this Microsoft project was Hydra.
Terminal Services allows the operation of standard Windows software in a mainframe model centralized computing vs. distributed computing. Users log onto the server using thin client hardware and the server creates a session in memory dedicated to that user. Any GUI commands that would normally be sent to a local graphics card are instead compressed and sent to the client. Likewise, user keyboard and mouse movements are sent back to the user's task running on the server.
It is likely that the term "thin client" started to be used instead of "graphical terminal" for the following reasons:
A thin client (or a lean client) is a network computer without a user writable long term storage device, which, in client/server applications, is designed to be especially small so that the bulk of the data processing occurs on the server. The embedded OS in a thin client is stored in a "flash drive", in a Disk on Module (DOM), or is downloaded over the network at boot-up. The embedded OS in a thin client usually uses some kind of write filter so that the OS and its configuration can only be changed by administrators.
Thin client (computing): A server-centric computing model in which the application software, data, and CPU power resides on a network server rather than on the client computer.
A thin client as an application program communicates with an application server and relies for most significant elements of its business logic on a separate piece of software, an application server, typically running on a host computer located nearby in a LAN or at a distance on a WAN or MAN.
A thin client does most of its processing on a central server with as little hardware and software as possible at the user's location, and as much as necessary at some centralized managed site.
Other definitions of thin versus thick/fat client application program try to draw the line at whether the deployment of the application requires the installation of additional software at the user site or not. Unfortunately, this is also arguable, since e.g., a browser used for a client application might be part of one client platform, but not the other. So on one platform no additional software installation is required, while another client platform requires it. The only objective definition would seem to be whether the boot image that is normally used to start the user's computer needs to be modified in any way before the client can be used: if not, then, the client is probably thin. Another criterion is related to the management of the thin client device or program. If it can be centrally managed, it is probably thin.
However, a great deal of software is today typically included in a base boot image, specifically to support various user applications, so that it need not be reinstalled on every computer. Often, a departmental boot image is prepared to include applications specific to a department.
A thin client as a device is designed to provide just those functions which are useful for user-interface programs. Often such devices do not include hard disk drives, which may become corrupted by the installation of misbehaved or incompatible software, but instead, in the interests of low maintenance cost and increased mean-time between failures (MTBF) the thin client device will use read-only storage such as a CD-ROM, Network Virtual Drive or flash memory.
Ideally the user will have only a screen, keyboard, a pointing device (if needed) and enough processing power to handle display and communications. Numerous companies develop and market these devices.
"Thin client" has also been used as a marketing term for computer appliances designed to run thin client software. The NEC US110, IGEL Technology Universal Desktops, Wyse Winterms, Neoware's Appliances, Hewlett-Packard HP Compaq t-series, Chip PC Jack PC and Xtreme PC Series, SaaS style Nexterm, NEXterminal , Sabertooth TC , ACP's ThinManager Ready Thin Clients, X terminal, ClearCube, Koolu, ThinCan or web kiosk might be considered thin clients in this sense.
A more recent concept in this genre is 'Ultra Thin Client' technology – which takes the 'thin' concept one step further by running the connection client software (Citrix, Windows Terminal Services, telnet etc) directly from the appliance's hardware. This is a marked difference to legacy thin-client hardware architecture which ran an operating system, often Windows CE or Linux between the hardware and connection client software.
The latest concept is a 'Zero Client' which is a hardware only appliance that runs no software at all and directly connects the user to a virtualized desktop. This approach completely eliminates the need to manage thin client software of any kind. Pano Logic introduced such a device in 2007.
Many thin clients are software-only however, and run on standard PC hardware.
Examples of this type of software-only thin client include AnywhereTS, PXES and Thinstation. Knoppix, Puppy Linux and Lan Core is also pursuing this market. Hopnetix is a diskless, network-booting thin client operating system that runs from a Windows server platform, similar to (and potentially licensed from) Neoware Image Manager software. Neoware Image Manager allows a diskless client to stream a complete system disk "on-demand" from a file server. In that perspective, it replaces the IDE or SCSI cable that connects a disk drive to a motherboard by a network protocol and a server that shares virtual disk drives to the client.
Another example on the Windows platform is the BeTwin program by the Thinsoft company based in Hong Kong. The program, downloadable over the net, uses extra VGA/DVI graphics ports, or separate cards, in the host PC plus standard USB/PS2 connected keyboards, mice and sound systems to enable additional workstations and supports up to 5 user stations per PC. Each user has simultaneous and independent utilization of the operating system, installed programs and peripheral devices connected to the shared PC system. Running Windows XP on an adequate PC system plus use of dual head video cards and the correct drivers, the software even supports two users per installed video card, additionally hardware efficient.
The advocates of both architectures tend to have contentious relationships. In practice, there seems to be little to choose between the two approaches for many applications. A few situations may clearly call for one or the other. Distributed computing projects such as the SETI@home project (whose whole point is to pass off computationally intensive analysis to a large collection of remote computers) are applications that require thick/fat clients. A classroom environment, with desk real-estate at a premium, large numbers of students in a single room, and the need to have every workstation run the exact same software in the exact same way, would definitely lend itself to a thin client solution. Several companies now sell thin-client laptops that can access internal resources through a Virtual private network so the connection between client and server passes through an encrypted tunnel. This can allow mobile workers to access security-sensitive databases with less risk of lost or compromised data should the laptop be lost or stolen since it has no local storage.
Since 2006 there has been a growing interest in using Thin Client technology in hazardous areas, such as oil & gas exploration, military mobile use to monitor gen sets and mobile missile installations, and in industry in Zone 1 areas where hardened industrial computers can be prohibitively expensive. Thin Client hardware is easier to seal against environmental hazards and contamination, and can sometimes withstand a wider temperature and vibration level, due to simplified components and lack of moving parts, such as hard drives and cooling fans.
Thin Clients are also a natural choice for operator interface displays in manufacturing areas where a rugged tamper proof solution is required, and where limited "stateless" applications such as Human Machine Interface (HMI) and Supervisory Control and Data Acquisition (SCADA) software is the standard. "PC" on a plant floor usually stands for "Problem Center".
Flash memory is a critical component of these Thin Client terminals. With sufficient Flash (solid-state) memory and the ability to install user firmware and PCI cards, a Thin Client can perform most of the functions that a user would want to perform at these locations, typically called "point-of-attack" or "rig floor". The Thin Client can be equipped with a sealed membrane keypad for operator interface, and I/O (input/output) connectors that allow the Thin Client to be hooked up to a remote location and perform data recording of processes or history of connected device readings, that could be anything from radar to pumps to drilling apparatus.
Industrial Thin Clients typically operate on embedded software systems such as Linux, Windows CE.net, or Windows XP Embedded Ethernet. Industrial clients typically prefer an OS (operating system) environment in which the Thin Client can easily interface with proprietary firmware (software embedded in memory on a PCB card), and communications often interface with a hosting central computer in a safe environment control room. Ethernet protocol is a preferred network means of interfacing the computers, Thin Clients, and devices being monitored and controlled.
Some Thin Client solutions (such as ACP's ThinManager Ready Thin Clients) are tightly coupled with specialized management software that enhances the basic features offered by server operating systems. These enhancements include, but are not limited to, failover to backup servers, client device independence, redundant Ethernet, multiple sessions on a single client and automatic client configuration.
Fiber networks and wireless interface are also popular to enhance versatility in location and installation of Thin Clients. Encryption of data is usually a priority, and wireless options need to rely on a fast Ethernet radio transmitter using 11Mbit/s 2.4 GHz 802.11 output, otherwise data transmission speed and security can be compromised.
Ruggedized Thin Client products therefore enable easy-to-employ industry standard network creation and control at hazardous area zones for less cost and with less risk of failure than full computer systems. In fact, in the first quarter of 2007, mandates have been created by the US Armed Forces to look at Thin Client solutions in all field applications. The military is primarily interested in Thin Client technology in the field due to its improved cost control, more robust construction, less vulnerability to failure and security breaches, lesser weight and greater mobility, and lower incidence of OS failures.
Obviously, boot image control is much simpler when only thin clients are used – typically a single boot image can accommodate a very wide range of user needs, and be managed centrally, resulting in:
Most moderate to large Thin Client deployments use some type of management software. Companies such as Citrix (XenApp), IGEL Technology (Remote Management Suite), Wyse (Rapport), 2X (ThinClientServer), ACP (ThinManager), ThinSoftInc and Symantec (Altiris Deployment Solution) have products that vary in cost and functionality. These products all enhance the basic server operating system, adding features such as Server Load Balancing, Application Publishing, Shadowing, Local Storage Options and Session Reconnection.
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