瘦客户端
维基百科，自由的百科全书
瘦客户端（Thin Client）指的是在客户端-服务器网络体系中的一个基本无需应用程序的计算机终端。 它通过一些协议和服务器通信，进而接入局域网。瘦客户端将其鼠标、键盘等输入传送到服务器处理，服务器再把处理结果回传至客户端显示。不同的客户端可以同时登录到服务器上，模拟出一个相互独立又在服务器上的工作环境。与此相反，普通客户端会尽可能多地进行本地数据处理，与服务器（或其他客户端）的通信中只传送必要的通信数据。
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[编辑] 简介
设计一个客户端-服务器应用程序的时候，要分配好客户端和服务器需要分别完成的任务。这个分配工作将会极大地影响客户机和主机的成本，应用程序的健壮性和安全，和后期修改、移植灵活度。
其中一个难点是客户端软件应该多大程度上特定于应用程序。使用标准化的客户端软件，如网络浏览器或X11，能节省许多设计上的成本，但是缺陷是标准的客户端有很多局限性而无法针对用户个人化。
[编辑] 与网络计算机的区别
和网络计算机（Network Computer、NetPC）最大的不同在于，瘦客户端无需硬盘，所有的数据处理均由服务器完成。
[编辑] 应用程序
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 局域网 or at a distance on a 广域网 or 城域网.
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 possible at some centralized managed site.
The meaning of the words "significant elements", "core function", "most" and "as little" are arguable.
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 criteria 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.
[编辑] 用户界面设备
把瘦客户端作为一个设备，指的是把它设计成只提供对用户界面程序有用的功能。通常这些设备都不配备硬盘，因为在安装恶意程序或非兼容程序时，硬盘将有可能损坏。出于低维护成本和增加平均设备失败间隔时间的需要，瘦客户端使用只读存储器如光盘驱动器, 网络虚拟驱动器或闪存.
理想情况下，用户将只拥有一台显示器、一个键盘、一个触碰式设备(如有需要)和足够的计算设备以完成显示和通信功能。目前生产并推销这些设备的公司包括Chip PC，升阳公司，BOSaNOVA和惠普公司。
[编辑] Device for running a thin client application program
"Thin client" has also been used as a marketing term for computer appliances designed to run thin client software. Chip PC Xtreme PC, Chip PC Jack PC, X terminal, Wyse Winterm, Neoware Appliance, Clearcube or Web kiosk might be considered thin clients in this sense.
The latest 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. There are many benefits in not requiring an operating system, lower cost, higher performance and non-vulnerablity to viruses.
[编辑] Software thin client
Most thin clients are software-only however, and run on standard PC hardware. One example of this software-only thin client is PXES Universal Linux Thin Client and Pilotlinux. Knoppix is also actively pursuing this market. (See also Puppy Linux). An example on the Windows platform is the BeTwin program, which uses extra VGA/DVI graphics ports, or cards, in the host PC plus USB connected keyboards and mice to enable additional workstations.
[编辑] Examples of thin client and thick client usage
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 clients. On the other hand multicasting entertainment or educational material to a number of clients might best be done with thin clients since exactly the same material is to be presented at each.
Some technologies such as Network Virtual Disks or Disk Drive Images are aimed to provide central management and security/reliability/affordability comparable to what is found in Thin Clients devices: the OS that operates the client is stored on a shared virtual disk image (a disk image file) that is housed on a server in the clients' LAN. This virtual disk drive is "write protected" and shared by several clients at the same time. The content of this virtual disk drive is streamed on demand to the clients. Yet, all the computing is done by the client itself. Instead of centralizing a complete computer (centralize the storage, CPU, memory, devices etc) this approach centralizes only the hard disk drives. Because the virtual disk drives can be shared and protected, the aim to reduce TCO is also achieved with this technology. This technology can be used to run typical Embedded Operating Systems such as Embedded Linux and Windows XP Embedded, but also to run desktop operating systems such as Microsoft Windows XP Pro, FreeBSD or Linux. Organizations that develop and market Network Virtual Disks include Linux Terminal Server Project, IBM (iBoot technology). Network Block Device support is another kind of Network Virtual Disk embedded in Linux.
[编辑] 瘦客户端的优点
瘦客戶端的應用大大簡化了起動映像(boot image)的控制。通常一個起動映像已能滿足非常廣泛的應用，而且能夠集中管理，帶來以下的好處：
- 管理成本較低 - 管理人員可以在服務器上集中管理瘦客戶端的，而瘦客戶端可能發生硬件故障的地方也較少。而由於本地環境被嚴格限制(通常還是沒有記憶的)，防止了惡意軟體的攻擊。
- 數據的保護較容易 - 經過適當設計，應用數據完全不會存放在瘦客戶端上(它只是把運算結果繪製出來)；對惡意軟體防衛也可以集中進行。
- 硬體成本較低 - 因為沒有硬碟，記憶體，也沒有高性能的處理器，瘦客戶端一般是較便宜，而且它們也不會常常需要升級或很快老化。即使把服務器的價錢計算在內，使用瘦客戶端的總體價錢也往往比傳統客戶端低。其中一個原因是前者能把硬體物盡其用：傳統客戶端的處理器雖然強大，但往往只是被閒置。另外，瘦客戶端之間可以共用記憶體，若是多個用戶同時使用一應用程式，那程式要只需在中央服務器上載入，這情況下胖客戶端需各自把程式載入到本機的記憶體。
- 耗能量低 - 瘦客戶端專用的硬件耗能量比胖客戶端的低。除了節省了電費外，還可能免去空調或不須要額外的空調，顯著地減省了成本又達到節能的目的。
- 不會引起賊人興趣 - 由於瘦客戶端電腦是以專用的硬件或以較舊的硬件組裝，它們不能在客戶端-服務器以外的環境使用。賊人偷了後很難賣出，即使賣了價錢也不高。
- 適合惡劣環境 - 因為沒有散熱扇，瘦客戶端可在多塵的環境使用，不用擔心塵埃積聚阻礙散熱扇的運行。
- 使用較少帶寬 - 若使用胖客戶端，用戶開啟10MB的檔案，便需要把這10MB傳送到用戶的電腦上，儲存時又要把這10MB傳送到檔案服務器去，列印時亦要把10MB傳送到印表機上，這是沒有效率的。在瘦客戶端的環境下，由於終端機服務器和檔案服務器通常以高速的主軸網絡相連，開啟或儲存檔案的通訊得以局限在服務器室裡。在服務器和用戶之間，只有滑鼠和鍵盤活動，以及屏幕的更新需要傳送。使用如ICA等的協議，傳送這些訊息所需的帶寬可低至5kbps。
[编辑] 传统客户端的优点
- 對服務器的要求較低 - 因為大部份的應用處理都在客户端進行，所以传统客户端不像瘦客戶端那樣需要高性能的服務器，服務器成本大大降低。
- 多媒體性能較佳 - 在運行多媒體應用程式時，传统客户端較為佔優，因為這時瘦客戶端需要大量的帶寬。例如电子游戏製就較適合在传统客户端上運行。
- 彈性較大 - 在某些作業系統上（例如視窗），軟件是被設計在擁有本地資源的個人電腦上運行的。嘗試在瘦客戶端的環境下執行這些軟件可以是困難的。
[编辑] 虚拟网络磁盘终端的优点
- Lower IT admin costs. See above
- Easier to secure. See above
- Lower hardware costs. No hard disk drive, no flash-disk.
- Worthless to thieves. See above
- Fewer server requirements. The server acts only as a file server, not an application server.
- Better multimedia performance. The client can use its own hardware resources
- More flexibility. See above
- More scalable. See above
[编辑] 虚拟网络磁盘终端的缺点
- 100Base-T LAN required. Because of the amount of data streamed to the clients, it is usually difficult to have a Network Virtual Disk client connected through WAN link to the server that house its virtual disk drive.
[编辑] 协议
在客户端-服务器通信方式中几种常用的通信协议：
- NX technology
- X11协议的压缩版，以提供高高效服务
- Virtual Network Computing
- 提供虚拟桌面共享功能
- Citrix ICA
- with Citrix MetaFrame
- 远程桌面协议（Remote Desktop Protocol，RDP）
- the default remote desktop access mechanism for MS-Windows
[编辑] 瘦客户端的主要生产商
- eLux NG
- INFOTRONIC
- Rangee Thin Clients
- Neoware Systems
- Igel Technology
- Wyse
- ThinCCo AG
- LinWare Thin Clients
- Thinner
- LISCON
- AXEL
- levigo ThinClient Suite
- HP Compaq ThinClient
[编辑] 参见
- 局域网整合终端（LAN Intergrated Terminals）
- 哑客户端
