Client Access License

A Client Access License ("CAL") is a proprietary software license distributed by software companies such as Microsoft to allow clients to connect to its server software and use the software's services.

Software Licensing Introduction
Most commercial software, including Microsoft's, is licensed to end users or businesses. In a legally binding agreement between the proprietor of the software (the "licensor") and the end user or organisation (the "licensee"), the licensor gives permission to the licensee to use the software under certain limitations, which are set forth in the license agreement. Contrary to somewhat popular understanding, agreeing to a software license does not constitute ownership of the software in question; if this were the case then it would be permissible for the licensee to do whatever they want with the software, such as selling it (for more information on this see software licensing) or copying it and redistributing it. Microsoft usually has notices on the packaging of their products which state that removing the notice or using the software constitutes agreement of the license terms.

Microsoft's consumer retail or "off-the-shelf" products generally use very similar licences, allowing the licensee to use the software on one computer, subject to the usual terms and conditions. For businesses however, Microsoft offers several types of licensing schemes for a range of their products, which are designed to be cost effective, flexible, or both.

Server software, such as Windows Server 2003 and SQL Server 2005 require licenses that are more expensive than those which are purchased for desktop software like Windows Vista. In the more recent versions of many of their server products, Microsoft require that all clients that connect to these server applications have a license to connect to and use the services of that software. These special purpose licenses come in the form of a Client Access License.

Client Access Licenses
A Client Access License (CAL) is a kind of software license that legally permits client computers to connect to Microsoft server software. They usually come in the form of a Certificate of Authenticity (CoA) and a license key, which is sometimes attached to the certificate itself. The various editions of most of Microsoft's server software usually include a small number of CALs, and this allows the software to be used by either a few users or a few computers, depending on the CAL licensing mode. If more connections to the server are needed, then additional CALs must be purchased.

Some Microsoft Server products require one CAL per concurrent connection, whereby one CAL is required for each unique client connection at any point in time. For example, consider a small business network where the computers are used by ten people, but there are never more than five people on site using the computers at any one time. In this scenario, only five CALs are needed. However, some of Microsoft's products and licensing modes require a CAL for each unique client regardless of how many will be connecting at any single point in time. Some of Microsoft's server software programs do not require CALs at all, as is the case of Windows Server Web Edition.

CAL Enforcement
Most CALs are paper only and are distributed for the sole reason of representing compliance to the software license agreement. Technically, any individual or business can use the software with an unlimited number of users and buy no CALs at all - but they will be in breach of the license agreement, (and the law in most jurisdictions), and large penalties will apply if they are caught.

Some CALs, however, are electronically enforced: the server will refuse to service clients if there are not enough CALs to cover them all. In order for the CALs to be used, either the CAL files must be imported into the software or the CAL licence keys must be entered in. The software will not service clients unless there are valid CALs installed, and it will authenticate and serve the number of clients licensed under the CALs. Once that number is met, the server can either accept a small number of additional connections and then warn administrators before refusing to serve further connections, however usually it will simply refuse to authenticate additional clients until one or more of them disconnects. In this way, the server electronically enforces the Client Access licensing.

At present, there are only two Microsoft products that use this form of electronic license enforcement; Windows Small Business Server 2003 and Terminal Services. Presumably, the electronic enforcement in SBS is due to the fact that most small businesses that run the Small Business Server do not have dedicated IT staff to ensure compliance, whereas larger organizations usually have dedicated IT staff that monitor CAL usage and organize the acquisition of additional CALs when necessary. Windows Small Business Server 2008 does not track CAL usage.

Per-User vs. Per-Device
CALs apply to either a "device" (as defined in the license agreement) or a "user". A business is free to choose either mode. In Per-User mode, a CAL is purchased to allow one user to connect to the server software. Any user can connect, but only one user may use a given CAL at any given time. Any number of CALs can be purchased to allow five, five hundred, or any number of users to simultaneously connect to the server. Any number of devices may connect to the server software, but only a set number of users can connect to it at once.

Per-Device mode operates in much the same way, but limits connections made by devices, rather than users. One CAL enables one device to connect to and use the server software, regardless of how many users are connecting.

Although User and Device CALs are the same price, they may not be used interchangeably, and cannot be switched without buying new CALs.

Much more in-depth information can be found on Microsoft's CAL Guide website.

Core CAL
The Core CAL is a special CAL offered by Microsoft through corporate license agreements such as Enterprise, or Open Value Company Wide. The Core CAL, is a combination of Windows Server, Exchange Server, SharePoint Server and System Center Configuration Manager licenses. It is a more cost effective option of purchasing all four CALs separately saving approximately 30 percent. With the release of the new 2007 products, Microsoft now offers the Enterprise CAL Suite. The Enterprise CAL Suite combines 11 CALs, including the Enterprise functionality of Exchange, Office Communications and SharePoint Servers, as well as the Windows Server and System Center Configuration Manager licenses.

Software Editions
CALs usually enable connectivity to server software regardless of the edition of the software. For example, CALs purchased to enable client connectivity with Windows Server 2003 Enterprise Edition can be used with Windows Server 2003 Datacenter Edition, and vice-versa.

Terminal Services
Terminal Services is a function of Microsoft Windows that allows several types of connections to the server components of the system. Windows Server versions prior to 2003 do not necessarily require the use of specialized Terminal Services CALs; rather, clients which are of at least the same or lower version of the operating system are allowed access automatically. For example, Windows NT 4.0 clients may connect to Windows NT 4.0 terminal servers but not Windows 2000 or later; Windows 2000 or Windows XP clients may connect to Windows NT 4.0 or Windows 2000 terminal servers. This is called the equivalency license.

Windows Server 2003 requires separate CALs for all clients in Terminal Server mode, regardless of the client operating system.

A new type of CAL was introduced in Windows Server 2003: the "Per User" CAL. However, in Windows Server 2003 SP1, there is no technical enforcement of these. This means, any number of users can technically connect, and the Windows Server 2003 will not decrease the number of available CALs. Reportedly, this is because Microsoft did not have time to finalize technical enforcement before the release of the operating system.