I get asked a lot about what licenses customer need when they want to deploy Exchange & Office Communications Server, in order to keep themselves legal & compliant. It's sometimes a bit confusing that there are several versions of the core products, and often add-on licenses such as external connectors and the likes.
Like most Microsoft server products, both Exchange and OCS operate on a "Server/CAL" model, where you buy the actual server software, then acquire the access license to give you the rights to use that software from a client machine. CALs can be assigned to people ("users"), meaning the holder of a CAL can access the software from any machine, or they're assigned to a machine ("device"), which could allow any number of people to use that machine.
In businesses, the "per user" model is the most common model, since you could license users to be able to connect to the server from their home PC or from an internet cafe, or several devices at a time (including PCs, browsers, phones, Blackberry devices etc). In some circumstances (eg shift workers, or students sharing lab PCs), it makes more sense to license "per device", and you can mix the two together - so you might have 200 users licensed "per user" but then buy 25 "per device" licenses for the call-centre workers who might actually number 75, but working in shifts and only 25 at a time. Clear?
Along with Sharepoint, Microsoft introduced a new CAL type to Exchange & OCS in the 2007 wave of servers - the Enterprise CAL. The deal here is that some of the most advanced, new, functionality in the server software needs an Enterprise CAL to be in possession by the user or device, and it is an add-on to the Standard CAL which everyone will have anyway. You don't need to buy Enterprise CALs for everyone - only the users or devices which will make use of that additional functionality.
There's no actual installation of a CAL, and there's little real tracking of CAL usage: it's a legal requirement for the organisation operating the software to ensure that you have enough licenses, and that in itself can sometimes be a challenge. Using software like System Centre Configuration Manager, you can keep check on what users are doing, and with partner services such as Software Asset Management, you can get help with keeping track of what you've bought and who's using what.
Standard vs Enterprise Edition servers & CALs
Where some confusion sometimes lies is that, for years, we've had Standard & Enterprise Edition servers, where the more advanced functionality (like clustering) was often part of Enterprise Edition, and cost more. Now that there are Standard & Enterprise CALs, things start to look murky. Some literature even refers to the CALs as "Client Access License Standard/Enteprise Edition" which only heightens that confusion.
There is no dependence on CAL versions vs Server versions: ie you could use clustering in the Enterprise Edition server, but still use just Standard CALs to access it. Or you could deploy a single, Standard Edition server, and have all the users taking advantage of the most advanced functionality that comes as part of the Enterprise CAL. And, of course, you can have a mixture of all of the above, as you see fit.
The Standard edition of Exchange 2007 is a good bit more capable than Standard Edition previously - there is now effectively no data storage limit to the server (compared to a 16Gb and later, 75Gb, limit in Exchange 2003), though you can only have 5 databases per server (compared to a single one in earlier versions at Standard Edition, and a 50-database limit in Exchange 2007 Enterprise Edition). Apart from some exceptions in how Messaging Records Management works, the only other real difference is that Standard Edition server doesn't support clustering.
If you want to run clustered Exchange, you need Exchange Enterprise Edition on top of Windows Enterprise Edition (which actually provides the clustering technology that Exchange uses) for the clustered mailbox servers themselves, but all other Exchange boxes can be Exchange Standard Edition running on top of Windows 2003 Standard Edition.
When it comes to CALs, the Standard CAL gives you everything (and more) that Exchange had in the past; but some of the new functionality, like Unified Messaging or Managed Folders, requires the Enterprise CAL. See the CAL Comparison for more information
Office Communication Server 2007
OCS follows a very similar model to Exchange; Standard Edition server does everything that Enterprise Edition does, except it isn't clusterable and isn't designed to scale out to the same degree.
OCS Standard CAL gives you the basics of Instant Messaging & Presence/identity, whereas Enterprise CAL adds voice capabilities (which were previously a separate license for LCS2005), along with new stuff like on-premise Live Meeting data conferencing.
There are other options with OCS... if you want to extend the presence/identity piece out to the public networks (AOL, MSN and Yahoo), there's a subscription license called Public IM Connectivity. PIC subscriptions are collected by Microsoft then paid to the public networks in lieu of the adverts that you'd be seeing if you'd been using their own client, rather than Office Communicator).
There are also external connectors for both OCS and Exchange which could allow you to provide services to external users who aren't part of your organisation (eg giving your clients a mailbox/presence entity).
When Microsoft people say "Enterprise CAL" they don't always mean it
I often hear MS folk talk about "Enterprise CAL" or "E-CAL", but they don't mean the Exchange Enterprise CAL which allows you to use Unified Messaging, or the OCS Enterprise CAL which gives you voice & data conferencing. They're talking about something that should really be referred to as the Enterprise CAL Suite. It's a collection of both the Standard and Enterprise CALs for a number of different products, available to buy as a package, depending on what licensing agreement you have with Microsoft.
The idea with Enterprise CAL Suite is that if you decided you wanted the full gamut of Unified Communications, rather than having to buy Exchange Standard CAL + Enterprise CAL (since the Enterprise CAL is an "additive" to the Standard), and also buy OCS Standard + Enterprise CALs, you could acquire all of them along with various others (like Sharepoint Enterprise CAL, Forefront Client Security and many more), for a packaged cost.
In true economic terms, the more you want to buy, the lower the unit costs of each becomes. In buying OCS Standard + Enteprise CAL and Exchange Standard + Enteprise CAL, you'll have almost spent as much as the Enterprise CAL Suite costs, so going to the Suite will add a whole slew of additional licenses and services that you could take advantage of.
Now, I hope that's all clear. I think I'm going to go off and lie down now.
Explore the Microsoft Enterprise CAL Suite by