Original text the below quote is here.
If System Center licensing required a spreadsheet and a cheat-sheet, to truly understanding VDI you’re going to need a young priest, an old priest and possibly a bottle or six of scotch. To start with, VDI has a few different flavours.
The first is Remote Desktop Services (RDS) - otherwise known as Terminal Server - in which multiple users log into a single Windows Server instance. With the new Fair Share feature and a lot of the enhancements to RemoteFX in RDS, this is a method worth considering if only for the simplicity of it all. You licence the appropriate number of Windows Server virtual instances to host the number of users you require, you buy a Windows Server CAL and an RDS CAL for each user and you’re done.
The extension to this is that you can give each user their own virtual machine by simply buying a Windows Server Datacenter license and spinning up one Server VM per user. This method was popularly considered a neat way to sidestep the byzantine complexity that Microsoft’s Vista/Windows-7-era VDI licensing was, as WS2012 allows two administrators to connect to the operating system without needing RDS installed.
Microsoft has explicitly forbidden this approach; if you are using instances of WS2012 as individual VMs for your users, you must get an RDS CAL for each user. Unlike the traditional RDS approach, however, your users can have their own VMs.
Microsoft’s official stance is that VDI is best done by using a Windows Client operating system. This is where things get dark. The intuitive licensing approach - buying a copy of Windows XP, 7, or 8, installing it in a VM and letting your users have at it - is explicitly outside of compliance. As soon as you put Windows Client inside a virtual machine - or remotely access a physical machine through RDP - you must licence the client device as well.
There are some basics to VDI licensing for Windows Client operating systems to know. Microsoft offers a licence called Virtual Device Access (VDA) which costs $100 per year. It is available only through volume licensing; signing a volume licensing agreement means agreeing to have Microsoft audit you each year, so make sure you have your ducks in a row. VDA licenses apply to client devices. Windows endpoints covered under Software Assurance (SA) have VDA rights and so do not need to pay this fee.
A single Windows VDA license allows that endpoint to be RDPed into a maximum of four Windows Client instances simultaneously. You must hold as many VDA licenses as you have thin clients and non-SA-covered endpoints accessing the VDI environment.
Extended Roaming Rights are another critical concept. Any device covered under VDA or SA allows the user to access a Windows Client from up to four x86 devices, so long as those devices are outside the corporate firewall. This is designed to allow users to access their work PC from home, or a personal laptop.
If that personal device - the canonical example is the personal Macbook - is brought into the office, then it is considered part of the work environment and must be covered by VDA or SA.
To summarise: if you have a PC at work covered by SA or VDA then you can access up to 4 VDI instances of a Windows Client operating system. You can use up to four computers not owned by the company outside of the corporate premises to access those same VDI instances. If you stop using your Macbook at the Starbucks across from the office, walk into the office with it and use it to RDP into a work VM, then that Macbook must be covered by VDA or SA.
ARM devices are licensed differently. If your company covers its PCs with Software Assurance and buys you a Windows RT tablet, then you can access a Windows Client instance from that Windows RT device.
Corporate-owned iOS, Android, Blackberry or other ARM devices must purchase a Windows Companion Subscription Licence. CSLs are per-user and cover up to four companion devices. Personally-owned ARM devices - including Windows RT devices - must be licensed under a CSL to access a Windows Client operating system.
None of the above brings Intune or Office 365 into the discussion, considers diskless PCs, or attempts to explain licensing Office in a virtual environment. It is not intended to offer exacting guidance: VDI licensing can change at any moment, and making sure you get the best possible set of licenses for your deployment can involve factors beyond those discussed above. It is always best to discuss your VDI plans with your VAR or Microsoft rep.
If possible, try to get a stamp of approval from MS on your deployment before you deploy. Remember that the burden of auditing is on the business - not Microsoft - and you may be asked to verify your compliance every year. It is strongly recommended that anyone considering VDI invest heavily in automated compliance tools. You’ll need them.
The long and short for me: I clocked my access in 2012 to my home XP VM at just over 300 devices. At $100/device/month for "remote usage rights" Microsoft would have me pay $30K/year to access my home VM. Of course, I can't access my home VM because I can't get remote usage rights without SA. I can't have SA without being a company. So I the initial cost for just that one licence is up a few thousand as well.
If I want to stand up the ability to allow my staff to access Windows 7 instances from any device anywhere in the world it is going to cost, and cost and cost. Every new endpoint that connects is $100/device/year, and Office is yet more.
Microsoft says "just use Server + TS and use shims to fool apps." Sadly, I have a number of apps that won't work on Server and for which shims have not been - and likely never will be - written. Microsoft's response is "tough titties." (That's not even getting into a Server licence + CALs, etc is a hell of an entry-level buy in for someone to access a home VM, or an SMB worker to use a centrally provided desktop on the road.)
For me, I am dumping every last spare dollar into both supporting Weyland/Weston with the new FreeRDP compositor and porting my apps to Linux (for graphics intensive stuff) and standards/HTML delivery (for everything else.) Until Weyland/Weston/FreeRDP are ready to rock on RHEL/CentOS (a few years yet) then VDI just isn't something we can afford.
Your mileage will vary.
We are the Beast of Redmond.
Lower your expectations and surrender your productivity.
You will use both biological and technological means to accomplish what should doable with technology alone (we won't licence the technology to the likes of you.)
Your business will adapt to service us.
Resistance is futile.