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USB redirection makes virtual desktops more palatable to users because it enables them to connect to local USB devices, but administrators have to decide which devices are worth the hassle to support.
Network limitations and device requirements make it difficult, if not impossible, to redirect certain devices. Yet supporting USB peripheral devices is important enough to the user experience that many IT teams are willing to take on the trials and tribulations of redirecting devices. USB redirection technology allows a device plugged in to the client computer's USB port to connect to the user's virtual desktop. In many cases, redirection requires device drivers on the client operating system and the virtual desktop.
In theory, admins can redirect just about any type of USB device, but most redirection tools block certain device types by default, and for good reason. The device might not be suited to redirection, such as a USB-to-Ethernet adapter. In that case, the user can't redirect his network connection, or he wouldn't be able to access his virtual desktop in the first place. Other examples include if the device is already integrated into the VDI technology, such as a mouse or keyboard, or if the device is impractical to use with redirection, such as an external hard drive or other mass storage device. Still, admins can simply override the default settings and redirect whatever devices they choose.
The desktop virtualization industry is also rapidly evolving to support previously unapproved devices. For instance, as of Horizon 6.1, VMware supports remote desktop USB redirection for mass storage devices to Windows Server 2012 R2 Remote Desktop Session Hosts. Of course, IT also needs to make sure the network can support such scenarios, which means assessing bandwidth usage.
Which USB peripheral devices not to support
Admins should determine which device types to approve on a case-by-case basis. For example, scanners are notorious for causing redirection problems because they require a large amount of bandwidth and run operations sequentially. An unreliable connection can result in scans taking a significant amount of time. Redirecting a scanner requires a top-notch local area network that's fast and reliable. Even then, users might run into issues. For instance, Microsoft's RemoteFX protocol supports scanner redirection, but Microsoft says that advanced features on the device might not work.
VMware has attempted to address some of the challenges with scanner redirection by adding a functionality in Horizon View that captures and compresses the entire scanned image on the client and then sends the image to the data center.
Audio and video devices also require a large amount of bandwidth. For example, a webcam can use more than 60 Mbps when streaming a video from the client to the virtual desktop. Redirecting a large number of audio and video devices at once can eat up the network and cause latency for other users as well.
For this reason, VDI products often limit the ability to redirect video and audio devices. For example, VMware does not support redirection for webcams, and Citrix blocks audio devices in XenApp. RemoteFX supports webcams and voice over IP phones, but Microsoft warns that they are very sensitive to latency and may not work correctly over slow connections.
Constraints of remote desktop USB redirection
USB device redirection comes with other limitations. For example, many of today's device types work for redirection, but VDI software often doesn't support legacy or boot devices.
Plus, biometric devices are generally limited to use within the current session, which means workers can't use them to log in to a virtual desktop. In addition, once a USB peripheral device redirects to the virtual desktop, it is often unavailable to the underlying client OS.
Admins must take into account the network, devices and supporting systems before enabling remote desktop USB redirection, but problems can also pop up with specific devices or VDI environments.
For example, USB flash drives formatted with FAT32 tend to operate a lot slower than those configured with NTFS, regardless of the VDI vendor. Admins might also encounter device-specific challenges, such as reported problems redirecting Philips SpeechMike and Nuance's Dragon Power Mic II dictation devices to Horizon View desktops.
An organization's best bet is to implement USB redirection on its most reliable network, control which devices users can redirect, and make sure its VDI software is current. Virtual desktop users will be a lot happier with redirection capabilities than without, but admins have to set restrictions to avoid getting bogged down by the many ways USB redirection can go wrong.
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