Kirill Kedrinski - Fotolia


Remote desktop USB redirection is still a tightrope act

Workers want to use their USB devices with virtual desktops, but to match the performance of a physical PC, IT has to provide a strong network connection and overcome the various hurdles of VDI software.

USB redirection is a functionality many virtual desktop users will appreciate, but beware that device performance depends on network strength and the company's VDI software.

USB redirection makes it possible for users to plug their USB devices into local computers and access them from their virtual desktops. VDI software forwards a USB device's functions from a physical endpoint to a virtual desktop via the network that connects the two, but not all networks are created equal.

The evolving state of USB standards -- such as the transition from USB 2.0 devices to USB 3.0 -- only adds to the challenges of remote desktop USB redirection, as does the need to maintain current server operating system (OS) and VDI software environments. Products such as VMware Horizon View and Citrix XenApp and XenDesktop support USB redirection to some extent, but each platform has its own hang-ups.

Before administrators jump into enabling remote desktop USB redirection, they should have some sense of the issues they might be up against.

USB in the WAN and LAN universe

One of the first things admins should do when contemplating USB redirection is to figure out what each type of network can handle. Organizations often use local area networks (LANs) in offices and wide area networks (WANs) for users across different geographic locations. Redirection can work in either environment, but admins will have better results implementing USB redirection via a LAN connection.

From the front end, USB redirection probably appears less taxing to a network than it actually is.

Consider Microsoft's RemoteFX technologies, which enhance the Remote Desktop Protocol. RemoteFX USB redirection will work in both LAN and WAN environments but is only optimized for LAN connections. To support redirection, the network needs a latency rate of less than 20 milliseconds, which is often possible over a LAN connection, but difficult to achieve over a WAN connection.

Latency and network reliability play a critical role in remote desktop USB redirection. According to documentation for its Horizon View VDI platform, a read request for a redirected storage device requires three round trips between the client and the virtual desktop, and retrieving a complete file can require multiple read operations.

A weak or unreliable network connection affects most aspects of end-user computing to some extent. USB devices are especially susceptible to performance issues because of all the behind-the-scenes activity required in redirection. Not surprisingly, devices that require high throughput are the most susceptible to latency and reliability issues over a WAN connection.

The evolving world of USB redirection

USB redirection and the technologies that support it are in a constant state of flux. For example, Horizon View supports USB 3.0 devices, but users are unlikely to see any performance benefits compared to USB 2.0 devices because of the usual network culprits: bandwidth, latency and reliability.

Citrix limits USB 3.0 devices even further. Currently, the generic USB redirection functionality in XenDesktop does not support devices connected to USB 3.0 ports. Users can connect a USB 3.0 device to a 2.0 port using an adapter, or Citrix provides optimized virtual channels for specific device types.

To make USB redirection work, admins also need up-to-date VDI software and the right server OS environment. VMware does not support USB redirection on Windows Server 2003 or 2008 systems or for Horizon View desktops managed by Microsoft Terminal Services. Citrix XenApp doesn't support generic redirection on anything earlier than Windows Server 2012 R2. And Microsoft's RemoteFX protocols only became available in the Windows Server 2008 R2 Service Pack 1.

Admins also need to be aware of the differences in VDI software versions. Prior to Horizon View 5.2, redirecting audio devices resulted in quality degradation, which is one of the reasons Horizon used to disable USB audio devices by default. Horizon View 6 added improvements, such as the ability to redirect USB storage device to Remote Desktop Session Host desktops and applications. View Agent 6.0.1 and Horizon Client 3.1 added support for USB 3.0 devices.

From the front end, USB redirection probably appears less taxing to a network than it actually is. Users access their virtual desktops on physical endpoints and it seems like the USB port is inches away, but with so many back-end operations involved, admins need to create the right environment for USB redirection to succeed.

Next Steps

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