6.7

The quality of the physical cables, hubs, devices, and power conditions can affect USB device
performance. To ensure the best results, keep the client computer USB bus topology as simple as
possible for the target USB device, and use caution when you deploy new hubs and cables into the
topology. The following conditions can affect USB behavior:
n
Connecting or chaining multiple external USB hubs increases device enumeration and response time,
which can make the power support to the connected USB devices uncertain.
n
Chaining hubs together increases the chance of port and hub error, which can cause the device to
lose connection to a virtual machine.
n
Certain hubs can cause USB device connections to be unreliable, so use care when you add a new
hub to an existing setup. Connecting certain USB devices directly to the client computer rather than to
a hub or extension cable might resolve their connection or performance issues. In some cases, you
must remove and reattach the device and hub to restore the device to a working state.
USB Compound Devices
For compound devices, the virtualization process filters out the USB hub so that it is not visible to the
virtual machine. The remaining USB devices in the compound appear to the virtual machine as separate
devices. You can add each device to the same virtual machine or to different virtual machines if they run
on the same host.
For example, the Aladdin HASP HL Drive USB dongle package contains three devices (0529:0001 HASP
dongle, 13fe:1a00 Hub, 13fe:1d00 Kingston Drive). The virtualization process filters out the USB hub. The
remaining Aladdin HASP HL Drive USB dongle devices (one Aladdin HASP dongle and one Kingston
Drive) appear to the virtual machine as individual devices. You must add each device separately to make
it accessible to the virtual machine.
Connect USB Devices to a Client Computer
You can connect multiple USB devices to a client computer so that virtual machines can access the
devices. The number of devices that you can add depends on several factors, such as how the devices
and hubs chain together and the device type.
USB physical bus topology defines how USB devices connect to the client computer. Support for USB
device passthrough to a virtual machine is available if the physical bus topology of the device on the client
computer does not exceed tier seven. The first tier is the USB host controller and root hub. The last tier is
the target USB device. You can cascade up to five tiers of external or internal hubs between the root hub
and the target USB device. An internal USB hub attached to the root hub or built into a compound device
counts as one tier.
The quality of the physical cables, hubs, devices, and power conditions can affect USB device
performance. To ensure the best results, keep the client computer USB bus topology as simple as
possible for the target USB device, and use caution when you deploy new hubs and cables into the
topology. The following conditions can affect USB behavior:
n
Connecting or chaining multiple external USB hubs increases device enumeration and response time,
which can make the power support to the connected USB devices uncertain.
vSphere Virtual Machine Administration
VMware, Inc. 158