View Single Post
Old 14. Sep 2013, 01:22 AM   #2 (permalink)
Senior Member
Join Date: Jul 2010
Location: New Zealand
Posts: 1,741

If you have multiple USB 3 drives attached to one USB 3 controller on a USB network where all controllers and devices are USB 3 then overall bandwidth is limited and the throughput of individual drives would be constrained.

There are several factors that would also need to be considered in most real world scenarios including:
  • the USB device class of the devices
  • having drivers that support all features and therefore max throughput
  • the communications path for the end-to-end connection including the USB version of intervening controllers
A device's bandwidth will depend upon what it does as USB devices have classes that determine how they will communicate. These aren't the correct technical terms but they make it easy to understand:
  • set speed e.g. real-time video or audio that need an assured bandwidth
  • immediate response e.g. mouse or other devices that want to interrupt everything else
  • bandwidth hogs e.g. file transfers that don't mind interruptions or changes in bandwidth
Throughput also depends upon what a device is communicating with and where that device is located as USB uses a star topology with up to five tiers. At each tier, including the top, there is a controller and devices are attached in series on each tier. A controller has a limit of 127 devices so that is also the limit for the network.

USB 3 is full-duplex, transfers both ways at the same time, rather than half-duplex, transfers one direction at a time, as used by USB 1/2. That is why USB 3 devices can initiate communication with the USB host, ie can speak up, whereas USB 1 & 2 controllers poll the devices which cannot initiate anything, ie only speak when they are spoken to (polled).

USB 3 hubs have separate controllers for USB 3 and USB 1/2 devices so there is little impact on USB 3 throughput from legacy devices.

A USB device has a limit of 32 endpoints so it is unlikely but theoretically possible that throughput would be constrained by trying to communicate with too many devices at the same time.

For others who are reading here, note that newer USB specifications introduce new functions and features and not just improvements in raw bandwidth. USB 1 provided two speeds Low-Speed for devices like mice and Full-Speedfor devices like memory and disk. USB 2 has two types of controller: you need the Hi-Speed one to get the higher raw bandwidth but I've seen very few without USB 2 ports that aren't Hi-Speed. All USB 2 devices have other features like the ability to communicate with one another without a separate host.

Here's the maximum raw speeds anyway:
USB 1 1.5Mbps Low
USB 1 12Mbps Full
USB 2 12Mbps Full
USB 2 480Mbps Hi
USB 3 5Gbps Super
Better to light a candle ... than to curse the darkness.
Remah is offline   Reply With Quote