USB slows down with more devices?!

Discussion in 'Mac Accessories' started by dmelgar, Oct 16, 2008.

  1. dmelgar macrumors 68000

    Apr 29, 2005
    I was doing some testing between firewire and USB and came up with a surprising result. If I connect a hard drive either firewire or USB, I get transfer speeds up to 35MB/sec. HOWEVER, with USB, as I add more devices, even though they're not active, the speed to the hard drive drops significantly as I add more devices. Roughly 4MB/device speed reduction but it varies by device.

    If I attach the hard drive, a DVD burner, Blu-ray burner, HDTV tuner, iPhone the maximum data transfer speed to the hard drive drops to 20MB/sec or less. That almost half the original speed. Even though none of the other devices is being used.

    I don't have a firewire hub and can only daisy chain two firewire devices, but with both of them in the chain I still get 35-36MB/sec.

    Is this a known USB phenomenon? All attached devices are USB 2.0. When I have all the devices connected they are going through a USB hub but I have noticed the same drop attaching a second device without using the hub.

    This is on an original Macbook Core Duo.

    Is this known USB behavior that you can't avoid?
    Is this a limitation of the USB hardware in the Macbook?
    Is this a limitation of the USB software in the Macbook?
    Could a better hub improve the situation?

  2. Borjan macrumors regular

    Sep 28, 2004
    Were all the devices connected on the same hub, ie from one port?

    I'm curious as to what will happen if you place one HD on one port, then have a USB hub, with all the peripherals, on the other port.
  3. dmelgar thread starter macrumors 68000

    Apr 29, 2005
    I have an original Macbook. Macbooks have 2 USB ports. I don't know if they are really separate so that each can achieve 480mbit or if that total between the two of them.

    To answer your question, I tried all sorts of permutations of devices hooked up to the two ports and through a hub. The same speed reduction occurs regardless of the location of the additional devices.

    Plugging one device into each native port on the Macbook results in the same speed slowdown as if I have the two devices plugged into a hub and the hub plugged into one USB port on the Macbook.
  4. PlaceofDis macrumors Core

    Jan 6, 2004
    this is in part because USB relies on the Processor to help transfer data, as opposed to FW which has its own controller built in at the end of each device, hence the faster and more consistent data transfers that are always seen with FW.
  5. dmelgar thread starter macrumors 68000

    Apr 29, 2005
    I don't see how that explains the original problem.

    The problem that I've witnessed is that as devices are added to the USB bus, the speed drops. Even if those devices are not being accessed.

    As an example, having an iPod or iPhone plugged into USB, charging, severely drops the access speed of an external hard drive.

    This is surprising to me. I may try the test on Vista to see if its a software issue. I haven't seen any mention of this performance problem.

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