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USB Hubs: Daisy Chaining, Bandwidth and Power Demands

USB hubs are useful devices that split one USB port on a laptop or desktop computer into multiple ports that can each accommodate its own peripheral device. This is a common solution for computer users with several extra devices, like external hard drives, thumb drives, DVD burners, keyboards, mice and other gadgets like lamps and plasma balls. When a user needs more USB ports than can be found in a single USB hub, they will often connect multiple hubs together in a process colloquially known as  daisy chaining.  This can be a viable solution when several USB ports are needed and many manufacturers list the maximum number of ports that can be made available with this method when using their hubs. Frequently, as many as 127 devices can be hooked up to a computer via a single daisy chain of USB hubs. However, this process raises a few important concerns that you will need to plan for if you plan to implement this method of USB expansion. We ve assembled a few key points to remember when connecting powered USB hubs in this manner.

Power Consumption
USB devices draw power from the ports they are connected to, receiving at most 5.25V from a single port; the same is true for ports made available by USB hubs. Most devices will not require the full power load that these ports make available, but there are several devices that will make use of all of the power this connection facilitates. A select few devices, like high speed external hard drives, will require more power than a single USB hub port can provide. Typically, these devices come with a Y-shaped cord that can be plugged into, and draw power from, two ports simultaneously. When connecting multiple high-demand peripheral devices, it may be necessary to have a powered USB hub rather than a hub with no external power. All of the devices we reviewed come with a power adapter for these situations. If you have too many demanding external devices connected at once through the daisy chain process, your devices may not receive sufficient power and may turn themselves off to preserve sensitive data or equipment. These problems are generally easy to overcome by attaching the appropriate power supplies to each hub you place in the sequence.

Data Transfer Rates
The maximum rate at which data can be transferred via a USB connection is 480Mbit/s. This data rate holds true for each USB controller on the motherboard. Several motherboard models incorporate multiple controllers on what is called the  southbridge,  a chip in the motherboard that facilitates communication between different parts of the computer. This transfer rate remains constant, regardless of the number of devices you have attached to the computer. Therefore, the more peripheral devices you have connected to the USB hubs you have daisy chained to a single port, the lower the data transfer rate will be to those devices. It creates a bottleneck in the process, slowing the overall transfer rate to these devices proportionate to the demands of the devices attached to it. So, no matter how many USB hubs you have lined up and connected, the single port that you have it plugged into will only be able to provide a total of 480Mbit/s per USB controller. The number of controllers depends on your motherboard.

When connecting multiple USB hubs together, be sure that you'll have sufficient power for each peripheral device by attaching the included power supply. Also, realize that the data transfer rates to your peripheral devices will be lower if you have connected multiple demanding electronics. Daisy chaining can be a useful process if you need more ports than are traditionally available with a USB hub, but take note of these drawbacks before connecting your sensitive backup drive.