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Why are USB hubs provided in 4- and 7-port varieties. What's special about these numbers? Why don't I see 3-, 5-, 6-, and 8-port hubs?

Is there something special about 4- and 7-port configurations that makes them more attractive?

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I would speculate that the 4-port controller is a cheap and widely available component. You could then build a 7-port hub by internally connecting two 4-port hubs (with the second hub connecting to one of the first hub's ports, "using up" one of them). – coneslayer Jun 28 '10 at 16:49
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3 Answers

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If you're tying up one port to only really gain two, why do it? 4 port hubs are common, and an seven port hub is simply a doubled up four port with one port gone because you're using it to extend the four port again.

It's cheap manufacturing.

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up vote 4 down vote

Five port hubs
Six port hubs

Three probably isn't enough to justify making a product when 4 ports are readily available, so I'm sure that's why companies skip them. 8-port is a lot of ports, I would estimate the demand is small. Five and six port hubs do exist though.

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Why the downvotes? – MDMarra Jun 28 '10 at 16:55
+1 to counter mysterious downvote. – goblinbox Jun 28 '10 at 18:06
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Down voted because it doesn't make sense. "Three isn't enough to justify when four ports are readily available" – So you're saying that because four ports are available they don't make three ports. That's like saying we don't sell half gallon milk because we already sell one gallon milk. It doesn't address the issue of why they are sold in gallons (or four ports in this case) in the first place. – Josh K Jun 28 '10 at 18:18
@Josh K - I doubt there is much of a market for a 3 port USB hub. You would have a net-gain of two ports in that situation. Doesn't make much sense when a 4-port is dirt cheap and nets you three ports and the manufacturing could would only differ by the cost of a single USB port. – MDMarra Jun 28 '10 at 19:34
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@Josh K - right, and neither did you, but I did not downvote your answer. – MDMarra Jun 28 '10 at 19:53
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up vote 1 down vote

Here's another aspect that is easily overlooked: Power!

USB 2.0 hub with 4 ports can directly run form host computer and provide minimum power to each port just form host.

On 7 port USB 2.0 hub, external power source is required to provide power for each port. Since each powered port needs 500 mA plus, we get 3.5 A power supply for external components (I'll assume that host computer can power the hub itself). At this point power supplies tend to get big and expensive and just the power supply can cost few times more that the hub itself.

As we are slowly moving to USB 3.0, power situation gets worse, since most basic unit load is 150 mA (up from 100 mA in USB 2.0) and each port can take up to 900 mA. So for 4 port hub, we'd need 3.6 A power supply.

Another interesting point is saturation of host port. As we get more and more ports on the hub, more and more devices will be connected meaning that the chances that several devices will try to use port at the same time increase. This could easily lead to performance problems.

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