Why are usb hubs only available up to 7 ports?

Question

Why are usb hubs only available up to 7 ports? Is there something technical that prevents that? I thought USB addressing allowed 127 USB devices to be connected, so that's not it.

EDIT: Thanks for the links for hubs with more ports. There are a lot of them with 7 ports, and I'd never seen one with more. I didn't think 7 was the technical limit, I just wondered why 7 seemed to be so universal.

Here's what I have that plugs into USB:

1. mouse
2. keyboard
3. ipod
4. ipod shuffle
5. camera
6. joystick
7. rudder pedals
8. throttle quadrant
9. gps
10. external HD
11. memory stick
12. cell phone

Answer 1

Why should the theoretical port number be limited to only seven ports? There are hubs with more ports on the market.

Look at this monster example: 49-port USB hub is just plain practical

_{(source: crunchgear.com)}

Answer 2

I think the main issue is power.

I'm not going to look up the technical numbers and figures, but if you put too many devices on a hub, The USB port won't be able to supply enough power to all the devices.

Here is a USB hub with 16 ports (8 on each side) and it uses an external power source.

edit: the 13 port hub posted earlier also has an external power supply...

Answer 3

The 7 comes from the spec. The theoretical limit for USB is 127 devices which is (2^7)-1. It would be 128 because there are 7 bits for the address, but address 0 is reserved. So we have 127. Did you notice how the 49 port hub is a multiple of 7? Also, there is an upper limit of 7 tiers of devices which is explained in these articles:

http://www.totalphase.com/support/kb/10047/

http://www.usblyzer.com/usb-topology.htm

Answer 4

Here is a 13 port usb hub.

13 port USB hub

Answer 5

Maybe because no one (as in 99.9999%) has a use for more then 7 ports ?

Answer 6

There are hubs with more then 7 ports, I've seen hubs with 10, 13 ports usually. However, a USB host can have only 7 devices attached to it. Therefore, when there are more ports then 7 in a USB hub, there are several logical hubs inside.

So, I think, the 13 port hub internally is organized as two hubs connected one to the other:

a) primary 7 port hub, of which 1 port is used for uplink for the secondary hub, so has 6 free ports

b) secondary second 7 port hub, with 7 ports.

Together, it is 6+7=13 ports.

Therefore, the 13 ports are not equal, 6 are closer to the root hub, and 7 ports are 1 level further away. This can be important in some cases when many hubs are daisy chained. Recently I got a warning in Windows 8 saying I have too many USB hubs daisy chained, and devices more then 5 level away from the root will have problems, so I had to reconfigure them.

The practical consequence of this is that it's better to have one hub which acts as a "source" for all other hubs, instead of plugging one hubs sequentially, to minimize the distance from the root.

I have tried many USB 2.0 hubs recently, in the end I have settled with an EXSYS EX-1177. It provides sufficient power for external HDD drives, and non-powered USB hubs. I use it as the lowest level hub, and other hubs are attached to this one.

Answer 7

I suspect that power is a concern, as suggested, but also that the market for hubs over a certain size isn't big enough to make it worth pushing them as a common item.