Powered Hubs

David, I'll try to summarize it for you.

A USB hub connects to a single upstream host port and expands it into two or more downstream host ports.  Full-size host port connectors are normally "type A" while a hub's upstream connector is "type B", which is why hubs require a type-A-to-type-B lead (sometimes called "A-B") to connect them to the upstream host.  The type B side is the chunky squarish end, while type A is the usual USB flat connector such as found on USB mice.

USB hubs are typically powered in one of two different ways.  A self-powered hub takes its power from a dedicated external power supply, which is typically a mains wall adapter connected to the hub via a barrel connector.  In contrast, a bus-powered hub takes its power from the upstream host port to which the hub is connected, through the A-B lead.  In addition, when a self-powered hub does not have its external power cable attached (or the external supply isn't running), it typically behaves like a bus-powered hub and hence takes its power from its upstream USB connection.

The Raspberry Pi model B has a micro-USB connector through which it takes power from a USB charger.  That connector does not have any data lines connected, so it is not a "USB port", just a power connector.

The Pi model B also has two type A host sockets, into which you can plug USB type A plugs such as your wifi dongle, or the type A plug of a type-A-to-type-B lead such as connects to your USB hub.  If you connect your dongle directly into a Pi's type A host socket, then the dongle will take its power from this socket and hence indirectly from the Pi's power supply.  In contrast, if you plug the dongle into a self-powered hub that is properly powered from its external power adapter, then the dongle will take its power from this external supply and hence not draw power from the Pi.

Because the Pi's power design is rather poor and USB chargers are typically quite dreadful compared to how a regulated power supply should behave (compounded by the high resistance of many micro-USB leads), it is not advisable to power high-current peripherals such as wifi dongles directly from the Pi.  This is where your self-powered hub ("powered hub" for short) comes in --- it provides your dongle with data access to the Pi through the upstream USB A-B lead, but powers the dongle from the external power supply instead of from the Pi's host port.

A bit long-winded, but hopefully this helps to explain the relationship between dongles and powered hubs.

Morgaine.

So the USB Type-A side is where data is accessed and the Type-B side is where the data is disseminated? 

The Wikipedia page on USB (http://en.wikipedia.org/wiki/USB) is good, but the USB 2.0 standard itself (available for free at http://www.usb.org/developers/docs/ as a .zip file) has IMO much better introductory material.  Start with Chapter 4 of the "USB Specification rev 2.0" = usb20.pdf, and skip back to Chapter 2 for terminology.

David Harrison wrote:

 

So the USB Type-A side is where data is accessed and the Type-B side is where the data is disseminated? 

USB data goes in both directions on a USB cable, but only one direction at a time (half duplex).  The host controller in RasPi's BCM2835 SoC initiates all USB transactions: a USB device only "speaks when spoken to".  A USB network is a tree, with the host controller at the root.  On a standard USB A-to-B cable, the A end is closer to the host controller and either plugs into the host controller itself or to one of a hub's downstream A jacks.  The B end plugs into a USB device, either the single B jack on (for example) a USB printer or the upstream B jack on a hub.  The B plug and jack may be mini USB or micro USB instead of standard B.  There are also mini-A and micro-A, but full size A are much more common.

Many devices don't have physical B jacks.  For example, a USB keyboard or mouse connects the B end of the cable directly to its microcontroller, so only the A end is a physical plug.  This is also true of USB hubs that have an upstream cable instead of an upstream B jack.  A USB Flash drive doesn't have a cable at all -- it just provides an A plug and the rest of the cable is just a few short wires connected to the USB Flash drive's microcontroller.

The Wikipedia page on USB (http://en.wikipedia.org/wiki/USB) is good, but the USB 2.0 standard itself (available for free at http://www.usb.org/developers/docs/ as a .zip file) has IMO much better introductory material.  Start with Chapter 4 of the "USB Specification rev 2.0" = usb20.pdf, and skip back to Chapter 2 for terminology.

David Harrison wrote:

 

So the USB Type-A side is where data is accessed and the Type-B side is where the data is disseminated? 

USB data goes in both directions on a USB cable, but only one direction at a time (half duplex).  The host controller in RasPi's BCM2835 SoC initiates all USB transactions: a USB device only "speaks when spoken to".  A USB network is a tree, with the host controller at the root.  On a standard USB A-to-B cable, the A end is closer to the host controller and either plugs into the host controller itself or to one of a hub's downstream A jacks.  The B end plugs into a USB device, either the single B jack on (for example) a USB printer or the upstream B jack on a hub.  The B plug and jack may be mini USB or micro USB instead of standard B.  There are also mini-A and micro-A, but full size A are much more common.

Many devices don't have physical B jacks.  For example, a USB keyboard or mouse connects the B end of the cable directly to its microcontroller, so only the A end is a physical plug.  This is also true of USB hubs that have an upstream cable instead of an upstream B jack.  A USB Flash drive doesn't have a cable at all -- it just provides an A plug and the rest of the cable is just a few short wires connected to the USB Flash drive's microcontroller.