Peripheral Server Installation Guide (Rev. A) – Page 8
Understanding Hubs
Hubs, critical components in the plug-and-play architecture, are wiring concentrators that enable
the attachment of multiple devices, thus converting a single attachment point into multiple
attachment points. USB architecture allows a cascaded multiple hub configuration with certain
power limitations (explained later in this section). See figure 1.
Device
USB Director
P C
Host
H U B
b u s-powered hub
scanner
joystick
mouse
Figure 1: Example of a Typical Hub Configuration
Device
Device
Each hub has an upstream port, connecting to the host, and multiple downstream ports,
connecting to downstream devices, including other hubs. A hub can detect attachment and
detachment of downstream devices and enable and monitor the distribution of the power to
downstream devices via their integral hardware and the operating system.
Each USB device reports its power requirements to the operating system, which then enables
and disables the device as a function of its power requirements and the amount of available
power. High-speed devices typically need to be connected to a self-powered hub, which obtains
power from its external power supply and provides up to 500 mA for each downstream port. Only
simple devices, such as a mouse, can be connected to a bus-powered hub, which obtains power
from its upstream host and provides up to 100 mA for each downstream port.
Due to the limited available power for bus-powered hubs, cascading two bus-powered hubs is an
illegal topology, and devices connected to the second hub will not function.
(USB specifications
limit the connection of a bus-powered hub to a self-powered hub or host only.)
According to the USB Specification, the maximum limit of hubs cascaded in series cannot exceed
five. In other words, you may have a maximum of five hubs between any device and the host.
This does NOT mean that the maximum number of hubs in a system is five. Indeed, up to seven
hubs can be connected parallel
at any given level
. You must tally both external and embedded
hubs when counting downstream hubs.
