There are 3 main components of a USB system: the Host, the Hub, and the Peripheral

· The Host
o In any USB system the Host is composed of the hardware and software on the base computer using the system. It is responsible for controlling and maintaining all of the simultaneous connections on the system.  It is at this level that the available bandwidth is framed and split between devices.
o A key part of this process is called enumeration.  This is the live control of all peripheral connects and disconnects of the system.  As a new device is connected and recognized by the host the device drivers are installed and a mask address is assigned by the host to identify the flow of communication to the device.  Also, during this process a peripheral will request a particular type of data transfer interface it wishes to have with the host. These could be one of three broad categories:
§ Interrupt:  Only brief concise bits of data are sent to the host ( much like an interrupt on a microcontroller).  Used for peripherals like keyboards.
§ Bulk:  Large blocks of data can be sent to peripherals using this method and allows error checking in-between transitions. Used for peripherals like printers.
§ Isochronous:  This is a streaming protocol that transfers data in real-time and does not use error checking.
o At any given moment the host is monitoring the type of and amount of connections it has.  It will limit the total amount of bandwidth allocated to isochronous and interrupt connections to 90% of the available bandwidth.

· The Hub
o Hubs have two main functions.  The first is to provide an access point to the Host.  They act as repeaters in both directions replicating any signal from one side to the other side.  They also communicate with the host itself and keep track of the devices connected to it.
o The other major function of the Hub is to power any devices attached to it.  It will provide at least 0.5W of power to each peripheral attached and can provide up to 2.5W peak power to an individual peripheral.  Often this is the sole source of system power for the attached peripheral, but there are devices (such as printers and scanners) that have there own power supply and use the hub only as a communication port.  Because of the possible power drains on a USB Hub, external Hubs are most often self-powered and will be hooked in series to allow the dispersion of peripherals that require more power across the network.

· The Peripheral
o The Peripheral is the simplest part the system.  Once it is connected to the Hub, its signals are detected by the Host and it becomes a slave to the Host.  The Peripheral is programmed with reflex responses to the commands sent to it by the Host and will only communicate when told to by the Host.