A USB device must indicate its speed by pulling either the D+ or D-
line high to 3.3 volts. A full speed device, pictured below will use a
pull up resistor attached to D+ to specify itself as a full speed
device. These pull up resistors at the device end will also be used by
the host or hub to detect the presence of a device connected to its
port. Without a pull up resistor, USB assumes there is nothing
connected to the bus. Some devices have this resistor built into its
silicon, which can be turned on and off under firmware control, others
require an external resistor.
The series resistance are for line termination
The Universal Serial Bus (USB) line termintation is specified in the
USB 1.1 specification to insure proper terminations so that signal
integrity is maintained. The termination requirement varies depending
on what driver chipset is used, whether the port is upstream or
downstream, and if the transceiver operates in full or low speed.
When the USB 1.1 specification was written, a series resistor between
the range of 28Ω to 44Ω was required. Most USB drivers will require a
termination resistor of 16 - 33Ω.