What is the difference between Charging Downstream Ports (CDP) and Standard Downstream Ports (SDP)?

Many Sealevel USB Hubs add support for USB 2.0 Battery Charging Specification version BC1.2 by adding Charging Downstream Ports (CDP) alongside Standard Downstream Ports (SDP).

USB 2.0 Standard Downstream Port (SDP)

An SDP is a traditional USB 2.0 port that provides for data communications as well as a minimum of 500mA of power. The host controller must be active to allow charging and the attached device will charge at a slower rate due to the limited power available.

USB 2.0 Charging Downstream Port (CDP)

A CDP is a USB 2.0 port that provides for data communications and has the enhanced capability to provide up to 1.5A to the attached downstream device. For devices compatible with the USB Battery Charging BC1.2 specification a handshaking process occurs during attachment that allows the device to determine the amount of power available. Once a CDP port is detected the device is then aware that extra power is available and can be used to charge the device faster and at a higher rate.