Each hub has exactly one upstream port and a number of downstream ports. The upstream port connects the hub (directly or through other hubs) to the host. Other hubs or devices can be attached to the downstream ports. During normal transmission, hubs are essentially transparent: data received from its upstream port is broadcast to all devices attached to its downstream ports; data received from a downstream port is generally forwarded to the upstream port only. This way, what is sent by the host is received by all hubs and devices, and what is sent by a device is received by the host but not by the other devices (an exception is resume signalling). Downstream routing has been changed in USB 3.0 with the addition of Point to Point routing: A route string sent in the packet header allows a USB 3.0 host to only send a downstream packet to a single destination port, decreasing congestion and power consumption.