Unlike USB 2.0, the buffering in the USB 3.0 hub (and the host controller) allows a USB 3.0 SuperSpeed transfer to continue immediately with the next packet, without needing to wait for an acknowledgment of successful receipt of a previous packet.
![]() The total bandwidth of the downstream ports together cant be greater than the bandwidth available on the upstream port. The ports on the PC are known as root ports, and the root ports are designated as tier 1. Tiers 2 through 6 represent additional levels made possible by hubs, and tier 7 is the final level of devices supported by a hub at Tier 6. There are physically separate pins in the connectors and separate wires in the USB 3.0 cable for USB 3.0 SuperSpeed traffic and USB 2.0 traffic. The USB 3.0 SuperSpeed path operates at a raw bit rate of 5.0 Gbitss, while the USB 2.0 path operates at 480 Mbitss (High Speed), 12 Mbitss (Full Speed), or 1.5 Mbitss (Low Speed). Within the hub, only the port power control logic is shared between the USB 3.0 path and the USB 2.0 path, since there is only one 5-V power path in either USB 2.0 or USB 3.0. The additional pins are arranged mechanically so a USB 2.0 connector or cable can be used in place of a USB 3.0 connector or cable in nearly all cases to allow USB 2.0 data traffic (at USB 2.0 speeds) even if there is no available path for USB 3.0 SuperSpeed traffic. The main mechanical incompatibility arises when attempting to use a USB 3.0 cable for a USB 2.0 device, due to the physical size of the USB 3.0 Standard-B plug on a USB 3.0 cable. Conversely, a USB 2.0 cable can be used with USB 3.0 ports to allow USB 2.0 data flow at USB 2.0 speeds. ![]() Unfortunately, the 8-bit device address used in USB limits USB topologies to a maximum of 255 devices. Data throughput considerations will usually limit the practical number of devices further, and there is also usually a limit in the host controller on the number of device slots (one slot per device) that the host controller can support. To enable point-to-point USB 3.0 packet routing, packets originating in the host contain a 20-bit route string field (Fig. The route string consists of five 4-bit subfields signifying the port numbers on the hubs to which the packet should be routed. For example, a hub residing at a depth of 3 (tier 5) and assigned depth 3 during enumeration will use the port number in the depth 3 field of the route string to determine the intended downstream port for the packet. A port number of zero means the packet is targeted for the hub itself, not for any of the hubs downstream ports. The host is always the final destination for any packet transmitted by a device. Packets moving upstream are not broadcast to other devices or USB links. In USB, there is no mechanism for one device to transmit a packet to another device instead of transmitting it to the host. There is always one host and one or more devices (if any data flow is occurring). Packet flow is from host to device or vice versa, never device-to-device. For example, the host controller may still be finishing a USB 2.0 transmission or packet reception at the same time that a USB 3.0 packet begins to flow from a USB 3.0 device through a USB 3.0 hub and finally to the USB 3.0 host controller. This is only possible with USB 3.0 hubs. USB 2.0 hubs dont have separate data paths to allow this kind of simultaneous data flow.
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