【正文】 of the USB specification), mechanical (Chapter 5), and SuperSpeed physical (Chapter 6) specifications for the buses. The SuperSpeed physical layer is described in Section . USB MechanicalThe mechanical specifications for USB cables and connector assemblies are provided inChapter 5. All USB devices have an upstream connection. Hosts and hubs (defined below) have one or more downstream connections. Upstream and downstream connectors are not mechanically interchangeable, thus eliminating illegal loopback connections at hubs.USB cables have eight primary conductors: three twisted signal pairs for USB data paths and a power pair. Figure 32 illustrates the basic signal arrangement for the USB cable. In addition to the twisted signal pair for USB data path, two twisted signal pairs are used to provide the SuperSpeed data path, one for the transmit path and one for the receive path.32. USB CableUSB Architectural Overview33USB receptacles (both upstream and downstream) are backward patible with USB connector plugs. USB cables and plugs are not intended to be patible with USB upstream receptacles. As an aid to users, USB mandates standard coloring for plastic portions of USB plugs and receptacles.Electrical (insertion loss, return loss, crosstalk, etc.) performance for USB is defined withregard to raw cables, mated connectors, and mated cable assemblies, with pliance requirements using industry test specifications established for the latter two categories. Similarly, mechanical (insertion/extraction forces, durability, etc.) and environmental (temperature life, mixed flowinggas, etc.) requirements are defined and pliance established via recognized industry test specifications. USB PowerThe specification covers two aspects of power:? Power distribution over the USB deals with the issues of how USB devices consume power provided by the downstream ports to which they are connected. USB power distribution is similar to USB , with increased supply budgets for devices operating at SuperSpeed.? Power management deals with how hosts, devices, hubs, and the USB system software interact to provide power efficient operation of the bus. The power management of the USB bus portion is unchanged. The use model for power management of the SuperSpeed bus isdescribed in Appendix C. USB System ConfigurationUSB supports USB devices (all speeds) attaching and detaching from the USB at any time. Consequently, system software must acmodate dynamic changes in the physical bus topology. The architectural elements for the discovery of attachment and removal of devices on USB are identical to those in USB . There are enhancements provided to manage the specifics of the SuperSpeed bus for configuration and power management.The independent, dualbus architecture allows for activation of each of the buses independently and provides for the attachment of USB devices to the highest speed bus available for the device. USB Architecture SummaryUSB is a dualbus architecture that incorporates USB and a SuperSpeed bus. Table 31summarizes the key architectural differences between SuperSpeed USB and USB .Table 31. Comparing SuperSpeed to USB CharacteristicSuperSpeed USBUSB Data RateSuperSpeed ( Gbps)lowspeed ( Mbps), fullspeed (12 Mbps),and highspeed (480 Mbps)Data InterfaceDualsimplex, fourwire differential signalingseparate from USB signalingSimultaneous bidirectional data flowsHalfduplex twowire differential signalingUnidirectional data flow with negotiateddirectional bus transitionsCable signal countSix: Four for SuperSpeed data pathTwo for nonSuperSpeed data pathTwo: Two for lowspeed/fullspeed/highspeeddata pathBus transactionProtocolHost directed, asynchronous traffic flowPacket traffic is explicitly routedHost directed, polled traffic flowPacket traffic is broadcast to all devices.PowermanagementMultilevel link power management supportingidle, sleep, and suspend states. Link, Device,and Functionlevel power managementPortlevel suspend with two levels of en