4.1.1 Bus Topology
Due to timing constraints allowed for hub and cable propagation times, the maximum number of tiers
allowed is seven (including the root tier).
由于集线器和电缆传播时间允许的时间限制，允许的最大层数为7层（包括根层）。
Only functions can be enabled in tier seven.
只有功能可以在第七层启用。
4.1.1.1 USB Host
There is only one host in any USB system.
4.1.1.2 USB Devices
USB devices are one of the following:
Hubs, which provide additional attachment points to the USB
集线器，为USB提供附加连接点
Functions, which provide capabilities to the system, such as an ISDN connection, a digital joystick, or speakers
为系统提供功能的功能，如ISDN连接，数字游戏杆或扬声器
USB devices present a standard USB interface in terms of the following:
4.2 Physical Interface
4.2.1 Electrical
There are three data rates:
The USB high-speed signaling bit rate is 480 Mb/s.
The USB full-speed signaling bit rate is 12 Mb/s.
A limited capability low-speed signaling mode is also defined at 1.5 Mb/s.
4.2.2 Mechanical
4.3 Power
4.3.1 Power Distribution
4.3.2 Power Management
4.4 Bus Protocol
The USB is a polled bus. USB是一个轮询总线。
The Host Controller initiates all data transfers.
主机控制器启动所有数据传输。
Most bus transactions involve the transmission of up to three packets.
大多数总线事务涉及传输多达三个数据包。
4.5 Robustness
4.5.1 Error Detection
4.5.2 Error Handling
4.6 System Configuration
4.6.1 Attachment of USB Devices
4.6.2 Removal of USB Devices
4.6.3 Bus Enumeration
Bus enumeration is the activity that identifies and assigns unique addresses to devices attached to a bus.
总线枚举是标识和分配连接到总线的设备的唯一地址的活动。
Because the USB allows USB devices to attach to or detach from the USB at any time, bus enumeration is
an on-going activity for the USB System Software.
由于USB允许USB设备随时连接或脱离USB，因此总线枚举是USB系统软件的持续活动。
Additionally, bus enumeration for the USB also includes the detection and processing of removals.
此外，USB的总线枚举还包括检测和处理清除。
Due to timing constraints allowed for hub and cable propagation times, the maximum number of tiers
allowed is seven (including the root tier).
由于集线器和电缆传播时间允许的时间限制，允许的最大层数为7层（包括根层）。
Only functions can be enabled in tier seven.
只有功能可以在第七层启用。
4.1.1.1 USB Host
There is only one host in any USB system.
4.1.1.2 USB Devices
USB devices are one of the following:
Hubs, which provide additional attachment points to the USB
集线器，为USB提供附加连接点
Functions, which provide capabilities to the system, such as an ISDN connection, a digital joystick, or speakers
为系统提供功能的功能，如ISDN连接，数字游戏杆或扬声器
USB devices present a standard USB interface in terms of the following:
4.2 Physical Interface
4.2.1 Electrical
There are three data rates:
The USB high-speed signaling bit rate is 480 Mb/s.
The USB full-speed signaling bit rate is 12 Mb/s.
A limited capability low-speed signaling mode is also defined at 1.5 Mb/s.
4.2.2 Mechanical
4.3 Power
4.3.1 Power Distribution
4.3.2 Power Management
4.4 Bus Protocol
The USB is a polled bus. USB是一个轮询总线。
The Host Controller initiates all data transfers.
主机控制器启动所有数据传输。
Most bus transactions involve the transmission of up to three packets.
大多数总线事务涉及传输多达三个数据包。
4.5 Robustness
4.5.1 Error Detection
4.5.2 Error Handling
4.6 System Configuration
4.6.1 Attachment of USB Devices
4.6.2 Removal of USB Devices
4.6.3 Bus Enumeration
Bus enumeration is the activity that identifies and assigns unique addresses to devices attached to a bus.
总线枚举是标识和分配连接到总线的设备的唯一地址的活动。
Because the USB allows USB devices to attach to or detach from the USB at any time, bus enumeration is
an on-going activity for the USB System Software.
由于USB允许USB设备随时连接或脱离USB，因此总线枚举是USB系统软件的持续活动。
Additionally, bus enumeration for the USB also includes the detection and processing of removals.
此外，USB的总线枚举还包括检测和处理清除。