Android USB Camera(2) : UVC协议分析

1. 前言

本文对USB的功能类协议USB Video Class（UVC）的具体设计进行介绍，但不会介绍USB基础协议，所以需要对USB基础协议有一定的了解，包括USB四大描述符以及四种传输方式。分析usb协议的重点在于描述符，这里将以实际设备的描述符为线索，贯穿、分析uvc协议。

2. UVC功能概述

UVC设备都是多Interface设备，这点同普通的u盘不同。UVC设备最起码有两个Interface，VideoControl（VC）Interface和VideoStream(VS) Interface； 这也是最常见的UVC设备。 Spec明确要求一个具有可用的，具有实际UVC功能的设备要有一个VC Interface，一个或多个VS Interface。

VCInterface用于进行配置，操控，设置UVC设备进入不同的功能状态，而VSInterface则负责视频数据流的传输；完整的UVC功能需依赖VS，VC Interfaces的配合才能实现。

3. IAD以及Interfaces

UVC功能需要使用一个VC Interface和一个或多个VS Interface的配合。Spec明确要求UVC设备必须使用一个Interface Association Descriptor（IAD）来描述这个包含了VC和VS的Interfaces集合。

此外USB协议中并没有专门朝设备索取IAD的命令，IAD需作为Configuration描述符的一部分发送给Host端。在该Configuration描述符的排列顺序中，IAD要放在它所包含的VC,VS描述符之前，且同一个IAD中所有的VC,VS Interface的Interface number必须连贯。

===>Device Descriptor<===...===>Configuration Descriptor<===...===>IAD Descriptor<=== bLength: 0x08 bDescriptorType: 0x0B bFirstInterface: 0x00 bInterfaceCount: 0x02 bFunctionClass: 0x0E -> Video Interface Class bFunctionSubClass: 0x03 -> Video Interface Collection bFunctionProtocol: 0x00 -> PC_PROTOCOL_UNDEFINED protocol iFunction: 0x00 ===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x00 bAlternateSetting: 0x00 bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x01 -> Video Control Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00...===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x00 bNumEndpoints: 0x00 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00...===>IAD Descriptor<=== bLength: 0x08 bDescriptorType: 0x0B bFirstInterface: 0x02 bInterfaceCount: 0x02 bFunctionClass: 0x01 -> Audio Interface Class bFunctionSubClass: 0x01 -> Audio Control Interface SubClass bFunctionProtocol: 0x00 iFunction: 0x04 English (United States) "USB2.0 MIC" ===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x02 bAlternateSetting: 0x00 bNumEndpoints: 0x00 bInterfaceClass: 0x01 -> Audio Interface Class bInterfaceSubClass: 0x01 -> Audio Control Interface SubClass bInterfaceProtocol: 0x00 CAUTION: This may be an invalid bInterfaceProtocol iInterface: 0x04 English (United States) "USB2.0 MIC"...===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x03 bAlternateSetting: 0x00 bNumEndpoints: 0x00 bInterfaceClass: 0x01 -> Audio Interface Class bInterfaceSubClass: 0x02 -> Audio Streaming Interface SubClass bInterfaceProtocol: 0x00 CAUTION: This may be an invalid bInterfaceProtocol iInterface: 0x04 English (United States) "USB2.0 MIC"...

可以看到我所使用的uvc设备包含两个IAD，第一个IAD是视频输入设备，也就是摄像头。第二个IAD是音频输入设备，也就是麦克风，暂时忽略这个IAD。重点分析第一个IAD，它包含了两个Interface，一个VC Interface和一个VS Interface

4. VC Interface

VC主要用于控制UVC功能中的Units和Terminals，通过对不同的Units和Terminals发下不同的指令，从而影响UVC设备的当前工作状态；Host端发下的所有对UVC设备的操纵控制命令都是经由此Interface完成的

===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x00 bAlternateSetting: 0x00 bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x01 -> Video Control Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Class-Specific Video Control Interface Header Descriptor<=== bLength: 0x0D bDescriptorType: 0x24 bDescriptorSubtype: 0x01 bcdVDC: 0x0100 wTotalLength: 0x0033 -> Validated dwClockFreq: 0x02DC6C00 = (48000000) Hz bInCollection: 0x01 baInterfaceNr[1]: 0x01 USB Video Class device: spec version 1.0===>Video Control Input Terminal Descriptor<===...===>Video Control Processing Unit Descriptor<===...===>Video Control Output Terminal Descriptor<===...===>Endpoint Descriptor<=== bLength: 0x07 bDescriptorType: 0x05 bEndpointAddress: 0x81 -> Direction: IN - EndpointID: 1 bmAttributes: 0x03 -> Interrupt Transfer Type wMaxPacketSize: 0x0010 = 1 transactions per microframe, 0x10 max bytes bInterval: 0x0A ===>Class-specific VC Interrupt Endpoint Descriptor<=== bLength: 0x05 bDescriptorType: 0x25 bDescriptorSubtype: 0x03 wMaxTransferSize: 0x0010 = (16) Bytes

这个设备包含了一个Processing Unit、一个Input Terminal和Output Terminal，VC Interface需要对它们下发不同的命令，为此VC Interface需要包含一个控制端点（强制性要求），它使用的就是每个USB设备中默认的端点0。

另外一个Interrupt端点则是可选的，用来返回或通知Host端当前的UVC设备内部状态有变化。大部分情况下，一个UVC设备的VC Interface不需一定要实现此端点，但一旦UVC设备需要实现某些特定feature时，Spec会强制性要求实现该interrupt端点。

5. UVC设备内的各种Entity

UVC的Spec在制定时考虑到了诸多不同UVC设备可能存在的特性，为保证Spec具有最大范围的适用性和兼容性，将UVC设备内可能存在的每个功能模块划分为逻辑意义上的功能单元（Entity），并要求具体UVC设备的开发者必须为每个Entity指定ID号，且该ID号能唯一标识某一指定的功能单元，以便Host端的UVC驱动能依此对设备内不同的功能单元进行操作和配置。

Entity粗略可划分为Unit，Terminal两种。这都是逻辑意义上的划分，它们对应UVC设备内的某个功能单元。

5.1 Unit

Unit可以理解为构建出UVC设备功能的各功能单元，多个Unit按照一定的规则连接后就是一个完整的UVC功能设备。Spec规定Unit有一个或多个入口，一个出口；也就是说可以有多个Unit或Terminal作为一个Unit的输入源头，而数据在流经Unit后仅能作为其他Unit或Terminal的一个输入源头。

Spec中定义了三种Unit：Selector Unit (SU)，Processing Unit (PU)，Extension Unit (EU)。

===>Video Control Processing Unit Descriptor<=== bLength: 0x0B bDescriptorType: 0x24 bDescriptorSubtype: 0x05 bUnitID: 0x02 bSourceID: 0x01 wMaxMultiplier: 0x0000 bControlSize: 0x02 bmControls : 0x3F 0x06 D00 = 1 yes - Brightness D01 = 1 yes - Contrast D02 = 1 yes - Hue D03 = 1 yes - Saturation D04 = 1 yes - Sharpness D05 = 1 yes - Gamma D06 = 0 no - White Balance Temperature D07 = 0 no - White Balance Component D08 = 0 no - Backlight Compensation D09 = 1 yes - Gain D10 = 1 yes - Power Line Frequency D11 = 0 no - Hue, Auto D12 = 0 no - White Balance Temperature, Auto D13 = 0 no - White Balance Component, Auto D14 = 0 no - Digital Multiplier D15 = 0 no - Digital Multiplier Limit iProcessing : 0x00

Processing Unit则代表了对采集的数据进行处理的单元，负责对采集到的图像特性进行调整设置，当Host端要求设定这些Processing Unit负责范围内的功能特性时，Processing Unit作为指定的命令响应者就需要对之进行响应和调整。

Spec定义中要求PU单元负责响应Host端对如下图像特性方面的设置调整要求（在该UVC设备确实支持这些特性可变，可供设置的前提下）
Usr Controls
• Brightness
• Contrast
• Hue
• Saturatio
• Sharpness
• Gamma
• Digital Multiplier (Zoom)
Auto Controls
• White Balance Temperature
• White Balance Component
• Blcklight Compensation
Other
• Gain
• Power Line Frequency
• Analog Video Standard
• Analog Video Lock Status

5.2 Terminal

Terminal是指整个UVC功能的入口和出口，逻辑意义上讲Terminal只有一个出口或者入口，它要么代表整个UVC功能的数据输入点，要么代表数据在流经UVC功能内部的各Unit功能单元后的最终流入点。

Spec中定义了两种Terminal：Input Terminal，Output Terminal。

===>Video Control Input Terminal Descriptor<=== bLength: 0x12 bDescriptorType: 0x24 bDescriptorSubtype: 0x02 bTerminalID: 0x01 wTerminalType: 0x0201 = (ITT_CAMERA) bAssocTerminal: 0x00 iTerminal: 0x00 ===>Camera Input Terminal Data wObjectiveFocalLengthMin: 0x0000 wObjectiveFocalLengthMax: 0x0000 wOcularFocalLength: 0x0000 bControlSize: 0x03 bmControls : 0x00 0x00 0x00 D00 = 0 no - Scanning Mode D01 = 0 no - Auto-Exposure Mode D02 = 0 no - Auto-Exposure Priority D03 = 0 no - Exposure Time (Absolute) D04 = 0 no - Exposure Time (Relative) D05 = 0 no - Focus (Absolute) D06 = 0 no - Focus (Relative) D07 = 0 no - Iris (Absolute) D08 = 0 no - Iris (Relative) D09 = 0 no - Zoom (Absolute) D10 = 0 no - Zoom (Relative) D11 = 0 no - Pan (Absolute) D12 = 0 no - Pan (Relative) D13 = 0 no - Roll (Absolute) D14 = 0 no - Roll (Relative) D15 = 0 no - Tilt (Absolute) D16 = 0 no - Tilt (Relative) D17 = 0 no - Focus Auto D18 = 0 no - Reserved D19 = 0 no - Reserved D20 = 0 no - Reserved D21 = 0 no - Reserved D22 = 0 no - Reserved D23 = 0 no – Reserved 

Input Terminal可以理解为整个UVC功能的数据源头，它仅有一个outPin，可以连接到其他Unit的Input Pin作为该Unit的输入源。

===>Video Control Output Terminal Descriptor<=== bLength: 0x09 bDescriptorType: 0x24 bDescriptorSubtype: 0x03 bTerminalID: 0x03 wTerminalType: 0x0101 = (TT_STREAMING) bAssocTerminal: 0x00 bSourceID: 0x02 iTerminal: 0x00 

Output Terminal则可以理解为当图像数据流程UVC设备内部的整个处理流程后的输出点，它仅有一个Input Pin， 整个UVC处理流程上的最后一个Unit会将它的Output Pin与Output Terminal的Input Pin连接在一起。

通过如上描述符所获得的信息，我们可以看出该USB Camera大致有三个逻辑功能意义上的Entity：CT(代表硬件上的图像数据采集源，Sensor)，PU（代表Sensor中可对采集所得图像数据进行加工的功能单元），OT(代表实际中USB模块的的ISO In端点)，并无SU单元来控制图像数据采集源的切换和选择（只有一个输入源），此外也没有对采集所得的数据进行个性化处理的功能，故也没有EU单元的存在（无EU描述符）。

6. VS Interface

VS Interface则专注与负责传输UVC设备的Video数据到Host端。若一UVC设备支持n种格式的Video数据，则它需要实现n个VS Interface，每个Interface对应一种专门的数据格式；而每个VSInterface则必须包含一个ISO或bulk端点来传输Video数据，一个可选的bulk端点专门用于传输静态图片数据（在实现了第三种静态图片拍摄机制的前提下）。

===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x00 bNumEndpoints: 0x00 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Video Class-Specific VS Video Input Header Descriptor<=== bLength: 0x0E bDescriptorType: 0x24 bDescriptorSubtype: 0x01 bNumFormats: 0x01 wTotalLength: 0x010F -> Validated bEndpointAddress: 0x83 -> Direction: IN - EndpointID: 3 bmInfo: 0x00 -> Dynamic Format Change not Supported bTerminalLink: 0x03 bStillCaptureMethod: 0x02 -> Still Capture Method 2 bTriggerSupport: 0x01 -> Hardware Triggering Support bTriggerUsage: 0x00 -> Host will initiate still image capture bControlSize: 0x01 Video Payload Format 1 0x00 D00 = 0 no - Key Frame Rate D01 = 0 no - P Frame Rate D02 = 0 no - Compression Quality D03 = 0 no - Compression Window Size D04 = 0 no - Generate Key Frame D05 = 0 no - Update Frame Segment D06 = 0 no - Reserved D07 = 0 no - Reserved ===>Video Streaming MJPEG Format Type Descriptor<=== bLength: 0x0B bDescriptorType: 0x24 bDescriptorSubtype: 0x06 bFormatIndex: 0x01 bNumFrameDescriptors: 0x05 bmFlags: 0x01 -> Sample Size is Fixed bDefaultFrameIndex: 0x01 bAspectRatioX: 0x00 bAspectRatioY: 0x00 bmInterlaceFlags: 0x00 D00 = 0 non-Interlaced stream or variable D01 = 0 2 fields per frame D02 = 0 Field 1 not first D03 = 0 Reserved D4..5 = 0 Field patterns -> Field 1 only D6..7 = 0 Display Mode -> Bob only bCopyProtect: 0x00 -> Duplication Unrestricted ===>Video Streaming MJPEG Frame Type Descriptor<=== --->This is the Default (optimum) Frame index bLength: 0x2A bDescriptorType: 0x24 bDescriptorSubtype: 0x07 bFrameIndex: 0x01 bmCapabilities: 0x00 wWidth: 0x0280 wHeight: 0x01E0 dwMinBitRate: 0x0001F400 dwMaxBitRate: 0x00A8C000 dwMaxVideoFrameBufferSize: 0x00096000 dwDefaultFrameInterval: 0x00051615 bFrameIntervalType: 0x04 ===>Additional Discrete Frame TypeData dwFrameInterval[1]: 0x00028B0A dwFrameInterval[2]: 0x00051615 dwFrameInterval[3]: 0x000A2C2A dwFrameInterval[4]: 0x001E8480 ===>Video Streaming MJPEG Frame Type Descriptor<=== bLength: 0x2A bDescriptorType: 0x24 bDescriptorSubtype: 0x07 bFrameIndex: 0x02 bmCapabilities: 0x00 wWidth: 0x00A0 wHeight: 0x0078 dwMinBitRate: 0x0001F400 dwMaxBitRate: 0x00A8C000 dwMaxVideoFrameBufferSize: 0x00009600 dwDefaultFrameInterval: 0x00051615 bFrameIntervalType: 0x04 ===>Additional Discrete Frame TypeData dwFrameInterval[1]: 0x00028B0A dwFrameInterval[2]: 0x00051615 dwFrameInterval[3]: 0x000A2C2A dwFrameInterval[4]: 0x001E8480 ===>Video Streaming MJPEG Frame Type Descriptor<=== bLength: 0x2A bDescriptorType: 0x24 bDescriptorSubtype: 0x07 bFrameIndex: 0x03 bmCapabilities: 0x00 wWidth: 0x00B0 wHeight: 0x0090 dwMinBitRate: 0x0001F400 dwMaxBitRate: 0x00A8C000 dwMaxVideoFrameBufferSize: 0x0000C600 dwDefaultFrameInterval: 0x00051615 bFrameIntervalType: 0x04 ===>Additional Discrete Frame TypeData dwFrameInterval[1]: 0x00028B0A dwFrameInterval[2]: 0x00051615 dwFrameInterval[3]: 0x000A2C2A dwFrameInterval[4]: 0x001E8480 ===>Video Streaming MJPEG Frame Type Descriptor<=== bLength: 0x2A bDescriptorType: 0x24 bDescriptorSubtype: 0x07 bFrameIndex: 0x04 bmCapabilities: 0x00 wWidth: 0x0140 wHeight: 0x00F0 dwMinBitRate: 0x0001F400 dwMaxBitRate: 0x00A8C000 dwMaxVideoFrameBufferSize: 0x00025800 dwDefaultFrameInterval: 0x00051615 bFrameIntervalType: 0x04 ===>Additional Discrete Frame TypeData dwFrameInterval[1]: 0x00028B0A dwFrameInterval[2]: 0x00051615 dwFrameInterval[3]: 0x000A2C2A dwFrameInterval[4]: 0x001E8480 ===>Video Streaming MJPEG Frame Type Descriptor<=== bLength: 0x2A bDescriptorType: 0x24 bDescriptorSubtype: 0x07 bFrameIndex: 0x05 bmCapabilities: 0x00 wWidth: 0x0160 wHeight: 0x0120 dwMinBitRate: 0x0001F400 dwMaxBitRate: 0x00A8C000 dwMaxVideoFrameBufferSize: 0x00031800 dwDefaultFrameInterval: 0x00051615 bFrameIntervalType: 0x04 ===>Additional Discrete Frame TypeData dwFrameInterval[1]: 0x00028B0A dwFrameInterval[2]: 0x00051615 dwFrameInterval[3]: 0x000A2C2A dwFrameInterval[4]: 0x001E8480 

可以看出这个设备只支持一种视频格式”MJPEG”，所以它只有一个VS Interface，而同一种视频格式下有很多参数可以调整，比如说图像分辨率，每秒的帧速率等。其参数组合信息就是紧跟在Format Type Descriptor之后的Frame Type Descriptor。

在采用不同的参数组合时，UVC设备进行数据传输时，所需占用的USB数据带宽显然有很大的不同；因此一个VS Interface下需要有多个setting，每个settnig对应一个固定的Video参数设置组合，也就对应了一个固定参数值的Video数据，该setting下每秒要传输的帧数（帧速率），图像分辨率等均固定，进而对USB的带宽要求也固定。

===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x01 bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Endpoint Descriptor<=== bLength: 0x07 bDescriptorType: 0x05 bEndpointAddress: 0x83 -> Direction: IN - EndpointID: 3 bmAttributes: 0x05 -> Isochronous Transfer Type Synchronization Type = Asynchronous Bulk Transfer Type wMaxPacketSize: 0x00C0 = 1 transactions per microframe, 0xC0 max bytes bInterval: 0x01 ===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x02 bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Endpoint Descriptor<=== bLength: 0x07 bDescriptorType: 0x05 bEndpointAddress: 0x83 -> Direction: IN - EndpointID: 3 bmAttributes: 0x05 -> Isochronous Transfer Type Synchronization Type = Asynchronous Bulk Transfer Type wMaxPacketSize: 0x0180 = 1 transactions per microframe, 0x180 max bytes bInterval: 0x01 ===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x03 bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Endpoint Descriptor<=== bLength: 0x07 bDescriptorType: 0x05 bEndpointAddress: 0x83 -> Direction: IN - EndpointID: 3 bmAttributes: 0x05 -> Isochronous Transfer Type Synchronization Type = Asynchronous Bulk Transfer Type wMaxPacketSize: 0x0200 = 1 transactions per microframe, 0x200 max bytes bInterval: 0x01 ===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x04 bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Endpoint Descriptor<=== bLength: 0x07 bDescriptorType: 0x05 bEndpointAddress: 0x83 -> Direction: IN - EndpointID: 3 bmAttributes: 0x05 -> Isochronous Transfer Type Synchronization Type = Asynchronous Bulk Transfer Type wMaxPacketSize: 0x0280 = 1 transactions per microframe, 0x280 max bytes bInterval: 0x01 ===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x05 bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Endpoint Descriptor<=== bLength: 0x07 bDescriptorType: 0x05 bEndpointAddress: 0x83 -> Direction: IN - EndpointID: 3 bmAttributes: 0x05 -> Isochronous Transfer Type Synchronization Type = Asynchronous Bulk Transfer Type wMaxPacketSize: 0x0320 = 1 transactions per microframe, 0x320 max bytes bInterval: 0x01 ===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x06 bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Endpoint Descriptor<=== bLength: 0x07 bDescriptorType: 0x05 bEndpointAddress: 0x83 -> Direction: IN - EndpointID: 3 bmAttributes: 0x05 -> Isochronous Transfer Type Synchronization Type = Asynchronous Bulk Transfer Type wMaxPacketSize: 0x03C0 = 1 transactions per microframe, 0x3C0 max bytes bInterval: 0x01 ===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x07 bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Endpoint Descriptor<=== bLength: 0x07 bDescriptorType: 0x05 bEndpointAddress: 0x83 -> Direction: IN - EndpointID: 3 bmAttributes: 0x05 -> Isochronous Transfer Type Synchronization Type = Asynchronous Bulk Transfer Type wMaxPacketSize: 0x0A80 = 2 transactions per microframe, 0x280 max bytes bInterval: 0x01 ===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x08 bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Endpoint Descriptor<=== bLength: 0x07 bDescriptorType: 0x05 bEndpointAddress: 0x83 -> Direction: IN - EndpointID: 3 bmAttributes: 0x05 -> Isochronous Transfer Type Synchronization Type = Asynchronous Bulk Transfer Type wMaxPacketSize: 0x0B20 = 2 transactions per microframe, 0x320 max bytes bInterval: 0x01 ===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x09 bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Endpoint Descriptor<=== bLength: 0x07 bDescriptorType: 0x05 bEndpointAddress: 0x83 -> Direction: IN - EndpointID: 3 bmAttributes: 0x05 -> Isochronous Transfer Type Synchronization Type = Asynchronous Bulk Transfer Type wMaxPacketSize: 0x0BE0 = 2 transactions per microframe, 0x3E0 max bytes bInterval: 0x01 ===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x0A bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Endpoint Descriptor<=== bLength: 0x07 bDescriptorType: 0x05 bEndpointAddress: 0x83 -> Direction: IN - EndpointID: 3 bmAttributes: 0x05 -> Isochronous Transfer Type Synchronization Type = Asynchronous Bulk Transfer Type wMaxPacketSize: 0x1380 = 3 transactions per microframe, 0x380 max bytes bInterval: 0x01 ===>Interface Descriptor<=== bLength: 0x09 bDescriptorType: 0x04 bInterfaceNumber: 0x01 bAlternateSetting: 0x0B bNumEndpoints: 0x01 bInterfaceClass: 0x0E -> Video Interface Class bInterfaceSubClass: 0x02 -> Video Streaming Interface SubClass bInterfaceProtocol: 0x00 iInterface: 0x00 ===>Endpoint Descriptor<=== bLength: 0x07 bDescriptorType: 0x05 bEndpointAddress: 0x83 -> Direction: IN - EndpointID: 3 bmAttributes: 0x05 -> Isochronous Transfer Type Synchronization Type = Asynchronous Bulk Transfer Type wMaxPacketSize: 0x13E8 = 3 transactions per microframe, 0x3E8 max bytes bInterval: 0x01 

此外，对于每个VS Interface来讲，必须实现setting0，且setting0不代表任何实际的Video流和参数组合，当指定setting0时就是要求UVC设备暂停数据传输，放弃对USB带宽的占用。

7. 总结

UVC同MSC一样，系USB框架下的功能类协议，但却与MSC有着较大差异。MSC功能采用Control+Bulk传输完成，其枚举流程，描述符配置较为清晰，控制传输阶段简单，定义的类功能控制命令较少。而BULK传输阶段则较为复杂繁琐，出错机制，续传机制等要求较为严格。

而UVC则刚好相反，它采用Control+ISO传输机制实现（BULK和INTR机制为可选特性），其枚举流程，描述符配置较为复杂，繁琐，定义了诸多的类控制命令，Entity等；而具体的数据传输阶段即ISO传输较为简单。