USB protocol layer describes the grammar and protocol used in the communication of USB host with USB devices,and presents the USB hierarchy consisting of fields ,packets, transactions and transfers. Furthermore, it defines the dynamic proce d utes of data-link set-up and normal or abnormal transfer processing. Six field types are SYNC Field, Packet Identifier Field, Address Field, Frame Number Fiel d, Data Field and CRC Field. Four packet trpes are Token,Data, Handshake and Spe cial. Four transaction types are Bulk Transaction, Set-Up Transaction,Interrupt Transaction and Isochronous Transaction. Four transfer types are Bulk Transfer, Control Transfer, Interrupt Transfer and Isochronous Transaction.Essentially, t he name and definition of fields, packets, transactions and transfers are the en suring measures in data-link set-up,data transfer and data-link disconnection for satisfying various data transmission.%USB协议层描述了USB主机与USB设备交互时的语法和协议，从中定义了字段、包、事务和传送的结构，以及由字段到包、由包到事务、由事务到传送所组成的组织层次关系。协议同时还规范了数据链路的建立，正常或异常传送处理的动态过程。字段中分同步、包标识符、地址、帧号、数据和CRC校验等字段。包中分标记、帧起始、数据、握手4类包。事务中有批处理、控制、中断和同步4类事务。传送可分为控制、批处理、中断和同步4类传送。字段、包、事务和传送的命名和定义，实质上是为了满足不同需求的传送内容，在链路建立、数据传送及链路拆除的可靠执行中所采用的一系列保障措施。
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