蓝牙core spec v4.0研读笔记

蓝牙core spec v4.0研读笔记（一）

1.     Broadcast links have no feedback route, each packet is transmitted several times to increase the probability that the receiver is able to receive at least one of the copies successfully. So these links are considered unreliable.即广播连接没有反馈的回路，每个包都是通过多次的传输来使得接受者至少又一次能够收到正确的包，这种连接时不可靠的。

2.     The transmitter may remove packets from the transmit queue such that the receiver does not receive all the packets in the sequence. If this happens detection of the missing packet is delegated to the L2CAP layer.即发送者可能从发送队列中移除一些包，这样的话接受者就不能够接受到队列中所有的包，这种情况下，对丢包的检查是由L2CAP层来负责的。

3.     The reliability of each AMP depends on the underlying MAC/PHY technology. The Bluetooth Core maintains a level of reliability for protocols and profiles above the Core by mandating the use of Enhanced Retransmission Mode or Streaming Mode for any L2CAP channel used over the AMP.即AMP的可靠性取决于底层的MAC/PHY技术。蓝牙核心通过授权AMP上层的L2CAP通道使用加强的重传模式或者流模式来保证核心之上的协议和配置的可靠性。

4.     About BR/EDR generic packet structure. It should include the channel access code. And there is no direct field within the BR/EDR packet structure that represents or contains information relating to physical links, this information is implied in the logical transport address (LT_ADDR) carried in the packet header.即对于BR/EDR通用包结构而言，通道访问代码是必须的。（主要用来鉴别同一RF下的特定的物理通信通道）。同时没有直接的包含物理链路信息，它是包含在逻辑传输地址中的（LT_ADDR位于包头）。

5.     The payload header is present in all packets on logical transports that support multiple logical links.即负载头存在于所有支持多路逻辑链接的逻辑传输包中。

6.     All LE packets include the Access Address. This is used to identify communications on a physical link, and to exclude or ignore packets on different physical links that are using the same PHY channels in physical proximity. 即所有的LE包包含访问地址。它用于辨别物理链路上的交互，排除使用同样PHY通道的不同物理链接。

7.     All LE advertising physical links use a fixed Access Address. LE active physical links use a randomly generated 32-bit value as their Access Address.即所有的LE广播物理链接使用一个固定的访问地址，LE获得物理链接使用一个随机的通用32bit值作为他们的访问地址。

8.     All LE packets include a PDU header. It determines the type of advertisement broadcast or logical link carried over the physical channel.即所有的LE包包含一个PDU头。它决定了广告广播或者逻辑链路转到物理通道的类型。

9.     In the BR/EDR core system, peer devices use a shared physical channel for communication.即在BR/EDR核心系统中，同级的设备使用共享的物理通道进行通信。

10.  Four BR/EDR channels are defined. Two of these physical channels (the basic piconet channel and adapted piconet channel) are used for communication between connected devices and are associated with a specific piconet. The remaining BR/EDR physical channels are used for discovering Bluetooth devices and for connecting Bluetooth devices.即DR/EDR物理通道有四个，两个（基本的微微网通道和适应微微网通道）用于连接设备之间的通信。其他的两个通道用于发现和连接蓝牙设备。

11.  A Bluetooth device can only use one BR/EDR physical channel at any given time. In order to support multiple concurrent operations the device uses time division multiplexing between the channels.即一个蓝牙设备在任何时候只能使用一个BR/EDR物理通道。为了支持多重并行操作，这个设备需要在多个通道间使用时分复用技术。

12.  On the basic piconet channel the master controls access to the channel. The master starts its transmission in even-numbered time slots only. Packets transmitted by the master are aligned with the slot start and define the piconet timing. Packets transmitted by the master may occupy up to five time slots depending on the packet type.即一个基本的微微网通道是由主设备控制访问的。主设备只在偶数的时间跳跃点上传输。每次数据包的传输都是发生在时间跳跃点的开始。主设备传输包最多占据5个时间跳跃点，这和包的类型是有关系的。

13.  The adapted piconet channel differs from the basic piconet channel in two ways. First, the frequency is not recomputed between master and subsequent slave packets. Second, the adapted piconet channel may be based on fewer than the full 79 frequencies.即适应微微网通道和基本微微网通道有两点不同：1，主从设备的频率是一样的。2，适应微微网通道没有全部的79个频率点。

14.  In order for a device to be discovered, an inquiry scan channel is used.即查询扫描通道用于发现一个设备。

15.  A connectable device (one that is prepared to accept connections) does so using a page scan channel. A connectable device listens for a page request on its page scan channel and once received, enters into a sequence of exchanges with this device.即一个可连接的设备使用呼叫扫描通道。它在呼叫扫描通道上等待呼叫请求，一旦接受到呼叫请求，立即与该设备进行有序交互。

16.  Following a successful conclusion of the page procedure both devices switch to the basic piconet channel that is characterized by having the paging device as master.即呼叫过程建立之后，两个设备将会切换到由呼叫设备作为主设备的基本微微网通道。

17.  Two LE physical channels are defined. The LE piconet channel is used for communication between connected devices and is associated with a specific piconet. The LE advertisement broadcast channel is used for broadcasting advertisements to LE devices. These advertisements can be used to discover, connect or send user data to scanner or initiator devices.即LE物理通道有两种：1.LE微微网通道是用于两个连接的设备通信并且与一个特定的微微网相关联。2.LE广告广播通道用于广告广播给LE设备。这些广告用于发现，连接或者发送用户数据到扫描或者发起的设备。

18.  The LE piconet channel is characterized by a pseudo-random sequence of PHY channels and by three additional parameters provided by the master. The first is the channel map that indicates the set of PHY channels used in the piconet. The second is a pseudo random number used as an index into the complete set of PHY channels. The third is the timing of the first data packet sent by the master after the connection request.即LE微微网通道由一个PHY通道的伪随机队列和三个主设备的额外参数来表征。第一个参数是通道映射表用来表示微微网中使用的PHY通道。第二个参数是一个用来索引全部PHY通道的伪随机数。第三个参数是在连接请求之后主设备的第一个数据包发送的时间。

19.   There are 37 LE piconet channels.有37个LE微微网通道。

20.  At present an LE device is only permitted to belong to one piconet at a time.现在一个LE设备在同一时间仅允许存在于一个微微网中。

21.  The LE advertising channel is used to set up connections between two devices or to communicate broadcast information between unconnected devices.即LE广告通道是用于在两个设备之间建立连接或者在两个没有连接的设备之间交换广播信息。

22.  A physical link represents a baseband connection between Bluetooth devices. A physical link is always associated with exactly one physical channel (although a physical channel may support more than one physical link.).即物理连接表现了蓝牙设备间的基带连接。一个物理连接通常与一个特定的物理通道相关联（尽管一个物理通道可能支持多个物理连接）。

23.  The basic and adapted piconet physical channels support a physical link which may be active or parked.即基本和适应微微网物理通道支持一个活动的或者休眠的物理连接。

蓝牙core spec v4.0研读笔记（二）

1.     The physical link between a master and a slave device is active if a default ACL logical transport exists between the devices.即主从设备之间若有一个默认的ACL逻辑传输存在，则认为这个物理链路是活动的。

2.     The physical link between a master and a slave device is parked when the slave remains synchronized in the piconet, but has no default ACL logical transport.即从设备在微微网中保持同步，但是没有默认的ACL逻辑传输，则认为主从设备的物理链路是休眠的。

3.     A parked slave broadcast (PSB) logical transport is used to allow communication of a subset of LMP signaling and broadcast L2CAP to parked slaves.即PSB逻辑传输用来允许LMP信号和广播L2CAP的子集与休眠从设备通信。

4.     The physical link between a master and a slave device is active if a default LE ACL logical transport exists between the devices. Active physical links are identified by the randomly generated Access Address used in the Link Layer packet.即两个设备若存在默认的LE ACL 逻辑传输，则它们之间的物理链路就是活动的。活动的物理链路由链路层包中的随机通用访问地址所标识。

5.     An advertising physical link between an advertising device and an initiating device for the purposes of forming a connection can exist for a relatively short period of time.即在广播设备和一个以建立连接为目的的启动设备之间可以存在短时间的一个广播物理链路。

6.     About logical transport types, the classification of each link type follows from a selection procedure within three categories. The first category is that of casting. This may be either unicast or broadcast. The second category relates to the scheduling and acknowledgement scheme of the link, and implies the type of traffic that is supported by the link. These are synchronous, isochronous or asynchronous. The final category is related to the class of data that is carried by the link. This is either control (LMP or PAL) data or user data. The user data category is subdivided into L2CAP data and stream data.即逻辑传输类型的分类是由三个策略的选择过程产生的。第一个策略是casting（角色分配？），它有可能是单一通道或者广播。第二个是与链路的调度和确认计划有关，它表示了链路支持的数据类型。它们是同步，等时或者异步。最后一个策略是链路传输数据的类型。它分为控制数据（LMP 或者 PAL）和用户数据。用户数据再被细分为L2CAP数据和流数据。

7.     The main difference from synchronous links, the data rate on isochronous links need not be constant.即等时和同步的最大差别在于，等时的数据传输速率不需要固定。

8.     The ACL logical transport can be identified by the LT_ADDR field in the packet header in combination with the packet type field.即ACL逻辑传输能够通过包头中的LT_ADDR域和包的类型一起来确定。（SCO其实也包含LT_ADDR，所以不能仅用LT_ADDR来确定）

9.     The synchronous connection-oriented (SCO) logical transport is a symmetric, point-to-point transport between the master and a specific slave. SCO logical transports carry 64 kb/s of information synchronized with the piconet clock.即SCO是主从设备间的一个对称的点对点的传输。SCO逻辑传输能达到64kb/s的与微微网时钟同步的信息。

10.  eSCO links offer a number of extensions over the standard SCO links, in that they support a more flexible combination of packet types and selectable data contents in the packets and selectable slot periods, allowing a range of synchronous bit rates to be supported.即eSCO相对于标准的SCO连接提供了一系列的扩展，它支持更加灵活的包的类型的组合，可选择的数据内容和可选择的跳跃周期，支持一系列同步比特率。

11.  The ASB logical transport is inherently unreliable because of the lack of acknowledgement.即ASB逻辑传输是不可靠的，主要原因是缺少确认。

12.  The ASB channel may be used for L2CAP group traffic, and is never used for L2CAP connection-oriented channels, L2CAP control signaling or LMP control signaling.即ASB通道可以被L2CAP组传输使用，但是不能被面向连接的L2CAP通道，L2CAP控制信号或者LMP控制信号使用。

13.  The parked slave broadcast link is the only logical transport that exists between the piconet master and parked slaves.即休眠从设备广播连接是微微网主设备与休眠从设备间唯一存在的逻辑传输。

14.  The PSB logical transport is more complex than the other logical transports as it consists of a number of phases, each having a different purpose. These phases are the control information phase (used to carry the LMP logical link), the user information phase (used to carry the L2CAP logical link), and the access phase (carrying baseband signaling.)即PSB逻辑传输比其他逻辑传输要更加复杂，因为他有一系列相位组成，并且每一个都有不同的目的。这些相位是控制信息相位（用于运输LMP逻辑链路），用户信息相位（用于运输L2CAP逻辑链路）和访问相位（运输基带信号）。

15.  The PSB logical transport is identified by the reserved LT_ADDR of 0.即PSB逻辑传输是由保留的LT_ADDR=0来鉴别的。

16.  The ACL Control Logical Link (ACL-C) is used to carry BR/EDR LMP signaling between devices in the piconet. The ACL-C link is always given over the ACL-U link when carried on the same logical transport.即ACL控制逻辑链路用来搬运微微网设备间的BR/EDR LMP信号。在同样的逻辑传输上，ACL-C的优先级比ACL-U的优先级高。

17.  Synchronous (SCO-S) and extended synchronous (eSCO-s) logical link are used to support isochronous data delivered in a stream without framing. These links are associated with a single logical transport, where data is delivered in constant sized units at a constant rate.即SCO-S和Esco-s逻辑链路用来支持无帧流传输的同步数据。这些链路和以固定速率，固定大小传输的单个逻辑传输相关。

18.  L2CAP provides a multiplexing role allowing many different applications to share an ACL-U or AMP-U logical link.即LCAP提供了多路复用技术允许许多不同的应用分享ACL-U或者AMP-U逻辑链路。

19.  L2CAP channel endpoints are identified to their clients by a Channel Identifier (CID). It may be configured to provide an appropriate Quality of Service (QoS) to the application. It supports channels that are connection-oriented and others that are group-oriented.即L2CAP端点是由他们的CID来鉴别的。它能够被配置成一个对应用提供QoS服务。它支持面向连接和面向组的通道。

20.  Apart from the creation, configuration and dismantling of channels, the main role of L2CAP is to multiplex service data units (SDUs) from the channel clients onto ACL-U, LE-U, or AMP-U logical links, and to carry out a simple level of scheduling, selecting SDUs according to relative priority.即除了创建，配置和销毁通道，L2CAP的主要任务是把客户通道的服务数据单元复用到ACL-U,LE-U或者AMP-U逻辑链路，并且根据相关的优先级进行一个简单的调度和选择SDU。

21.  In the case where an HCI is present, the L2CAP is also required to segment L2CAP SDUs into fragments that will fit into the baseband buffers, and also to operate a token based flow control procedure over the HCI, submitting fragments to the baseband only when allowed to do so.即在有HCI的时候，L2CAP用于把L2CAP SDU分成小块以适应基带buffer，并且对HCI进行基础的流控，仅当基带允许接受时才提交小块给基带。

22.  A Bluetooth device may participate concurrently in two or more piconets. It does this on a time-division multiplexing basis. A Bluetooth device can never be a master of more than one piconet.即一个蓝牙设备能够同时参与多个微微网。它以时分复用为基础，但是一个蓝牙设备不能成为多个微微网的主设备。

蓝牙core spec v4.0研读笔记（三）

1.     An Extended Inquiry Response can be used to provide miscellaneous information during the inquiry response procedure.即扩展查询相应用来在查询相应过程中提供其它的信息。

2.     During the time that a slave device is actively connected to a piconet there is always a default ACL logical transport between the slave and the master device. There are two methods of deleting the default ACL logical transport. The first method is to detach the device form the piconet physical channel, at which time the entire hierarchy of L2CAP channels, logical links, and logical transports between the device is deleted. The second method is to place the physical link to the slave device in the park state, at which time it gives up is default ACL logical transport.即当一个从设备连接到微微网中，通常会有一个默认的ACL传输在主从设备之间。有两种方法可以去除这个默认的传输，第一个方法就是把设备从微微网的物理通道中分离，这时候整个的L2CAP通道层，逻辑链路层和和逻辑传输层均被删除。第二个方法就是把刀这个设备的物理链路置为休眠状态，这时它会放弃默认的ACL逻辑传输。

3.     Hold mode is not a general device mode, but applies to unreserved slots on the physical link. When in this mode, the physical link is only active during slots that are reserved for the operation of the synchronous link types SCO and Esco. All asynchronous links are inactive.即保持模式不是一个通用的设备模式，它应用物理链路上的一些unreserved的跳跃点。在这种模式下，物理链路只在SCO和ESCO时工作，所有的异步连接均是不工作的。

4.     Sniff mode is not a general device mode, but applies to the default ACL logical transports. Sniff mode only affects the default ACL logical transports, and does not apply to any additional SCO or ESCO logical transports that may be active.即呼吸模式不是一个通用的设备模式，但是应用于默认的ACL 逻辑传输。它仅仅影响默认ACL 逻辑传输，不会应用于那些可能活动的额外SCO或者ESCO逻辑传输。

5.     A slave device may remain connected to a piconet but have its physical link in the parked state. In this state the device cannot support any logical links to the master with the exception of the PSB-C an PSB-U logical links that are used for all communication between the piconet master and the parked slave.即一个连接中微微网中的从设备的物理链路可能处于休眠状态。这种状态下，设备不能支持任何除了PSB-C和PSB-U（用于微微网中的主设备和休眠从设备交互）的逻辑链路。

6.     Enhanced Data Rate can be enabled only for ACL-U, ESCO-S logical transports and cannot be enabled for ACL-C, SCO-S, and the broadcast logical transports.即加强数据传输仅能够在ACL-U, ESCO-S逻辑传输中使能，不能够在ACL-C, SCO-S和广播逻辑传输中使能。

7.     The device filtering procedure is a method for controllers to reduce the number of devices requiring communication responses.即设备过滤过程是一个控制器减少设备请求响应数目的方法。

8.     An advertiser uses the advertising procedure to perform unidirectional broadcasts to devices in the area.即广播者使用广播过程去完成一个向区域内特定设备的单向广播。

9.     Advertising devices can be connected to other LE devices in an LE piconet. However advertisers are limited to using non-connectable advertising events if they are already connected to another LE device.即一个在LE微微网中的广播设备能够连接到别的LE 设备。然而，假如它连接到别的LE设备，则不能使用未连接广播事件。

10.  A scanning device uses the scanning procedure to listen for unidirectional broadcasts of user data from advertising devices using the advertising physical channel.即一个扫描设备使用扫描步骤去收听使用广告物理通道的广告设备发出的单向广播的用户数据。

11.  The AMP Manager is responsible for discovering local AMP Controller(s) and maintaining that list over time as AMPs may be added or removed dynamically from the system.即AMP管理用来发现本地AMP控制器和维持可能动态加入和移出系统的AMP列表。

12.  Each AMP is identified with an ID and a type.每一个AMP是由一个ID 和一个类型来鉴别的。

13.  To set up an L2CAP channel with a remote device over an AMP physical link, the AMP Manager first discovers the remote AMP, collects the necessary information about the remote AMP to set up a physical link in an optimal way and then initiates the physical link creation. This is done over a dedicated L2CAP channel.即在一个AMP物理链路上建立一个L2CAP通道，首先，AMP管理发现这个外部AMP，收集必要的信息去建立一个物理链路，然后初始化物理链路的建立。这些都是在一个特定的L2CAP通道上完成的。

14.  The AMP Manager provides the remote AMP information that it collected about the remote AMP to the local AMP PAL. The local AMP PAL then uses this information to create the AMP physical link.即AMP管理向本地AMP PAL提供了外部的AMP信息。然后本地的AMP PAL使用这些信息去创建AMP物理连接。

15.  Secure Simple Pairing has two security goals: protection against passive eavesdropping and protection against man-in-the-middle (MITM) attacks.即安全简单匹配有两个安全目标：保护不被被动窃听和保护不被中途攻击。

16.  Secure Simple Pairing uses Elliptic Curve Diffie Hellman (ECDH) public key cryptography as a means to thwart passive eavesdropping attacks. ECDH provides a very high degree of strength against passive eavesdropping attacks but it may be subject to MITM attacks, which however, are much harder to perform in practice than the passive eavesdropping attack.即安全简单匹配使用ECDH公共密钥作为阻扰被动窃取攻击的方法。ECDH能够很好地方志被动窃取攻击，但是在MITM攻击中表现并不够理想。当然MITM相比于被动窃取攻击在实际中时比较难以实现的。

17.  To prevent MITM attacks, Secure Simple Pairing offers two user assisted numeric methods: numerical comparison or passkey entry.为了防止MITM攻击，安全简单匹配提供了两个用户辅助数字方法：数字匹配或者密钥进入。

18.  Secure Simple Pairing uses four association models referred to as Numeric Comparison, Just Works, Out Of Band, and Passkey Entry.即安全简单匹配使用四个关联模型：数字匹配，仅工作，带外数据和Passkey Entry（万能钥匙进入？？）。

19.  The numeric comparison serves two purposes. First, since many devices do not have unique names, it provides confirmation to the user that the correct devices are connected with each other. Second, the numeric comparison provides protection against MITM attacks.即数字匹配达到了两个目的。第一，因为许多设备没有一个固定的名字，它向用户确认了就是这两个设备相互连接了。第二，数字匹配防止了MITM攻击。

20.  Knowing the displayed number is of no benefit in decrypting the encoded data exchanged between the two devices.即知道了屏幕显示的数字对解密两个设备间传输的数据毫无帮助。

蓝牙core spec v4.0研读笔记（四）

1.     The Out of Band (OOB) association model is primarily designed for scenarios where an Out of Band mechanism is used to both discover the devices as well as to exchange or transfer cryptographic numbers used in the pairing process.即OOB的关联模型最初是设计用来使用在发现设备和匹配过程中交换和传输密码数据的情景中的。

2.     The OOB association model supports any OOB mechanism where cryptographic information and the Bluetooth Device Address can be exchanged. The OOB association model does not support a solution where the user has activated a Bluetooth connection and would link to use OOB for authentication only.即OOB的关联模型支持任何一个密码信息和蓝牙设备地址能够交换的OOB机制。它不支持当用户拥有一个活动链接，只想使用OOB进行鉴别身份的场合。

3.     The Passkey Entry association model is primarily designed for the scenario where one device has input capability but does not have the capability to display six digits and the other device has output capabilities.即万能钥匙输入关联模型是用在一个设备有输入能力但是没有显示六位数字的能力而另外一个设备有输出能力的情形下。

4.     Bluetooth LE uses three association models referred to as Just Works, Out of Band and Passkey Entry. Bluetooth LE does not have an equivalent of Numeric Comparison. Just Works and Passkey Entry do not provide any passive eavesdropping protection. This is because Secure Simple Pairing uses Elliptic Curve DiffiHellman and LE does not.即蓝牙LE使用的的关联模型有Just Works，OOB和万能钥匙输入。蓝牙LE没有一个相等数字匹配关联模型。Just works和Passkey Entry没有提供任何被动攻击的保护。主要是由于简单安全匹配使用了ECDH而LE没有使用。

5.     Bluetooth LE supports the ability to send authenticated data over an unencrypted ATT bearer between two devices with a trusted relationship.即蓝牙LE支持传输非加密数据通过两个信任设备间非加密的ATT bearer。

6.     Bluetooth LE supports a feature that reduces the ability to track a LE device over a period of time by changing the Bluetooth device address on a frequent basis.即蓝牙LE支持一个按一定频率定期改变蓝牙设备地址的方式来减少LE设备被跟踪的可能性。

7.     The privacy feature also defines a reconnection address which allows for bonded device to reconnect while also filtering devices to known devices.即隐秘特性也定义了一个重连地址允许连接的设备在过滤已知设备的时候进行重连。

8.     In BR/EDR, GAP defines a single role with functionality that may be present in each device.即在BR/EDR中，GAP定义了一个每个设备都可能存在的单一功能的规则。

9.     In LE, GAP defines four specific roles: Broadcaster, Observer, Peripheral, and Central.即在LE中，GAP定义了四个特殊的角色：广播者，观察者，外围和核心。

10.  The Broadcaster role is optimized for transmitter only applications. The Observer role is optimized for receiver only applications. The Peripheral role is optimized for devices that support a single connection and are less complex than central devices. Devices supporting the peripheral role only require Controllers that support the Controller’s slave role. The Central role supports multiple connections and is the initiator for all connections with devices in the peripheral role. Devices supporting the central role require a Controller that support the Controller’s master role and generally supports more complex functions compared to the other LE GAP roles.即广播者角色用来优化只传输的应用。观察者角色用来优化只接受的应用。观察者用来优化支持单链接且没有核心设备复杂的设备。支持观察者角色的设备要求控制器支持控制器从角色。核心角色支持多路链接并且是所有外围角色从设备的发起者。支持核心角色的设备要求控制器支持控制器主角色并且通常支持相比于其他LE GAP角色更加复杂的功能。

11.  Generic Attribute Profile (GATT) is built on top of the Attribute Protocol (ATT) and establishes common operations and a framework for the data transported and stored by the Attribute Protocol. GATT defines two roles: Server and Client. GATT and ATT are not transport specific and can be used in both BR/EDR and LE. However, GATT and ATT are mandatory to implement in LE since it is used for discovering services.即GATT在ATT上层，为ATT协议的数据的传输和存储建立了一些通用的操作和架构。GATT定义了两个角色：服务器和客户。GATT和ATT不是和传输相关的，它能够使用在BR/EDR和LE上，但是，GATT和ATT被强制使用在LE上，因为它被用来发现服务。

12.  A service is a collection of data and associated behaviors to accomplish a particular function or feature of a device or portions of a device.即一个服务是一些为了实现相关设备的特定功能和特性的数据和相关操作的结合。

13.  There are two types of services: primary and secondary.即有两种服务类型：主要的和次要的。

蓝牙core spec v4.0研读笔记（五）

1. L2CAP provides connection-oriented and connectionless data services to upper layer protocols with protocol multiplexing capability and segmentation and reassembly operation.    即L2CAP向上层协议提供了面向连接和无连接的具有协议复用能力和分段重组操作的数据服务。

2. Many multiplexed applications are better served if L2CAP has control over the PDU length. a) Segmentation will allow the interleaving of application data units in order to satisfy latency requirements. b) Memory and buffer management is easier when L2CAP controls the packet size. C) Error correction by retransmission can be made more efficient. D) The amount of data that is destroyed when an L2CAP PDU is corrupted or lost can be made smaller than the application’s data unit. E) The application is decoupled from the segmentation required to map the application packets into the lower layer packets.即假如L2CAP控制PDU的长度，那么许多复用的应用将能够更好得被服务。A）分段将会允许用户数据单元为了满足延时要求而交叉存取。B)当L2CAP控制数据包的大小，mem和buffer的管理将会更加简单。C)通过重发进行的纠错将会更加有效。D）L2CAP PDU的丢包大小将会比用户数据单元小。E)应用将不会被低层对分段大小的要求所限制。

3. Streaming mode is used to stop HCI and Controller based flow control from being applied on the receiving side.即流模式是用来阻止基于HCI和控制器的流控制作用于接收端。

4. The Flow and Error Control block provides four modes. Enhanced Retransmission mode and Retransmission Mode offer segmentation, flow control and L2CAP PDU retransmissions.即流和错误控制模块提供了四种模式。加强重传模式和分组重传模式，流控制和L2CAP PDU重传。

5. L2CAP does not transport synchronous data designated for SCO or eSCO logical transports.即L2CAP不传输SCO或者eSCO逻辑传输的同步数据。

6. L2CAP does not support a reliable broadcast channel.即L2CAP不支持可靠的广播通道。

7. Although described within the L2CAP layer, fragmentation may actually occur in an HCI host driver, and/or in a Controller, to accommodate the L2CAP PDU transport to HCI data packet or Controller packet sizes.即虽然是在L2CAP层描述的，分片真实发生在HCI主机驱动或者一个控制器，去使得L2CAP PDU传输适应HCI数据包或者控制器包的大小。

8. The null identifier (0x0000) shall never be used as a destination endpoint. Identifiers from 0x0001 to 0x003F are reserved for specific L2CAP functions. These channels are referred to as Fixed Channels. Fixed channels shall only run over ACL-U or LE-U logical links and shall not be moved.即null标识符不能够被一个目的端点使用。从0x0001到0x003f是为了特殊的L2CAP功能所保留的。这些通道称之为固定通道。固定通道应当只工作在ACL-U或者LE-U逻辑链路上并不不应当被移出。

9. A dynamically allocated CID is allocated to identify the local endpoint and shall be in the range 0x0040 to 0xFFFF.即动态申请的CID应当在0x0040到0xffff之间。

10. L2CAP may operate in one of five different modes as selected for each L2CAP channel. A) Basic L2CAP Mode B) Flow Control Mode C) Retransmission Mode D) Enhanced Retransmission Mode E) Streaming Mode. 即L2CAP工作在以下五个不同的模式：a）基本的L2CAP模式。B)流控制模式。C)重传模式。D)加强的重传模式。E)流模式。

11. In Flow Control Mode no retransmissions take place, but missing PDUs are detected and can be reported as lost.即在流控制模式，没有重传发生，但是丢失的PDU会被探测到并且会被报告为丢失。

12. L2CAP maps channels to Controller logical links, which in turn run over Controller physical links.即L2CAP映射通道到控制器逻辑链路，它轮流地去查询控制器物理链路。

13. When a Guaranteed channel is created or moved to a Controller, a corresponding Guaranteed logical link shall be created to carry the channel traffic. Creation of a Guaranteed logical link involves admission control. 即当一个有保证的通道建立或者移动到一个控制器，一个响应的有保证的逻辑链路应当被建立来运输通道的内容。一个有保证的逻辑链路的建立包含允入控制。

14. A channel represents a data flow between L2CAP entities in remote devices. Fixed channels other than the L2CAP connectionless channel (CID 0X0002) and the two L2CAP signaling channels (CIDs 0X0001 AND 0X0005) are considered connection-oriented. All channels with dynamically assigned CIDs are connection-oriented.即一个通道代表了L2CAP实体和外部设备之间的一个数据流。固定通道除了L2CAP无连接通道（CID 0X002）和两个信号通道（CIDs 0x0001和0x0005）其他的都是面向连接的。所有的动态申请CID的通道都是面向连接的。

15. All L2CAP layer packet fields shall use Little Endian byte order with the exception of the information payload field. The endian-ness of higher layer protocols encapsulated within L2CAP information payload is protocol-specific.即所有的L2CAP层的数据包字段应当使用小印第安序除了负载字段。L2CAP信息负载中包含的更高层协议的印第安情况是根据各个协议不同的。

16. In basic L2CAP mode, the L2CAP PDU on connection-oriented channel is also referred to as a “B-frame”.即在基本的L2CAP模式下，面向连接通道上的L2CAP PDU也被称为“B帧”。

17. L2CAP PDU within a connectionless data channel is also referred to as a “G-frame”.即无连接数据通道中L2CAP PDU也被称为“G帧”。

18. Implementations shall support a connectionless MTU of 48 octets on the connectionless channel.即在无连接通道上应当支持48比特的无连接MTU。

19. To support flow control, retransmissions, and streaming, L2CAP PDU types with protocol elements in addition to the Basic L2CAP header are defined. The information frames (I-frames) are used for information transfer between L2CAP entities. The supervisory frames (S-frames) are used to acknowledge I-frames and request retransmission of I-frames.即为了支持流控制，重传和流传输，L2CAP PDU除了基本的L2CAP头之外还定义了一个协议元素。信息帧（I帧）用来在L2CAP实体之间传输信息。监督帧（S帧）用来确认I帧和请求I帧的重传。

20. The Control Field identifies the type of frame. There are tree different Control Field Formats: the Standard Control Field, the Enhanced Control Field, and the Extended Control Field. Information and Supervisory frames types are distinguished by the least significant bit in the Control Field. 即控制字段是用来表明帧的类型的。这里有三种不同控制字段格式：标准的控制字段（应当用于重传模式和流控制模式），加强的控制字段（应当用于加强的重传模式和流传输模式）和扩展控制字段（可能用于加强的重传模式和流传输模式）。信息和监督帧的类型是由控制字段的最后一个有效位来区分的。

21. Defined types of S-frames are RR (Receiver Ready), REJ (Reject), RNR (Receiver Not Ready) and SREJ (Selective Reject).即S帧有以下几个类型：RR（接收就绪），REJ（拒绝接收），RNR（接收未就绪）和SREJ（有选择性地拒绝）。

22. For Retransmission mode and Flow Control mode, a received PDU shall be regarded as invalid, if one of the following conditions occurs: 1.Contains an unknown CID. 2.Contains an FCS error. 3.Contains a length greater than the maximum PDU payload size (MPS). 4.I-frame that has fewer than 8 octets. 5.I-frame with SAR=01 (start of L2CAP SDU) that has fewer than 10 octets. 6.I-frame with SAR bits that do not correspond to a normal sequence of either unsegmented or start, continuation, end for the given CID. 7.S-frame where the length field is not equal to 4.即在重传模式和流控制模式下，有下列情况之一的认为是无效的PDU：1.包含未知的CID。2包含FCS错误。3包含长度比最大的PDU负载长度还大。4.I帧小于8比特。5.SAR=01的I帧比10比特小。6.SAR不是四种之一。7.S帧的长度段不为4.