blocks|key|712518|text|我不是蓝牙通信方面的专家，但我目前正在从事基于L2CAP的移动应用程序和基于linux的服务器之间的通信。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|712519|在这项开发工作中，我在蓝牙SIG规范-+Core+4.2中遇到了一种叫做面向连接的通道和无连接的数据通道。在此链接中部分理解：https://www.bluetooth.com/specifications/bluetooth-core-specification|offset|length|712520|据我从阅读中了解到的部分内容是，连接导向的渠道，顾名思义，是面向连接的。也就是说，每个服务都与一个通道相关联。|712521|在无连接信道的情况下，psm充当多路复用器，方便多个服务使用无连接信道:+0x0002。因此，psm(协议/服务复用器)基本上类似于该信道中的端口号。|712522|这些只是我的理解和观察。我是新来的，也许是错的。我努力写这个，因为我没有看到真正的文档或论坛，人们都想解释他们对这个概念的理解。越多越快乐。|712523|entityMap|0|LINK|mutability|MUTABLE|url|https://www.bluetooth.com/specifications/bluetooth-core-specification^0|0|1R|1X|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|U|8|@]|9|@$D|V|E|W|1|X]]|A|$]]|$1|F|3|G|5|6|7|Y|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|Z|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|10|8|@]|9|@]|A|$]]|$1|L|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|M|$N|$5|O|P|Q|A|$R|S]]]]

I am no expert in Bluetooth communication, but I am currently working on an L2CAP LE based communication between a mobile app and a linux based server.

During this development work, I came across something called Connection-oriented Channels and Connectionless Data Channel in the bluetooth SIG specification - Core 4.2. Particluarly in this link: <a href="https://www.bluetooth.com/specifications/bluetooth-core-specification" rel="nofollow noreferrer">https://www.bluetooth.com/specifications/bluetooth-core-specification</a>

As far as what I have understood from reading it partly is that the connection-oriented channels are, as the name suggests, connection oriented. That is, each service is associated with a channel. 

In case of a connectionless channel, the psm acts as a multiplexer and facilitates multiple services to use the connectionless channel: 0x0002. Thus, psm(protocol/Service multiplexer) basically is similar to a port number in this channel.

These are just my understanding and observations. I am new to this and maybe wrong. I made an effort to write this since I see no real documentation or forum where people have cared to explain their understanding of this concept. The more the merrier.

blocks|key|4105618|text|请参阅蓝牙核心规范v4.2第3卷A部分第4.2节，它指出：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4105619|blockquote|4105620|协议/服务多路复用器-+PSM+(最小2倍频)+PSM字段长度至少为两个八位数。PSM字段的结构基于地址字段的ISO+3309扩展机制。所有PSM值都应该是奇数，也就是说，最不重要的八进制中最不重要的位必须是1。而且，所有PSM值都应该有最重要的八进制中最小的位等于0。这允许将PSM字段扩展到16位以上。PSM值被分为两个范围。第一个范围内的有效值由蓝牙SIG分配，并指示协议。第二个值范围与服务发现协议(，SDP)一起动态分配和使用。动态分配的值可用于支持特定协议的多个实现。|unordered-list-item|4105621|它还提供了l2cap连接请求包的概要。|4105622|📷|atomic|offset|length|4105623|​|4105624|基本上把CID看作是端口，或者一般的通信端点。连接在特定CID上接收，您可以在特定CID上请求连接。甚至还有蓝牙SIG保留的CID，您可以在第3卷A部分2.1节中看到。|4105625|另一方面，PSM可以被看作是试图连接到您的特定服务或设备的标识符(如果您正在进行连接，则为您自己的设备)。例如，请求与您连接的设备可以在文档中指定以查找PSM为0x80。|4105626|entityMap|0|IMAGE|mutability|IMMUTABLE|imageUrl|https://developer.qcloudimg.com/http-save/yehe-900000/cc55a5b898c13462a6a6e08de05e97f9.jpg|imageAlt^0|0|0|0|0|0|1|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|-4|5|C|7|13|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|14|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|15|8|@]|9|@]|A|$]]|$1|I|3|J|5|K|7|16|8|@]|9|@$L|17|M|18|1|19]]|A|$]]|$1|N|3|O|5|6|7|1A|8|@]|9|@]|A|$]]|$1|P|3|Q|5|6|7|1B|8|@]|9|@]|A|$]]|$1|R|3|S|5|6|7|1C|8|@]|9|@]|A|$]]|$1|T|3|-4|5|6|7|1D|8|@]|9|@]|A|$]]]|U|$V|$5|W|X|Y|A|$Z|10|11|-4]]]]

Check out the Bluetooth Core Specification v4.2 [Vol 3, Part A, Section 4.2] it states:

<blockquote>
 <ul>
 <li>Protocol/Service Multiplexer - PSM (2 octets (minimum))
 The PSM field is at least two octets in length. The structure of the PSM field
 is based on the ISO 3309 extension mechanism for address fields. All PSM
 values shall be ODD, that is, the least significant bit of the least significant
 octet must be 1. Also, all PSM values shall have the least significant bit of
 the most significant octet equal to 0. This allows the PSM field to be
 extended beyond 16 bits. PSM values are separated into two ranges. Valid
 values in the first range are assigned by the Bluetooth SIG and indicate
 protocols. The second range of values are dynamically allocated and used
 in conjunction with the Service Discovery Protocol (SDP). The dynamically
 assigned values may be used to support multiple implementations of a
 particular protocol.</li>
 </ul>
</blockquote>

It also provides an outline of an l2cap connection request packet
<a href="https://i.stack.imgur.com/5bqbg.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/5bqbg.jpg" alt="l2cap Connection Request Packet"></a>

Basically think of the CID as the port, or general communication endpoint. Connections are received on a specific CID and you can request connections on a specific CID. There are even Bluetooth SIG reserved CIDs which you can see in [Vol 3, Part A, Section 2.1]

The PSM on the other hand can be thought of as an identifier for a specific service or device that is trying to connect to you (or for your own device if you are doing the connecting). For example a device requesting a connection to you may specify in documentation to look for a PSM of 0x80.

blocks|key|1191732|text|从2002年开始，在合成主题的蓝牙应用程序开发人员指南中发现了以下内容|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|1191733|问:+L2CAP+PSM值和L2CAP+CID之间有什么区别？
A:协议服务复用器(PSM)值标识用于在L2CAP信道上通信的协议。实际上，这定义了使用通道的更高层。同一高层的多个实例可能使用不同的L2CAP通道，但它们都将由相同的PSM值标识。每个单独的通道都通过其信道ID+(CID)进行唯一标识。高层可以通过指定0x0003的PSM值来请求到远程RFCOMM实体的L2CAP连接。然后，本地和远程L2CAP层为该链接分配CID。CID用于实际识别RFCOMM层之间发送的通信量。|blockquote|1191734|entityMap^0|F|C|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@$9|L|A|M|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|N|8|@]|D|@]|E|$]]|$1|I|3|-4|5|6|7|O|8|@]|D|@]|E|$]]]|J|$]]

Found the following in Syngress' Bluetooth Application Developer Guide from 2002

<blockquote>
 Q: What is the difference between an L2CAP PSM value and an L2CAP CID?
 
 A: Protocol Service Multiplexor (PSM) values identify the protocol
 used to communicate over an L2CAP channel. In effect, this defines the
 higher layer that uses the channel. Multiple instances of the same
 higher layer may use different L2CAP channels, but they will all be
 identified by the same PSM value. Each separate channel is uniquely
 identified by its Channel ID (CID). A higher layer may request an
 L2CAP connection to a remote RFCOMM entity by specifying a PSM value
 of 0x0003. The local and remote L2CAP layers then assign CIDs to this
 link. The CIDs are used to actually identify traffic sent between
 RFCOMM layers.
</blockquote>

I've been teaching myself how to write programs in C for BLE devices using the BlueZ API. However, I seem to be stumped on one particular aspect of the sockaddr_l2 struct. The definition I am using is the following:

<pre><code>struct sockaddr_l2 {
 sa_family_t l2_family;
 unsigned short l2_psm;
 bdaddr_t l2_bdaddr;
 unsigned short l2_cid;
 uint8_t l2_bdaddr_type;
};
</code></pre>

Now for the part I don't understand: what is the difference between l2_psm and l2_cid? This is what I've gathered so far:

PSM stands for "Protocol Service Multiplexers", and are the L2CAP connection's "port". Source: Bluetooth for Programmers, by Albert Huang

CID stands for "Channel Identifier", and are local names representing a logical channel end-point on the device. Source: <a href="http://ecee.colorado.edu/~ecen4242/marko/Bluetooth/Bluetooth/SPECIFICATION/L2CAP.html" rel="noreferrer">http://ecee.colorado.edu/~ecen4242/marko/Bluetooth/Bluetooth/SPECIFICATION/L2CAP.html</a>

Now I'm sure the answer is in those definitions, but I just can't seem to wrap my head around the definition of CID. Can anyone explain to me the differences between PSM and CID in a more comprehensive manner?

Thanks :)

Bluetooth LE L2CAP CID vs. PSM

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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我一直在教自己如何使用BlueZ API用C语言为BLE设备编写程序。然而，我似乎对sockaddr_l2结构的一个特定方面感到困惑。我所用的定义如下：struct sockaddr_l2 {    sa_family_t l2_family;    unsigned short  l2_psm;    bdaddr_...

问蓝牙LE L2CAP CID与PSM
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