1 Introduction
Since its launch by Ericsson in 1994, Bluetooth technology has gone through 20 years. From the original Bluetooth V1.0 to Bluetooth V4.0 (the latest is V4.1, released at the end of 2013), it has developed into the current situation after nearly 9 revisions.
To be honest, today's Bluetooth 4.1 is a hodgepodge: BR/EDR follows the old Bluetooth specification; LE copies 802.15.4; AMP uses 802.11 directly. The purpose of all this is to be in a dilemma between power consumption and transmission rate based on compatibility and ease of use. Snail thought that this was not an elegant design.
But it doesn't matter, it is reasonable to exist. Therefore, WoWu opened such a topic, hoping to sort out the knowledge of Bluetooth technology from top to bottom, so as to deepen their understanding of Bluetooth technology, and to inspire readers who are engaged in Bluetooth-related work.
This article is the first article of this topic, mainly based on the Bluetooth 4.1 specification ( Core_V4.1.pdf ), describing the basic concepts of Bluetooth technology.
2. Overview of Bluetooth Technology
2.1 Two Bluetooth technologies: Basic Rate (BR) and Low Energy (LE)
The Bluetooth protocol includes two technologies: Basic Rate (BR for short) and Low Energy (LE for short). These two technologies include mechanisms such as discovery management and connection management, but they cannot communicate with each other ! This is also one of the reasons why the snail complains that the Bluetooth protocol is not elegant.
Manufacturers either implement one of these two technologies, and then they can only communicate with devices that also implement this technology, but cannot communicate with devices that implement the other technology. If manufacturers want to ensure that they can communicate with all Bluetooth devices, they can only implement two technologies at the same time, regardless of whether they are really needed, so that people can talk about it!
2.1.1 Basic Rate(BR)
Basic Rate is the authentic Bluetooth technology, which can include optional (optional) EDR (Enhanced Data Rate) technology, and alternately used (Alternate) MAC (Media Access Control) layer and PHY layer extension (AMP for short). It's a mouthful to say, but through the application scenarios behind it, it's easy to understand:
At the beginning of the birth of Bluetooth, BR technology was used. At this time, the theoretical transmission rate of Bluetooth can only reach 721.2Kbps. In that era, the 56Kbps Modem was very tall, and the speed was amazing! But technology is changing so fast that BR technology is obsolete in a blink of an eye. then what should we do? Repair the seams, enhance the speed, and the Enhanced Data Rate will appear.
Using Bluetooth with EDR technology, the theoretical rate can reach 2.1Mbps. This time the upgrade is quite elegant, because there is no change in any hardware architecture, software architecture and usage.
As you may have guessed, EDR is outdated again. Look at other people's WIFI (WLAN), dozens of Mbps, hundreds of Mbps, we are only 2.1Mbps, it's too shabby! then what should we do? The bluetooth group had a bad idea: hey, WIFI! Lend me your physical layer and MAC layer to use! This is AMP (Alternate MAC and PHY layer extension). Emma, finally relieved, we can hit 54Mbps.
However, due to the obvious difference between the physical layer of Bluetooth itself and AMP technology, this extension can only be used alternately (Alternate), that is to say, there is me (BR/EDR) but not you (AMP). Um! Not elegant!
Bury a question: can only be used interchangeably, how do they switch? In the follow-up content, WoWo will discuss this problem based on the solutions of mainstream Bluetooth chips.
[Note 1: Attentive readers may notice that the words optional and alternate are particularly emphasized here, which are the original words of the Bluetooth Spec. It means that BR and EDR can exist at the same time, but BR/EDR and AMP can only be selected. 】
2.1.2 Low Energy（LE）
The evolution route of the BR technology mentioned above is the acceleration, acceleration, and acceleration of the transmission rate. But energy is conserved. If you want to transmit faster, the price is to consume more energy. There are many application scenarios that do not care about the transmission rate, but are very concerned about the power consumption. This is the background that Bluetooth LE (called Bluetooth Low Energy) came into.
Compared with BR technology, LE technology is very different, or it is two different technologies, which happen to be prefixed with "Bluetooth". We will explain this difference in detail later, as well as the behavioral characteristics of LE.
2.2 The composition of the Bluetooth system
The composition of the Bluetooth system involves terms such as Bluetooth Application, Bluetooth Core, Bluetooth Host, Bluetooth Controller, etc. I don't know if it is because of the ambiguity of English understanding or because of the ambiguity defined by the Bluetooth specification itself. It feels a bit awkward when WoWa understands these words. Therefore, in this chapter, the related concepts and the meaning behind them are explained.
The above figure describes the composition of the Bluetooth system, we need to pay attention to the following characteristics:
1) The components of the Bluetooth system described in the figure, such as Bluetooth Core and Bluetooth Application, such as Host and Controller, all refer to " logical entities ". The so-called "logical entity" needs to be separated from the "physical entity" in daily life. For example, when doing circuit design, a Bluetooth chip and a main control CPU refer to physical entities. These "logical entities" described by the Bluetooth protocol may not necessarily correspond to physical entities. For example, in practical applications, the Host and the Bluetooth Application may be located in the same physical entity (the main control CPU), while the Controller is located in the same physical entity. In another physical entity (Bluetooth chip).
2) The Bluetooth protocol specifies two levels of protocols, namely Bluetooth Core and Bluetooth Application. The Bluetooth core protocol focuses on the description and specification of the Bluetooth core technology. It only provides basic mechanisms and does not care about how to use these mechanisms; the Bluetooth application layer protocol is based on the Bluetooth core protocol, according to specific application requirements. Define various policies, such as FTP, file transfer, LAN, etc.
3) Bluetooth Core consists of two parts, Host and Controller. These two parts have slightly different roles in different Bluetooth technologies (BR/EDR, AMP, LE), but the general functions are the same. The Controller is responsible for defining the specifications of partial hardware such as RF and Baseband, and abstracting the logical link (Logical Link) for communication on top of it; the Host is responsible for more friendly encapsulation based on the logical link, so that the The details of the Bluetooth technology can be shielded to make the Bluetooth Application more convenient to use.
4) In a system, there is only one Host, but there can be one or more Controllers. For example: separate LE Controller; separate BR/EDR Controller; separate LE+BR/EDR Controller; one or more additional AMP Controllers are added on the basis of separate BR/EDR Controller or LE+BR/EDR Controller.
[Note 2: The detailed description of the Bluetooth Core will be described in the next article, and this article will not introduce it in depth. 】
3. BR/EDR vs LE vs AMP
Let's first have a further understanding of the three technologies of BR/EDR, AMP and BLE from the following pictures (click here to view the enlarged original picture):
This picture is an overview of the Bluetooth Core, from the RF Physical Channel, to the Baseband Physical Link, Logical Link, LMP, L2CAP and other concepts, there are some rough introductions. It can be seen from this picture that BR/EDR, AMP, BLE and other technologies have the following characteristics:
1) BR/EDR technology focuses too much on "point-to-point" communication, so that although the concepts of multicast (Unidirectional) and broadcast (Broadcast) are mentioned at the bottom layer of the protocol (such as Logical Link), in the upper-layer application scenario, There are few (and no) corresponding applications.
2) But with the development of the Internet of Things, the industry's demand for simple multicast or broadcast communication that does not require connection is more and more urgent. Therefore, BLE technology has been modified in the RF and Baseband protocols to adapt to this. This requirement is to modify the frequency hopping method of the original 79 channels, reduce the number of channels to 40, and retain no less than 3 fixed channels for broadcast communication. For frequency hopping only on the remaining 37 data channels.
3) Because of this change, the original concepts of search/connection/pairing no longer exist on BLE, and replaced by concepts such as Advertisor and Initiator. But at the level of subsequent data communication, try to keep it as consistent as possible.
4) For AMP, it is a BR/EDR-based controller. After completing the usual point-to-point connection, the two Bluetooth devices negotiate whether the subsequent data communication needs to be transferred to the AMP controller. This is the AMP technology introduced by Bluetooth 3.0.
For the time being, we will make a perceptual understanding of Bluetooth technology in this article. In subsequent articles, we will expand and advance step by step based on protocols at various levels, and strive to analyze Bluetooth technology thoroughly.