Content deleted Content added
m →The Bluetooth Data Transport Architecture: Added Figure 2 to second image. |
Misleading acronym lead to incorrect interpretation of 'ACL', fixed following the official Bluetooth documentation's definitions |
||
| (5 intermediate revisions by 5 users not shown) | |||
Line 1:
▲ACL is an informal acronym which refers to the Bluetooth Asynchronous Connection-oriented Logical transport. ACL is used as a shorthand to refer to one of two types of logical transport defined in the Bluetooth Core Specification, either BR/EDR ACL or LE ACL. BR/EDR ACL is the ACL logical transport variant used with Bluetooth Basic Rate/Enhanced Data Rate (BR/EDR, also known as Bluetooth Classic) whilst LE ACL is the ACL logical transport variant used with Bluetooth Low Energy (LE).
The ACL transports are part of the Bluetooth data transport architecture.
Note that all definitions of Bluetooth terminology, protocols and procedures including ACL are defined in the Bluetooth Core Specification<ref>{{cite web |title=Bluetooth Core Specification |date=12 July 2021 |url=https://www.bluetooth.com/specifications/specs/core-specification-5-3/}}</ref> which is published by the standards development organisation, the [[Bluetooth Special Interest Group]] (Bluetooth SIG).
The architecture section of the Bluetooth Core Specification defines a number of concepts which collectively constitute the ''Bluetooth data transport architecture''. Key amongst these concepts are the Physical Channel, Physical Link, Logical Link and Logical Transport. Certain combinations are intended for use in different application types which have particular requirements regarding issues such as topology, timing, reliability and radio channel use.
Line 19 ⟶ 18:
Both ACL variants are designed to provide reliable, bi-directional, point to point communication.
A Bluetooth LE Central device may establish a connection with an advertising Peripheral device by responding to a received connectable advertising packet with a PDU that requests a connection. A number of parameters are specified in the request. Amongst these parameters are ''connection interval'', ''supervision timeout'', ''peripheral latency'' and ''channel map''.
Line 41 ⟶ 40:
[[File:Bluetooth LE-ACL connection with Peripheral Latency 1.png|thumb|upright=2.5|Figure 4 - A Bluetooth LE-ACL connection with Peripheral Latency=1 (C denotes the Central device, P denotes the Peripheral device)]]
Link layer data packets contain three important fields which contribute to communication being reliable. These fields are called the Sequence Number (SN), Next Expected Sequence Number (NESN), and the More Data field. All three of these fields are single bit fields and their use provides a system of acknowledgements and a method for checking for the correct ordering of received packets.
Line 62 ⟶ 61:
[[File:CRC failure at the Bluetooth link layer.png|thumb|Figure 7 - Behaviour of the Bluetooth link layer when a CRC failure occurs]]
LE-ACL employs a spread spectrum scheme known as ''adaptive frequency hopping''. At the start of each connection event, ''frequency hopping'' occurs, with one of the 37 general purpose Bluetooth LE radio channels being selected from the set of available channels using a ''channel selection algorithm''. Each device in the connection will then switch to the selected channel and over time and a series of connection events, communication will take place using a frequently changing series of different channels, distributed across the 2.4 GHz band, thereby significantly reducing the probability of collisions occurring.
A ''channel map'' is maintained by the Central device and may also be maintained by the Peripheral device. This is a table of data which indicates which channels are available for use and which are not. Implementations mark channels as ''used'' or ''unused'' according to how well each channel is performing in terms or errors and evidence of interference. Unused channels are not selected by the channel selection algorithm. In this way, adaptive frequency hopping dynamically adjusts the channels used for active communication according to the prevailing RF conditions in the environment.
A number of control procedures relating to LE ACL connections are defined. A selection of examples appears in Table 1.
Line 92 ⟶ 91:
|}
Subrated connections are LE ACL connections which have additional properties assigned to them and behave differently in some ways. The additional properties are called the ''subrate factor'', ''subrate base event'', and ''continuation number''.
Line 107 ⟶ 106:
The Bluetooth Core Specification Version 5.3 Feature Enhancements paper has a substantial chapter dedicated to the subject of subrated connections and is recommended as a source of further information.
Communication using a BR/EDR ACL logical transport is similar to the LE ACL variant and provides an asynchronous point-to- point communication mechanism for exchanging data between a Central device and a Peripheral.
Line 119 ⟶ 118:
A BR/EDR ACL connection is established by a Peripheral device paging a Central device. The Central device performs ''page scanning''.
1-bit header fields ARQN and SEQN are used to allow positive or negative acknowledgements to be made and to verify that the order of packets received is as it should be.
The RX buffer associated with BR/EDR ACL connections may become full. The header field FLOW is used to provide a simple flow control mechanism with values in responses indicating STOP or GO.
Adaptive frequency hopping (a responsibility of the underlying physical channel) is in effect when using the BR/EDR ACL logical transport, with a channel selected at each reception or transmission event. 79 channels are defined for use with Bluetooth BR/EDR and there are a number of different possible hopping patterns defined.
The Link Manager Protocol (LMP) defines a series of PDU types which allow the control of and negotiation over details of the BR/EDR ACL logical transport to be carried out. LMP PDUs are sent over an ACL-C logical link.
== References ==
| |||