Description
The Asynchronous Connection-Oriented Link (ACL) is a fundamental Bluetooth transport mechanism that provides reliable, connection-oriented data communication between a master device and one or more slave devices in a piconet. Unlike Synchronous Connection-Oriented (SCO) links designed for isochronous voice traffic, ACL links handle asynchronous data transfer with built-in error detection and retransmission capabilities. ACL operates on a master-slave architecture where the master device controls timing and polling of slave devices, allocating time slots for bidirectional or unidirectional data transmission.
ACL connections use a time-division duplex (TDD) scheme where the master and slave alternate transmission in consecutive time slots. The master initiates communication in even-numbered slots, and the addressed slave responds in the following odd-numbered slot. This polling mechanism allows the master to maintain connections with up to seven active slaves in a piconet. ACL packets support various payload sizes and data rates, with the base rate offering 723.2 kbps asymmetric or 433.9 kbps symmetric data transfer. Enhanced Data Rate (EDR) modes introduced in later Bluetooth versions significantly increase throughput.
Error control in ACL links employs Automatic Repeat Request (ARQ) mechanisms with cyclic redundancy check (CRC) and forward error correction (FEC) for certain packet types. The master device uses a stop-and-wait ARQ protocol where each transmitted packet must be acknowledged before sending the next. Unacknowledged packets are retransmitted until successful delivery or until a timeout occurs. ACL supports multiple packet types with different levels of error protection, allowing trade-offs between reliability and throughput based on application requirements.
ACL links coexist with SCO links in Bluetooth networks through time multiplexing. The master device schedules both link types within the same piconet, with SCO links receiving priority for their time-critical voice traffic. ACL traffic fills the remaining time slots, providing efficient bandwidth utilization. This dual-link architecture enables Bluetooth devices to simultaneously support voice calls and data transfer applications. ACL also forms the foundation for advanced Bluetooth profiles and services requiring reliable data transport.
Purpose & Motivation
ACL was created to provide reliable data communication in Bluetooth networks, addressing the need for error-free transfer of application data alongside voice traffic. Before Bluetooth standardization, wireless data transfer often relied on proprietary protocols with inconsistent reliability mechanisms. ACL introduced a standardized approach to asynchronous data transport with guaranteed delivery, enabling interoperable data applications across different manufacturers' devices.
The technology solves the problem of maintaining data integrity over error-prone wireless channels while efficiently sharing bandwidth with synchronous voice connections. Previous wireless data solutions either lacked proper error correction or used inefficient retransmission schemes that degraded overall network performance. ACL's combination of ARQ, FEC, and flexible packet types provides adaptable error protection suitable for various data types and channel conditions.
ACL's creation was motivated by the growing need for cable replacement technologies that could handle both voice and data applications reliably. By providing connection-oriented data transport with sequencing and acknowledgment mechanisms, ACL enabled Bluetooth to support file transfer, network access, synchronization, and other data-intensive applications. This made Bluetooth suitable for broader use cases beyond simple voice headsets, contributing to its widespread adoption in mobile devices, computers, and IoT applications.
Release Timeline
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (8 CRs across 3 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the specification for the ACL function was updated to extend its descriptive text. Specifically, the text within the EF ACL clause was modified to also include a reference to DNN (Data Network Name). This change ensures the ACL documentation aligns with the core network elements used for 5G connectivity.
- Extend text in EF ACL clause to also include DNN TS 31.102CR0816
In Release 16, the new functionality for the Asynchronous Connection-Oriented Link (ACL) introduced support for dynamic ACL establishment. This was specifically enabled over the S1 and X2 interfaces, as detailed in the respective Change Requests. Furthermore, the release added support for managing dynamic ACL during critical procedures like handover and dual connectivity.
In Release 17, the ACL function was enhanced to support dynamic establishment over both the S1 and X2 interfaces. This was complemented by work to address missing use cases for this dynamic capability. Furthermore, remaining issues for ACL were resolved through updates to the NGAP protocol.
Explore further
Broader topics and technologies where ACL plays a role.
Defining Specifications
3GPP specifications that define or reference ACL, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TR 21.905 vj00 | 3GPP Technical Terms and Definitions | Rel-19 |
| TS 23.234 vd10 | 3GPP-WLAN Interworking Index | Rel-13 |
| TS 23.722 vf10 | Common API Framework (CAPIF) for 3GPP Northbound APIs | Rel-15 |
| TS 24.147 vj00 | IMS Conferencing Protocol Details | Rel-19 |
| TS 24.235 vc10 | I-WLAN Interworking Management Object | Rel-12 |
| TS 24.302 vj00 | Access to EPC via non-3GPP networks; Stage 3 | Rel-19 |
| TS 24.305 vj00 | Selective Disabling of 3GPP UE Capabilities | Rel-19 |
| TS 24.368 vj40 | NAS Configuration Management Object | Rel-19 |
| TS 24.483 vj20 | Mission Critical Services Management Object | Rel-19 |
| TS 28.602 vc00 | CN & non-3GPP NRM IRP Information Service | Rel-12 |
| TS 29.199 v1900 | Multimedia Messaging Web Services | Rel-9 |
| TS 31.102 vj40 | USIM Application Specification | Rel-19 |
| TS 31.121 vi50 | UICC-terminal interface test specification | Rel-18 |
| TR 31.901 ve00 | USIM/ISIM/USAT Feature Review Study | Rel-14 |
| TS 33.320 vj00 | H(e)NB Subsystem Security Architecture | Rel-19 |
| TS 33.812 v920 | M2M Remote Subscription Management Security | Rel-9 |
| TS 33.820 v1830 | Home NodeB/eNodeB Security Architecture | Rel-8 |
| TS 36.413 vj10 | S1 Application Protocol (S1AP) | Rel-19 |
| TS 36.423 vj10 | X2 Application Protocol (X2AP) Specification | Rel-19 |
| TS 36.816 vb20 | In-device coexistence interference avoidance | Rel-11 |
| TS 38.413 vj10 | NG Application Protocol (NGAP) | Rel-19 |
