Overview of BR/EDR PHY OperationV5.2 (2019-12-31)

Note:
  • this page is based on Bluetooth core 5.2, for details please check the specification directly.

GENERAL DESCRIPTION

Bluetooth wireless technology is a short-range communications system intended to replace the cable(s) connecting portable and/or fixed electronic devices.

There are two forms of Bluetooth wireless technology systems, both including device discovery, connection establishment and connection mechanisms.

  1. Basic Rate (BR) includes optional Enhanced Data Rate (EDR) Alternate Media Access Control (MAC) and Physical (PHY) (AMP) layer extensions. The Basic Rate system offers synchronous and asynchronous connections with data rates of 721.2 kb/s for Basic Rate, 2.1 Mb/s for Enhanced Data Rate and high speed operation up to 54 Mb/s with the 802.11 AMP.
  2. Low Energy (LE) includes features designed to enable products that require lower current consumption, lower complexity and lower cost than BR/EDR. The LE system is also designed for use cases and applications with lower data rates and has lower duty cycles. The LE system includes an optional 2 Mb/s physical layer data rate and also offers isochronous data transfer in a connection-oriented and connectionless mechanism that uses the isochronous transports.

OVERVIEW OF BR/EDR OPERATION

BR/EDR operates in the unlicensed ISM band at 2.4 GHz. The system employs a frequency hopping transceiver to combat interference and fading. BR uses a shaped, binary frequency modulation (GFSK) to minimize transceiver complexity. The symbol rate is 1 Msym/s. The bit rate is 1 Mb/s for BR; 2 Mb/s or 3 Mb/s for EDR.

During typical operation a physical radio channel is shared by a group of devices that are synchronized to a common clock and frequency hopping pattern:

  • Master: the device that provides the synchronization reference
  • Slave: all other devices synchronized to a master’s clock and frequency hopping pattern
  • Piconet: a group of devices synchronized in this master/slave fashion form a piconet.

Devices in a piconet use a specific frequency hopping pattern:

  • The basic hopping pattern is a pseudo-random ordering of the 79 frequencies, separated by 1 MHz, in the ISM band.
  • The hopping pattern can be adapted – on a per-slave basis – to exclude a portion of the frequencies that are used by interfering devices. The adaptive hopping technique improves Bluetooth co-existence with static (non-hopping) ISM systems when they are co-located.

The physical channel is sub-divided into time units known as slots:

  • Data is transmitted between Bluetooth devices in packets that are positioned in these slots.
  • a number of consecutive slots may be allocated to a single packet.
  • Frequency hopping may take place between the transmission or reception of packets.
  • Bluetooth technology uses Time-Division Duplex (TDD).

Typically within a physical channel, a physical link is formed between a master device and slave devices. Exceptions to this include Inquiry scan and Page scan physical channels, which have no associated physical link.

The physical link provides bidirectional packet transport between the master and slave devices, except in the case of a Connectionless Slave Broadcast physical link. In that case, the physical link provides a unidirectional packet transport from the master to a potentially unlimited number of slaves. Since a physical channel could include multiple slave devices, there are restrictions on which devices may form a physical link. There is a physical link between each slave and the master. Physical links are not formed directly between the slaves in a piconet.

OVERVIEW OF AMP OPERATION

Alternate MAC/PHYs (AMP) are secondary Controllers in the Bluetooth core system.

  • The BR/EDR radio, the primary radio, is used to perform discovery, association, connection establishment, and connection maintenance.
  • Once an L2CAP connection has been established between two devices over the BR/EDR radio, the AMP Managers can discover the AMPs that are available on the other device. When an AMP is common between the two devices, the Core system provides mechanisms for moving data traffic from BR/EDR Controller to an AMP Controller.

Each AMP consists of a Protocol Adaptation Layer (PAL) on top of a MAC and PHY. The PAL is responsible for mapping the Bluetooth protocols and behavior (as specified by HCI) to the specific protocols of the underlying MAC and PHY. L2CAP channels may be created on, or moved to, an AMP. L2CAP channels may also be moved back to the BR/EDR radio when those capabilities are not necessary or when the AMP physical link has a link supervision timeout. A link supervision timeout on an ACL link connecting two BR/EDR devices forces a disconnection of all AMP physical links between those devices.

AMPs may be enabled or disabled as needed in order to minimize power consumption in the system.

DATA TRANSPORT ARCHITECTURE

The Bluetooth data transport system follows a layered architecture. This description of the Bluetooth system describes the Bluetooth core transport layers up to and including L2CAP channels. All Bluetooth operational modes follow the same generic transport architecture, which is shown below:

bluetooth data transportBluetooth generic data transport architecture

Reliability

Bluetooth is a wireless communications system. In poor RF environments, this system should be considered inherently unreliable. To counteract this the system provides levels of protection at each layer.

  • The baseband packet header uses forward error correcting (FEC) coding to allow error correction by the receiver and a header error check (HEC) to detect errors remaining after correction.
  • Certain Baseband packet types include FEC for the payload.
  • Furthermore, some Baseband packet types include a cyclic redundancy error check (CRC).

On ACL logical transports the results of the error detection algorithm are used to drive a simple ARQ protocol. This provides an enhanced reliability by retransmitting packets that do not pass the receiver’s error checking algorithm.

The L2CAP layer provides an additional level of error control that is designed to detect the occasional errors not detected by the baseband and request retransmission of the affected data. This provides the level of reliability required by typical Bluetooth applications.

Broadcast links have no feedback route, and are unable to use the ARQ scheme (although the receiver is still able to detect errors in received packets). Instead each packet is transmitted several times in the hope that the receiver is able to receive at least one of the copies successfully. Despite this approach there are still no guarantees of successful receipt, and so these links are considered unreliable.