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It is known that Bluetooth Low Energy transmits data only during short time intervals called Connection Events. Connection events occur regullary with predefined period. The rest of the time Bluetooth LE device is does not transmit nor receive data. That is how low energy consumption is reached.

Meanwhile, classical Bluetooth BR/EDR has Park state. In this mode the parked slave wakes up at regular intervals to listen to the channel in order to re-synchronize and to check for broadcast messages. The rest of the time Bluetooth BR/EDR device is does not transmit nor receive data.

So, why Bluetooth LE consumes less energy?

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There are many factors that make BLE low power, and I have attempted to address as many of them as possible.

In order to better understand the power consumption differences between Bluetooth classic and BLE, it would helpful to look at some of the differences between the Bluetooth technologies. This would help appreciate the difference in power consumption. For starter Bluetooth classic consist of Bluetooth 1.0-3.0. These include Bluetooth BR (Basic Rate) round 1.2Mbit/sec, Bluetooth EDR (Enhance Data Rate) at 3Mbits/Sec and Bluetooth HS.

Bluetooth operates in 2.4 GHz ISM band, with Bluetooth classic uses 79 channels from 2.4GHz to 2.4835 GHz each spaced 1Hhz apart whereas BLE uses 40 channels from 2.402 GHz 2.480 GHz each spaced 2MHz apart. Of the 40 channels 3 of them are dedicated to advertising requests. Initial parameters are exchanged using same channel used for the connection request. Upon successful discover and connection, regular data channels are used for communication. Also note that advertising channels don’t overlap with Wifi Direct-sequence spread spectrum (DSSS) channels 1, 6, and 11. So Bluetooth uses a 2.4 GHz frequency band but implements a simpler Gaussian Frequency Shifting Protocol to reduce power as well as DSSS modulation.

rf spectrum br edr vs bluetooth smart
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advertising and data channels
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Bluetooth LE channel arrangements
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BLE has many different modes of which the main modes of operation are advertising mode, scanning mode, master device, and slave device. In the advertising mode the BLE base device will receive responses from other BLE devices for advertising events. On the scan mode the BLE device will scan for advertising request from other BLE devices and will respond with additional information depend on the status of the active scan status. There is also the passive mode, scanner only as well as advertiser only in which case the receiver and transmitter function of the RF module is required respectively. Some understanding of the Link Layer State machine is beneficial to understanding management of power consumption. There are five states and they are

  1. Standby: Can be entered into from any other state and no transmitting or receiving packets
  2. Advertising: This state can be entered from standby state. In this state the link layer will be transmitting advertising packets as well as responding to advertising related data exchanges
  3. Scanning: Scanning state can be entered from standby state, which listens for advertising channel packets from devices
  4. Initiating: The link layer in this state initiates a connection with another device responding to advertising channel packets from specific devices
  5. Connection: The connection state has two defined roles, namely master and slave. A device in the master role will define timing for transmission

State Diagram

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A connection is established by one device being in advertiser mode and other in initiator mode. The initiator becomes the master and the advertiser becomes the slave. This master slave data exchange defines critical connection parameters such as defining the channel and timing, which includes connection interval and salve latency. The slave latency is important because this determines the number of connection intervals the slave can ignore without losing connection. This helps the slave to optimize and preserve power consumption. The slave can request to update the communication parameters to better fit the slave’s application.

In your question you had referenced connection event. The diagram below describes a connection event.

Connection Event

The power consumption during a connection event will be discussed later.

A CONNECT_REQ PDU is sent by the initiator or received by the advertiser at which point the connection parameters are exchanged. These parameters have a profound effect on power consumption.

  • Connection Interval determines the time between two connections. This can be as low as 7.5 ms or as high as 4 sec. As one can imagine longer connection intervals mean low power consumption, but also means low data rates.
  • Slave Latency defines the number of the consecutive connection events the slave can ignore from the master which once again have an effect on low power consumption
  • Supervision Timeout is the timeout between two received data packets before the connection is lost.

Bluetooth Low Energy framework too contributes to low power consumption. The shortest packet transmitted can be 80 bits with a transmission time of 80usec. The longest packet can be 376 bits with a transmit time of around 0.3 mSec. These are very important for single mode BLE devices.

Packet Breakdown

In order to manage power consumption as well as maintain legacy designs bluetooth 4.0 standards were developed. Bluetooth 4.0 effectively has two modes, single and dual. The single mode supports low power slave device using the standard better known as BLE. The dual mode as one might guess support both Bluetooth BR/EDR and BLE.

BLE 4.0 Configuration

Another power saving option is white lists. This allows the link layer to filter advertisers, initiators and scanners.

So BLE technology scans only 3 advertising channels bluetooth has to scan 32 channels. This is around 0.6 to 1.2 ms of discovery time for BLE as oppose to 22.5 ms of discovery time for Bluetooth, this the power savings for BLE.

Also BLE devices in 3 ms can scan, connect, send data, confirm validate reception and terminate where is Bluetooth take over 100 ms to accomplish he the same tasks.

Also the BLE packets are much shorter than Bluetooth classic data packets which also contribute to power savings.

To conclude this response below are scope measurements of connection events and related power consumption for bluetooth low energy which have been done on a TI CC2541.

Power Consumption - 1

Power Consumption - 2

Power Consumption - 3


References

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