Advertising process

Advertising and discovery

When a Bluetooth LE device is in an advertising state, it sends out advertising packets to announce its presence and potentially connect to another device. These advertising packets are sent out periodically at advertising intervals.


Advertising intervals: The interval at which an advertising packet is sent. In the range of 20 ms to 10.24 s, with a step increase of 0.625 ms.

The smaller the advertising interval is, the more frequently the advertising packets are sent, and consequently, the more power is consumed. Therefore, the tradeoff here is power consumption vs. how fast the advertiser’s advertisement packets will be received by a scanner, commonly referred to as discoverability. To avoid packet collisions, a random delay of 0-10 ms is added before each advertisement packet. This ensures that devices with the same advertising interval do not end up with advertisement packet collisions all the time.

Advertisement channels

Bluetooth LE devices communicate through 40 different frequency channels. These channels are divided into three primary advertisement channels and 37 secondary advertisement channels. Primary advertisement channels are the channels mainly used for advertisement purposes. Secondary advertisement channels can sometimes be used for advertisement purposes as well, but are mainly used for data transfer after establishing a connection. Throughout this course we will focus only on primary advertisement channels.

To ensure a certain degree of redundancy, advertising packets are sent on all three primary advertising channels, channels 37, 38, and 39. Simultaneously, a scanning device will scan these three channels to look for advertising devices.

Since advertising packets are essential to establishing connectivity, the primary advertising channels are carefully chosen. Channels 37, 38, and 39, despite being consecutive numbers, are not actually neighboring channels, as you can see in the figure above. The separation between the three channels serves to avoid adjacent-band interference. Additionally, these three specific channels suffer the least from noise from other technologies using the ISM band, such as Wi-Fi.

Scan interval and scan window

Similar to an advertising interval, a scan interval refers to how often a scanner will scan for advertisement packets. The scan window refers to the time the scanner spends scanning for packets, which in practice represents the duty cycle in which the device is scanning vs not scanning during each scan interval.


Scan interval: The interval at which a device scans for advertisement packets.
Scan window: The time that a device spends scanning for packets at each scan interval.
Both range from 2.5 ms to 10.24 seconds with a step increase of 0.625 ms.

Since a device advertises on three different advertisement channels, the scanner will rotate though the advertisement channels, by switching the channel after each scan interval.

Example of an advertiser (peripheral) and a scanner (central)

Short advertising intervals and scan intervals lead to shorter discovery times but increase power consumption. A good practice is to set a relatively short scan interval with a longer advertising interval, so that the scanning process is more likely to receive the advertising packets. When done like this, the increased power consumption is on the scanner (central device), which usually has a larger battery or is connected to a power source.

Scan request and response

When a peripheral is advertising, a central can also choose to send a scan request to the peripheral, asking for additional information that is not included in the advertisement packets. If the scan request is accepted, the peripheral will respond with what’s called a scan response, also transmitted over the three primary advertisement channels.


Scan request: A message sent by a central device to a peripheral to request additional information not present in the advertisement packet.
Scan response: A message sent as a response to a scan request, containing additional user data.

This is a way for the peripherals to send additional data without having to establish a connection with the central first. The peripheral can alternatively choose to send back an empty scan response if it has no more additional information to provide.

Another way to increase the amount of data a peripheral can advertise at once is with a feature called extended advertising. With this feature, the advertisement packets broadcast on the primary advertisement channels are pointing to supplementary information that is being advertised on the secondary advertisement channels. Extended advertising is beyond the scope of this course, and we will be focusing only on legacy advertisement.

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