max number of broadcasters scanned from a central device

[daveln]

Hi,

I'm using nrf51822 with s110 softdevice v6

My question is the following. How many devices sending advertising messages can be detected at a time by a central unit(pca10000 or pca10004)?

The specs gives information about BLE topology (star), or maximum number of active devices connected to a master (limitless, when on previous versions of BT, the maximum where 7), but they dont give information about the maximum number of devices that a master can scan & get advertising messages at a time.

[John]

I don't believe there is a limit. It is whatever the central can keep up with. For example if I fire up the Master Control Panel in our building with close to 80 or more BLE devices, the MCP can see them all advertise. The same goes for our app on a iOS device.

[daveln]

Regarding advertisements, theoretically there is no limit in BLE. What happens in practice is that, since i have only 3 advertising channels, a lot of BLE devices advertising themselves at same time will result in collisions. So I think the max advertisers in a single ADV / SCAN interval was limited.

Any idea of avoiding collisions?

[John]

The following quote comes from the 4.1 Bluetooth specification Volume 1, Section 3.3.2.2 Advertising Broadcast channel:

"The channel is divided intoadvertising events where each advertising event can hop on all three advertising PHY channels. Consecutive advertising events begin on the first advertising PHY channel. The advertising events occur at regular intervals which are slightly modified with a random delay to aid in interference avoidance."

And should address your concern.

[daveln]

Thanks for your response. So the softdevice will make random delay itself while broadcasting? Or there are some api needed to be used?

[John]

I believe the softdevice deals with this for you. I am not aware of any API needed to make this happen.