BLE: Max number of packets in Connection interval

Question

Is there a limit on maximum number of packets (LE_DATA) that could be send by either slave or master during one connection interval?

If this limit exists, are there any specific conditions for this limit (e.g. only x number of ATT data packets)?

Are master/slave required or allowed to impose such a limit by specification?

Answer 1

From my understanding, the limit is: min{max master event length, max slave event length, connection interval}.

To clarify, both the master and slave devices (specifically, the BLE stack thereof) typically have event length or "GAP event length" times. This time limit may be used to allow a central and/or advertiser and/or broadcaster to schedule the "phase offset" of more than one BLE radio activity, and/or limit the CPU usage of the BLE stack for application processing needs. E.g. a Nordic SoftDevice stack may have a default event length of 3.75ms that is indefinitely extendable (up to the connection interval) based on other demands on the SoftDevice's scheduler. In Android and iOS BLE implementations, this value may be opaque or not specified (e.g. Android may set this value to "0", which leaves the decision up to the controller implementation associated with the BLE chip of that device).

Note also that either the master or the slave may effectively "drop out" of a connection event earlier than these times if their TX/RX buffers are filled (e.g. Nordic SoftDevice stack may have a buffer size of 6 packets/frames). This may be implemented by not setting the MD bit (if TX buffer is exhausted) and/or "nacking" with the NESN bit (if RX buffer is full). However, while the master device can actually "drop out" by ending the connection event (not sending any more packets), the slave device must listen for master packets as long as at least one of master and slave have the MD bit set and the master continues to transmit packets (e.g. the slave could keep telling the master that it has no more data and also keep NACKing master packets because it has no more buffer space for the current connection event, but the master may keep trying to send for as long as it wants during the connection interval; not sure how/if the controller stack implements any "smarts" regarding this).

If there are no limits from either device in terms of stack-specified event length or buffer size, then presumably packets could be transmitted back and forth the entire connection interval (assuming at least one side had data to send and therefore set the MD bit). Just note for throughput calculation purposes that there is a T_IFS spacing (currently specified at 150us) between each packet and before the end of the connection interval.

Answer 2

(I hope I'm not reviving a dead post. But I think the section 4.5.1 is better suited to answer this than 4.5.6.)

The spec doesn't define a limit of packets. It just states the following:

4.5.1 Connection Events - BLUETOOTH SPECIFICATION Version 4.2 [Vol 6, Part B]

(...)

The start of a connection event is called an anchor point. At the anchor point, a master shall start to transmit a Data Channel PDU to the slave. The start of connection events are spaced regularly with an interval of connInterval and shall not overlap. The master shall ensure that a connection event closes at least T_IFS before the anchor point of the next connection event. The slave listens for the packet sent by its master at the anchor point.

T_IFS is the "Inter Frame Space" time and shall be 150 μs. Simply put it's the job of the master to solve this problem. As far as I know iOS limits the packet number to 4 per connection event for instance. Android may have other hard coded limits depending on the OS version.

Answer 3

I could find the following in Bluetooth Spec v4.2:

4.5.6 Closing Connection Events

The MD bit of the Header of the Data Channel PDU is used to indicate that the device has more data to send. If neither device has set the MD bit in their packets, the packet from the slave closes the connection event. If either or both of the devices have set the MD bit, the master may continue the connection event by sending another packet, and the slave should listen after sending its packet. If a packet is not received from the slave by the master, the master will close the connection event. If a packet is not received from the master by the slave, the slave will close the connection event.

Two consecutive packets received with an invalid CRC match within a connection event shall close the event.

This means both slave and masters have self-imposed limit on number of packets they want to transmit during a CI. When either party doesn't wish to send more data, they just set this bit to 0 and other one understands. This should usually be driven by number of pending packets on either side.

Since I was looking for logical limits due to spec or protocol, this probably answers my question.

Physical limits to number packets per CI would be governed by data rate, and as @morynicz mentioned, on MTU etc.

Answer 4

There is max data rate that can be achieved both on BT and BLE. You can tweak this data rate by changing MTU (maximum transmission unit - packet size) up to max MTU both ends of transmission can handle. But AFAIK there is no straight constraint on number of packets, besides physical ones imposed by the data rate.

You can find more in the spec