What distinguishes an ACK packet on-air from a regular packet?

[Ant Skelton]

I would like to implement a simple wireless sensor network, using Enhanced Shockburst with Auto-ACK. The idea is that there will be many sensor nodes (SNs) reporting to one master node (MN). The MN will listen on address P0 (say 0x0011223344) and all SNs will transmit to this address on an intermittent, periodic basis.

Assume I have auto-ack enabled everywhere.

1. Node SN0 transmits a packet with PID=1, it begins listening on P0 for an ACK from MN.
2. Before the ACK can be sent, a second node SN1 transmits a packet with PID=1 also.
3. Since SN0 is listening on P0 while SN1 is transmitting on P0, will SN0 mistakenly identify SN1's data packet as an ack from MN?

As far as I can tell, for the same PID, there is no difference between a regular data packet and an ack-with-payload: I am assuming that ACKs are always sent with the NO_ACK bit of the PCF set to 1, and that a PTX detects a valid ack if ((ack_pid == last_tx_pid) && (ack_nak == 0)) ???

If this is the case, then I can assume that the answer to my question above is "no", but what about this:

1. Node SN0 transmits a packet with PID=1, it begins listening on P0 for an ACK from MN.
2. Before the ACK can be sent, a second node SN1 transmits a packet with PID=1 also, and listens
3. MN sends ACK on P0 with PID=1.
4. Presumably both SN0 and SN1 will interpret this as a valid ACK, even though it was meant for only one of them?

This would seem to make sense; I am happy to burn some payload bytes to identify individual transmitters, I'd just like to verify that my assumptions are correct.

[Ant Skelton]

Hi thanks for the response, it is useful and informative. However, I would still like to know what distinguishes an ACK on-air from a regular data packet.

Imagine a node has just sent a packet:

PKT: { addr=[0xaa,0xbb,0xcc], PCF={len:1, NAK:0}, payload=[0x55], CRC=x}

According to the ESB/PTX flowchart, the PTX is now in "RX mode --> is it an ACK?"

Imagine that the ACK to this packet also has a payload, also one byte, also the value '0x55'.

Does the ACK look like this:

ACK: { addr=[0xaa,0xbb,0xcc], PCF={len:1, **NAK:0**}, payload=[0x55], CRC=x}

Or does it look like this:

ACK: { addr=[0xaa,0xbb,0xcc], PCF={len:1, **NAK:1**}, payload=[0x55], **CRC=y**}

In other words, how is the decision "Is it an ACK?" actually made?

And, depending on the answer to that, can the PTX ACK-check be fooled by an identical PKT sent from a different node?

[I am using a single data-pipe in an attempt to keep the question simple; for my network I plan to use all available pipes as you suggest.]

cheers

ant

[Håkon Alseth]

If you have not enabled dynamic payload length, the payload will be equal to any other payload, except it is '0' in length.

The whole PFC field will remain the same unless EN_DPL is set.

Let's say that two devices send almost at the same time. Host receives payload from Device#1. Both devices goes into RX mode, and receives the host's ACK. In this scenario, both devices will get a TX_DS interrupt, while the host only got one of the payloads.

If you plan to have more devices communicate on the same pipe, with an extended address byte in firmware, then you will have to disable the hardware acking and do this in firmware instead.

-H