Nano-Gateway Doubt

[MJ]

Hi there, i am trying to understand the code using in https://docs.pycom.io/chapter/tutorials/lora/lora-mac-nano-gateway.html which is for Gateway and Node. I not understand in the Gateway part :

while (True):
    recv_pkg = lora_sock.recv(512)
    if (len(recv_pkg) > 2):
        recv_pkg_len = recv_pkg[1]
        device_id, pkg_len, msg = struct.unpack(_LORA_PKG_FORMAT % recv_pkg_len, recv_pkg)

What is it mean by "(len(recv_pkg) > 2)" , "recv_pkg_len = recv_pkg[1]" ?

In the node :

 if (len(recv_ack) > 0):
            device_id, pkg_len, ack = struct.unpack(_LORA_PKG_ACK_FORMAT, recv_ack)
            if (device_id == DEVICE_ID):
                if (ack == 200):
                    waiting_ack = False
                    # If the uart = machine.UART(0, 115200) and os.dupterm(uart) are set in the boot.py this print should appear in the serial port
                    print("ACK")

How about this?

Anyone would explain how it works? Since i need to understand to implement my own code in it. Thank you

[seb]

@mj said in Nano-Gateway Doubt:

Hi there, i am trying to understand the code using in https://docs.pycom.io/chapter/tutorials/lora/lora-mac-nano-gateway.html which is for Gateway and Node. I not understand in the Gateway part :

while (True):
    recv_pkg = lora_sock.recv(512)
    if (len(recv_pkg) > 2):
        recv_pkg_len = recv_pkg[1]
        device_id, pkg_len, msg = struct.unpack(_LORA_PKG_FORMAT % recv_pkg_len, recv_pkg)

What is it mean by "(len(recv_pkg) > 2)" , "recv_pkg_len = recv_pkg[1]" ?

what's happening here is as follows:

if (len(recv_pkg) > 2):

First it is checking if the received packet of data is at least 3 bytes big because in this example packets are expected to have the following format:

[Device ID (1 byte)] [Size of data in bytes (1 byte)] [Data (as many bytes as in the previous section)]

recv_pkg_len = recv_pkg[1]

Now that we know the packet has enough data to be valid, we get the length of the data which is in the second byte of recv_pkg (python array indexes start at zero)

device_id, pkg_len, msg = struct.unpack(_LORA_PKG_FORMAT % recv_pkg_len, recv_pkg)

This line is a bit complex so let me break it down, firstly _LORA_PKG_FORMAT % recv_pkg_len creates a format string that the struct.unpack understands. For example if recv_pkg_len is 4 the format string will be BBB4s. Finally the struct is then unpacked such that device_id = the first byte, pkg_len = the second byte and msg = a string containing the rest of the bytes.

If in your application you need to send something that is not a string you can modify this format string to match the datatypes you need to send as per: https://docs.python.org/3/library/struct.html#format-characters

In the node :

 if (len(recv_ack) > 0):
            device_id, pkg_len, ack = struct.unpack(_LORA_PKG_ACK_FORMAT, recv_ack)
            if (device_id == DEVICE_ID):
                if (ack == 200):
                    waiting_ack = False
                    # If the uart = machine.UART(0, 115200) and os.dupterm(uart) are set in the boot.py this print should appear in the serial port
                    print("ACK")

How about this?

Anyone would explain how it works? Since i need to understand to implement my own code in it. Thank you

So this code is similar to the previous code. When the gateway successfully receives a packet, it sends back a "ack" (acknowledgement) to the node.

if (len(recv_ack) > 0):
             device_id, pkg_len, ack = struct.unpack(_LORA_PKG_ACK_FORMAT, recv_ack)

This is the same as before, although the > 0 should probably be == 3 because the ACK has a fixed format

if (device_id == DEVICE_ID):

Now that we know we have received a packet we need to check if it was meant for this node if not ignore it

if (ack == 200):
    waiting_ack = False

If the packet was for this node, check if the ack byte has a value of 200. The actual value of 200 has no significance, 200 was picked to match the HTTP OK status code. Once we get a ACK we set waiting_ack so the node will stop waiting for one.

Hope this all makes sense

[MJ]

@seb

Can you explained more details on the

if (len(recv_pkg) > 2): recv_pkg_len = recv_pkg[1]

what i understand is if the length of received package is more than 2byte as mentioned earlier " _LORA_PKG_FORMAT = "BB%ds" and how about "recv_pkg_len = recv_pkg[1] ? The number 1 indicate? Thankyou.

[robert-hh]

@mj The answer is in any Python book: recv_pkg is a byte array, and the index [1] points to the second element of that array. In the data structure of the recv_pkg, this contains the intended length of the packet.

[MJ]

@robert-hh What is the intention on " recv_pkg_len = recv_pkg[1]" means the received package length is equal to the received package of the package length? Am i correct or wrong? Please do correct me, thank you.

device_id, pkg_len, msg = struct.unpack(_LORA_PKG_FORMAT % recv_pkg_len, recv_pkg)

[seb]

@mj both recv_pkg_len and pkg_len are the same value, the second line could well have been written like this also:

device_id, _, msg = struct.unpack(_LORA_PKG_FORMAT % recv_pkg_len, recv_pkg)