LTE_LC_RRC_MODE_CONNECTED timeout

[allard.p]

I've the following situation:

1. Send CoAP message trough send(), this causes the the nRF9160 to start RCC connection

2. Wait for RCC connected mode (LTE_LC_RRC_MODE_CONNECTED)

3. Start CoAP retransmit timers

Two questions:

- Is it possible to 'force' RCC connection without sending something trough a socket?

- What would be a reasonable timeout for step 2? I tried to extract this from the 3GPP TS 38.331 document, but I did not find the timeout.

[helsing]

Hi Allard,

Are you using eDRX, PSM or RAI?

If we ignore current consumption for a moment: One does not need to stay in RRC Connected mode. The modem will listen for paging from the network in RRC Idle, however, the latency will somewhat increase (a typical paging interval might be 2.56s).

After a while you will get trouble with NAT timers. For UDP these are often 50-60 seconds, however, we have seen them as short as 16s. After this the timeout will have forgotten how it is routing packages to the device and the reply from the server will not reach the device.

When using eDRX, then the device will only listen for pages in Active Time, before turning the radio completely off. This way it becomes important that Active Time is long enough for the server to reply.

The network should buffer data for devices in eDRX mode or PSM, so the response from the server should arrive even if the device is in eDRX mode or PSM, as long as the response happened before the NAT timeout.

[allard.p]

Thanks for the replies. We are using NB-IOT with PSM and RAI, so power usage is very important. eDRX could solve the 'separate response' problem but is to costly in power usage. We decided to follow Achim's advice, this is currently testing and working great.

[Achim Kraus]

:-) Great!