Using CoAP/DTLS 1.2 CID keeps the security context (DTLS) even over ip-address changes in deep sleeps. So no need for handshakes or TCP connections on wakeup.
Half the truth. If you want to send data via any TLS protocol, you need a TLS handshake first. That adds at least 2000 bytes. So, you can collect and postpone the data into 1000 bytes resulting in about 3000. Or you use DTLS 1.2 CID and use the 10x (100 (+ 60 DTLS header)) resulting in 1600 bytes in real time.
> CAT-M1 vs. CAT-NB1
In my experience this largely depends on the supported features by the network provider. My provider supports PSM for both, but seems not to support RAI for CAT-M1. Using rather small messages (200-250 bytes in order to stay below the 330 bytes for one CAT-NB1 package) results then in a better ratio for CAT-NB1, because my CAT-M1 stays "connected for about 10s" and NB-IoT goes "Idle in less than 1s".
My recommendation is therefore different:
- if your application is considered to send small messages (<250 bytes) a couple of times a day, use NB-IoT
- if your application is considered to send larger messages, CAT-M1 is your friend. But be careful about the costs for data-budget of your SIM-card
For all, who want to build up their own mind, just try out the above example. I'm very sure, the most will report, that CoAP/DTLS 1.2 CID will deliver the messages more reliable, while the TLS based will struggle more. For CoAP/DTLS 1.2 CID only two messages must reach their destination, for TLS that's more, even up to 20 (if a TLS handshake is required).
