Data consumption af MQTT over GPRS/3G

[Adrien Margueron]

Hello everybody,

I'd like to know how many data a MQTT subscriber client consume per month for a permanent TCP/IP connection over GPRS or 3G. This client will subscribe to a topic an wait for commands to be posted on that topic.
I also don't know if connections over 3G or GPRS need a long or short keepalive. This is quite an important parameter in the evaluation of data consumption!

Thanks

[Dave Locke]

From an MQTT bytes perspective.  Once the subscribe is in place the MQTT bytes that are flowed will  be:

• ping req and ping resp every keepalive interval which is 2 bytes in each direction.  (If other messages flow then a keepalive is not needed while there is other activity)
• for each event that matches the subscription a message will be published to the client.  The number of bytes will depend on QOS. For example using QOS 0 the number of bytes will be 2 bytes for the fixed header + UTF encoded topic string (including length)  + message payload.

This does not include TCP bytes including headers and keepalive.

What keepalive period to use comes down to 1) What the network requires to keep the connection alive 2) max time to wait before being notified a connection has broken.

For 1) some networks are configured to drop connections if no data flows in a finite time. Where this is the case the keepalive needs to be configured to be less than the network no data kill time.  
For 2) In many cases the "network" will inform the MQTT client / server the connection has broken without needing to rely on the MQTT keepalive - the keepalive is really a fall back / safety mechanism


All the best
Dave  

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