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HelloMQTT is an example of using the MQTT API. The MQTT API is portable across network interface stacks. MQTT is designed to be used with TCP/IP, but any transport with similar characteristics should be suitable.

HelloMQTT uses the EthernetInterface to show how this works. The MQTT library contains an MQTTEthernet.h header, which is a wrapper around the mbed ethernet interface. To use the MQTT API with ethernet, include the following two headers:

#include "MQTTEthernet.h"
#include "MQTTClient.h"

then instantiate an MQTT client like this:

MQTTEthernet ipstack = MQTTEthernet();             
MQTT::Client<MQTTEthernet, Countdown> client = MQTT::Client<MQTTEthernet, Countdown>(ipstack);

Countdown is a timer class supplied in the MQTT library that is specific to mbed, so you should not need to change that.

If you want to use the MQTT API with a different network stack, create an interface header similar to MQTTEthernet.h. The class you create must have the following two methods at a minimum:

    int read(char* buffer, int len, int timeout);
    int write(char* buffer, int len, int timeout);

where the timeout is in milliseconds, and the return value is the number of characters read from or written to the network, or -1 on failure.