CodeIgniter's Approach to Transactions Isolation levels

[eluser]Unknown[/eluser]
Hi

Someone can tell me what level of Isolation is used by default for example:

$this->db->trans_start();
$this->db->query('AN SQL QUERY...');
$this->db->query('ANOTHER QUERY...');
$this->db->query('AND YET ANOTHER QUERY...');
$this->db->trans_complete();

And how cam it be customized.

In mind that the isolation available in mysql are:
REPEATABLE READ
| READ COMMITTED
| READ UNCOMMITTED
| SERIALIZABLE

Thanks