This description does not read well. When these test cases fail the error message will be:
```
A SMPP-PDU checks whether pdu is a Delivery Acknowledgement (deliver_sm)
```
I think it would be clearer if this read more like:
```
An SMPP PDU with the 'deliver_sm' command can detect a delivery acknowledgment.
```
The idea is that the test cases are describing the behavior that we want to ensue -- this is called [Behavior Driven Development](http://en.wikipedia.org/wiki/Behavior-driven_development) or BDD.