The CELL library make extensive use of what is called "unconnected
messages".  An unconnected message is any message that does not rely on an
established data path into the PLC.  Unconnected messages are used mostly
for simplicity sake as there are some items within the connected message
structures that are not clearly defined, making that route more
"experimental".

There are certain tradeoff's to using unconnected messages.  First of all,
every message must contain the complete path to the object you are trying to
send the message to.  In most cases, this is simply ethernet -> backplane
-> cpu_slot (as is 95% of the cases where CELL will be used), but can be as
complex as ethernet -> backplane -> slot with controlnet card -> node 31 on 
controlnet -> backplane -> cpu_slot.  

Second, a PLC can only handle a small number of unconnected sessions -
something like 4-5 sessions.  An unconnected session is typically reserved
for such things as network browsing (doing a "who" inside RSLinx) and so
forth.

Lastly, an unconnected session receives a lower priority from the PLC than a
connected session.  This, in theory, results in slower answers to your
inquiries from the PLC.  In practice, it is highly doubtful that most people
will notice the speed difference.

On the plus side, it appears that there is no limitation to the types of
messages that can be dealt with while using unconnected messages.
