A source node needs information about the whole network topology to check if the flow is admissible. However, only links between MPRs and MPR selectors are advertised and thus each node has only partial topology information about the whole network (all the nodes but only some links). So, CAC at the source cannot be used with the QOLSR routing protocol.
A CAC at each hop is more suitable for proactive protocols. Each intermediate node on the selected path checks local QoS constraints using local (vs. global) total (vs. partial) topology information (using information from HELLO messages).
A node can proceed to (soft and later hard) resource reservation on the links during the CAC process. This ensures that no two flows initiated simultaneously will contend for the same resource.