Complexity Driver 2: Per-Session QoS, cont


What are these invariants?
The destination subscriber is not alerted unless the QoS resources are available for all data flows  
Billing does not commence until the destination subscriber answers, and 
QoS cannot be obtained on transmitted data until billing starts

What does all of this mean?
First, it requires reserve/commit phasing in the QoS signaling and pre-condition testing in the application signaling 
And note that while IMS has adopted a bunch of SIP extensions to do the reserve/commit phasing (cf. RFC3312), suffice it to say that both the implementation costs and the operational complexity are considerable.