th a short session duration. Most present evidence suggests that in a multimedia network, sessions are substantially longer than traditional telecom voice calls and are increasing over time. This suggests that any supposed efficiency benefits achievable from having a connectionless end-to-end channel are becoming increasingly invalid. This also affects the overall efficiency of call-setup protocols such as SIP. A SIP message generates about 8 kilobytes of signalling load. A change in session property (a new user or additional content stream) requires a new SIP message exchange. As sessions become more complex over time, SIP becomes progressively less efficient and requires substantially higher signalling bandwidth than existing SS7 signalling. 17.9.5 The Hidden Costs of IP MMS An IP MMS (IP multimedia subsystem) enabled multiservice network is rather like a postal or courier service but instead of two classes of service (first class or second class for postal services, priority or standard for couriers) there are 4 levels of service. Note that the postal service in the UK loses money on second-class mail and makes money on first-class mail. This is partly because first-class mail has a higher value but also because second-class mail costs more to deliver. The additional costs are incurred because the storage costs (and related storage administration) now exceed the benefits of holding back mail to fully fill increasingly inexpensive trucks (rather bizarrely the real costs of running a large delivery van up and down the motorway are going down rather than up over time). With international carriers, the same tipping points apply and as aviation costs have reduced (larger, cheaper more efficient aeroplanes) it becomes cheaper not to store and forward but just to forward. The same principle applies to indirect routing. Although the cost of indirect routing may decrease over time, the process still consumes additional bandwidth. This is also true of wireless mesh networks, which can be remarkably spectrally inefficien