ass stands at least a chance of arriving the next day. A Introduction second-class letter will take longer to arrive as it will be stored at any point where there is not enough delivery bandwidth, lorries, aeroplanes or bicycles, for the onward journey. In theory, this means that the delivery bandwidth can be loaded at a point closer to 100% utilisation, thus reducing cost. This is true but fails to take into account the real-estate cost of storing those letters and packets in some dark corner of a sorting office and managing their timely reintroduction in to the delivery path. Depending on how these costs are calculated it can be argued that a second- class letter or packet probably costs the postal service rather more to deliver than a first-class letter or packet with of course less revenue to cover the extra cost. An end-to-end packetised channel is the post-office equivalent of having first-, second-, third- and fourth-class stamps on every letter and packet. The routers have to inspect each packet to determine priority and store lower priority packets as needed. Packet storage needs fast memory, which is expensive and energy hungry and constitutes a directly associated cost that needs to be factored in to the end-to-end cost equation. If there is insufficient storage bandwidth available the packets will have to be discarded or other time-sensitive offered traffic will need to be throttled back at source. This introduces additional signalling load that in itself absorbs available bandwidth and may compromise end-user experience. As end-user experience degrades, churn rates and customer support costs increase. On the radio part of the network, the additional address overhead needed to discriminate between delay-tolerant and delay-sensitive traffic combined with the signalling bandwidth needed to maintain a reasonably optimum trade off between storage bandwidth and delivery bandwidth utilisation comes directly off the radio link budget and will therefore show up as a capacity loss or coverage loss, effectively