Figure 12.3 AXE digital switch³ with additional knitting. Reproduced with permission of Ericsson. As with postal systems, packets can be stored (buffered) during the delivery process and as with postal systems packets can be lost or have to be deliberately discarded if the storage or buffer space overflows. This results in a variable customer experience. Post offices traditionally manage this by introducing differential pricing, first- and second-class stamps and guaranteed or signed for delivery. IP networks manage this by introducing multiple service levels based on multilevel packet prioritisation. In theory, this increases the multiplexing efficiency of the network, trading variable delay against network utilisation efficiency and trading delivery bandwidth efficiency against memory bandwidth occupancy. However, this requires routers to read and interpret IP addresses together with the priority labels embedded with the address. This information then provides the basis for a complex buffering and routing decision. This introduces variable delay into the delivery budget. The overall delay and the variability of the delay can be reduced by using hardware accelerators and high-speed memory. These are costly energy-absorbent devices. Even more frustratingly, just as in the traditional postal system, many of the packets delivered over the network are unwanted and have negative value - they annoy the receiver. These unwanted packets get in the way of latency-sensitive end-to-end services including voice communication. In a cellular network these costs are multiplied in several ways. IP address overhead has to be accommodated within the RF link budget and the unwanted and unneeded negative value packets cost money to deliver. Wireless marketing teams have decided that users want and need to be offered high peak data rates even if this compromises average data throughput. High peak data rates, can only be achieved by using higher-level "http://www.ericssonhistory.com/templates/Ericsson/Article.aspx?id=2095&ArticleID=1