 in a postal system refers to the aggregation of mail collected within a region or the disaggregation of mail collected elsewhere for delivery within a region. This sortation or wholesale level includes regional trans- portation warehouses and both inward and outward sortation. Finally, the distri- bution stage in postal service refers to delivery to individual customer premises by truck and on foot and to collection from customer premises. The distribution stage also includes local post offices that provide local sortation and serve the delivery and collection functions. The three stages of production or service layers are represented in Figure 5.4. Consider now the three service layers or stages of production in telecommu- nications networks. First, the transmission stage uses the facilities of the backbone network, including all the nodes and links that connect the large-scale portion of the network. The transmission stage provides transport services for aggregate traffic. The Networks in Telecommunications aggregate traffic carries telephone calls or data packets in a combined form, much as large containers are used in long-distance freight transportation. The backbone network uses high-capacity trunk lines and switches. The technology of large-scale data pipes generally is fiber optics with various methods of data aggregation and transmission. Second, sortation employs facilities, including switches and routers, that aggre- gate traffic that is received from the distribution stage or disaggregate traffic that is received from the transmission stage. Traffic originating from the distribution stage must be directed to the proper long-distance transmission facilities, and traf- fic received from transmission must be directed to the proper local distribution facilities. The sortation stage provides routing services that intermediate between access services and transmission services. In a circuit-switched network, we use the term sortation to refer to connections between local distribution and long- distance tran