work managers of the access network. This layer also provides interfaces to higher order management functions. For the various access network parts the following terms may be used: A customer drop between customer and customer access unit. A feeder part between access node and customer access unit. In mobile access a lower radio access network is sometimes used. Access Network Common network Lower radio access network management Site sharing Service network/ GSM RBS/ applications WCDMA RBS Combined radio Control layer resources Higher radio access network Backbone Shared transmission Node reuse Transmission hub site Figure 13.13 A common mobile access network structure An access transport network part (if necessary) between an access node and the edge node In mobile access a higher radio access network is sometimes used. A mobile access network can look like Figure 13.13. The higher radio access network is usually fibre based. The lower radio access network is often based on point-to-point microwave links or point-to-multipoint microwave of the LMDS type. These (fixed) radio technologies are briefly described at the end of this chapter. The general access network design is based on economical criteria, such as ROCE. All possible synergies between different accesses should be used. Concentration and multiplexing are other key features in saving money. For mobile systems the initial con- centration is achieved by means of the attach-detach procedures ('concentration in the air'), as opposed to for example an ATM access multiplexer in the fixed network, dif- ferentiating between idle cells and traffic carrying cells from the subscriber side. See Figure 13.14. The efficiency at transmission level can be measured in bits per bandwidth unit. A summary and example of common structures is given in Figure 13.15. 13.7 THE LAYERED DIMENSION A specific transfer mode can use different transmission systems. This is a main reason for the richness in implementation of fixed and mobile access systems. One of the access vendors ha