TABLE OF CONTENTS The unifying principle in the UTRAN development work has been to keep the mobility management (MM) and connection management (CM) layers independent of the air interface radio technology. This idea has been realized as the access stratum (AS) and nonaccess stratum (NAS) concepts (Figure 7.1). The AS is a functional entity that includes radio access protocols between the UE and the UTRAN. These protocols terminate in the UTRAN. The NAS includes core network (CN) protocols between the UE and the CN itself. These protocols are not terminated in the UTRAN, but in the CN; the UTRAN is transparent to the NAS. The MM and CM protocols are GSM CN protocols; GMM and SM are GPRS CN protocols. Just as the NAS tries to be independent of the underlying radio techniques, so also have the MM, CM, GMM, and SM protocols tried to remain independent of their underlying radio technologies. The apparent dependence of these higher-layer protocols on the radio access protocols will be clarified later in this chapter.
The NAS protocols can be kept the same, at least in theory, regardless of the radio access specification that carries them. Thus it should be possible to connect any 3G radio access network (RAN) to any 3G CN. This is a nice principle and a worthy goal, but in practice its implementation is not simple. True independence among the layers in a protocol stack is difficult and expensive to implement. Time and budget constraints usually...
