This chapter gives a more detailed and extensive introduction of the
UMTS terrestrial radio access network
(UTRAN) including its basic network elements, interfaces, transport channels, protocol stacks, and key management functions. Along with the evolution of UMTS, the UTRAN is under development aiming for an efficient network architecture, much higher transport bandwidths, improved transport network cost-efficiency, and enhanced support for the packet data services. This chapter starts with a detailed introduction of the UTRAN from UMTS Rel99 and then proceeds further towards evolved UTRAN networks along the UMTS evolution path.