This chapter deals with the architecture and the protocol level details of GSM, GPRS and UMTS. GPRS leverages GSM infrastructure to establish a packet based mobile cellular network. It can be viewed as an overlay network, on GSM, through a IP based network consisting two new type of nodes with certain enhancements to BSS, besides software upgrades. UMTS also provides a number of extensions over the core GSM network for maintaining connectivity with the user’s equipment. Knowing about the radio resources and its management are key to understanding of the core GSM protocols. Control channels and traffic channels have different roles in exchange of information between a Mobile Handset (MH) and Base Station Subsystem (BSS). Establishing a call to an MH in a GSM network is a complex and involved procedure. It requires control channels to perform certain specific tasks before a traffic channel can be assigned to a call. This chapter addresses all the key issues related to management of the radio resources for establishment of call and mobility. The protocol stacks of all three networks also have been discussed in appropriate details.