The development of telecommunication systems during the past two decades has beenrnfrom the circuit switched towards the packet switched paradigm. Initially a distinctionrnwas made between telecommunications and computer networks which is hardlyrnapplicable in today's reality. Incumbent network operators are running legacy circuitswitchedrntelephony networks which inhibit them from enjoying the benefits of packetrnswitching. Many operators have now started the paradigm shift in the telephony networkrnby moving to packet based technologies. This new approach is often called NextrnGeneration Networks (NGN). According to ITU Next Generation Network (NGN) is arnpacket-based network able to provide telecommunication services and able to make usernof multiple broadband, QoS-enabled transport technologies and in which service-relatedrnfunctions are independent from underlying transport-related technologies. It offersrnunrestricted access by users to different service providers. It supports generalizedrnmobility which will allow consistent and ubiquitous provision of services to usersrnNGN enables the network operators to run all services on one network, i.e. voice, datarnand video.rnIn this thesis the transformation of the legacy network to NGN in general and thernEthiopian Telecom migration in particular will be examined. This work reviews thernevolution of voice telecommunications and attempts to analyze the network and servicernpoint of view as the industry migrates from circuit switched networks to packet switchedrnnetworks. For the network part, the work tries to asses the legacy PSTN networkrnelements with the NGN softswitched network. In the service part, a service modelingrntool from Alcatel have been used to analyze and model next generation network servicesrnfor the case of Ethiopian Telecommunication Corporation