Research On Key Technologies Of PoC

Accompanied with the advances of information technology and the pace of the society, network convergence and service convergence have been propelled and can not be reversed. Many CMA (Collaborative Multimedia Application) services emerged from the next generation network, of which IMS (IP Multimedia Subsystem) has been recognized as one of the most principal and essential building blocks. PoC (Push To Talk over Cellular) is the most typical and important service of IMS, which has many advantages such as short delay of session creation, high efficiency for network resource, wider coverage of users’geographical distribution, etc. PoC has been deemed as a novel service which has the most potential ability to be next killer application after the Short Message Service.PoC is a half-duplex, one to one or one to many mode supported, bi-way talking, multimedia conversation system. In a PoC session, only one user is permitted to speak and the others listen.If one wants to speak, he or she has to apply for the speak right first, which is called’floor’ in this paper. The contention race for floor application has to be mediated by PoC. Floor control takes the responsibility to coordinate all new requests from all users in the session. Theortically, PoC can be used by all the users distributed throughout all signal covered areas, which is the main advantage of PoC than other group based talking system. At the present phase, several main obstacles for large deployment of PoC can be summaried as:a long delay for voice packets transportation, the heavy dealing burden for the central control server in a session, too excessive controlling signals for session handling, etc. The research interests for PoC mainly focus on how to optimize redundant media and controlling traffic in a session, how to shorten the transferring delay of voice packets, how to improve the efficiency of the central controlling point, how to balance the heavy loads between different PoC servers.In this paper, first we go through all PoC related research outcomes and classfy them into several categories according to the specific traits of PoC. At the same time, all floor control related research has also been summed up and classified into several categories. We may learn what we should do nextly, and we know where the hot aspects for PoC research lay. Based on OMA’s (Open Mobile Alliance) PoC criterion version1.0and2.0, we put forward a novel RTP (Real-time Transport Protocol) packets diverting scheme to reduce the redundant packets in a session. We also bring forward a new transferring scheme to balance the heavy load between different PoC servers. Next, based on OMA’s central control floor control mechanism, TBCP (Talk Burst Control Protocol), the algorithms of’distributed queue’and’mobile queue’ have been imported into TBCP and two distributed floor control mechanisms have been proposed to improve the robustness and scalability of a PoC session.The thesis describes the research works in detail. The principle contributions of the work presented in this thesis are:1. Distributed Media Traffic Optimization mechanism in PoC. In criterion version1.0, OMA defined a RTP packets transmitting scheme for PoC-4interface, which we called MTP (Media Transport Protocol). In order to reduce the redundance voice traffic through PoC-4interface, MTO (Media Traffic Optimization) has been provided as the enhancement version of version1.0in criterion version2.0. Based on the above two schemes, we put forward a distributed controlling based optimization mechanism by taking loose-coupling and distributed-controlling ideas into consideration. Under the new mechanism, media traffic through PoC-4interface can be reduced greatly, network bandwidth can be saved much and waiting delay of RTP packets in PoC server can also be shortened obviously. Theoretical analysis and simulation results show that the new mechanism acts more effectively than MTP and MTO.2. RTP stream transferring scheme with load balancing and delay optimization in PoC. After modeling the RTP stream transferring scheme proposed by OMA for PoC server, an improved scheme is presented. Under the new scheme media traffic through CS(Controlling Server) can be reduced largely and the transferring delay of RTP packets could also be shortened obviously by enhancing PS(Participating Server) controlling capabilities and importing distributed-controlling ideas into PoC system. Analyzing and simulation results show that the new scheme always performs better than the original one in fields of load balancing and media transferring delay optimization.3. A distributed floor control mechanism based on the distributed queue algorithm. TBCP has been defined as the floor control mechanism for PoC by OMA. It is a centralized controlling mechanism which has several limitations as other central approaches have, such as bottleneck effect, bad scalability, high maintenance cost of the central node, etc. In order to overcome the deficiencies of TBCP, we put forward a novel distributed queue based floor control mechanism TBCP/DQ (Distributed Queue). Under TBCP, request waiting queue has been distributed into several sub queues which locate at all participating PoC servers in a session. The global queue has been maintained by the cooperation of all PoC servers. By this means, the scalability and robustness of a PoC session has been improved.4. A distributed floor control mechanism based on mobile queue algorithm. The mobile queue algorithm has been imported into TBCP and TBCP/MQ (Mobile Queue) has been brought forward for PoC. Unlike TBCP, under TBCP/MQ the request waiting queue has been moved from one PoC server to another according to the changes of current speaker location. In other words, the global queue will always be maintained by the PoC server of the subnetwork of which the current floor holder belongs to. Through analytical and experimental evaluations, we find the new approach may reduce the heavy burden at the central point and improve system scalability of a PoC session.