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In general, there are different approaches that aim to solve the problem of access service creation for residential networks. From pure SDN technology, moving across special hybrid architectures and getting to virtualization of specific access functions, different solutions have been designed, simulated and some of them implemented. This chapter defines the general and specific BRAS system requirements considered as part of several approaches that will serve as a base to establish the essential functions to be implemented in this thesis. Additionally, chapter ef{ch:related_work} presents the related work where diverse approaches may make use of different terminology and therefore, next sections also establish a base terminology, which will help the reader to set a base language in order to avoid contextual disorientation. It is utilized in the document and particularly used to describe the related work approaches.

This section aims to define the services that comprehend a BRAS system. It describes their functionalities using a consistent terminology. It establishes the definition of BRAS, as well as the services on BRAS functionalities based on NFV architecture.The extit{broadband remote access server (BRAS)} is a system composed of one or multiple devices, which jointly aim to grant access services to end customers. This system delivers specific configuration parameters and assigns network resources including policy restrictions, usually influenced by contractual constraints, which together allow customer traffic to be forwarded through the network infrastructure of an ISP citep{SPARC4.3}.

In spite of the fact that a BRAS platform, in classical networks architectures, is encountered as a single entity at the edge of the core network, when referring to NFV and specifically to the function virtualization of a BRAS, a multi-platform composition may occur for two main reasons: First, other platforms in the subscriber path can be involved in the service provisioning process or because of a distribution of access services may be intended and second due to the fact that the architecture of NFV implies a control and data plane separation which leads to ending up with a likely multi-platform implementation. In general, the extit{Access service provisioning} process is determined according to four general phases: User discovery and authentication, parameters assignment, access control and features enforcement, and connection monitoring citep{SPARC3.3}. A large group of services must be carried out in order to achieve the provision of access services to subscribers, e.g. to provide registration configuration parameters to users.

Figure ef{fig:Access_service_creation_tree} presents a typical flow for access services creation for a BRAS system. Starting from the top-left and moving down, the table makes evident how the process goes through the different functional services that the BRAS system must provide.

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