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A core network, or network core, is the central part of a telecommunications network that provides various services to customers who are connected by the access network. One of the main functions is to route telephone calls across the PSTN.
Typically the term refers to the high capacity communication facilities that connect primary nodes. A core/backbone network provides paths for the exchange of information between different sub-networks. For enterprise private networks serving one organization, the term backbone is more commonly used, while for service providers, the term core network is more common.
Core/backbone networks usually have a mesh topology that provides any-to-any connections among devices on the network. Many main service providers would have their own core/backbone networks that are interconnected. Some large enterprises have their own core/backbone network, which are typically connected to the public networks.
The devices and facilities in the core / backbone networks are switches and routers. The trend is to push the intelligence and decision making into access and edge devices and keep the core devices dumb and fast. As a result, switches are more and more often used in the core/backbone network facilities. Technologies used in the core and backbone facilities are data link layer and network layer technologies such as SONET, DWDM, ATM, IP, etc. For enterprise backbone network, Gigabit Ethernet or 10 Gigabit Ethernet technologies are also often used.
Core networks typically provide the following functionality:
- Aggregation: The highest level of aggregation in a service provider network. The next level in the hierarchy under the core nodes is the distribution networks and then the edge networks. Customer-premises equipment (CPE) do not normally connect to the core networks of a large service provider.
- Authentication: The function to decide whether the user requesting a service from the telecom network is authorized to do so within this network or not.
- Call Control/Switching: call control or switching functionality decides the future course of call based on the call signalling processing. E.g. switching functionality may decide based on the "called number" that the call be routed towards a subscriber within this operator's network or with number portability more prevalent to another operator's network.
- Charging: This functionality handles the collation and processing of charging data generated by various network nodes. Two common types of charging mechanisms found in present-day networks are prepaid charging and postpaid charging. See Automatic Message Accounting
- Service Invocation: Core network performs the task of service invocation for its subscribers. Service invocation may happen based on some explicit action (e.g. call transfer) by user or implicitly (call waiting). Its important to note however that service "execution" may or may not be a core network functionality as third party network/nodes may take part in actual service execution.
- Gateways: Gateways shall be present in the core network to access other networks. Gateway functionality is dependent on the type of network it interfaces with.
Physically, one or more of these logical functionalities may simultaneously exist in a given core network node.
Besides above mentioned functionalities, the following also form part of a core network:
- O&M: Operations & Maintenance centre or Operations Support Systems to configure and provision the core network nodes. Number of subscribers, peak hour call rate, nature of services, geographical preferences are some of the factors which impact the configuration. Network statistics collection, alarm monitoring and logging of various network nodes actions also happens in the O&M centre. These stats, alarms and traces form important tools for a network operator to monitor the network health and performance and improvise on the same.
- Subscriber Database: Core network also hosts the subscribers database (e.g. HLR in GSM systems). Subscriber database is accessed by core network nodes for functions like authentication, service invocation etc.
There exist basically two core network types for mobile telephony:
- The Mobile Application Part (MAP) used for GSM and UMTS
- The IS-41 core network used for D-AMPS (TDMA), cdmaOne and CDMA2000.
Both variants have evolved over time to integrate new services and air interfaces.