 components, the circuit core and the packet core, which are responsible for voice and data respectively. In GSM, core or circuit-core network planning is also known as switch planning, because it is mainly related to the mobile switching centre (MSC), also called 'Switch'. Core network planning in GSM consists of network elements such as MSC, VLR, HLR, AC and EIR. 4.1.2 Elements of the Core Network MSC/VLR The MSC is the core element of the network sub-system. It is responsible for the switching of subscriber calls. It is also responsible for traffic management, paging, and the collection of charging-related information. Apart from this, the MSC also acts as an interface between the network and the networks of other operators and PSTN networks. Another important element that is hosted in an MSC is the visitor location register (VLR). The VLR contains information on the subscribers who are being handled by the MSC at a given moment. Owing to the nature of its functionality, the VLR participates in call processing and mobility management functions of the network such as the use of temporary mobile identification, IMSI attach/detach, etc., apart from location registration of the mobile subscriber. The Fundamentals of Cellular Network Planning & Optimisation A.R. Mishra, C 2004 John Wiley & Sons, Ltd. ISBN: 0-470-86267-X CORE NETWORK PLANNING AND OPTIMISATION HLR/AC/EIR Ater' MSC/VLR O&M Interface Figure 4.1 Scope of core network planning VLR is also involved in security functions such as IMEI (International Mobile Equipment Identification) checking. HLR/EIR/AC The home location register (HLR) is responsible for keeping all permanent information regarding subscribers and their equipment. It is also responsible for creating, modifying, deleting or managing these data. Apart from this, the HLR also participates in call processing functions such as routing, supporting the incoming call barring service/unconditional call forwarding service, etc. The HLR consists of an EIR and an AC. The EIR (equipment identification reg