ls between the fixed networks and Introduction to Telecommunications Network Engineering network elements of PLMN. The GMSC also controls echo cancellers, which are needed between the fixed and cellular network because of long speech-coding delays. 5.5.1.3 HLR All subscriber parameters for each mobile user are permanently stored in one HLR. The HLR provides a well-known and fixed location for variable rout- ing information. The main functions of the HLR are as follows: Storage of the subscriber data, for example, services available for this subscriber; Location registration and call handling, central store for subscriber location data; Support for encryption and authentication; Handling of supplementary services (e.g., barring or call transfer); Support for the short message service. The HLR is implemented by an efficient real-time database system that may store the subscriber data of 1 million subscribers. 5.5.1.4 VLR The VLR provides local storage for all of the variables and functions needed to handle calls to and from the mobile subscribers in the area related to that VLR. The information is stored in the VLR as long as the mobile station stays in that area. The VLR communicates with the HLR to inform it about the location of a subscriber and to obtain subscriber data that includes infor- mation about, for example, what services should be provided to this specific subscriber. The main functions of the VLR are as follows: Storage of data for subscribers located in its area; Management and allocation of the local identity codes to avoid frequent use of a global identity on the radio path for security reasons; Location registration and call handling; Authentication; Support of encryption; Support for handover; Mobile Communications Handling of supplementary services; Support for SMS. The VLR is a database system that is usually integrated in each mobile exchange MSC. 5.5.1.5 Authentication Center (AuC) The security data of a subscriber are stored in the AuC that contains a subscriber-specific security key, encry