Improving the Fault Tolerance of GSM Networks

A PCS network constantly tracks the locations of the mobile stations so that incoming calls can be delivered to the target mobile stations. In general, a two-level database system is used to store location information of the mobile stations. When the location databases fail, incoming calls may be lost. This article describes the standard GSM database failure restoration procedure which reduces the number of lost calls. Then we propose an efficient VLR identification algorithm for the HLR failure recovery procedure, which utilizes mobile station movement information to speed up the recovery procedure. personal communications services (PCS) network tracks the locations of mobile stations (MSs) or mobile phones so that incoming calls can be delivered to subscribers. To exercise location tracking, a PCS service area is partitioned into several location areas (LAs). Each LA consists of a group of base stations that communicate with the MSs through radio contact. The major task of mobility management is to update the location of an MS when it moves from one LA to another. The location update procedure is referred to as registration, which is initiated by the MS as follows. The base stations continuously broadcast the corresponding LA addresses to the MSs. When an MS receives a different LA address, it sends a registration message to the network. The location information is stored in the PCS mobility databases called the home location register (HLR) and the visitor location register (VLR). For every LA, there is a corresponding VLR. When an MS visits the LA, a temporary record of the MS is created in the VLR to indicate its location (i.e., the LA address). For every MS, there is a permanent record stored in the HLR. The record stores the address of the VLR visited by the MS. We use Global System for Mobile Communications (GSM) [l] as an example to illustrate the network architecture for PCS mobility management (Fig. 1). In this architecture, the base stations of an LA are connected to a mobile switching center (MSC), a telephone switch tailored for PCS services). Thus, an MSC covers several LAs. One or more MSCs are connected to a VLR, and exchange location information with the VLR through the Signaling System No. 7 (SS7) network [2]. Similarly, the VLR communicates with the HLR to exchange location information using SS7 messages. The GSM mobility management protocol, Mobile Application Part (MAP), is implemented using the SS7 platform. If the location databases fail, the location information loss or corruption will seriously degrade the service offered to subscribers. Thus, fault tolerance of location databases becomes an important issue for PCS network management. This article describes the failure restoration procedures in GSM, and proposes an algorithm to speed up the HLR failure recovery procedure.