tailieunhanh - Enabling Technologies for Wireless E-Business phần 5

được gọi là cho một máy chủ tập trung mạnh mẽ hơn, đó là HLR / AUC (Home Location Đăng ký / chứng thực Trung tâm ). Hình 3,16 cho thấy cơ sở hạ tầng thông tin liên lạc GSM và, nói chung, AUC sẽ được gắn với một HLR trong một môi trường an toàn. Trung tâm xác thực (AUC) một cách an toàn lưu trữ khóa bí mật của | 7 Data Management for Mobile Ad-Hoc Networks 151 or of the implementation details of each requested transaction. A hopping property is added to model the mobility of the transactions. Each subtransaction represents the unit of execution at one base station and is called a joey transaction JT . The authors define a Pouch to be the sequence of global and local transactions which are executed under a given KT. Each KT has a unique identification number consisting of the base station number and unique sequence number within the base station. When a mobile unit moves from one cell to another the control of the KT changes to a new DAA at another base station. The DAA at the new base station creates a new JT as result of the hand-off process. JTs have sequenced identification numbers consisting of both the KT identification number and an increasing number. The mobility of the transaction model is captured by the use of split transactions. The old JT is committed independent of the new JT. If a failure of any JT occurs which in turn may result in undoing the entire KT a compensation for any previously completed JTs must be assured. Therefore a KT could be in a split mode or in a compensating mode. A split transaction divides an ongoing transaction into serialized subtransactions. Earlier created subtransaction may be committed and the remaining ones can continue in its execution. However the decision on as to abort or commit a currently executing subtransaction is left to the main DBMS. Previous JTs may not be compensated so that neither splitting mode nor compensating mode guarantees serializability of KTs. Although compensating mode assures atomicity isolation may be violated because locks are obtained and released at the local transaction level. With the compensating mode joey subtransactions are serializable. The MTM keeps a transaction status table on the base station DAA to maintain the status of those transactions. It also keeps a local log into which the MTM writes

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