Mobile Database Systems Vijay Kumar Computer Sc. Telecommunications University of Missouri-Kansas City 5100 Rockhill Road Kansas City, MO 64110, USA kumar@cstp. umkc. edu This Presentation has been Modified by Young Chul Park Kyungpook National University, Korea
Mobile Database Systems What is a Mobile Database System (MDS)? An Information management system on a mobile platform which is capable of providing full transaction management and database functionality from anywhere and at anytime. è è How nicely database technology and wireless mobility can be mingled together? Mobile Transaction Models and Query Processing. A system with the following structural and functional properties v Distributed system with mobile connectivity v Full database system capability v Complete spatial mobility v Built on PCS/GSM platform v Wireless and wired communication capability
Mobile Database Systems What is a mobile connectivity? A mode in which a client or a server can establish communication with each other whenever needed. Intermittent connectivity It is a special case of mobile connectivity. A mode in which only the client can establish communication whenever needed with the server but the server cannot do so. This mode is inadequate to satisfy global consistency requirements of MDS.
Mobile Database Systems Outline v Fully Connected Information Space : Chapter 1 v Personal Communication System (PCS) Mobile Database Systems (MDS) Transaction Management Data Caching Query Processing Data Classification Conclusion v v v
Mobile Database Systems Fully connected information space
Mobile Database Systems Fully connected information space v Each node of the information space has some communication capability. v Some node can process information. v Some node can communicate through voice channel. v Some node can do both All functional units are fully connected with each other through wireless links.
Mobile Database Systems Types of Mobility 1. Terminal Mobility: The connection is established between two network connection points and not between the two persons calling each other. 2. The mobility of terminal is supported; that is the same terminal can be used to connect to the other party from anywhere by any user. 2. Personal Mobility: A user can use any communication device for establishing communication with the other party. This facility requires an identification scheme to verify the person wishing to communicate.
Mobile Database Systems Fully connected information space It can be created and maintained by integrating legacy database systems, and wired and wireless systems (PCS, Cellular system, and GSM) è Users desire that a mobile unit (laptop, cell phone, PDA, etc. ) should have transaction management capability. è The fully connected information space needs transactional services.
Mobile Database Systems Wireless Technology The First-Generation (1 G): in the 1980’s. Analog. : Voice only. The Second-Generation (2 G): in early 1990’s. Digital. GSM : Voice, limited amount of data (SMS: Short Message Service). The 2. 5 G: in early 2000’s. : Voice, limited amount of data, e-mail. The Third-Generation (3 G): : Voice, e-mail, multimedia data transfer, unrestricted internet access, video streaming, etc. PCS – Personal Communication System GSM – Global System for Mobile Communication
Personal Communication System (PCS) Part 1 v v Architecture Wireless communication Bandwidth limitations Frequency reuse
Personal Communication System (PCS) v Radio Frequency (RF) v Spectrum : The entire range of (electromagnetic) waves. v (Frequency) Band : A range of RFs usable for a particular task. v Bandwidth : The size of a band. Ex. Band = between 80 MHz and 180 MHz Bandwidth = 180 MHz – 80 MHz = 100 MHz Most wireless communication today use a frequency range of 800 MHz to 3 GHz.
Personal Communication System (PCS) v v Attenuation Communication Slot: a geographic area within which the RF can be used to set up communication. v In cellular terminology this communication slot is referred to as a cell, and the entire communication infrastructure is known as cellular communication. v v Footprint : A cell’s actual radio coverage. Cell Site : the point in the cell where the transceiver (i. e. , base station, BS) is installed. v See Fig. 2. 2 Location of cell site. v Cell site at the junction of three cells. v Cell site located at the center of a cell. The transceiver covers a portion, referred to as “sector” of each cell, and provides each sector with its own channels. v v v Communication path, communication link: See Fig. 2. 3 on Page 17. v Actual communication link. v User’s perception of the link
Personal Communication System (PCS) v In a cellular architecture a number of wireless and wired components are required to establish the desired point-to-point or point-to-multipoint communication. v A BS functions under the supervision of a telecommunication switch called Mobile Switching Center (MSC) and connected to it through wired line. v It is the MSC which connects the entire mobile system with PSTN (Public Switched Telephone Network).
Personal Communication System (PCS) v Continuous connectivity during communication is highly desirable for voice communication but essential for data exchange. v A communication session uses a channel to exchange information. v A channel is a communication path between any two devices such as computers, mobile units, base stations, etc. v In mobile discipline two frequencies are required to establish communication; one from mobile to base station and one from base station to mobile. For this reason a channel is defined as a pair of radio frequencies, one to receive and one to transmit. The frequency which is used by mobile unit (MU) for transmission to base station is called reverse channel (uplink channel) and the frequency used by the base station to transmit to mobile unit is called forward channel (downlink channel).
Personal Communication System (PCS) v See Fig. 2. 4 downlink and uplink channels. v v v Two channels. One channel has the following characteristics. v A reverse channel: Mobile transmits at 825. 030 MHz. v A forward channel: The BS transmits at 870. 030 MHz. v The channel separation is 45 MHz. v Each voice channel has 30 KHz bandwidth. See Table 2. 3 General mobile frequency table v American cellular 832 channels (33 + 333 + 50 + 83) v 824 to 849 MHz mobile unit to base station v 869 to 894 MHz base station to mobile unit v GSM v 890 to 915 MHz mobile unit to base station v 930 to 960 MHz base station to mobile unit
Personal Communication System (PCS) v v v A cell has a fixed number of voice channels available for communication. The MSC allocates on demand a channel through the BS to the MU when it tries to make a call. The strength (expressed in decibels (d. B)) of the communication channel reduces as the MU moves away from the BS. Signal Fading. When a MU crosses its cell boundary and enters a neighboring cell, the MSC transfers the ongoing communication session to a new channel which belongs to the new BS. The migration of a MU from one cell to another is managed by a process referred to as handoff. A handoff makes sure that the MU gets the necessary channel in the new cell to continue its communication and relieves the channel it was using without any interruption. Moving from the cell of BS 1 to the cell of BS 2 while 1. 2. actively communicating with somebody, or in a doze mode or
Personal Communication System (PCS) v Two communication sessions may interfere with each other if the frequencies they use are quite close to each other or even identical. v v v v The presence of an interference could give rise to cross talk, where one party can hear the communication of the other party in the back ground. Co-channel interference, Adjacent channel interference Co-channel interference occurs when the same frequency is used for communication in two nearby cells. Co-channel interference is solved by keeping the cells, which plan to use the same set of frequencies, apart by a distance called frequency reuse distance. The frequency reuse distance is the minimum safe distance between two cells which can reuse the same frequencies without interference. Frequency reuse is implemented through cell clustering. A cell cluster is made up of a number of same size cells and the cluster size depends on the entire coverage area, the frequency plan, acceptable cochannel interference, and mobile commerce (M-commerce) strategies. In M-commerce it is important for a mobile user to know in which cluster he or she is currently residing.
Personal Communication System (PCS) v v A cell cluster may be made up of 3, 4, 7, 9, 12, 13, 16, etc. , cells; but of these, 7 -cell and 4 -cell clusters are most commonly used to cover an area. In reality, an area is covered by a number of clusters, each one composed of different size cells as illustrated by Figure 2. 11 e.
Personal Communication System (PCS) A system where wired and wireless networks are integrated for establishing communication.
Personal Communication System (PCS) Wireless Components Base Station (BS): A switch, which serves as communication link between MU and the entire network Mobile Units (MU): Also called Mobile Systems (MS) or Mobile Hosts (MH). A mobile component, which communicates with BS through a limited number of wireless channels. It is a wireless device which is composed of (a) antenna, (b) transceiver, and (c) user interface.
Personal Communication System (PCS) MU A MU has its own permanent memory where it stores (a) Mobile Identification Number (MIN), (b) Electronic Serial Number (ESN), and (c) Station Class Mark (SCM). MIN is a 10 -digit subscriber’s telephone number. The ESN is a unique 32 -bit number which is used by the cellular system to identify the unit. SCM identifies the power-handling capability of a mobile unit, and it is a 4 -bit number. 1. When a call arrives to a mobile unit a paging message is sent by MSC with the subscriber’s MIN to all base stations. 2. When the mobile unit receives the page on a forward control channel, it sends a response with its own MIN using the reverse control channel. 3. When MSC receives mobile unit’s response with its MIN, it then directs the BS to assign a voice channel for setting up the communication.
Personal Communication System (PCS) BS The BS consists of a pair of transmitter and receiver. Each cell is managed by one BS only, and the size of the cell is determined by its power. In PCS architecture the activities of a BS such as setting up a communication, allocating a channel for communication, etc. , are directly managed by the MSC. 1. When a mobile subscriber dials a number, the BS of that cell reaches its MSC for a voice channel for establishing that call. 2. At the end of the call, it returns the channel to the MSC. In GSM architecture a BS is referred to as a BTS (Base Transceiver Station).
Personal Communication System (PCS) Base Station Controller (BSC) This is a GSM component which manages the activities of a number of BS or BTS. Its main functions are RF frequency administration, BTS, and handover management. At one end a BSC is connected to many BTSs and to a MSC on the other end. Mobile Switching Center (MSC) It is a switch and is referred to by many names such as MTSO, mobile switch (MS), or Mobile Switching Center (MSC). They all process mobile telephone calls. Large systems may have two or more MSCs and are connected with one or many base stations through wired line.
Personal Communication System (PCS) Home Location Register (HLR) It is a large database which stores necessary information such as geographic location of a subscriber, call processing, billing, service subscription, service restrictions, etc. Its contents and structure are vendor specific but the schema developed by Lucent Technologies is used by almost all network companies. In a centralized scheme, HLR is usually stored at one MSC, and in proposed distributed approaches it is distributed to multiple MSCs. See Fig. 2. 20 HLR database
Personal Communication System (PCS) Visitor Location Register (VLR) A VLR is a current subset of HLR for a particular cell. Whenever a subscriber enters a new cell, its current location is stored in VLR representing that cell. The entry is removed when the subscriber leaves that cell. The information about a subscriber is replicated in HLR and VLR to achieve faster location search which begins from VLR and ends in HLR if the entry is not found in the VLR. The VLR data are not used for any administrative purpose; it merely serves as a location identifier. For example, Motorola uses more than 60 HLRs nationwide. The processing of HLR is not complex, so it does not need sophisticated functionality. The recent trends is to distribute HLRs and use distributed processing approaches for its management.
Personal Communication System (PCS) Authenticating Center (AC or AUC) The AC is a processor system, which authenticates subscribers. It is colocated with HLR. Thus AC and HLR together are stored in MSC or somewhere nearby it. An AC may serve more than one HLR. A complete authentication process is performed only when the first call is made. In all subsequent calls from the same subscriber, if made within a system defined time period, the stored information is used for authentication. Equipment Identify Register (EIR) It is a database which stores information for the identification of mobile units. For example, it maintains a database of Electronic Serial Number which is unique to a mobile unit which prevents its theft and malicious use. GSM maintains three types of list identified as white, black, and gray. Any call originating from a black list unit is blocked by the network.
Personal Communication System (PCS) Short Message Entity (SME) This unit manages short text messages (200 bytes or less). Message Center (MC) This unit stores and forwards short messages to mobile destination. If the destination is unavailable for any reason, it stores the message for later dispatch.
Personal Communication System (PCS) Interface A number of interfaces are required to establish link among functional units. These links could be one to one, one to many, many to many, and one to zero.
Personal Communication System (PCS) Call Processing When a mobile unit is switched on, it goes through a set of validating steps before a call can be made and then a channel is assigned to a mobile unit to initiate the call. Essential Steps a. Determine the type of home system from the stored parameter. b. If type A is identified, then the mobile unit begins scanning type A channels and tunes to the strongest signal. The mobile unit stores System Identification (SID), which is a 15 -bit number assigned to a cellular system by the FCC, and stores paging channels and their range. It tunes to the strongest paging channel and receives an overhead train message. The mobile based on SID being transmitted on the paging channel determines its status (roaming or not) and sets the roam indicator accordingly.
Personal Communication System (PCS) Call Processing Essential Steps b. The mobile enters the idle mode, where it periodically receives an overhead train message and captures system access information, system control information, and system configuration information. c. The mobile starts a timer for each operation. For example, it sets 12 seconds for call orgination, 6 seconds for page response, and so on.
Personal Communication System Call Processing (Mobile-to-Land Call) a. The mobile user is in idle mode. b. It dials the number. When the caller presses the SEND key, then only the numbers are transmitted. c. The mobile unit goes into the system access mode as the call originator. d. The mobile unit tunes to the strongest control channel and attempts to send an origination message through this channel. e. The base station receives the message, validates it, and sends a subset of fields for further validation to MTSO validates user subscription type, service profile, etc. f. After a successful validation, the MTSO sends a message to the BS to assign a voice channel to the user. In this assignment the base station sends channel number to the mobile unit via an IVCD (Initial Voice Designation) message. g. The base station transmits the SAT (Supervisory Audio Tone) to the mobile unit. The unit tunes to the voice channel and returns the SAT on the reverse voice channel back to the base station. h. The base station receives the SAT from the unit and sends it to the MTSO, which then dials the called number via PSTN. i. The mobile user receives the ringing tone and begins conversation. j. At the end of transmission, it sends an ST (Signaling Tone) for 1. 8 seconds and terminates the session. k. The base station detects the ST and sends a disconnect message to MTSO. The MTSO releases the connection with PSTN and the land user. l. If the land user terminates the call, then PSTN initiates the release process.
Personal Communication System Call Processing (Land-to-Mobile Call) a. The land user keys in the mobile telephone number. b. MSC receives this number. First it has to make sure that the mobile unit is available to take this call, and also it must know the location. The MSC may have some information about the location of the unit from its recent registration in which case it sends a page request to the BS of the registration area. The BS, after receiving this page request, pages the MU through the paging channel. If the location of MU is not known, then the location manager is invoked. c. The MU acknowledges the receipt of this page. d. The BS informs the MSC that it has received the acknowledgement from the MU. e. The MSC validate user subscription type, service profile, etc. f. After a successful validation, the MSC sends a message to the BS to assign a voice channel to the mobile unit. In this assignment the BS sends a channel number to the MU via an IVCD (Initial Voice Channel Designation) message and asks MU to tune to the channel. g. The BS transmits the SAT to the MU. The unit tunes to the voice channel and returns the SAT on the reverse voice channel to the BS. h. The BS receives the SAT from the unit and sends it to the MSC and sends an alert order to MU. i. In response to this alert order, the MU begins ringing to inform the user about the incoming call. j. The MU generates a signaling tone on the reverse channel which BS detects and informs MSC that MU has been alerted. k. The MSC generates a ring-back tones for the land user. l. The MU connects to the voice channel and answers the phone. m. The MSC stops ringing the land user and the conversation begins.
Personal Communication System (PCS) Wireless channels are limited Item Europe (MHz) US (MHz) NMT: 453 -457, 463 -467 GSM: 890 -915, 935 -960, 1710 -1785, 1805 -1880 AMPS, TDMA, CDMA 824 -849, 869 -894 GSM, TDMA, CDMA 1850 -1910, 1930 -1990 PDC: 810 -826 940 -956, 1429 -1465, 1477 -1513. Cordless CT 1+: 885 -887, 930 -932 PACS Phones CT 2: 864 -868 1850 -1910, 1930 -1990; DECT: 1880 -1900 PACS-UB: 1910 -1930 PHS 1895 -1918; JCT: 254 -380 Mobile Phones NMT: Nordic Mobile Telephone PDC: Pacific Digital Cellular PACS: Personal Access Communications System PHS: Personal Handyphone System PACS-UB: PACS Unlicensed Band JCT: Japanese Cordless Telephone (Taken from Mobile Communications by Jochen Schiller) Japan (MHz)
Personal Communication System (PCS) Limited channels must be utilized efficiently. It is done so by Frequency reuse The same radio frequency is used for communication by more than one cell sessions. Mobile cells To achieve frequency reuse, the entire wireless coverage area is divided into cells.
Personal Communication System (PCS) Mobile cells
Personal Communication System (PCS) Mobile cells The entire coverage area is a group of a number of cells. The size of cell depends upon the power of the base stations.
Personal Communication System (PCS) Frequency reuse D = distance between cells using the same frequency R = cell radius N = reuse pattern (the cluster size, which is 7). Thus, for a 7 -cell group with cell radius R = 3 miles, the frequency reuse distance D is 13. 74 miles.
Personal Communication System (PCS) Problems with cellular structure v How to maintain continuous communication between two parties in the presence of mobility? Solution: Handoff v How to maintain continuous communication between two parties in the presence of mobility? Solution: Roaming v How to locate of a mobile unit in the entire coverage area? Solution: Location management
Personal Communication System (PCS) Handoff A process, which allows users to remain in touch, even while breaking the connection with one BS and establishing connection with another BS.
Personal Communication System (PCS) Handoff To keep the conversation going, the Handoff procedure should be completed while the MS (the bus) is in the overlap region. A handoff must be initiated and completed before the ongoing call is lost due to reduction in signal strength.
Personal Communication System (PCS) Handoff issues v Handoff detection v Channel assignment v Radio link transfer
Personal Communication System (PCS) Handoff detection strategies v Mobile-Controlled handoff (MCHO) v Network-Controlled handoff (NCHO) v Mobile-Assisted handoff (MAHO)
Personal Communication System (PCS) Mobile-Controlled Handoff (MCHO) In this strategy, the MU continuously monitors the radio signal strength and quality of the surrounding BSs. When predefined criteria are met, then the MU checks for the best candidate BS for an available traffic channel and requests the handoff to occur. MCHO is used in DECT and PACS.
Personal Communication System (PCS) Network-Controlled Handoff (NCHO) In this strategy, the surrounding BSs, the MSC or both monitor the radio signal. When the signal’s strength and quality deteriorate below a predefined threshold, the network arranges for a handoff to another channel. NCHO is used in CT-2 Plus and AMPS.
Personal Communication System (PCS) Mobile-Assisted Handoff (MAHO) It is a variant of NCHO strategy. In this strategy, the network directs the MU to measure the signal from the surrounding BSs and to report those measurements back to the network. The network then uses these measurements to determine where a handoff is required and to which channel. MAHO is used in GSM and IS-95 CDMA.
Personal Communication System (PCS) Handoff types with reference to the network v Intra-system handoff or Inter-BS handoff The new and the old BSs are connected to the same MSC.
Personal Communication System (PCS) Handoff types with reference to the network v Intersystem handoff or Inter-MSC handoff The new and the old BSs are connected to different MSCs.
Personal Communication System (PCS) Handoff types with reference to link transfer v Hard handoff The MS connects with only one BS at a time, and there is usually some interruption in the conversation during the link transition. In hard handoff the MU does experience momentary silence in voice communication. v Soft handoff The two BSs are briefly simultaneously connected to the MU while crossing the cell boundary. As soon as the mobile's link with the new BS is acceptable, the initial BS disengages from the MU.
Personal Communication System (PCS) Handoff types with reference to link transfer Hard handoff 1. MU temporarily suspends the voice conversation by sending a link suspend message to the old BS. 2. MU sends a handoff request message through an idle time slot of the new BS to the network. 3. The new BS sends a handoff ack message and marks the slot busy. 4. The MU returns the old assigned channel by sending a link resume message to the old BS.
Personal Communication System (PCS) Handoff types with reference to link transfer Hard handoff 5. MU continues voice communication while the network prepares for the handoff. 6. Upon receipt of a handoff request message, the new BS sends a handoff ack message and reconfigures itself to effect the handoff. 7. The MSC inserts a bridge into the conversation path and bridges the new BS. 8. Finally, the network informs the MU to execute the handoff via both the new and old BSs by sending the handoff execute message.
Personal Communication System (PCS) Handoff types with reference to link transfer Hard handoff 9. MU releases the old channel by sending an access release message to the old BS. 10. Once the MU has made the transfer to the new BS, it sends the network a handoff complete message through the new channel, and resumes the voice communication. The network removes the bridge from the path and frees up the resources associated with the old channel.
Personal Communication System (PCS) Handoff types with reference to link transfer Soft handoff 1. MU sends a pilot strength measurement message to the old BS, indicating the new BS to be added. 2. The old BS sends a handoff request message to the MSC. If the MSC accepts the handoff request, it sends a handoff request message to the new BS. 3. The BS sends a null traffic message to the MU to prepare the establishment of the communication link.
Personal Communication System (PCS) Handoff types with reference to link transfer Soft handoff 4. The new BS sends a join request message to the MSC. The MSC bridges the connection for the two BSs, so that the handoff can be processed without breaking the connection. 5. The new BS sends a handoff ack message to the old BS via the MSC. The old BS instructs the MU to add a link to the new BS by exchanging the handoff command handoff complete messages.
Personal Communication System (PCS) Handoff types with reference to link transfer Soft handoff 6. The old BS and the MSC conclude this procedure by exchanging the required handoff information. The quality of the new link is guaranteed by the exchange of the pilot measurement request and the pilot strength measurement message pair between the MU and the new BS.
Personal Communication System (PCS) Roaming is a facility, which allows a subscriber to enjoy uninterrupted communication from anywhere in the entire coverage space. A mobile network coverage space may be managed by a number of different service providers. They must cooperate with each other to provide roaming facility. Roaming can be provided only if some administrative and technical constraints are met.
Personal Communication System (PCS) Roaming Administrative constraints v Billing. v Subscription agreement. v Call transfer charges. v User profile and database sharing. v Any other policy constraints.
Personal Communication System (PCS) Roaming Technical constraints v Bandwidth mismatch. For example, European 900 MHz band may not be available in other parts of the world. This may preclude some mobile equipment for roaming. v Service providers must be able to communicate with each other. Needs some standard. v Mobile station constraints.
Personal Communication System (PCS) Roaming Technical constraints v Integration of a new service provider into the network. A roaming subscriber must be able to detect this new provider. v Service providers must be able to communicate with each other. Needs some standard. v Quick MU response to a service provider’s availability. v Limited battery life.
Personal Communication System Location Management Three Fundamental Tasks 1. Location update It is initiated by the MU. 1. The current location of the MU is recorded in HLR and VLR databases. 2. Location lookup It is initiated by the MSC. 1. 3. 4. It is a database search to obtain the current location of the MU. Paging. It is initiated by the MSC. Through paging the system informs the caller the location of the called unit in terms of its current database station. To reduce the location update frequency, a number of neighboring cells are grouped together to form a location area, and the paging area is constructed in a similar way.
Personal Communication System Location Management HLR: Home Location Register A HLR stores user profile and the geographical location. VLR: Visitor Location Register A VLR stores user profile and the current location who is a visitor to a different cell that its home cell. Two-Tier Scheme The first tier provides a quick location lookup. The second tier search is initiated only when the first tier search fails.
Personal Communication System (PCS) Location Management Two-Tier Scheme steps. MU 1 wants to talk to MU 2.
Personal Communication System (PCS) Location Management Two-Tier Scheme steps. MU 1 wants to talk to MU 2. 1. VLR of cell 2 is searched for MU 2’s profile. 2. If it is not found, then HLR is searched. 3. Once the location of MU 2 is found, then the information is sent to the base station of cell 1. 4. Cell 1 establishes the communication.
Personal Communication System (PCS) Location Management Two-Tier Scheme steps location update 1. MU 2 moves from cell 1 to cell 2. MU 2’s location is changed so new location must be recorded. 3. HLR is updated with the new location address. 4. MU 2’s entry is deleted from the VLR of cell 1 and new entry is made in cell 2’s VLR.
Personal Communication System Location Management Two-Tier Scheme steps location search ls : location server, mms : base station HLR VLR
Personal Communication System Location Management Two-Tier Scheme steps location update It is performed when a MU enters a new registration area. HLR
Personal Communication System Location Management Two-Tier Scheme It has very high search and update costs, which increase significantly in the presence of frequent cell crossing because every registration area crossing updates HLR. Forward Pointer Location Management Scheme
Fundamentals of Database Processing Conventional Database Architecture Centralized DBMS Distributed DBMS Data distribution: Database Partition, Partial Replication, Full Replication. A federated database system is a type of meta-database management system which transparently integrates multiple autonomous database systems into a single federated database. The constituent databases are interconnected via a computer network, and may be geographically decentralized. Since the constituent database systems remain autonomous, a federated database system is a contrastable alternative to the (sometimes daunting) task of merging together several disparate databases. A federated database (or virtual database) is the fully-integrated, logical composite of all constituent databases in a federated database system. Through data abstraction, federated database systems can provide a uniform front-end user interface, enabling users and clients to store and retrieve data in multiple noncontiguous databases with a single query--even if the constituent databases are heterogeneous. To this end, a federated database system must be able to deconstruct the query into subqueries for submission to the relevant constituent DBMS's, after which the system must composite the result sets of the subqueries. Because various database management systems employ different query languages, federated database systems can apply wrappers to the subqueries to translate them into the appropriate query languages.
Fundamentals of Database Processing Transaction A transaction is defined as unit of processes which change a database from one consistent state to another consistent state. A flat transaction is a sequence of database operations with the following ACID properties: Atomicity, Consistency, Constraints Isolation, and <Serializability Theory, Two-Phase Locking> Durability. SQL: Commit, Rollback; (Partial or Total Rollback) Database Recovery: Transaction, Restart, Media
Fundamentals of Database Processing Serialization of transactions Serial Execution Serialization of concurrent transactions Read – Read, compatible Read – Write, Unrepeatable Read Problem Write – Read, Dirty Read Problem Conflict Write – Write Lost Update Problem Operations Phantom Record Problem Conflict Serializability practical View Serializability, State Serializability not practical level_of_isolation : = READ UNCOMMITTED | // Dirty Read: no lost update, dirty read: long X / no S READ COMMITTED | // Cursor Stability: no lost update, no dirty read: long X / short S REPEATABLE READ | // no lost update, no dirty read, repeatable read: long X / long S SERIALIZABLE // no lost update, no dirty read, repeatable read, no
Fundamentals of Database Processing Advanced Transaction Models Advanced 0. Flat Transaction Model : strict enforcement of ACID properties. Advanced transaction models which relaxed ACID properties. 1. Nested Transaction Model : Fig. 4. 7 : Transaction – {Subtransaction} : Tree v Subtransaction : Semi-independent v A subtransaction can commit or rollback independent to other subtransactions or the parent of the tree. However such a commitment or rollback takes effect only if the parent commits or rolls back. v A subtransaction has only A, C, and I properties because its durability depends only on its parent. If the parent rolls back, then all its subtransactions must also be rolled back. 2. SAGA : Transaction – {(fine) transaction, Compensating transaction} : Chain v Fine transaction : Independent v A transaction is divided into a number of independent transactions and are chained together, then each transaction can execute independently. 3. Cooperative Transaction : Fig. 4. 8: Transaction Group (TG) – Cooperative Transaction Members : Tree 4. Con. Tract 5. Flex Transaction
Fundamentals of Database Processing Concurrency Control Mechanisms Two-phase locking (2 PL) protocol Conflicts Rollback and Blocking operations Locking Phase (the growing phase) and Unlocking phase (the shrinking phase) (1) Simultaneous Locking and simultaneous Unlocking protocol Fig. 5. 1 (2) Incremental Locking and simultaneous Unlocking protocol Fig. 5. 2 (3) Simultaneous Locking and incremental Unlocking protocol Fig. 5. 3 (4) Incremental Locking and incremental Unlocking protocol Fig. 5. 4 Deadlock: Deadlock detection and resolution Deadlock Prevention Multiple Granularity Locking (MGL) protocol Locking Granularity: System – Database – Table – Page – Record Lock Modes : S, X, IS, IX, SIX Fig. 5. 6 Multiversion Approach : Every write operation generates a new version of the data. Distributed Database System (1) Centralized 2 PL: One site is responsible for managing all locking activities. (2) Primary Copy 2 PL (3) Distributed 2 PL
Mobile Database Systems (MDS) Part 2 v Architecture v Data categorization v Data management v Transaction management v Recovery
Mobile Database Systems (MDS) A Reference Architecture (Client-Server model)
Mobile Database Systems (MDS) MDS Applications v Insurance companies v Emergencies services (Police, medical, etc. ) v Traffic control v Taxi dispatch v E-commerce v Etc. Mobile Commerce (M-Commerce) Example 6. 1 MU Cooperate Database Server Example 6. 2 Car Police, Hospital, etc. : Wireless Notification System NOW (Notification on Wireless) : Unit. Of Missouri – Kansas City Example 6. 3 Car : Find a nearby restaurant, hospital Example 6. 4 Auction : Broadcast based data dissemination.
Mobile Database Systems (MDS) MDS Limitations v Limited wireless bandwidth v Wireless communication speed v Limited energy source (battery power) v Less secured v Vulnerable to physical activities v Hard to make theft proof.
Mobile Database Systems (MDS) MDS capabilities v Can physically move around without affecting data availability v Can reach to the place data is stored v Can process special types of data efficiently v Not subjected to connection restrictions v Very high reachability v Highly portable
Mobile Database Systems (MDS) Objective To build a truly ubiquitous information processing system by overcoming the inherent limitations of wireless architecture.
Mobile Database Systems (MDS) MDS Issues v Data Management l Data Caching l Data Broadcast (Broadcast disk) l Data Classification v Transaction Management l Query processing l Transaction processing l Concurrency control l Database recovery
Mobile Database Systems (MDS) MDS Data Management Issues How to improve data availability to user queries using limited bandwidth? Possible schemes v Semantic data caching: The cache contents is decided by the results of earlier transactions or by semantic data set. v Data Broadcast on wireless channels
Mobile Database Systems (MDS) MDS Data Management Issues How to improve data availability to user queries using limited bandwidth? Semantic caching v Client maintains a semantic description of the data in its cache instead of maintaining a list of pages or tuples. v The server processes simple predicates on the database and the results are cached at the client.
Mobile Database Systems (MDS) MDS Data Management Issues Data Broadcast (Broadcast disk) A set of most frequently accessed data is made available by continuously broadcasting it on some fixed radio frequency. Mobile Units can tune to this frequency and download the desired data from the broadcast to their local cache. A broadcast (file on the air) is similar to a disk file but located on the air.
Mobile Database Systems (MDS) MDS Data Management Issues Data Broadcast (Broadcast disk) The contents of the broadcast reflects the data demands of mobile units. This can be achieved through data access history, which can be fed to the data broadcasting system. For efficient access the broadcast file use index or some other method.
Mobile Database Systems (MDS) MDS Data Management Issues How MDS looks at the database data? Data classification v Location Dependent Data (LDD) v Location Independent Data (LID)
Mobile Database Systems (MDS) MDS Data Management Issues Location Independent Data (LID) The class of data whose value is functionally independent of location. Thus, the value of the location does not determine the value of the data. Example: Person name, account number, etc. The person name remains the same irrespective of place the person is residing at the time of enquiry.
Mobile Database Systems (MDS) MDS Data Management Issues Location Dependent Data (LDD) The class of data whose value is functionally dependent on location. Thus, the value of the location determines the correct value of the data. Location Data value Examples: City tax, City area, zip code, telephone area code, etc. Example: Hotel Taj has many branches in India. However, the room rent of this hotel will depend upon the place it is located. Any change in the room rate of one branch would not affect any other branch. Schema: It remains the same only multiple correct values exists in the database. LDD must be processed under the location constraints. Thus, the tax data of Pune can be processed correctly only under Pune’s finance rule. Needs location binding or location mapping function. Location binding or location mapping can be achieved through database schema or through a location mapping table.
Mobile Database Systems (MDS) MDS Data Management Issues Location Dependent Data (LDD) Distribution MDS could be a federated or a multidatabase system. The database distribution (replication, partition, etc. ) must take into consideration LDD. One approach is to represent a city in terms of a number of mobile cells, which is referred to as “Data region”. Thus, Pune can be represented in terms of N cells and the LDD of Pune can be replicated at these individual cells.
Mobile Database Systems (MDS) MDS Data Management Issues Concept Hierarchy in LDD In a data region the entire LDD of that location can be represented in a hierarchical fashion.
Mobile Database Systems (MDS) MDS Query processing Query types v Location dependent query v Location aware query “What is the distance between Dallas and Kansas City? ” v Location independent query
Mobile Database Systems (MDS) MDS Query processing Location dependent query A query whose result depends on the geographical location of the origin of the query. Example What is the distance of Pune railway station from here? The result of this query is correct only for “here”. “What is the distance from the airport to here? ” “How far is the airport now? ”, “Where am I? ” “What is the sales tax of this city? ”
Mobile Database Systems (MDS) MDS Query processing Location dependent query Situation: Person traveling in the car desires to know his progress and continuously asks the same question. However, every time the answer is different but correct. Requirements: Continuous monitoring of the longitude and latitude of the origin of the query. GPS can do this.
Mobile Database Systems (MDS) MDS Transaction Management Transaction properties: ACID Consistency, Isolation, and Durability). (Atomicity, Too rigid for MDS. Flexibility can be introduced using workflow concept. Thus, a part of the transaction can be executed and committed independent to its other parts.
Mobile Database Systems (MDS) MDS Transaction Management Effect of Connectivity on Transaction Processing. 1. Connectivity mode. In this mode an MU is continuously connected to the database server. 2. Disconnected mode. In this mode an MU voluntarily disconnects from the server after refreshing the cache and continues to process workload locally. At a fixed time it connects and sends its entire cache to the server. 3. Intermittent connected mode. Does not have any fixed time for connecting and disconnecting an MU. Ex. , insurance agents, UPS or Fed. Ex, postal delivery, etc.
Mobile Database Systems (MDS) MDS Transaction Management Transaction fragments for distribution.
Mobile Database Systems (MDS) MDS Transaction Management Transaction fragments for distributed execution Execution scenario: User issues transactions from his/her MU and the final results comes back to the same MU. The user transaction may not be completely executed at the MU so it is fragmented and distributed among database servers for execution. This creates a Distributed mobile execution.
Mobile Database Systems (MDS) MDS Transaction Management A mobile transaction (MT) can be defined as Ti is a triple <F, L, FLM>; where F = {e 1, e 2, …, en} is a set of execution fragments, L = {l 1, l 2, …, ln} is a set of locations, and FLM = {flm 1, flm 2, …, flmn} is a set of fragment location mapping where j, flmi (ei) = li
Mobile Database Systems (MDS) MDS Transaction Management An execution fragment eij is a partial order eij = { j, j} where v i = OSj {Ni} where OSj = k. Ojk, Ojk {read, write}, and Nj {Abort. L, Commit. L}. v For any Ojk and Ojl where Ojk = R(x) and Ojl = W(x) for data object x, then either Ojk j Ojl or Ojl j Ojk. Definition 6. 6 An execution fragment, ei, satisfies a Location-Dependent Commit iff the fragment operations terminate with a commit operation and a location to data mapping exists. Thus all operations in ei operate on spatial replicas defined on the location identified by location mapping. The commit is thus associated with a unique location L.
Mobile Database Systems (MDS) MDS Transaction Management Mobile Transaction Models Kangaroo Transaction: It is requested at a MU but processed at DBMS on the fixed network. The management of the transaction moves with MU. Each transaction is divided into subtransactions. Two types of processing modes are allowed, one ensuring overall atomicity by requiring compensating transactions at the subtransaction level.
Mobile Database Systems (MDS) MDS Transaction Management Mobile Transaction Models Reporting and Co-Transactions: The parent transaction (workflow) is represented in terms of reporting and co-transactions which can execute anywhere. A reporting transaction can share its partial results with the parent transaction anytime and can commit independently. A co-transaction is a special class of reporting transaction, which can be forced to wait by other transaction.
Mobile Database Systems (MDS) MDS Transaction Management Mobile Transaction Models Clustering: A mobile transaction is decomposed into a set of weak and strict transactions. The decomposition is done based on the consistency requirement. The read and write operations are also classified as weak and strict.
Mobile Database Systems (MDS) MDS Transaction Management Mobile Transaction Models Semantics Based: The model assumes a mobile transaction to be a long lived task and splits large and complex objects into smaller manageable fragments. These fragments are put together again by the merge operation at the server. If the fragments can be recombined in any order then the objects are termed reorderable objects.
Mobile Database Systems (MDS) MDS Transaction Management Mobile Transaction execution.
Mobile Database Systems (MDS) MDS Transaction Management Serialization of concurrent execution. v Two-phase locking based (commonly used) v Timestamping v Optimistic Reasons these methods may not work satisfactorily v Wired and wireless message overhead. v Hard to efficiently support disconnected operations. v Hard to manage locking and unlocking operations.
Mobile Database Systems (MDS) MDS Transaction Management Serialization of concurrent execution. New schemes based on timeout, multiversion, etc. , may work. A scheme, which uses minimum number of messages, especially wireless messages is required.
Mobile Database Systems (MDS) MDS Transaction Management Database update to maintain global consistency. Database update problem arises when mobile units are also allowed to modify the database. To maintain global consistency an efficient database update scheme is necessary.
Mobile Database Systems (MDS) MDS Transaction Management Transaction commit. In MDS a transaction may be fragmented and may run at more than one nodes (MU and DBSs). An efficient commit protocol is necessary. 2 -phase commit (2 PC) or 3 -phase commit (3 PC) is no good because of their generous messaging requirement. A scheme which uses very few messages, especially wireless, is desirable.
Mobile Database Systems (MDS) MDS Transaction Management Transaction commit. One possible scheme is “timeout” based protocol. Concept: MU and DBSs guarantee to complete the execution of their fragments of a mobile transaction within their predefined timeouts. Thus, during processing no communication is required. At the end of timeout, each node commit their fragment independently.
Mobile Database Systems (MDS) MDS Transaction Management Transaction commit. Protocol: TCOT-Transaction Commit On Timeout Requirements Coordinator: Coordinates transaction commit Home MU: Mobile Transaction (MT) originates here Commit set: Nodes that process MT (MU + DBSs) Timeout: Time period for executing a fragment
Mobile Database Systems (MDS) MDS Transaction Management Protocol: TCOT-Transaction Commit On Timeout v MT arrives at Home MU. v MU extract its fragment, estimates timeout, and send rest of MT to the coordinator. Coordinator further fragments the MT and distributes them to members of commit set. MU processes and commits fragment and sends the updates to the coordinator for DBSs process their fragments and inform the coordinator. Coordinators commits or aborts MT. v v
Mobile Database Systems (MDS) MDS Transaction Management Transaction and database recovery. Complex for the following reasons v Some of the processing nodes are mobile v Less resilient to physical use/abuse v Limited wireless channels v Limited power supply v Disconnected processing capability
Mobile Database Systems (MDS) MDS Transaction Management Transaction and database recovery. Desirable recovery features v Independent recovery capability v Efficient logging and checkpointing facility v Log duplication facility
Mobile Database Systems (MDS) MDS Transaction Management Transaction and database recovery. v Independent recovery capability reduces communication overhead. Thus, MUs can recover without any help from DBS v Efficient logging and checkpointing facility conserve battery power v Log duplication facility improves reliability of recovery scheme
Mobile Database Systems (MDS) MDS Transaction Management Transaction and database recovery. Possible approaches v Partial recovery capability v Use of mobile agent technology
Mobile Database Systems (MDS) MDS Transaction Management Transaction and database recovery. Possible MU logging approaches v Logging at the processing node (e. g. , MU) v Logging at a centralized location (e. g. , at a designated DBS) v Logging at the place of registration (e. g. , BS) v Saving log on Zip drive or floppies.
Mobile Database Systems (MDS) Mobile Agent Technology A mobile agent is an independent software module capable of v Migrating to any node on the network v Capable of spawning and eliminating itself v Capable of recording its own history
Mobile Database Systems (MDS) Mobile Agent Technology A mobile agent can be used for the following activities, which are essential for recovery. v Centralized and distributed logging v Log carrier. A Mobile unit may need to carry its log with it for independent recovery v Log processing for database recovery v Transaction commit or abort
Mobile Database Systems (MDS) Mobile Agent Technology Possible approaches v Agent broadcast on a dedicated wireless channel v Pool of agents at every processing node v Agent migration to a required node.
Mobile Database Systems (MDS) Conclusions and summary Wireless network is becoming a commonly used communication platform. It provides a cheaper way to get connected and in some cases this is the only way to reach people. However, it has a number of easy and difficult problems and they must be solved before MDS can be built. This tutorial discussed some of these problems and identified a number of possible approaches.
Mobile Database Systems (MDS) Conclusions and summary The emerging trend is to make all service providing disciplines, such as web, E-commerce, workflow systems, etc. , fully mobile so that any service can be provided from any place. Customer can surf the information space from any location at any time and do their shopping, make flight reservation, open bank account, attend lectures, and so on. This is what the wireless technology driving us to.
Mobile Database Systems (MDS) References 1. Acharya, S. , Alonso, R. , Franklin, M. , and Zdonik, S. Broadcast Disks: Data management for Asymmetric Communication Environments. Proc. ACM SIGMOD Conf. , San Jose, May, 1995. 2. Alonso, R. , and Korth, H. Database Systems Issues in Nomadic Computing. Proc. ACM SIGMOD International Conf. on management of Data, May 1993.
Mobile Database Systems (MDS) References 3. Barbara, D. , and Imielinski, T. Sleepers and Workaholics: Caching Strategies in Mobile Environments. Proc. ACM SIGMOD Conf. , Minneapolis, May, 1994. Chrysanthis, P. K. , Transaction Processing in Mobile Computing Environment, in IEEE Workshop on Advances in Parallel and Distributed Systems, October 1993.
Mobile Database Systems (MDS) References 5. Dhawan, C. Mobile Computing. Mc. Graw-Hill, 1997. 6. Dunham, M. H. , Helal, A. , and Balakrishnan, S. , A Mobile Transaction Model That Captures Both the Data and Movement Behavior, ACM/Baltzer Journal on Special Topics in Mobile Networks and Applications, 1997. Forman, H. George and Zahorjan, J. The Challenges of Mobile Computing, Computers, Vol. 27, No. 4, April 1994. IEEE
Mobile Database Systems (MDS) References 8. Pitoura, E. and Bhargava, B. , Maintaining Consistency of Data in Mobile Distributed Environments. Proceedings of 15 th International Conference on Distributed Computing Systems. , 1995. 9. Pitoura, E. and Bhargava, Building Information Systems for Mobile Environments, Proc. 3 rd. Int. conf. on Information and Knowledge Management, Washington, DC, No. 1994.
Mobile Database Systems (MDS) References 10. Vijay Kumar, “Timeout-based Mobile Transaction Commit Protocol”, 2000 ADBISDASFAA Symposium on Advances in Databases and Information Systems, Prague, Sep. 5 -8, 2000. 11. Shaul Dar, Michael Franklin, Bjorn T. Johnsson, Divesh Srivastava, and Michael Tan, “Semantic Data Caching and Replacement”, Proc. Of the 22 nd VLDB Conference, Mumbai, India, 1996.
Mobile Database Systems (MDS) References 12. E. Pitoura and G. Samaras, “Data Management for Mobile Computing”, Kluwer Academic Publishers, 1998.
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