4. Data Transmitting Agent based strategy for

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1 120 Chapter 4 4. Data Transmitting Agent based strategy for Effective Cache Consistency 4.1 Architecture of Multicast-Data Dissemination Model Using Data Transmitting Agent The architecture of a mobile wireless communication network is illustrated in Fig. 4.1 and it is the motivation for the real model of the proposed system. It consists of a system with a wired network and is populated by various other components such as a number of mobile devices (Mobile Units-MUs or Mobile Hosts-MHs), multiple Original Servers and Base Stations (BS) also called Mobile Support Stations (MSS). A scenario consisting of two cells is depicted in the figure. Each cell is considered to be hexagonal shaped. The Base Stations in the corresponding cell are connected via a wired link to Mobile Switch Centre (MSC). Any BS is connected through the wired link to the Server on the communication network and serves multiple MHs through wireless channels. It maintains a Server Cache (SC) to store the data items. But storing the contents in the BS cache is not possible because many mobile devices are under the control of BS and the size of cache is small. Therefore, only data-identifiers corresponding to the most recent copy of the data-items are kept. Within this scenario, we consider read-only

2 121 communication, that is, all updates are performed at the server and the MHs are not allowed to alter the data-items individually. Even the MHs maintain a Mobile Host Cache (MHC) individually to keep some of the data-items that are retrieved from the server. A BS is usually responsible for maintaining cache consistency between the MHs and the servers, but this work introduces a novel method. In this method, one of the MH among several MHs is designated as DTA. DTA is completely delegated the responsibility of distributing the data-items pertaining to a specific group having similar interest using the multicast data dissemination strategy and thus helps to maintain cache consistency in wireless mobile communication environment. Hence, there are issues of cache consistency to be maintained at two levels i.e. SC- DTAC and DTAC- MHCs where DTAC represents the DTA Cache. This strategy assumes that there is cache consistency between SC and original servers present on the wired network. This strategy provides weak cache consistency for MHC with a remarkable cache hit probability is increased by the cache replacement algorithm discussed in chapter 6. The MHs requiring strong cache consistency can cross check the cached data item with the data entry present in the BS for each data access. In the existing strategies, the MH was probable to get a stale copy for two reasons: (1) It misses the IR of data item (2) the update time of a data item is smaller than expiration of TTL [27].

3 122 These cases can be improved by the use of the proposed scheme, where the client sends the very first request only to the server and the server redirects the query of the client to the DTA. From this point onwards, the DTA is responsible for offering data items to the client and thus the cache consistency between DTAC-MHC has to be considered. Even if the client sleeps, it can get the updated value of the data item it needs from the DTA, because the DTA always holds the latest copy of the data item. This is possible due to the fact that whenever the server broadcasts the information about the updates, the DTA checks the IR to see if there are items corresponding to all those MHs who are registered within the multicast group. The clients within the group have common data interests as that of DTA. Hence, even if the client sleeps and unexpectedly misses the broadcast, the DTA shares that copy with all such MHs. The DTA may have to store huge amount of data in the process of maintaining cache consistency. Therefore, a decision has to be made regarding which data should remain in the cache. The specific amount of time for which DTA stores the data is decided by the Cache Replacement Policy (CRP) discussed in section 6.3. The DTA multicasts the latest updates of data to all the MHs in its group periodically or on receiving a query from any of the MHs. If the data update is missed by any MH, then (i) It can send a request to DTA (ii) It can use its cached copy by checking the timestamp and Predicted-Deviation-in-Update- Interval(PDUI) that has been received in the previous multicast. The

123 success of the proposed model lies in selection of DTA. A DTA is selected from among the clients depending on the factors like lifetime of mobile client, energy and distance from base station.

4 123 success of the proposed model lies in selection of DTA. A DTA is selected from among the clients depending on the factors like lifetime of mobile client, energy and distance from base station. This set-up can conveniently provide cache consistency for dynamic data such as traffic information, sports, news, stock updates, online transaction information, etc. By incorporating sufficient security in the system, DTA can authenticate nodes on the network before providing the information to all the nodes registered under it. The IRs corresponding to dynamic data such as news, traffic information and sports are broadcast without acknowledgement whereas stock exchange information and online transaction information needs to be acknowledged.

5 DTA - based Multicast Data Dissemination for Effective Cache Consistency The proposed system is based on a simulation model depicted below: In this model, each client is a process and is divided into two subprocesses called Query Generator and internal Multicast-based Pull Manager. The client acts as a Data Transmitting Agent whenever the Multicast-based Pull Manager is active. The query generator loops continuously and requests for a page at random. To answer the query, it first checks the cache within the client, and an immediate reply follows, if a valid copy of the desired data item exists. Otherwise, it sends a request to the DTA and it multicasts the request suitable number of times. The clients are assumed to have a fixed size cache and make use of replacement Policy based on Semantic-Neural-Network Cache Replacement Policy (SNN-CRP). The DTA stores the data on behalf of the client s similar interests and multicasts to the clients on the wireless network. Since DTA downloads the complete data for the clients who are all registered with it, an enhanced version of CRP is required to manage the data on the client side. This CRP minimizes the access time further. Data in Cache can be accessed faster than that in Main memory of the client but at the same time the size of any cache is at least times smaller than the main memory and therefore the data has to be swapped

6 125 and mapped against the required data which is most frequently referred by the clients of common interests. Server Database Broadcast Messager Broadcast Channel Broadcast Program g b d a c e Push Pull Mux a c a f b a Message Manager Access Pattern Server Queue Page Reciever Client 1 DTA Client N No Bit Vector Yes Double No Dropped Query Generator Query Generator Query Generator O Full Yes Blocked Cache Thresh Filter Cache Thresh Filter Cache Thresh Filter Pull Manager Pull Manager MC-DTA Pull Manager Pull Request MC-DTA Pull Request Pull Request Uplink Channel Fig 4.2 Internal Architecture of Data Distribution in MWCN

7 Invalidation Report Generation and Query Request Invalidation Report (IR) generation strategy is one of the popular technique to achieve consistency in wireless communication environment. In this method MH is informed about the status of data whether (valid or invalid). The existing IR schemes are broadly classified into two categories: stateless and stateful server approach. The server does not keep track of contents of its mobile hosts in a stateless server approach instead the MH checks the data item for its validity prior to answering the request. The server keeps track of the contents of its MH in a stateful server approach and informs the MH whenever a data item is updated. A simple database management is required in stateless strategy which results in a huge traffic on the wireless communication network due to the query from MH to the Server. The performance of this strategy degrades with the addition of MHs (high scale factor) and the MH has to listen the channel to find the status of its cache. Thus losing energy and wasting power of the device. On the other hand the stateful method is scalable but there is server management overhead and the mobility of MH since the information of movement to a different cell should be notified to the server. In the proposed model if the data is updated at the server, BS is informed about the validity of data with the help of wired network. BS can now identify the invalid data item by using

8 127 the concept of flags. It clears the flag of DTA by placing a zero and then the IR is broadcasted to all the users and the users those receive the IR will invalidate data correspondingly. The main focus of the proposed technique is to maintain consistency of data at the DTA since it is the central point for all the disconnected and sleeping MHs. The DTA can request the updated version of the data items from the BS. In this way the DTA receives the correct copy of data. After receiving the broadcast DTA sends acknowledgement and immediately the BS resets the flag for corresponding data entry. In the next step the DTA can multicast the correct copy of the data to the MH that could not get the information due to reasons like sleep mode, network traffic congestion and switch off mode. Hence these clients need not uplink individually to get the latest updates of the data that they share with others even if they missed the broadcast before. 4.4 Cache Management at Base Station, DTA and Mobile Hosts Data Caching Each data item namely dk is identified by a unique identifier k. The Base Station Cache (SC) contains ( k, dk, tk, DTA-flag, Valid-flag, PDUI(AVI), Roam-flag, SDTA-flag ). An MHC contains (k, dk, tk, Valid-Flag, PDUI(AVI) and DTA-flag). If DTA-flag is set the corresponding MH is a DTA. On the other hand if DTA-flag = 0 then the corresponding MH is not a DTA. SDTA is necessary to select a successor DTA.

9 128 Mobile Host Cache Management MHC Management deals with maintenance of cache consistency through the Least Recently Used (LRU) Cache Replacement Policy (CRP) [24, 37, 38] or an enhanced CRP due to the fact that the MH designated as DTA has lot of data that is to be accessed by its peers. Chapter 6 discusses a new CRP namely SNN-CRP that further improves the performance of this strategy. 4.5 DTA - Roaming DTA notifies the BS about new location whenever it moves to a new cell. This situation can be resolved in two ways in (i) The BS can move the data under the DTA s charge to the corresponding successor(nominate a new MH as DTA) The BS will maintain details of the current DTA as well as the successor DTA if at all the DTA leaves the cell. (ii) BS can alternatively forward the requests from MHs to the new location where the DTA has moved. The latter method circumvents various issues and therefore the proposed strategy is based on the first technique. During the movement of the MH from one cell to another it may switch between sleep mode to awake-mode and vice-versa. An MH has to invalidate its cache while it is in the sleep state but if it is awake while roaming then the data in its cache is assumed to be valid. Also it

10 129 will be able to provide the new BS with its data and make it accessible whenever needed. 4.6 Query Description 1. The Mobile clients are PDA s, Laptops etc. The Base station (called server from here after) will identify one of the existing clients in the cell as the leader or DTA. 2. The factors for DTA selection are Energy level, Distance from server, Access rate. 3. The Clients send average of the values of the three factors to the server. 4. Server compares all three values sent by the clients and selects the highest among them and then broadcasts to the clients the port number of the client having the highest average value as the DTA. 5. Once the DTA is identified, that particular client requests the server for the latest updates. 6. After getting the latest updates from the server, the DTA creates a multicast group and multicasts the data. 7. Other clients join the multicast group and receive the multicast data 8. For further queries, clients uplink only to the DTA. 9. DTA gets updates from the server which it multicasts and clients get the updates from DTA.

11 Flowchart 1. The Clients send average of three factors (Distance from BS, Access rate, Energy) 2. BS selects DTA and announces it among the clients 3. Query comes in at client for data Check if data is available in cache? Yes 4. valid? ( check its AVI ) Yes 5. Return to User Stop Send request to BS/ DTA BS 6. Redirect request to DTA if registered user otherwise register 7. DTA multicasts updates to clients who are Group members and read data from group address and caches it Fig 4.3 Flowchart depicting sequence of activities to answer a query

12 Base Station Algorithm Begin Steps 1,2,3 & 4 are initial and are used to select the DTA 1. All the MH s sends average of the three parameters namely energy, distance, and lifetime to the BS. 2. BS receives average of the values of the three parameters from interested MH s. 3. BS Compares all three values sent by the MH s and designates the one having the highest average value among them as the DTA. Repeat the steps 4,5,6 & 7 for each time DTA gets nominated. 4. BS Broadcasts the information of the DTA to all MH s within the cell. 5. BS Broadcasts IR containing information of all those data items that are updated to all MH s. 6. If MH missed Broadcast(DTAnom,timex) If (BS gets query(x) from MH) then BS redirects the MH to DTA. 7. BS receives the query(x) from DTA for latest updates If (dx is available in BS cache ) Send Valid_data(x,dx,PDUI x, tx ) to DTA Else Send query(x) to server. 8. If (BS gets Update(x, d x,time x ) from the server) Update the database entry with ID x: as: dx = d x and tx = time x

13 132 end Client Algorithm Begin 1. If (MH receives query(x) message) If (dx is present in MH s cache and has valid AVI) Answer the query. 2. If dx is not present in MH s cache with a valid PDUI Send query(x) to BS 3. MHs register with DTA after the nomination of DTA to get specific service or updates from the DTA. 4. If dx is not present in MH s cache with a valid PDUI Send query(x) to DTA. 5. If (dx is valid in cache of DTA) Receive Multicast of dx periodically from DTA else DTA Sends query(x) to BS. end DTA Algorithm

14 133 Begin 1. If (DTA gets query(x) from MH) If (dx is valid in cache of DTA) Multicast dx to all clients in the multicast group else Send query(x) to BS i.e. uplink to BS on behalf of MH 2. The MHs interested in the same information need not send query(x) message to the BS later. They obtain updates from DTA only. end. The main idea in the proposed system is to maintain cache consistency between the DTA and the nodes in the group.

FLEXIBLE DATA DISSEMINATION STRATEGY FOR EFFECTIVE CACHE CONSISTENCY IN MOBILE WIRELESS COMMUNICATION NETWORKS

FLEXIBLE DATA DISSEMINATION STRATEGY FOR EFFECTIVE CACHE CONSISTENCY IN MOBILE WIRELESS COMMUNICATION NETWORKS Kahkashan Tabassum 1 Asia Sultana 2 and Dr. A. Damodaram 3 1 Department of CS & IT, Maulana