Data Caching Techniques For Mobile Adhoc Networks Computer Science Essay

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Data caching can remarkably improve the efficiency of information access in a wireless ad hoc network by reducing the access latency and bandwidth usage. The cache placement problem minimizes total data access cost in ad hoc networks with multiple data items. The adhoc networks are multihop networks without a central base station and are resource constrained in terms of channel bandwidth and battery power. By data caching the communication cost can be reduced in terms of bandwidth as well as battery energy. As the network node has limited memory the problem of cache placement is a vital issue. This paper attemptes to study the existing data caching techniques and their suitability in mobile adhoc networks.

Index Terms

Data Caching, MANET, Cooperative caching, cache consistency, cache placement.

1.Introduction

Improvements in wireless networks and small device processing power have increased mobile computing usage recently. New wireless network and telecommunication standards allow devices to communicate using higher speeds with longer range and more reliability.The adhoc networks have self organizing capability and can be effectively used where other technologies either fail or cannot be deployed effectively. This situation makes extensive use of mobile computing more attractive. It also gives a number of problems like message loss, unpredictable disconnections of mobile devices, and network partitioning. However, in order to offer high quality and low cost services, several technical challenges still need to be addressed in ad hoc networks.

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Most of the research on MANETs have focused on the development of Media Access Control protocols and routing protocols to increase connectivity among mobile hosts in a constantly varying topology. Because reliable and efficient access to information is the ultimate goal of networking, the design of higher layer protocols to meet user requirements is necessary. Although routing is an important issue in ad hoc networks, other issues such as data access are also very important since the ultimate goal of using such networks is to provide information access. In adhoc networks, due to frequent network partition, data availability is lower than that in traditional wired networks. With data caching the data retrieval performance in mobile networks has increased tremendously. Since the requests are served from the local cache the need of data transmission over wireless links is reduced and the data access delay is reduced.

Cooperative Caching for MANETs is the coordination of several nodes to share a cached data in an efficient way for all. In caching-based systems the nodes cache data or path to a data item in order to increase data availability and reduce query delay times.

As mobile clients in ad hoc net-works may have similar type of actions and common interest, cooperative caching, can be used to reduce the bandwidth and power consumption. Caching reduces communication cost of the system, which consequently results in the reduction of bandwidth and energy usage.

The remainder of the paper is structured as follows. Section 2 discusses the challenges in Data caching and Section 3 presents cooperative caching design and architecture .Section 4 surveys some related work carried out in the field . Section 5 is devoted for performance evaluation of different caching techniques Section 6 concludes the paper.

2.Challenges faced in Data Caching for MANET

Autonomous and infrastructure-less: MANET does not depend on any established infrastructure or centralized administration. Each node operates in distributed peer-to-peer mode, acts as an independent router and generates independent data. Network management has to be distributed across different nodes, which brings added difficulty in fault detection and management.

Multi-hop routing: No default router is available, every node acts as a router and forwards each other's packets to enable information sharing between mobile hosts.

Frequent disconnection of mobile hosts: Mobile hosts often get disconnected from the network due to various factors like power failure or their mobility. Servers which hold the data cannot provide services if they are disconnected from other mobile hosts.

Dynamically changing network topologies: In mobile ad hoc networks, because nodes can move arbitrarily, the network topology can change frequently and unpredictably, resulting in route changes, frequent network partitions, and possibly packet losses.

Variation in link and node capabilities: Each node may be equipped with one or more radio interfaces that have varying transmission/receiving capabilities and operate across different frequency bands.

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3. Cooperative Caching Design and Architecture

In a cooperative cache-based data access architecture mobile nodes cache the data or the path to the data to reduce query latency and to enhance data avaliability. Here the data is stored in a distributed way and is shared in the entire network. Cooperative caching takes advantages of local caching to facilitate data access by allowing geographically neighbouring hosts to share their caching contents. By doing so long-distance data accesses from the data center can be reduced. Here each node has to know wether there any node in the vicinity that has cached the data it requires and where it is, if any. These schemes allot every mobile node a certain amount of cache space. In addition to the local cache service, the network's nodes cooperate with their neighboring nodes on server data requests. In this way, when a request for a data item comes it can be served from a local or neighbouring cache without accessing the remote Data Center, or the gateways to the Internet.

Cache Replacement

Cache consistency

Cache management

Routing Protocols

Cooperative cache Services

Application Layer

A cooperative cache service should have the following modules.

Fig:1 Cooperative Caching in Mobile Adhoc Networks

A good cooperative cache management technique should have the following components.

1.Cache management module: This includes cache admission control module, which determines whether a received data item should be cached or not, and cache discovery module to find and deliver requested data items from the neighboring nodes .Since the network node has limited memory, it can only cache multiple data items subject to its memory capacity. The objective of cache management module is to minimize the overall access cost.

2. A cache consistency module :When the nodes in adhoc network caches data,problems may arise with inconsistent data. Maintaining cache consistency means ensuring that each node caching the data item is aware of the data update when the source data changes.

3. A cache replacement module: This module helps in deciding the items to be removed in the existing cache to make way for new ones. The hit ratio of a cache describes percentage of accesses that result in cache hits ie, how often a searched-for item is actually found in the cache. Replacement algorithms should keep track of the usage information to improve the cache hit ratio.

4. Data Caching Approaches for MANETS

To increase the data accessibility in ad hoc networks, several data caching schemes have been proposed in the literature.These schemes allocate every mobile node a certain amount of cache space. In a simple data caching scheme, a node that requests data always caches the received data and consecutive requests for the data are served from the cache until the cache expires. In case of a cache miss, it has to get the data from the data center which will increase the response time for the requests. In cooperative caching strategy, mobile hosts share their cache contents with each other to reduce both the number of server requests and the number of access misses.

In [1], three caching schemes, Cache Data, Cache Path and Hybrid Cache, were proposed to provide efficient data access in MANETs.In CacheData scheme the intermediate nodes in the routing path between the source and the destination caches the data item when it finds that the data item is popular, i.e., there were many requests for the data item, or it has enough free cache space.There is no cooperative caching among the mobile hosts. Each mobile host idependently performs the caching tasks such as placement and replacement. Cache Path works in a similar manner to Cache Data, but the forwarding nodes cache data path information for future use, instead of just the data. Hybrid Cache takes advantage of the above two schemes benefits to further improve caching performance. Here a mobile node caches the data or path based on some criteria. These criteria include the data item size and the Time to Live TTL. If data item size is small , CacheData is used since thedata item needs only a small portion of the available cache; otherwise, CachePath is used. If TTL is small, CacheData is used because the data item might soon be invalid,using CachePath can result in changing the path frequently.Here the cache consistency is ensured by a time to live (TTL) mechanism. A routing node considers a cached copy valid if its TTL hasn't expired.If theTTL has expired ,the node removes the cached data item.

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A caching scheme for Internet based MANETs was presented in [2]. In this scheme, an Aggregate Caching mechanism is proposed. Here each mobile node stores data item in its local cache and these local caches are aggregated to unified large cache. The data items can be received from local caches of the mobile nodes or through an a access point or a data center connected to the Internet. When a node needs a data item, it broadcasts a request to all of the adjacent nodes. The node which has the data item in its local cache will send a reply to the requester; otherwise, it will forward the request to its neighbors. Thus a request is flooded to the other connected nodes and eventually acknowledged by an access point or some nodes with cached copies of the requested data item. A broadcast based approach, called Simple Search (SS) algorithm, is implemented on the top of existing routing protocols, to locate the requested data items.

In [3], COOP a mobile node uses three schemes; adaptive flooding, profile-based resolution or forwarding node-based resolution, to search for requested data items in the network when it fails to find them in the local cache. In adaptive flooding, a node uses constrained flooding to search for items within the neighborhood. In profile-based resolution, a node uses the past history of received requests. In forwarding node-based resolution, forwarding nodes caching the requested item, reply to the requests instead of forwarding them to the remote data source.

In Zone Cooperative caching[4] each mobile client belongs to a zone and has a cache to store the frequently accessed data items. In this scheme each node has a local cache and these nodes share cache with other nodes that are in a zone.Zone consists of mobile hosts that are accessible in one-hop. So mobile hosts belonging to the zone of a given host forrms a cooperative cache system .The data items in the cache satisfy not only the client's own requests but also the data requests passing through it from other clients. When there is a miss in the local cache, the client first searches the data in its zone.If the data is found it is retrived otherwise the request is forwarded to the next client that lies on a path towards server. A cache replacement policy based on least VALUE has been used. VALUE of a data item is computed by considering the distance, size of the data coherence and popularity. A cache admission control based on distance and a cache consistency based on time to live parameter is also considered in this scheme.

The concept of Neighbor Caching (NC) [5] is to deploy the cache space of inactive neighbors for caching tasks. In this method a node uses its neighbor temporarily as secondary storage. The basic operations of NC are as follows. When a node takes data from a remote node, it puts the data in its local cache for reuse. A least recently used replacement algorithm is used to remove the data from the cache.When the data is removed it is stored in the idle neighbor node's storage. If the node needs the data again, it will retrive it from the near neighbor that keeps the copy of data. This scheme exploits the available cache space of neighbor to improve the caching performance. In order to find a neighbor node with the highest possibility to keep the data for a long time a ranking based prediction algorithm is used.

In [6] a scheme called Group Caching (GC) in which each node maintains a localized and group caching status of its 1-hop neighbors is proposed. Whenever a node wants to find dataor place data in cache the caching status of the group is checked. Each mobile host and its 1-hop neighbors form a group by using the "Hello" message mechanism. In order to utilize the cache space of each node in a group, the nodes periodically send their caching status in a group. By checking the cache status of the group,each mobile host knows the remaining available cache space of other mobile host in a group,the IDs and timestamps of their cached data object.Thus, the mobile host selects the proper group member for cache placement and replacement. Since the caching status of the group is recorded redundancy can be avoided and more cache space can be utilized.Each mobile host can store more different data objects in a group and thus increases the data accessibility.

In [7], a Cluster-based Cooperative Caching (COCA) scheme that uses a cross-layer approach to improve caching performance is proposed. COCA is a middleware which implements cross-layer optimization for cooperative caching. It consists of a stack profile, clustering, information search, cache management and prefetching modules. The stack profile module provides cross-layer information exchange, the clustering module for cluster formation and maintenance, the information search module for locating and fetching the data item requested by the client. The cache management module includes a cache admission control , cache replacement and cache consistency. The prefetching and cross-layering improves the performance significantly.

In [8], a Resource Efficient Adaptive Caching which uses an adaptive cache distribution and replication is implemented. The adaptive cache replication scheme known as Tidal Replication, considers a global demand for data for making replication decision.Here replication is done by caching the data items that consume significant portion of bandwidth for a control node towards a Dominant Request Path ie, the network path responsible for majority of request for a particular data item at the given node. When the demand wanes, the replicas are de-allocated and the burden for the data item returns to the control node. The cache distribution scheme, Magnetic Distribution redistributes data items according to their demand, balancing the demand in a place of equilibrium, in order to reduce hop count and response time.

5. Performance Comparison

Different schemes reviewed in the previous section provide their own terms of evaluation. Most of the evaluation metrics and parameters used are specific to each scheme and hence they mostly do not allow for a performance comparison. However, the performance of a caching scheme mostly depends on the parameters like cache hit ratio, average latency and hop count almost all the caching schemes have used these parameters for their performance evaluvation. Here we also compares the cache services offered by different schemes. Table 1 summarizes the features of the different approaches. R-Reduced; I-Increased;P-Present and X-Not considered.

Table. 1. Features of existing Data Caching Schemes in Mobile Adhoc Networks

Caching Scheme

Avg Query latency

Hop Count

Cache Hit Ratio

Cache admission control&Discovery

Cache Replacement

Cache Consistency

Cache Data

Cache Path

Hybrid Cache

Aggregate Cache

COOP

Zone Cooperative

Neighbour Caching

Group Cache

COCA

Adaptive Caching

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

R

I

X

X

I

X

I

X

I

X

P

P

P

P

P

P

P

P

P

X

P

P

P

P

X

P

X

X

P

X

P

P

X

6. Conclusion

In this paper we reviewed the challenges and basic concepts behind Data Caching in MANETs so that appropriate strategy may be selected for different applications. Existing techniques were compared based on how they addressed the identified issues. They all provide different mechanisms and techniques to tackle different challenges in design and development of various Data Caching for MANETs. It is observed that the different schemes referred above produce varying results with respect to the parameters like latency, hop count and cache hit ratio depending on the cache size, node density etc. None of the existing works give a combined and complete solution for efficient cache placement, discovery, replacement and consistency.