ABSTRACT:-
GPRS (General Packet Radio Service) is a packet based communication service for mobile devices that allows data to be sent and received across a mobile telephone network. GPRS is a step towards 3G and is often referred to as 2.5G. As the wireless technology evolves, one can access the Internet almost everywhere via many wireless access networks such as wireless LAN and GPRS. People would like to use the wireless networks with high data rate, large coverage and low cost. Some networks such as GPRS can provide large coverage, but they only provide low data rate; some networks like wireless LAN can provide high data rate, but the access points are not widely deployed. None of the wireless
Networks can meet all requirements of a mobile user. Heterogeneous networks solve parts of the problem. In heterogeneous networks, users can roam among different kind of networks such as 802.11 wireless LAN and GPRS through vertical handoffs. But in heterogeneous networks, each kind of wireless networks provide different quality of services. Users roaming among the wireless networks will suffer enormous change of quality of services. The paper proposed three access network selection strategies that keep mobile users staying in the wireless networks with higher quality services longer and thus improves the average available bandwidth and decreases the call blocking probability.
Introduction:
IEEE 802.11 wireless LAN is the most popular high data rate wireless network. But the coverage of an access point is too small, and the access points are not widely deployed and well organized. Users cannot receive the WLAN services ubiquitously and have to change their settings when they are in different WLAN.
On the other way, cellular systems like GPRS can provide services almost everywhere, but they cannot have a data rate like WLAN. Vertical handoffs in the heterogeneous works let users can get service from both GPRS and WLAN. Users who leave the coverage of an access point can vertically handover to the GPRS networks, and the Internet service. IEEE 802.11g has a 54 Mbps transmission rate while GPRS has only 171 kbps for optimal transmission rates for the users will not be terminated. The paper proposes new
Mobility strategies to extend the time mobile hosts staying in higher quality networks in the heterogeneous network environment by using ad hoc network. In an ad hoc network, mobile hosts relay messages for other mobile hosts. Such characteristic helps to extend the service range of an access point while there are mobile hosts available to form a path that are able to relay messages to the access point.
Interworking mechanisms:-
The integration of WLAN into GPRS will provide users in “hot-spot” areas to use the high-speed wireless network, and when outside a hot-spot coverage area, use the cellular data network. This is however not simple to implement as it must provide services such as: session continuity, integrated billing and authentication between networks, inter-carrier roaming, and most importantly, provide a seamless user experience.
Some Existing coupling methods:
1. Tight coupling methods:
In general, the proposed tight coupling architecture provides a novel solution for internetworking between 802.11 WLANs3 and GPRS, and features many benefits, such as:
• Seamless service continuation across WLAN and GPRS. The users are able to maintain their data sessions as they move from WLAN to GPRS and vice versa.
• Reuse of GPRS AAA.
• Reuse of GPRS infrastructure (e.g., core network resources, subscriber databases, billing systems) and protection of cellular operator’s investment.
• Support of lawful interception for WLAN subscribers.
• Increased security, since GPRS authentication and ciphering can be applied on top of WLAN ciphering.
• Common provisioning and customer care.
2.Loose Coupling Methods:
Loose coupling is another approach that provides internetworking between GPRS and WLAN. As can be seen, the WLAN network is coupled with the GPRS network in the operator’s IP network. Note that, in contrast to tight coupling, the WLAN data traffic does not pass through the GPRS core network but goes directly to the operator’s IP network.
Disadvantage of Existing Methods:
• After coupling between WLAN and GPRS Network cannot easily support third-party WLANs.
• Throughput capacities are very less.
• More important, tight coupling cannot support legacy WLAN terminals, which do not implement the GPRS protocols.
• Cost is more to implemententation.
The Proposed Strategies:
In the paper, the heterogeneous network is composed of WLAN, ad hoc WLAN and GPRS network. With the use of ad hoc WLAN network, mobile hosts can access Internet with others’ relaying to a WLAN AP. In original heterogeneous network environment, mobile hosts will prefer WLAN. But if no WLAN AP available, the mobile hosts will handover to the GPRS networks to keep the connections alive. With the use of ad hoc WLAN, mobile hosts have another alternative when there is no WLAN AP available. They can choose ad hoc WLAN. However, there may be more than one mobile host can relay packets to more than one access points. Mobile hosts may select one of the best relay mobile hosts, or decide not to use the ad hoc network. One of the best relay mobile hosts, or decides not to use the ad hoc network.
Mobile wireless network is the infrastructure less mobile network, commonly known as an ad hoc Network. Infrastructures less networks have no fixed routers. All nodes are capable of movement and can be connected dynamically in an arbitrary manner. Nodes of these networks function as routers which discover and maintain routes to other nodes in the network.
Selection strategies:-
Making such decisions will be a problem, and three selection strategies are proposed. The selection strategies are detailed below,
A. Fixed hop counts (FHC)
In the strategy, the ad hoc route cannot be longer than n hops; the mobile host first finds the access points, if no access point available, the mobile host will try to find a mobile host has a route shorter than n – 1 hops away from an access point. If more than one route shorter than n – 1 hops, select the shortest one. If more than one route is the shortest hop counts, select the AP has same IP range with itself. If no AP has same IP range, select arbitrary one. If no route is shorter than n – 1 hops, try to select GPRS network.
B. Any available route (AAR)
In the strategy, any ad hoc route will be chosen if there are no higher service networks available, the mobile host will try to find a mobile host that has a shortest route to an access point. If no route is available, try to select GPRS network.
C. Bandwidth pre-evaluation (BPE)
In the third strategy, the network status will be measured before selection; ad hoc networks will be select only if they have a higher quality of service than the GPRS network. In the proposed strategy, when a mobile host tries to initiate a call, it will look for WLAN AP, ad hoc WLAN relay host and GPRS networks sequently. And if none of the network can be selected, the connection is rejected. When a user leaves the coverage of a GPRS cell or an access point, a handoff occurred. The cases are more complicated than call initiation, and we discussed the three cases separately.
Call initiation in network:-
In the proposed strategy, when a mobile host tries to initiate a call, it will look for WLAN AP, ad hoc WLAN relay host and GPRS networks sequently. And if none of the network can be selected, the connection is rejected. When a user leaves the coverage of a GPRS cell or an access point, a handoff occurred. The cases are more complicated than call initiation, and we discussed the three cases separately.
A. Handoff from WLAN:-
First, try to find another WLAN AP. If no other AP is available, try to select an ad hoc WLAN network. And if no ad hoc WLAN is qualified, try to select the GPRS network. Finally, if no GPRS network is available, the connection will be forced terminated.
B. Handoff from ad hoc WLAN:-
First, try to find a WLAN AP. If no AP is available, try to select an ad hoc WLAN network. And if no ad hoc WLAN is qualified, try to select the GPRS network. Finally, if no GPRS network is available, the connection will be forced terminated.
C. Handoff from GPRS:-
First, try to find another GPRS base station. If no other base station is available, try to find a WLAN AP. If no AP is available, try to select an ad hoc WLAN network. And if no ad hoc WLAN is qualified, the connection will be forced terminated.
Conclusions:-
Proposed strategies can reduce the times a user changes his/her IP address. The advantage disappears with the increase of mobility, because the route cannot be maintained in a high mobility network. Here, three mobility strategies are proposed to improve the service quality for mobile hosts in heterogeneous networks by using ad hoc routing. Using the proposed strategies, the average available bandwidth can be two times more than no strategy applied, and the request-blocking rate can have a 94% reduction at most and a 50% reduction in average. The change of IP address is a serious problem for mobile users, and the proposed strategies can have a 9% improvement in the times of IP address changing. It helps to ease the impact of the mobile IP protocols to the real time applications.
However, the drawback of the ad hoc networks is inherited in the proposed strategies. The handoff opportunity rises due to the unstable of relaying host. This can be prevented by using an ad hoc routing protocol that considered the stability or reducing the length of an ad hoc route.
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