Twophase combined QoSbased handoff scheme

Learning objectives

After completing this chapter, you are able to

• demonstrate an understanding of a WATM network;

• explain hard and soft handoff;

• explain forward and backward handoff;

• explain combined QoS-based path optimization scheme; and

• explain different types of path optimization schemes.

Practice problems

12.1: What are the major components in a WATM network?

12.2: How is a hard handoff executed?

12.3: How is a soft handoff executed?

12.4: How is a forward handoff performed?

12.5: How is a backward handoff performed?

12.6: What is a handoff using full reestablishment?

12.7: What is a handoff using multicasting?

12.8: What is a handoff using connection extension?

12.9: What is a handoff using partial reestablishment?

12.10: What is a handoff using two-phase protocol?

12.11: What is a combined QoS-based path optimization scheme?

12.12: What are the types of path optimization schemes?

Practice problem solutions

12.1: There are two major components in a WATM network: a radio access layer providing high-bandwidth wireless transmission with appropriate MAC, DLC, and so on and a mobile ATM network for interconnection of BSs (APs) with appropriate support of mobility related functions, such as handoff and location management.

12.2: In a hard handoff, the MT switches the communication from the old link to the new link. Thus, there is only one active connection from the MT at any time. There is a short interruption in the transmission. This interruption should be minimized in order to make the handoff seamless.

12.3: In a soft handoff, the MT is connected simultaneously to two APs. As it moves from one cell to another, it 'softly' switches from one BS to another. When connected to two BSs, the network combines information received from two different routes to obtain better quality. This is commonly referred to as macrodiversity.

12.4: In a forward handoff, after the MT decides the cell to which it will make a handoff, it contacts the BS controlling the cell. The new BS initiates the handoff signaling to link the MT from the old BS. This is especially useful if the MT suddenly loses contact with the current BS.

12.5: In a backward handoff, after the MT decides the cell to which it attempts to make a handoff, it contacts the current BS, which initiates the signaling to handoff to the new BS.

12.6: A handoff using full reestablishment occurs in a connection-oriented wireless environment, in which virtual circuits are established from the source to the destination. The data follows the path that has been set up, and an in-order delivery is guaranteed. If a handoff is to occur, the old virtual connection is torn down, and an entirely new virtual circuit is set up from the current source to the current destination. Since both ends are explicitly involved, this handoff scheme is not transparent. Severe traffic interruptions are experienced and hence this scheme is not recommended.

12.7: A handoff using multicasting is used in both the connection-oriented and connectionless scenarios. In the case of a WATM environment, multicasting is used to establish links to all BSs that are neighboring the BS that is currently controlling a MT. Subsequently, in whichever direction the MT moves, a handoff path has already been established. Also, since the data is being multicast, it continues to flow without any interruption. This scheme ensures a lossless and seamless handoff. However, since data is being multicast to the entire set of nodes most of which is unused, bandwidth is being utilized very inefficiently. Also, if an MT is at the edge of two cells, it is very likely that it might get two copies of the data packets. This leads to other complications like BS synchronization.

12.8: The basic idea of the handoff using connection extension scheme is that the local paths are more affordable than the global paths. When an MT migrates from one BS to another, the old BS extends the connection to the new BS. The obvious disadvantage of this method is that the new path to the MT is not an optimal path.

12.9: A handoff using partial reestablishment uses the concept of a COS. The new BS does a partial reestablishment of the connection by opening a connection to the COS. This way it attempts to reuse as much of the existing connection as possible. The old partial path is then torn down and the resources are released. Buffering is done at the COS.

12.10: A handoff using two-phase protocol combines the connection extension and partial reestablishment schemes. The two-phase handoff protocol consists of two phases: path extension and path optimization. Path extension is performed for each interswitch handoff. Path optimization is activated when the delay constraint or other cost is violated.

12.11: A combined QoS-based path optimization scheme activates the path optimization when the delay constraint and path extension hops exceed a maximum value.

In the combined QoS-based path optimization algorithm, when there is a handoff request, the path extension is done first to ensure a seamless handoff. After the path extension, the new path's delay is the sum of the previous path delay and the delay between the new source node and previous source node.

12.12: Path optimization schemes can be classified into four types: QoS-based, network-based, time-based, and handoff-based. QoS-based path optimization schemes trigger path optimization of each mobile connection on the basis of its current QoS measures.

Mobile Telecommunications Protocols For Data Networks. Anna HaC Copyright © 2003 John Wiley & Sons, Ltd.

ISBN: 0-470-85056-6

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