Disadvantages Push And Pull Systems

Obeying nature's rule that "nothing is prefect," neither push nor pull alone can obtain the best-case performance. Both push and pull scheduling schemes have their own advantages and disadvantages. Push-based algorithms suffer when the number of data items in the system is large because then the broadcast cycle is long too, and hence the expected access time of the system is also large. For less-demanded items, average access time could be as high as half of the length of the broadcast cycle. A push scheduling is not only affected by the uplink channel constraints, it suffers from wasting resources in downlink wireless channels by repeatedly transmitting the less popular items. Figure 1.8 shows a disadvantage of push-based scheduling where the server broadcasts each item on a scheduled basis, no matter if that item is popular or unpopular among the clients. Thus for a huge set of data items the average length of the push-based broadcast schedule becomes quite high leading to a very high

Server

Clients

FIGURE 1.9 Disadvantage of a pull-based system.

Server

Clients

FIGURE 1.9 Disadvantage of a pull-based system.

Bluetooth Disadvantages

Hybrid Scheduling

FIGURE 1.10 Intuition for hybrid scheduling.

Push Scheduling

Pull

Scheduling g

Hybrid Scheduling

FIGURE 1.10 Intuition for hybrid scheduling.

average waiting time for the clients. Pull-based scheduling is an inefficient scheduling scheme when the system is highly loaded and the data items are very popular. Even though pull-based data dissemination scheme is performed on the basis of explicit clients' requests, such client requests are bounded by the uplink resource (bandwidth) constraints resulting in a bottleneck at the uplink. This is depicted in Figure 1.9.

Hence, neither push nor pull alone can achieve optimal performance [32]. These inefficiencies of push and pull were the main motivation for the hybrid approach of data scheduling. The thought was that if both push and pull scheduling were mixed in such a way that the benefits of both could be availed, then better performance could be gained. So, researchers started with the hybrid approach. This is shown in Figure 1.10.

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