Putting it together

Each of the synchronization protocols described above scale in some respects while not in others. In order to highlight these differences between them Figure 2.8 shows a set of radar diagrams that highlight the scalability characteristics of the protocols qualitatively. Each protocol is evaluated according to the following criteria: Communication - This refers to the amount of data exchanged between the synchronizing devices. Communication is usually important because of two reasons. First, it directly affects the time needed for synchronization and secondly it has a bearing on resources used while synchronizing such as the battery consumed in mobile devices. Both these factors directly affect the user. Slowsync for example transfers the entire database from the PDA to the PC for synchronization and scores poorly on this count as shown in Figure 2.8. Network size - The network size is the number of devices on the network that can synchronize. That is, the scalability of the protocol with the number of devices that synchronize with each other. The point to note about any synchronization protocol is the amount of information it has to store to synchronize with each device on the network. SyncML requires too much memory to store synchronization information. Fastsync does not allow multi-device synchronization. Robustness - The robustness here is characterized by the nature of the protocol itself - whether it is resilient to device failure in the network. Intellisync scales poorly because there is a central point of failure - a central intellisync server (see Figure 2.6) in the network whose failure will lead to disruption of the whole synchronization service for every device on the network. Computation - This is the amount of computation the devices need to do in order to synchronize. Mobile devices and PDAs generally have weak processors (though this is changing). CPISync is computationally intensive, though most of the computation can be transferred to the PC, saving the PDA slow and intensive number crunching. Memory - Memory is a precious resource on PDAs. Though the amount of memory on newer PDAs is rapidly increasingly, PDA memory is shared between the data and RAM. There is no separate secondary storage or ``hard disk" to store data. A synchronization protocol that takes up minimum real estate in the PDA memory is always a better choice. SyncML scores poorly on this count because it requires each device to keep status flags for every other device in the network. The ideal protocol with respect to the above criteria will correspond to the `full' pentagon as shown in Figure 2.2. It can possibly achieved by an amalgamation of the desirable characteristics of the protocols discussed in the previous section.

Figure 2.8: Scalability strengths and weaknesses of different synchronization protocols. An absolute qualitative scale is used in which high values indicate strengths and low values indicate weakness.