Synchronization is typically used in the following three physical ways or modes: local, pass-through, and remote. Each synchronization mode has its specific challenges, which are described below.
This is currently the most common way to use synchronization. A typical scenario, shown in Figure 1-5, has a user synchronizing data from the PC or laptop on his desk with a PDA that is connected to the PC or laptop through a serial, Universal Serial Bus (USB)™, infrared, or Bluetooth™ connection.
Figure 1-5. Local synchronization
An application running on the PC acts as the synchronization server. The data is usually retrieved from and stored in another application on the computer, such as Lotus Notes® or Microsoft Outlook®. This scenario requires the user to first synchronize his desktop application with the server (e.g. Lotus Domino™ or Microsoft Exchange®) to retrieve the latest updates from the server before synchronizing the PDA with the desktop.
This scenario can also be called "router." In this scenario, the synchronization client is connected locally to a computer, similar to local synchronization. But in this usage mode, the local computer is used as a router to pass the synchronization requests to another server acting as the "true" synchronization server. In this usage mode, shown in Figure 1-6, a device can synchronize with a remote server thanks to the pass-through service on the local computer bridging the technologies, even if this device only supports infrared or Bluetooth connectivity.
Figure 1-6. Pass-through synchronization
While in the office, the device can use the faster and cheaper network connection between a desktop computer and the server. A drawback is that the synchronization is now done with a remote server, even while in the office, which generates some additional network traffic compared to the local synchronization case. While on the road, the user uses the remote synchronization to directly synchronize with the synchronization server, as described in the next paragraph.
Remote synchronization, shown in Figure 1-7, is used if the synchronization client is connected to the synchronization server using a network connection. This might be a local area network (LAN) or a wireless network like GSM (Global System for Mobile Communications) or UMTS (Universal Mobile Telecommunications System).
Figure 1-7. Remote synchronization
With wireless networks like GSM, though, the data exchange rate available is obviously not as high as when connected to the synchronization server by a LAN. The advantage of a PDA and/or a mobile phone is that the user can access the server and synchronize the data from almost everywhere in the world.