for most navigational functions it is important to have in the vehicle an up-to-date map database, and for some functions it is critical, especially those related to active safety. a common strategy is to transfer update information to the vehicle whenever it becomes available over a wireless channel. the wireless channel might be bi-directional, such as wifi and cellular phone, broadcast, such as satellite radio, fm sub-carrier or atsc datacasting, or a combination of both. in any case it would be impractical or extremely inefficient to transmit the entire new database to replace an existing version, since it is likely to be several gigabytes in size.