Preservation of bits ensures that the values and order of digital bits is correct, undamaged and readable. The bits are the same as when they were received, and by managing them they will be available in the future. If the bits are changed, in the best case the object will appear different, and in the worst case the object will be unreadable in the future. Fixity can only ensure that the bits are the same; it is important along with bit preservation to plan for the logical preservation as well to make sure that the file can be rendered.
Bit security is based upon assessing the risks to the objects and then protecting the objects from events that will change the bits. The more you protect the bit integrity of the files, the more confidence you have that the files are accurately preserved.
The traditional method of file security is to make multiple copies. Those copies must be checked regularly for errors that would then need to be corrected. All copies are equally important and must be checked. You must also make sure that the copies will not be affected by the same failure event. It that happens and the error is not discovered, you could lose all copies. This is part of the risk assessment process, and you should consider the following items in order to make sure at least one copy is intact:
- Number of copies stored: The more copies stored, the more likely that at least one copy is intact
- Frequency of checking copies: The more often copies are checked, the more likely that at least one copy is intact
- Copies are stored independently, such as type of hardware, organizational custody, or geographical location, the greater chance that the copies won't be affected by the same problem
Media migration: Storage media do not last forever, so the digital content must be migrated regularly. It is important that the different copies are not exposed to the migration process at the same time.
Other considerations of bit preservation include understanding the cost; determining the level of object security desired; confidentiality of materials. The Royal Library, the National Archives and the National Library are working together to provide Bitmagasinet, a shared hosted service is to store data by cooperating with each other, with copies on different media, in different locations and at different organizations.