The DOBJ's in ADL are typically very large. For satellite images, a size of 150 MB is not uncommon and may exceed 2 GB. The collections are also large in size. The UCSB Map and Imagery Library has, for example, a collection of over two million air photos in analog form which, when scanned at 600dpi, require between 25MB and 100MB of storage space, resulting in a collection whose size exceeds 100 TB. Finally, there are a number of reasons why collections of such items must inevitably be distributed.
Three issues arising from these considerations include the difficulties faced by users in finding an appropriate item; the cost of examining or downloading large items over bandwidth-limited channels; and the provision of access to distributed sets of storage locations. The last issue is also related to questions of interoperability between DLs.
An important issue in distributed Internet applications is that there is currently no accepted standard for oid's. There are a number of alternative suggestions relating, for example, to URx's of various forms (where the ``x'' is an identifier, locator, or name) or handles [6].