PACKET WRITTEN CDS

3/16/2002

Packet Written CDs: CD packet writing is a technique for providing random access to (re)writable CDs. There are two kinds of packet written CDs. In one format, files are written in variable length packets -- one file per packet. In the other format, the CD is broken into fixed length packets similar to a floppy disk. Files are written to a series of not necessarily sequential or contiguous packets. The format used is Universal Data Format -- UDF.

Variable length packet written CDs can not be overwritten. But they can be extended if the CD isn't full. Fixed length packet written CDRWs can be overwritten. They format out to around 500mb rather than the 640mb of non-packet written CDs. Packets written on CDRWs using fixed length packets can be randomly accessed, overwritten and added to whereas data written on conventional CDRWs can not be overwritten except by rewriting the whole CDRW. Operating Systems do not provide drivers for packet written CDs. In order to read them one must use the packet CD writer (e.g. EZ CD Creator) or must use a special reader linked into the OS. As a result, a packet written CD may well be unusable even though the hardware is capable of reading it.

Adaptec's DirectCD seems to be the standard implementation for fixed length UDF. Some competing, incompatible, formats were used for a while. Variable length UDF is very close to the conventional ISO9660 CD format and can usually be "closed" to ISO9660 format by writing a Table Of Contents. It is (just barely) possible to create a variable length packet CDR or CDRW that can't be closed because it contains more data than can be accommodated by the Table Of Contents. Until the CDR(W) is closed, there is no TOC and a conventional CDROM driver can't make sense of the disk.

http://www.mrichter.com/cdr/primer/udf.htm

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