Optical Media Overview

See also Volumes.

Capacities

See wikipedia list of capacities of optical discs.

Assuming Blue-Ray will not be cheap enough for either drives or media in near future and some other potential formats are also not relevant, the largest single sided media likely to be used is DVD 9 (12 cm dual layer) at 7.95 GiB (8.54 GB).

An 8 GiB FAT32 partition would ensure such media can be flexibly handled, thus also supporting possible manipulation of double sided variants, one side at a time. Note that FAT16 does not support volumes larger than 4GB.

However currently DVD 5 (12 cm single layer) is more economical (cost per byte) and requires only 4.37 GiB (4.70 GB).

So either 4.5 GiB or 9.5 GB could be good choices for a spare FAT32 partition, with 9.5 GB supporting two DVD 5 copies or any of the double sided options that are not also dual layer.

Unfortunately FAT32 itself does not support individual file sizes exceeding 4 GiB so a full UDF or ISO 9660 file for either DVD 5 or DVD 9 cannot be copied directly and mounted from there.

Nevertheless reserving a partition for FAT32 should help with interchange. When larger files need to be stored in that partition it can be reformatted. When the space reserved is needed elsewhere it can be recovered using volume management. When compatability with FAT16 or less is needed (eg for interchange via USB drive or SD/MMC card), the partition can be shrunk.

Note that the most economical flash is currently between 8 GiB and 16 GiB. Much larger hard drives are quite relevant as interchange and backup media.

Compatability

For more flexible interchange ensure that devices using USB flash drives or flash cards implement FAT32 (not just FAT16) and also support partitioning. Flash card readers should support SDHC, not just SD. This includes support for MMC and also supports smaller mini and micro cards using adaptors. MMC cards may be more economical as media but the card reader should still support SDHC as well.

Actual files for interchange should be limited to 4GiB to avoid potential hassles, even though those hassles may be solvable. This policy would not result in any significant waste of space on either DVD 5 or DVD 9 media.

Some systems using optical drives may only support files up to 2 GiB for ISO 9660. Some may only support 8cm media and/or only support CDROM.

For 8cm CDROM the maximum file size would need to be less than 0.180 GiB (0.194 GB). This is unlikely to be relevant in practice, but would not be inconvenient or inefficient to adopt as a maximum fragment size for DTN networks. It is well below minimum practically available flash cards and USB thumb drives at 256 MiB (although there might be some still available at 128 MiB and the theoretical lower limit is 32 MiB.

A more likely to be relevant interchange limitation would be old 12 cm 74 minute CDROM drives at 0.635 GiB (0.682 GB). Keeping under that would also ensure compatability with 1 GiB flash drives and flash cards, which are still quite common.

For rewritable CDs the actual file space may be only about 500MB.

In practice an hour of MP3 audio is only 12 MB.

So a convenient fragment limit could be 128 MiB, (~ 134MB) with most bundles not likely to be fragmented at all.

Precise calculations will be needed taking into account file systems, spared sectors and bundle index overheads.

Meanwhile approximate fragment capacities of interchange media would be as follows:

CDROM 8cm

1

CDROM  74'

5

Flash 1 GiB

7

Fat32 file

31

DVD 5

31 (wasting small amount to avoid any hassles manipulating entire DVD as a FAT32 file)

Flash 8 GiB

63

DVD 9

63

Partition 9.5 GB

64

UDF

UDF could be used with rewritable media for some interesting possibilities. Care is required because of potential incompatabilities with Windows and Mac and for understanding the plain, VAT and Spared formats. It might be possible to circulate a rewritable CD or DVD through several nodes and then keep it as an archive tracing the state at each node despite both additions and deletions. At any rate it should be feasible to keep a FIFO queue of used rewritable media as backups/archives while still reusing them to reduce media costs.

Mass Production

For direct distribution to 800 area (local district) post offices or 45,000 schools mass replication at a manufacturing plant would be much cheaper and faster even if it has to be done in another country with the necessary facilities. The master would be sent via internet and the replicas could be efficiently packed and addressed to individual area or regional post offices. Convenient service for smaller volumes is readily available through the internet, though of course arrangements should be made with a larger manufacturer and related postal services for ongoing lowest cost.

Esoterica

Piggy Backed Direct Distribution

With some additional (one time) work, mass production distribution of the useful contents to end users could be piggy backed on mass production of DVDs for DTN distribution.

A DVD from which end users can directly access the useful content such as published audio, video, text and software files could be produced, keeping the DTN and DFR bundle information separate with references to the enclosed content, and with additional html indexes etc for browsing content (as with DVDs distributed together with many mass market magazines).

The bridge format combining ISO 9660 and UDF and hybrid could enable playback of video and audio on ordinary DVD players as well as access on Windows and Macintosh PCs as a hybrid disc with mutliple file systems (pointing to the same content files) and the usual autostart facilities.

Software installation, updates and rescue could also be facilitated by making the DVDs bootable using el torito and perhaps EFI.

Certificate revocation lists, public keys etc could also be included.

Pigeon Post

If pigeons do end up being used at all the loss rate for more expensive flash media could be a major issue.

Could a pigeon carry 8cm miniDVDs?

 If not, could they carry one cut to a smaller shape, like the bootable business card?

 These are readily available as miniDVDs cut to the shape of a (large) business card.

 Just in case, it would be desirable for DVD recorders to accept miniDVD format and odd shapes.