Information Technology Reference
In-Depth Information
and in 1983 released the standard for CD-ROM. Then in 1988, they released the Red Book
standard for recordable CD audio disks (CD-DA)
This served as a blueprint for the Yellow Book specification for CD-ROMs (CD-ROM
and CD-ROM-XA data format) and the Orange Book Parts 1 and 2 specifications for CD-
Recordable (CD-R/CD-E (CD-recordable/CD-erasable)). In the Red Book standard a disk is
organized into a number of segments:
•
Lead in - contains the table of contents for the disk that specifies the physical location of
each track.
•
Program area - contains the actual disk data or audio data and is divided up into 99
tracks, with a two-second gap between each track.
•
Lead out - contains a string of zeros which is a legacy of the old Red Book standard.
These zeros enabled old CD players to identify the end of a CD.
The CD is laid out in a number of sectors. Each of these sectors contains 2352 bytes, made
up of 2048 bytes of data and other information such as headers, sub-headers, error detection
codes and so on. The data is organised into logical blocks. After each session a logical block
has a logical address, which is used by the drive to find a particular logical block number
(LBN).
Within the tracks the CD can contain either audio or computer data. The most common
formats for computer data are ISO 9660, hierarchical file system (HFS) and the Joliet file
system.
The ISO 9660 was developed at a time when disks required to be mass replicated. It thus
wrote the complete file system at the time of creation, as there was no need for incremental
creation. Now, with CD-R technology, it is possible to incrementally write to a disk. This is
described as multisession. Unfortunately, after each session a new lead in and lead out must
be written (requiring a minimum of 13 MB of disk space). This consists of:
•
13.2 MB for the lead out for the first session and 4.4 MB for each subsequent session.
•
8.8 MB for lead in for each session comprising 8.8 MB.
Thus multisession is useful for writing large amounts of data for each session, but is not effi-
cient when writing many small updates. Most new CD-R systems now use a track-at-once
technique which stores the data one track at a time and only writes the lead in and lead out
data when the session is actually finished. In this technique the CD can be built up with data
over a long period of time. Unfortunately the disk cannot be read by standard CD-ROM
drives until the session is closed (and written with the ISO 9660 format). Another disadvan-
tage is that the Red Book only specifies up to 99 tracks for each CD.
Unfortunately the ISO 9660 is not well-suited for packet writing and is likely to be
phased out over the coming years.
6.9.5 Magneto-optical (MO) disks
As with CD-R disks, magneto-optical (MO) disks allow the data to be rewritten many times.
These disks use magnetic and optical fields to store the data. Unfortunately the disk must
first be totally erased before data is written (although new developments are overcoming this
limitation).
6.9.6 Transfer rates
Optical disks spin at variable speeds, they spin at a lower rate on the outside of the disk than
