Understanding a Digital Object: Basic Representation Information - Advanced Digital Preservation - page 92

Packet

Primary
Header
(48)

-Optional- Source Data

(variable)

.....

Secondary
Header
(variable)

discriminates

Source
Data
Length
(16)

Packet
Identification

Packet
Sequence
Control

Version
Number
(3)

Segmentation
Flag
(2)

Source
Sequence
Counter
(14)

discriminates

Type_Id
(1)

Secondary
Header
Flag
(1)

Application
Process ID
(11)

(x) : Length in bits

Fig. 7.9 Discriminants in a packet format

This has the EAST description shown in Fig. 7.10 .
EAST is used extensively in operational archives, most notably in the CDPP
[ 49 ] and other archives using the SITOOLS software [ 34 ]. Data deposited in CDPP
must have an EAST description and this allows automated processing including sub-
setting and transformations. For the latter one needs EAST descriptions of the two
formats and a mapping between the data elements of each.

7.3.4.2 DRB

The Data Request Broker [ 50 ]DRBAPI R is an Open Source Java applica-

tion programming interface for reading, writing and processing heterogeneous
data.
DRB API R

is a software abstraction layer that helps developers in programming
applications independently from the way data are encoded within files. Indeed, DRB
API R

is based on a unified data model that makes the handling of supported data
formats much easier. A number of implementations for particular cases are shown
in Fig. 7.11 .
Of particular interest is the SDF implementation which allows one to describe a
binary data file. The description is placed as an XML annotation element within an
XML Schema.
DRB-SDF is based on XML Schema [ 51 ] and XQuery [ 52 ] and uses some addi-
tional non-standard extensions to deal with binary data. The main restriction is that
the physical structure of data types cannot be defined explicitly as can be done