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DataTypes.xsd files. The difference consists in an optional 'chg' attribute, which is allowed for the attributes
of a feature when appearing in update messages, as discussed in 5.1.
When used in an AIXM-Update message, feature attributes such as <geoLat>, can have an optional 'chg'
attribute, as explained in 5.1.4.2. This attribute is declared at the level of the geoLat type, in the AIXMDataTypes.
xsd, through the intermediate of an attribute group, named 'Changes'. However, in order to
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enable the use of the 'chg' attribute only for Update messages and not for Snapshot messages, the 'Changes'
attribute group is left undefined in the AIXM-DataTypes.xsd file. It is defined only at a later stage, as
follows:
• in the AIXM-Update.xsd file, the 'Changes' attribute group is defined as containing a 'chg' attribute
• in the AIXM-Snapshot.xsd file, the 'Changes' attribute group is defined as an empty group
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Chapter 5. AIXM Message Types
Currently, the AIXM exchange format supports two types of data exchange:
• "Update" messages - containing data about new, changed or withdrawn aeronautical features;
• "Snapshot" messages - containing data about the versions of selected aeronautical features which
are valid at a given date and time.
Other message types are likely to be added in future versions of AIXM. An example could be an AIXMQuery
Schema, allowing to express queries with reference to AIXM features. Such queries would then
be interpreted by the receiving system and the result could come back, for example, in AIXM-Snapshot
format.
5.1. AIXM-Update
5.1.1. General
AIXM-Update messages are used to bring up to date previously exchanged data, without the need for
reissuing a complete new data set.
An AIXM-Update XML message has as root the <AIXM-Update> element, which contains a sequence
of one or more <Group> elements.
Figure 5.1. AIXM-Update Diagram
A system that implements AIXM may require a mechanism to guarantee that no messages are lost or
left unprocessed by the receiving system. Otherwise, this could result in a lack of synchronisation between
the sender and the receiver. The <AIXM-Update> element has two attributes that are intended to facilitate
the implementation of such a synchronisation control mechanism: 'origin' and 'number', which shall
contain the name of the issuing authority and a sequence number respectively. This allows a receiving
system to check that it has received and successfully processed all the messages. A system issuing AIXMUpdate
messages should foresee a mechanism that allows receiving systems to request a copy of a previously
sent message.
In the EAD implementation, the EAD System Interface (ESI1) is responsible for ensuring that all messages
are correctly received and processed. At the time of the writing of this document, the 'number' attribute
mentioned above was not foreseen to be used.
1For details, please see the "ESI Decision paper" at http://www.eurocontrol.int/ead/downloads.html.
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5.1.2. Groups
All changes included in one AIXM-Update message must have the same effective date, specified in the
'effective' attribute of the <AIXM-Update> element. Inside an Update message <Group> elements may
be used to collate multiple modifications which have a common cause (specified in the 'Reason' attribute).
A <Group> contains one or more modifications. Each modification must belong to one of the following
categories: new, changed or withdrawn. They occur as child elements of the <Group>, as shown in the
diagram above.
The use of <Group> elements is optional: if a system does not require or it is not able to group updates
by their common reason, it may create an AIXM-Update message that has only one <Group>, containing
all changes.
In the EAD, AIXM-Update Groups are used in connection with the 'slots mechanism'2. 'EAD slots' may
be regarded as containers, used to organise data updates issued by different originators, for the purpose
of applying validation rules. 'Private slots' are used to group together updates issued by the same originator,
having the same cause and the same effective date. 'Public slots' are then used to group together
private slots that have the same effective date. A <Group> element has two optional attributes, 'Name'
and 'SubName'. When used to send data to the EAD, these attributes can contain the identifier of the
public and private slot respectively, in which the updates must be included in the EAD.
5.1.3. New feature
The <New> element contains data about one new feature instance, such as a new VOR, a new RWY or
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