9.3.2 A-ASSOCIATE-RQ PDU Structure
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### 9.3.2 A-ASSOCIATE-RQ PDU Structure

An A-ASSOCIATE-RQ PDU shall be made of a sequence of mandatory fields followed by a variable length field. Table 9-11 shows the sequence of the mandatory fields.

The variable field shall consist of one Application Context Item, one or more Presentation Context Items, and one User Information Item. Sub-Items shall exist for the Presentation Context and User Information Items.

Table 9-11. ASSOCIATE-RQ PDU Fields

PDU bytes | Field name | Description of field
--- | --- | ---
1 | PDU-type | 01H
2 | Reserved | This reserved field shall be sent with a value 00H but not tested to this value when received.
3-6 | PDU-length | This PDU-length shall be the number of bytes from the first byte of the following field to the last byte of the variable field. It shall be encoded as an unsigned binary number
7-8 | Protocol-version | This two byte field shall use one bit to identify each version of the DICOM UL protocol supported by the calling end-system. This is Version 1 and shall be identified with bit 0 set. A receiver of this PDU implementing only this version of the DICOM UL protocol shall only test that bit 0 is set.
9-10 | Reserved | This reserved field shall be sent with a value 0000H but not tested to this value when received.
11-26 | Called-AE-title | Destination DICOM Application Name. It shall be encoded as 16 characters as defined by the ISO 646:1990-Basic G0 Set with leading and trailing spaces (20H) being non-significant. The value made of 16 spaces (20H) meaning "no Application Name specified" shall not be used. For a complete description of the use of this field, see Section 7.1.1.4.
27-42 | Calling-AE-title | Source DICOM Application Name. It shall be encoded as 16 characters as defined by the ISO 646:1990-Basic G0 Set with leading and trailing spaces (20H) being non-significant. The value made of 16 spaces (20H) meaning "no Application Name specified" shall not be used. For a complete description of the use of this field, see Section 7.1.1.3.
43-74 | Reserved | This reserved field shall be sent with a value 00H for all bytes but not tested to this value when received
75-xxx | Variable items | This variable field shall contain the following items: one Application Context Item, one or more Presentation Context Items and one User Information Item. For a complete description of the use of these items see Section 7.1.1.2, Section 7.1.1.13, and Section 7.1.1.6.

#### 9.3.2.1 Application Context Item Structure

An Application Context Item shall be made of a sequence of mandatory fields followed by a variable length field. Table 9-12 shows the sequence of the mandatory fields.

Table 9-12. Application Context Item Fields

Item bytes | Field name | Description of field
--- | --- | ---
1 | Item-type | 10H
2 | Reserved | This reserved field shall be sent with a value 00H but not tested to this value when received.
3-4 | Item-length | This Item-length shall be the number of bytes from the first byte of the following field to the last byte of the Application-context-name field. It shall be encoded as an unsigned binary number.
5-xxx | Application-context-name | A valid Application-context-name shall be encoded as defined in Annex F. For a description of the use of this field see Section 7.1.1.2. Application-context-names are structured as UIDs as defined in PS3.5 (see Annex A for an overview of this concept). DICOM Application-context-names are registered in PS3.7.

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