9.3 DICOM Upper Layer Protocol for TCP/IP Data Units Structure
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## 9.3 DICOM Upper Layer Protocol for TCP/IP Data Units Structure

### 9.3.1 General

The Protocol Data Units (PDUs) are the message formats exchanged between peer entities within a layer. A PDU shall consist of protocol control information and user data. PDUs are constructed by mandatory fixed fields followed by optional variable fields that contain one or more items and/or sub-items.

Items of unrecognized types shall be ignored and skipped. Items shall appear in an increasing order of their item types. Several instances of the same item shall be acceptable or shall not as specified by each item.

The DICOM UL protocol consists of seven Protocol Data Units:

1. A-ASSOCIATE-RQ PDU

2. A-ASSOCIATE-AC PDU

3. A-ASSOCIATE-RJ PDU

4. P-DATA-TF PDU

5. A-RELEASE-RQ PDU

6. A-RELEASE-RP PDU

7. A-ABORT PDU

The encoding of the DICOM UL PDUs is defined as follows (Big Endian byte ordering) :

### Note

The Big Endian byte ordering has been chosen for consistency with the OSI and TCP/IP environment. This pertains to the DICOM UL PDU headers only. The encoding of the PDV message fragments is defined by the Transfer Syntax negotiated at association establishment.

1. Each PDU type shall consist of one or more bytes that when represented, are numbered sequentially, with byte 1 being the lowest byte number.

2. Each byte within the PDU shall consist of eight bits that, when represented, are numbered 7 to 0, where bit 0 is the low order bit.

3. When consecutive bytes are used to represent a string of characters, the lowest byte numbers represent the first character.

4. When consecutive bytes are used to represent a binary number, the lower byte number has the most significant value.

5. The lowest byte number is placed first in the transport service data flow.

6. An overview of the PDUs is shown in Figure 9-1 and Figure 9-2. The detailed structure of each PDU is specified in the following sections.

### Note

A number of parameters defined in the UL Service are not reflected in these PDUs (e.g., service parameters, fixed values, values not used by DICOM Application Entities.)

Figure 9-1. Protocol Data Units Structure and Encoding

Figure 9-2. Protocol Data Units Structure and Encoding

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9.2.3 DICOM Upper Layer Protocol for TCP/IP State Transition Table | Home | 9.3.2 A-ASSOCIATE-RQ PDU Structure