7 Service overview

The DICOM Message Service Element supports communication between peer DIMSE-service-users. A DIMSE-service-user acts in one of two roles:

a) invoking DIMSE-service-user

  1. performing DIMSE-service-user

DIMSE-service-users make use of service primitives which are provided by the DIMSE-service-provider. The DIMSE-service-provider is an abstraction of the totality of those entities which provide DIMSE services to peer DIMSE-service-users. A service primitive shall be one of the following types:

a) request primitive

b) indication primitive

c) response primitive

d) confirmation primitive

These primitives (which are shown in Figure 7-1) are used as follows to successfully complete a DIMSE service:

⎯ The invoking DIMSE-service-user issues a request primitive to the DIMSE-service-provider.

⎯ The DIMSE-service-provider receives the request primitive from the invoking DIMSE-service-user and issues an indication primitive to the performing DIMSE-service-user.

⎯ The performing DIMSE-service-user receives the indication primitive from the DIMSE-service-provider and performs the requested service.

⎯ The performing DIMSE-service-user issues a response primitive to the DIMSE-service-provider.

⎯ The DIMSE-service-provider receives the response primitive from the performing DIMSE-service-user and issues a confirmation primitive to the invoking DIMSE-service-user.

⎯ The invoking DIMSE-service-user receives the confirmation primitive from the DIMSE-service-provider completing the DIMSE service.

7.1 Service types

DIMSE provides two types of information transfer services which are used by DICOM Application Entities:

a) a notification service

b) an operation service

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Figure 7-1 DIMSE SERVICE PRIMITIVES

Notification services enable one DICOM Application Entity to notify another about the occurrence of an event or change of state. The definition of the notification and the consequent behavior of the Application Entities is dependent upon the Service Class and Information Object Definitions. See PS 3.3 and PS 3.4.

Operation services enable one DICOM Application Entity to explicitly request an operation to be performed upon a SOP Instance managed by another DICOM Application Entity.

7.2 DIMSE-SERVICE-USER INTERACTION

The DICOM Message Service Element receives notification and operation requests and their related information from the DIMSE-service-user. Two DICOM Application Entities take the roles as peer DIMSE-service-users in order to exchange notifications and operations.

A notification or operation is implemented as a request/response interaction carried out within the context of an established application Association. Typically, one DIMSE-service-user requests that a particular operation be performed (or notification be processed) and the other DIMSE-service-user attempts to perform the operation (or process the notification) and then reports the outcome of the attempt.

When engaging in the operations or notifications, the DIMSE-service-user takes on one of two roles:

a) it performs operations (on SOP Instances for which it has responsibility) which were invoked by a peer DIMSE-service-user. It may also emit change-of-state notifications for SOP Instances to one or more peer DIMSE-service-users. These notifications may be invoked as a result of operations initiated by other DIMSE-service-users.

b) it invokes the performance of an operation on a peer DIMSE-service-user. It may also receive notifications from a peer DIMSE-service-user.

These roles are depicted in Figure 7.2-1.

Notes: 1. Role a) (called the Agent role in ISO terminology) is used by an implementation which conforms to a DICOM Service Class as an SCP.

2. Role b) (called the Manager role in ISO terminology) is used by an implementation which conforms to a DICOM Service Class as an SCU.

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Figure 7.2-1 OPERATION AND NOTIFICATION FLOW

7.3 Service modes

Operations and notifications, on an Association, are used in one of the following two modes:

a) synchronous

b) asynchronous

In the synchronous mode, the invoking DIMSE-service-user, on an established Association, requires a response from the performing DIMSE-service-user before invoking another operation or notification.

In the asynchronous mode, the invoking DIMSE-service-user, on an established Association, may continue to invoke further operations or notifications to the performing DIMSE-service-user without awaiting a response. In the asynchronous mode, the performing DIMSE-service-user may respond to the operations or notifications in a different order than they were received.

The mode selection (synchronous or asynchronous) is determined at Association establishment time. The synchronous mode serves as the default mode and shall be supported by all DIMSE-service-users. The asynchronous mode is optional and the maximum number of outstanding operations/notifications is negotiated during Association establishment. This negotiation is accomplished by Application Association Information as defined in Annex D.

7.4 Association services

The DICOM Message Service Element does not provide separate services for the establishment and termination of application Associations. This section provides an overview of how an Application Entity using the DIMSE service uses the Association Services defined in PS 3.8.

During the Association establishment phase, a DIMSE-service-user shall exchange initialization information using parameters of the A-ASSOCIATE Upper Layer Service (see Figure 7.4-1) which include:

⎯ Application context

⎯ Presentation and session requirements

⎯ DIMSE-specific user information

⎯ Application Association Information

The A-RELEASE and A-ABORT Services defined in PS 3.8 shall be used for the termination of an Association.

Note: The rules defining how the Association Services are used by a DIMSE-service-user are defined in Annex D.

7.4.1 ASSOCIATION ESTABLISHMENT

The A-ASSOCIATE Service is invoked by a DIMSE-service-user to establish an Association with a peer DIMSE-service-user. Association establishment is always the first phase of DICOM Message Exchange.

The initiating DIMSE-service-user and the responding DIMSE-service-user shall include Application Association Information on the request and response primitive respectively. The meaning of this parameter is Application Context specific. For more information on the use of the Application Association Information, see Annex D.

7.4.2 ASSOCIATION RELEASE

The A-RELEASE Service is invoked by a DIMSE-service-user to request the orderly termination of an Association between peer DIMSE-service-users. This part of the Standard does not specify any use of the parameters of the A-RELEASE service.

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Figure 7.4-1 DICOM APPLICATION ENTITY AND ASSOCIATION

The A-ABORT Service is invoked by a DIMSE-service-user to request the abrupt termination of the Association between peer DIMSE-service-users. The A-ABORT invoking DIMSE-service-user shall include (within the A-ABORT user information field) the Abort Source parameter. The Abort Source parameter indicates the initiating source of the abort. It takes one of the following symbolic values:

⎯ DIMSE-service-provider

⎯ DIMSE-service-user

Reference PS 3.8 for more information on the A-RELEASE and A-ABORT services.

7.5 DIMSE services

Because the manner in which operations applied to Composite SOP Instances differ from operations and notifications applied to Normalized SOP Instances, two groups of DIMSE services are defined:

⎯ DIMSE-N: those services applicable to Normalized SOP Instances

⎯ DIMSE-C: those services applicable to Composite SOP Instances

Table 7.5-1 DIMSE SERVICES

Name Group Type
C-STORE DIMSE-C operation
C-GET DIMSE-C operation
C-MOVE DIMSE-C operation
C-FIND DIMSE-C operation
C-ECHO DIMSE-C operation
N-EVENT-REPORT DIMSE-N notification
N-GET DIMSE-N operation
N-SET DIMSE-N operation
N-ACTION DIMSE-N operation
N-CREATE DIMSE-N operation
N-DELETE DIMSE-N operation

Note: Use of the Dialog command, supported in previous versions of this Standard, has been retired.

7.5.1 DIMSE-C SERVICES

The DIMSE-C services allow a DICOM Application Entity to explicitly request an operation by another DICOM Application Entity on Composite SOP Instances. The operations allowed are intended to be effectively compatible with those provided by previous versions of this Standard. DIMSE-C provides only operation services.

7.5.1.1 Operation services

DIMSE-C provides the following operation services which are all confirmed services and as such a response is expected:

a) The C-STORE service is invoked by a DIMSE-service-user to request the storage of Composite SOP Instance information by a peer DIMSE-service-user.

b) The C-FIND service is invoked by a DIMSE-service-user to match a series of Attribute strings against the Attributes of the set of SOP Instances managed by a peer DIMSE-service-user. The C-FIND service returns for each match a list of requested Attributes and their values.

c) The C-GET service is invoked by a DIMSE-service-user to fetch the information for one or more Composite SOP Instances from a peer DIMSE-service-user, based upon the Attributes supplied by the invoking DIMSE-service-user.

d) The C-MOVE service is invoked by a DIMSE-service-user to move the information for one or more Composite SOP Instances from a peer DIMSE-service-user, to a third party DIMSE-service-user, based upon the Attributes supplied by the invoking DIMSE-service-user

e) The C-ECHO service is invoked by a DIMSE-service-user to verify end-to-end communications with a peer DIMSE-service-user.

Notes: 1. The major differences between a C-GET and a C-MOVE operation are that the:

a. C-STORE sub-operations resulting from a C-GET are performed on the same Association as the C-GET. With a C-MOVE, the resulting C-STORE sub-operations are performed on a separate Association.

b. C-MOVE operation supports C-STORE sub-operations being performed with an Application Entity which is not the one that initiated the C-MOVE (third party move).

2. In the case where an Application Entity wishes to request that it receives one or more images for storage, it may use either a C-GET operation or a C-MOVE to itself. It is expected that in most environments the C-MOVE is a simpler solution despite the fact that two Associations are required. The use of the C-GET service may not be widely implemented. It may be implemented in special cases where a system does not support multiple Associations. It was left in this version of the Standard for backward compatibility with previous versions of the Standard.

7.5.2 DIMSE-N SERVICES

The DIMSE-N services provide both notification and operation services applicable to Normalized SOP Instances.

7.5.2.1 Notification service

DIMSE-N provides a single Notification Service, the N-EVENT-REPORT. The N-EVENT-REPORT service is invoked by a DIMSE-service-user to report an event about a SOP Instance to a peer DIMSE-service-user. This service is a confirmed service and a response is expected.

7.5.2.2 Operation services

DIMSE-N provides the following operation services which are all confirmed services and as such a response is expected:

a) The N-GET service is invoked by a DIMSE-service-user to request the retrieval of information from a peer DIMSE-service-user.

b) The N-SET service is invoked by a DIMSE-service-user to request the modification of information by a peer DIMSE-service-user.

c) The N-ACTION service is invoked by a DIMSE-service-user to request a peer DIMSE-service-user to perform an action.

d) The N-CREATE service is invoked by a DIMSE-service-user to request a peer DIMSE-service-user to create an instance of a SOP Class.

e) The N-DELETE service is invoked by a DIMSE-service-user to request a peer DIMSE-service-user to delete an instance of a SOP Class.

7.5.3 DIMSE PROCEDURES

All DIMSE operations and notifications are confirmed services. The performing DIMSE-service-user shall report the response of each operation or notification over the same Association on which the operation or notification was invoked.

Each DIMSE service is accomplished through the use of one or more service primitives. How the peer DIMSE-service-users utilize and react to the service primitives are defined by the service procedures.

7.5.3.1 Sub-operations

Some DIMSE services are atomic in that the service is performed by one operation or notification. In such a case the DIMSE service primitives are used by peer DIMSE-service-users to invoke and perform the operation or notification.

Other DIMSE services require the use of one or more sub-operations to perform the service. In such cases DIMSE service primitives are used by peer DIMSE-service-users to invoke and perform each sub-operation. How and when the sub-operation service primitives are used is defined by the procedures for the DIMSE service.

7.5.3.2 Multiple responses

Each DIMSE service requires one or more response primitives as a result of the invocation of the service. How and when the multiple response primitives are used is defined by the procedures for the DIMSE service. Whether multiple responses are returned is conditional upon the information included in the request primitive by the DIMSE-service-user.

7.5.3.3 Cancellation

Certain DIMSE services permit the cancellation of the service through the use of service primitives. This allows an invoking DIMSE-service-user to request termination of a DIMSE service after completion of the request service primitive but prior to completion of the confirm service primitive.

Table 7.5-2 lists each DIMSE service and its related procedure information. The complete specifications for the service procedures are defined in Sections 9 and 10 for DIMSE-C and DIMSE-N respectively.

Table 7.5-2 DIMSE SERVICES AND PROCEDURES

Name Sub-Operations Multiple Responses Cancel
C-STORE
C-GET M C M
C-MOVE M C M
C-FIND C M
C-ECHO
N-EVENT-REPORT
N-GET
N-SET
N-ACTION
N-CREATE
N-DELETE