The Expedited Data Service

The data transfer services of the Transport Layer allow data messages to be exchanged between two correspondent transport service users over a transport connection. One of these services allows a transport service user (TS-user) to send an item of data that bypasses the flow control procedure. It provides a means for expediting the delivery of (occasional) urgent data. Such urgent data might include an interrupt alarm, or abrupt connection termination. It is called the Expedited Data Service and has the following restrictions:

• A transport connection must be established before its use
• The two transport services users (TS-users) must agree to use this service when the TC is first setup.
• Must be used during the data transfer stage of a connection
• Only available when the transport entities use one of classes 1- 4

The diagram below shows where the expedited data appears in relation to other Transport User Services:

If, during connection setup, the calling user does not request the expedited data service, it is not available to either user at any time during that connection. When the calling user requests this service, the called user must agree to its use on this connection also, otherwise it will not be available.

The diagram below shows the Primitive sequence of expedited data transfer:

The transport service provider guarantees that an expedited data TSDU will be delivered before later TSDUs. Expedited data might overtake normal data which was sent earlier, but the amount that is bypassed cannot be predicted.

There are two TPDUs involved in expedited data transfer in the transport protocol:

• ED: expedited data
• EA: expedited acknowledge

It is worth looking at:

• How the transport entity sends ED data units urgently, and
• The actual contents of ED and EA TPDUs

The TS user data provided in a T.EXPEDITED_DATA request is sent within an ED TPDU (expedited data transport protocol data unit) as shown in the diagram below:

Sequence numbers are used within ED TPDUs. However no more than one ED-TPDU may remain unacknowledged at any time (flow control). The sender won't send another ED, until it receives the EA (Expedited Acknowledgement) for the last one.

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