OSI/TS Configuration and Management Manual

Managing a Transport Connection
OSI/TS Configuration and Management Manual—424831-001
6-11
Concatenation and Separation of TPDUs
Some services that transport peer entities provide during the data-transfer phase are:
The following subsections describe some of these services in detail.
Concatenation and Separation of TPDUs
In protocol classes 1, 2, 3, and 4, OSI/TS automatically concatenates outgoing TPDUs
and passes the concatenated string to the subnetwork so that several TPDUs can be
transferred in a single packet. No more than one TPDU that contains a variable part or
user data can be concatenated into the same packet. OSI/TS concatenates only TPDUs
that are ready when the subnetwork connection becomes available.
When OSI/TS receives incoming concatenated TPDUs, it separates the TPDUs and
routes them to the proper connections.
Segmenting and Reassembly of TSDUs
OSI/TS segments transport service data units (TSDUs) that are larger than the
negotiated TPDU size into multiple TPDUs. When OSI/TS receives a series of TPDUs
that contains a segmented TSDU, the TSDU is reassembled before it is passed to the
transport user.
Expedited Data
If the transport-expedited data option is selected when the transport connection is
negotiated, OSI/TS provides the following services:
Concatenation Concatenation of multiple TPDUs into a single packet. The
receiving transport entity separates the packet into TPDUs before
passing the data to the local transport-service user.
Segmentation Segmentation of user messages into smaller units that are
compatible with the subnetwork. The receiving transport entity
reassembles the message before passing the data to the local
transport-service user.
Expedited data Special priority in queuing by transport entities for data units
transferred over an expedited connection.
Flow control Control of data flow to ensure that data is not lost between
connection endpoints.
Error recovery Recovery from error conditions reported by the subnetwork.
Unsignaled error
detection
A protocol for detecting unsignaled subnetwork errors. This
feature is used when the Transport Layer is supported by a
subnetwork for which the probability of lost data is unacceptable.
Host-to-host
message
sequencing
Message sequencing service for subnetworks in which frames
might not always be received in the order in which they were
sent. The service also protects against asynchronous subnetwork
resets that set all Network Layer frame counters to 0.