When two applications communicate with each
other under the connection-oriented transmission
mode, they use the services provided by the serviceelements
(i.e. the layer-entities and the ASEs). Each
service-element has to established a logical connection
with its peer before actual data transmission between
the applications can take place.
The example shown in Figure 5 illustrates what
happens when an application process (VT-1) in system-
1 wants to set up a connection with another application
process (VT-2) in system-2. VT-1 sends a connection-
request to ACSE-1. Upon receipt of the
request, ACSE-1 sends a request to Presentation-
Entity-1 containing the connection-request the
ACSE-1 just received from VT-1. The connection-request
is passed down through the layer hierarchy in this
way until the operation arrives at the lowest layer for
transmission.
When the connection-request arrives at the target
system (System-2), it is passed up the layer hierarchy
towards the Application Layer. At each layer, the service-
element strips off the information created by its
peer and forwards the remaining information to the
next service-element up the hierarchy. By the time the
connection-request arrives at the targeted application
(VT-2), it is exactly the same as it was originally at
VT-1.
The target system can either accept or reject the
connection-request by sending a respond operation.
The respond operation is sent in the same way as the
request operation was, except in the opposite direction.