Transport

Transport is the abstract term used by the HL7 community for the infrastructure that serves the HL7 messages transfer from the Sending Application Role to the Receiving Application Role.

In other words, from the HL7 perspective Transport refers to OSI layers 1-6. The OSI layers 1-6 are perceived by the HL7 applications as the unified interface for message transfer, meaning that an HL7 Application is not required to (and usually doesn't) posses any specific knowledge for technology components residing at OSI layers 1-5.