HL7 Transport

Health Level Seven (HL7) is a reference to the OSI model of ideal networks. Back in the 1980's hospitals were working to "network" various computer system together so that they could communicate. Remember that this was before the "Web" and "Email" as we know it today. Many different companies offered "network" solutions and these were different and incompatible standards. The OSI model was created to explain the theoretical model of how networks should work.

The HL7 standards organization wanted to avoid implementation specifics of what kind of network was being used by limiting the discussion to the 7th (top) layer of the OSI model. Which simply means that HL7 would define how to format the message but not how it should be sent.

The advent of the modern Internet was made possible by an adoption of a standard networking protocol TCP/IP. The old proprietary protocols still exist but they are not hidden under standard TCP/IP protocols. Still HL7 Version 2 standards preserve the divide and do not specify transport standards.

HL7 Version 2 does provide a possible transport solution for TCP/IP networks called Minimum Lower Level Protocol (MLLP), and this has become the industry standard. Again, because HL7 is not supposed to move beyond message format, this is just a possible way to transport messages. But in practice it is so widely used that it is normally referred to and known as "TCP/IP".

This defacto standard has one flaw - it is not well suited to the Internet as it does not provide for security. This is why exchanging HL7 across the Internet has become clumsy and difficult, because there are no agreed upon national standards for doing so.