The Lower Layer Protocol (LLP) is one of the most straightforward TCP/IP protocols in the world to implement. Unfortunately this does not stop some creative programmers from implementing it in their own special way.

The symptoms for when the LLP protocol has not been correctly implemented is when the HL7 Listener receives a connection from an HL7 client but does not seem to pick up any messages.

The best way to diagnose the problem is to follow the testing procedures outlined in the transport section of the manual with the MSOCS tool. This tool allows you to accept an arbitrary feed of TCP/IP data and then copy and paste the data into a file. This file can then be opened in a hexadecimal editor which you can easily assess if the LLP protocol has been incorrectly implemented. This process is described in detail in the transport section.

	For more information about LLP, see LLP - Lower Layer Protocol. Another tool that you may find useful is called Wireshark. Wireshark is a powerful tool used for protocol analysis and network troubleshooting. It is useful for testing the structure of different network protocols like TCP/IP and can be run on a variety of platforms such as Windows, Linux and OS X.