What's a portNumber For? (Q69)

The information in this article applies to:

PortNumber is really only used in H.323 on ATM (C.3.7.2/H.323v4). The portNumber parameter was intended simply as a way to map physical or logical I/O ports on a terminal to the equivalent port at the far end. Suppose Terminal A has 4 RS-232 ports for data on the back, and these are used for generic data transfer of some kind through H.324, say using the V.42 protocol in a H.223 LC. Terminal B similarly has 4 ports. Suppose the ports are labeled on the back of the terminal "Port 1", "2", "3", and "4". How can we be sure that the data over "Port 3" on one end goes to "Port 3" on the other end, rather than another port? That's what portNumber is for. This applies equally, by the way, to anything that uses H.245, including H.324 and H.310.