Dear experts,

I am a PhD student working in the area of modelling and analysis of communication protocols with formal methods. Currently I am looking at the H.245 recommendation.

I have got a trivial question about the SDL keys used in H.245 (published in 05/99). In Figure C.1/H.245 -SDL key, "signal from protocol user" (the one with double vertical lines at the right side) and "local signal received" (the one with single vertical line) are defined as different SDL keys. But in the SDL specifications of some H.245 procedures, it seems that these two keys (as well as "signal to protocol user" and "local signal sent") are used undistinguishably. For example, in Figure C.9/H245 - Incoming CESE SDL, "local signal received" key is used for primitive "TRANSFER.response", which expected by me to be reprensentated as key "signal from protocol user" because it IS the primitive issued by the CESE user. There are some other simliar situations in this and other figures as well. I feel confused with these, is there anything inside the protocol which I have not got yet or is it the problem with misusing the SDL keys ?

Could anybody interested in this can give me some hints ? Looking forward to your reply.

Thanks a lot.

Lin Liu

================ PhD student School of Electrical and Information Engineering University of South Australia The Mawson Lakes Campus, SA 5095, Australia

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