msc {
	hscale=3;
	te [label="TE"], nt [label="NT"], an [label="AN"], le [label="LE"], l23 [label="L23"];

	--- [label="LE initiated activation of ISDN user port"];
	le <= l23 [label="MPH-AR\nActivate Request"];
	an <= le [label="FE101\nActivate access"];
	te <=> an [label="Activation procedure"];
	an => le [label="FE104\nAccess activated"];
	le => l23 [label="MPH-AI\nActivate Indication"];
	...;
	te => an [label="UL LAPDm frames"];
	an => le [label="Enveloped UL LAPDm frames"];
	le => l23 [label="UL LAPDm frames"];

	le <= l23 [label="DL LAPDm frames"];
	an <= le [label="Enveloped DL LAPDm frames"];
	te <= an [label="DL LAPDm frames"];
}