msc {
	hscale="1.2";
	sgsn [label="SGSN"], pcu [label="PCU"], bts [label="BTS"], bsc [label="BSC"];

	|||;
	bts box bsc [label="A-bis connection"];
	pcu <: bts  [label="Config. parameters"];
	pcu rbox pcu [label="listen on UDP socket"];
	pcu note sgsn [label="send to SGSN UDP (port 23000)"];
	pcu => sgsn [label="NS RESET"];
	pcu <= sgsn [label="NS RESET ACK"];
	...;
	pcu => sgsn [label="NS UNBLOCK"];
	pcu <= sgsn [label="NS UNBLOCK ACK"];
	pcu box sgsn [label="NS link established"];
	...;
	pcu => sgsn [label="BVC RESET"];
	pcu <= sgsn [label="BVC RESET ACK"];
	...;
	pcu => sgsn [label="BVC UNBLOCK"];
	pcu <= sgsn [label="BVC UNBLOCK ACK"];
	pcu box sgsn [label="BSSGP link established"];
	|||;
}