osmo-msc/doc/sequence_charts/inter_bsc_ho.msc

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msc {
hscale=2;
bsca [label="BSC-A"], i[label="MSC-I"], a[label="MSC-A"], t[label="MSC-T"], bscb[label="BSC-B"];
i note t [label="'MSC-A,I,T' are explained in 3GPP TS 49.008 '4.3 Roles of MSC-A, MSC-I and MSC-T'"];
i note i [label="I = Internal; the MSC that does BSSMAP to the BSC (may change to a remote MSC-B after HO)"];
a note a [label="A = first; the MSC that has MM control BSSMAP to the BSC (never changes)"];
t note t [label="T = transitory; the MSC that a handover is going towards (may be MSC-A for inter-BSC HO, may be a remote MSC-B)"];
bsca => i [label="BSSMAP Handover Required"];
i -> a [label="BSSMAP Handover Required"];
a -> t [label="MAP Prepare Handover"];
t => bscb [label="BSSMAP Handover Request"];
t <= bscb [label="BSSMAP Handover Request ACK"];
a <- t [label="MAP Prepare Handover Response"];
i <- a [label="MAP Prepare Handover Response"];
bsca <= i [label="BSSMAP Handover Command"];
--- [label="MS sends RACH to new cell"];
t <= bscb [label="BSSMAP Handover Detected"];
a <- t [label="MAP Access Signaling Request"];
t <= bscb [label="BSSMAP Handover Complete"];
a <- t [label="MAP Send End Signal"];
a abox a [label="MSC-A accepts the new BSC"];
i note t [label="previous MSC-I gets dropped, MSC-T becomes the new MSC-I"];
i abox i [label="discard"];
t abox t [label="MSC-I"];
bsca <= i [label="BSSMAP Clear Command"];
}