Telecom Tutorials by Samir Amberkar

Inter-MSC Handover [Under GSM >> Signaling >> Handover]

Upon receiving "HO Required" from BSC, MSC finds that the target BTS belong to different MSC and accordingly it decides to go for Inter-MSC HO.

BSC1

MSC1

MSC2

BSC2

BSSMAP - HO Required MAP - Prepare HO (HO Required) BSSMAP - HO Required SCCP connection confirmed BSSMAP - HO Required Ack MAP - Prepare HO Ack (HO Cmd) ISUP - IAM ISUP - ACM BSSMAP - HO Command BSSMAP - HO Detect MAP - Process Access Signaling (HO Det) ISUP - ANM Traffic connection is active BSSMAP - HO Complete MAP - Send End Signal (HO Cmp) BSSMAP - Clear Command BSSMAP - Clear Complete SCCP Release

Subsequent Handover

Note that after HO to MSC₂, MSC₁ still remain active in connection. So when inter-MSC HO is required again, MSC₂ contacts MSC₁ back. MSC₂ sends (MAP message) "Prepare Subsequent HO" to MSC₁. If target BTS is within MSC₁, MSC₁ sends "HO Required" to BSC where target BTS is located.

In case target BTS is with another MSC (say MSC₃), MSC₁ sends "Prepare HO" to MSC₃ as shown in diagram below.

MSC2

MSC1

MSC3

MAP - Prepare Subsequent HO (HO Rqd) MAP - Prepare HO MAP - Prepare HO Ack (HO Cmd) ISUP - IAM ISUP - ACM MAP - Prepare Subsequent HO Ack (HO Cmd) MAP - Process Access Signaling (HO Det) ISUP - ANM MAP - Send End Signal (HO Cmp) MAP - Send End Signal (HO Cmp) ISUP - REL ISUP - RLC

References: GSM Networks book by Heine, GSM book by Mouly and Pautet, Signaling diagrams from 24.007/24.008 (R99), 04.18 (RR procedures), 29.002 (MAP specification).

© Copyright Samir Amberkar 2010

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