Telecom Tutorials by Samir Amberkar

Voice Call continued [Under GSM >> Signaling]    §

Step 8: After some time, let us say MS₁ hangs up.

MS1

BTS1

BSC1

MSC1

MSC2

FACCH, CC - Disconnect RLM (Disonnect) DTAP (Disonnect) ISUP - REL DTAP (Release) RLM (Release) FACCH, CC - Release FACCH, CC - Release Complete RLM (Release Complete) DTAP (Release Complete) ISUP - RLC BSSMAP (Clear Command) RLM (Channel Release) FACCH, RR - Channel Release DCM - deact FACCH/SACCH L2 Disconnect1 RLM (Release) BSSMAP (Clear Complete) DCM Release SCCP Release

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MS2

BTS2

BSC2

MSC2

MSC1

ISUP - REL DTAP (Disconnect) RLM (Disconnect) FACCH, CC - Disconnect FACCH, CC - Release RLM (Release) DTAP (Release) ISUP - RLC DTAP (Release Complete) RLM (Release Complete) FACCH, CC - Release Complete BSSMAP (Clear Command) RLM (Channel Release) FACCH, RR - Channel Release DCM - deact FACCH/SACCH L2 Disconnect1 RLM (Release) BSSMAP (Clear Complete) DCM Release SCCP Release

1: To disconnect Layer 2, MS sends DISC frame and BTS acknowledges release with UA frame.

This completes signaling for voice call. In case call is being made to non-GSM network, MSC₂ would then be GMSC and signaling remain same in GSM network.

Note that in all signaling diagrams VLR is separate from MSC, but in practice, it MSC/VLR is a single entity with B interface an internal interface rather being an external one. MAP Specification 29.002 notes "The B-interface is not fully operational specified. It is strongly recommended not to implement the B-interface as an external interface."

References: GSM Networks book by Heine, GSM book by Mouly and Pautet, Signaling diagrams from 24.007/24.008 (R99).

© Copyright Samir Amberkar 2010

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