A-Interface » History » Version 1
laforge, 12/09/2017 10:30 AM
| 1 | 1 | laforge | h1. A-Interface |
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| 3 | The A interface exists between the [[BSC]] and [[MSC]] inside a GSM network. It uses SS7 as transport, particularly connection-oriented SCCP. |
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| 5 | The main specifications for A are 3GPP TS 48.006 and 48.008. |
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| 7 | h2. Transport |
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| 9 | h3. TDM |
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| 11 | Traditionally, it is spoken oder TDM links such as E1/T1. Voice channels are transcoded at the BSC and transmitted like ISDN voice calls in G.711 on 64kBps TDM slots. |
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| 13 | Osmocom doesn't support this out of the box, a media gateway would be required. |
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| 15 | h3. SCCPLite |
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| 17 | Long before 3GPP had sang standard for transporting A over IP, some companies implemented what is called SCCPLite. This uses the IPA multiplex over TCP as transport for SCCP, together with MGCP for configuring RTP media flow IP/Port information. |
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| 19 | Osmocom implements this so far only in osmo-bsc-sccplite and osmo-bsc_nat. |
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| 21 | h3. AoIP |
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| 23 | Later, 3GPP standardized a method on how to transport A interfaces over IP and called it AoIP. It uses real SCCP over M3UA/SCTP/IP for the signalling plane and replaces the G.711 TDM user plane with using untranscoded air interface codec payload in RTP/UDP/IP. See 3GPP TS 48.103 for the user plane (RTP) handling. |
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| 25 | The BSSMAP protocol was extended to be able to negotiate per-call IP/Port information between BSC and MSC. |
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| 27 | Osmocom [[OsmoBSC:]] works with 3GPP AoIP and has been tested to interoperate with several MSCs, not just [[OsmoMSC:]] |
