108 lines
4.0 KiB
Plaintext
108 lines
4.0 KiB
Plaintext
== AoIP message flow examples
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The flow diagrams / ladder diagrams of this section are intended to
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provide some examples on how AoIP procedures work. We hope they will be
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useful in understanding the interface better and aid in debugging any
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related issues.
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=== AoIP interface bring-up
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This Figure shows the exchange of messages of a BSC when it is
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establishing its AoIP interface from scratch, for example because it has
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just been started up. We assume the BSC/CN has already been fully
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brought up, so no SCTP/M3U*A bring-up between MSC and STP is dieplayed.
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The diagram shows only one possible scenario.
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Depending on the MSC implementation, in between the BSC and the MSC
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there may be either
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* a dedicated STP (or multiple replicated STPs)
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* no dedicated STP, as the functionality is implemented inside the MSC
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* an entire SS7 network between BSC and MSC, with multiple STP, SGW,
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elements switching messages from the BSCs to the MSCs.
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The configuration details that need to be known to the BSC at start-up
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time are:
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* at SCTP level
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** remote IP addresses to which it should establish a SCTP association
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** SCTP port number for M3UA at the STP
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* at M3UA level
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** routing key (0 for none)
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** local BSC-side SS7 point code
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** remote MSC-side SS7 point code
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There possibly may be more configuration details, such as
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* multiple local and/or remote IP addresses for SCTP multi-homing
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* a fixed local (BSC side) IP address and/or SCTP port (default:
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dynamic/random)
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.AoIP interface bring-up between BSC and MSC
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[mscgen]
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----
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include::../message-sequences/a_interface_bringup.msc[]
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----
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For the purpose of clairty, SCTP-level acknowledgement chunks are not
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shown. Those are automatically generated by the receiver for every
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DATA chunk received in order to confirm its reception and to allow the
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transmitter to re-transmit in case of packet loss.
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==== SCTP multi-homing
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If SCTP multi-homing is used, the additional IP addresses are typically
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exchanged via additional information elements in the INIT/INIT_ACK
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chunks at connection establishment. They may also change at a later
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point.
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==== MSC pooling
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If there is MSC pooling configured, there is typically still only one
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M3UA ASP / SCTP association. The different MSCs are addressed on the
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SCCP point-code level. It's the STPs job to route the messages based
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on point codes to the respective MSC.
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The BSC will try to establish BSSAP to each of the MSCs in the pool,
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using a separate BSSAP reset procedure to each of the pool members
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point code.
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See the Chapter _MSC Pooling_ in the OsmoBSC user manual for
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configuration examples of this situation.
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=== MO call establishment on AoIP with user plane
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The following figure shows a simplified version of the messages between
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MS, OsmoBTS, OsmoBSC, OsmoMGW@BSC, MSC[-Server] and MSC-MGW in during
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the establishment and release of a MO voice call. Particular focus
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is given on messages related to the establishment of the RTP based
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user plane.
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The fact whether or not the RAN or the CN use media gateways, how they
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control their respective media gateway, and whether there are multiple
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media gateways for load distribution is a private implementation detail
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of either RAN or CN. Either side does not need to know the
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internal structure of the other side, since the RTP endpoint parameters
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are signaled for each call individually over the A interface.
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The signaling between the BSC-colocated MGW and OsmoBSC is IETF MGCP
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(Media Gateway Control Protocol).
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The signaling between the MSC[-Server] and the MGW is internal to the
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CN. It is typically based on MEGACO/H.248.
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As only the BSC and the MSC exchange 3GPP specified signaling messages,
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there is no direct interaction between the RAN and the CN side MGW.
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They only exchange RTP and associated RTCP.
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In many real deployments, OsmoMGW will have a different IP address on
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the BTS/Abis facing interface than on the MSC/A facing interface. As
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a simplification, this has been omitted in the figure.
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[mscgen]
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----
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include::../message-sequences/mo_call-bsc-msc-mgw-aoip.msc[]
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----
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