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| IPA SCCP message flow in the BSC |
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| February, 2013 Holger Hans Peter Freyther |
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| CONTENTS |
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| 1. SCCP inside the IPA header |
| 2. Supported SCCP message types |
| 3. Receiving SCCP messages |
| 4. Sending SCCP messages |
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| 1. SCCP inside the IPA header |
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| Many Soft-MSCs implement something that is called SCCP/lite. This means |
| that SCCP messages are transported inside a small multiplexing protocol |
| over TCP/IP. This is an alternative to a full SIGTRAN implementation. |
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| The multiplexing protocol is the same as used with the sysmoBTS and the |
| ip.access nanoBTS. It is a three byte header with two bytes for the length |
| in network byte order and one byte for the type. The type to be used for |
| SCCP is 0xFD. |
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| struct ipa_header { |
| uint16_t length_in_network_order; |
| uint8_t type; |
| } __attribute__((packed)); |
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| 2. Supported SCCP message types |
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| To implement GSM 08.08 only a subset of SCCP messages need to be implemented. |
| For transporting paging and reset messages SCCP UDT messages are used. For |
| the connections with a Mobile Station (MS) a SCCP connection is opened. This |
| means that the SCCP CR, SCCP CC, SCCP CREF, SCCP RLC, SCCP RLSD, SCCP DT1 |
| and SCCP IT messages are supported. |
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| 3. Receiving SCCP UDT messages |
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| This is an illustration of the flow of messages. The IPA multiplexing protocol |
| is used for various protocols. This means there is a central place where the |
| multiplexing stream terminates. The stream is terminated in the osmo_bsc_msc.c |
| file and the ipaccess_a_fd_cb method. For SCCP messages the SCCP dispatching |
| sccp_system_incoming method is called. This function is implemented in the |
| libosmo-sccp library. |
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| To receive UDT messages osmo_bsc_sccp.c:osmo_bsc_sccp_init is using the |
| sccp_set_read function to register a callback for UDT messages. The callback |
| is msc_sccp_read and it is calling bsc_handle_udt that is implemented in the |
| osmo_bsc_bssap.c. This function will handle the GSM 08.08 BSSAP messages. |
| Currently only the reset acknowledge and the paging messages are handled. |
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| The BSC currently does not accept incoming SCCP messages and is only opening |
| SCCP connections to the MSC. When opening a connection the callbacks for state |
| changes (connection confirmed, released, release complete) are set and a routine |
| for handling incoming data. This registration is done in the osmo_bsc_sccp.c |
| file and the bsc_create_new_connection method. The name of the callback is |
| msc_outgoing_sccp_data and this will call bsc_handle_dt1 that is implemented |
| in the osmo_bsc_bssap.c file. This will forward the messages to the right |
| Mobile Station (MS). |
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| 4. Sending SCCP messages |
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| There are three parts to sending that will be explained below. The first part |
| is to send an entire SCCP frame (which includes the GSM 08.08 data) to the |
| MSC. This is done by first registering the low level sending. sccp_system_init |
| is called with the function that is responsible for sending a message. The |
| msc_sccp_write_ipa will call the msc_queue_write function with the data and |
| the right MSC connection. Below the msc_queue_write the IPA header will be |
| prepended to the msg and then send to the MSC. |
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| The BSC supports multiple different A-link connections, the decision to pick |
| the right MSC is done in this method. It is either done via the SCCP connection |
| or the ctx pointer. |
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| When the BSC is starting a BSS RESET message will be sent to the MSC. The reset |
| is created in osmo_bsc_msc.c:initialize_if_needed and sccp_write is called with |
| the GSM 08.08 data and the connection to use. The libosmo-sccp library will |
| embed it into a SCCP UDT message and call the msc_sccp_write_ipa method. |
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| When a new SCCP connection is to be created the bsc_create_new_connection |
| in the osmo_bsc_sccp.c file. The sccp_connection_socket method will create |
| the context for a SCCP connection. The state and data callback will be used |
| to be notified about data and changes. Once the connection is configured the |
| bsc_open_connection will be called that will ask the libosmo-sccp library to |
| create a SCCP CR message using the sccp_connection_connect method. For active |
| connections the sccp_connection_write method will be called. |
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