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-
-IPA SCCP message flow in the BSC
-
-February, 2013 Holger Hans Peter Freyther
-
-CONTENTS
-
-1. SCCP inside the IPA header
-2. Supported SCCP message types
-3. Receiving SCCP messages
-4. Sending SCCP messages
-
-
-1. SCCP inside the IPA header
-
-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.
-
-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.
-
- struct ipa_header {
- uint16_t length_in_network_order;
- uint8_t type;
- } __attribute__((packed));
-
-
-
-2. Supported SCCP message types
-
-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.
-
-
-3. Receiving SCCP UDT messages
-
-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.
-
-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.
-
-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).
-
-
-4. Sending SCCP messages
-
-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.
-
-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.
-
-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.
-
-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.
-
-
-