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diff --git a/doc/ipa-sccp.txt b/doc/ipa-sccp.txt deleted file mode 100644 index 5d6719e98..000000000 --- a/doc/ipa-sccp.txt +++ /dev/null @@ -1,94 +0,0 @@ - -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. - - - |