add bits and pieces of documentation
diff --git a/doc/e1-data-model.txt b/doc/e1-data-model.txt
new file mode 100644
index 0000000..8594fe4
--- /dev/null
+++ b/doc/e1-data-model.txt
@@ -0,0 +1,172 @@
+E1 related data model
+
+This data model describes the physical relationship of the individual
+parts in the network, it is not the logical/protocol side of the GSM
+network.
+
+A BTS is connected to the BSC by some physical link. It could be an actual
+E1 link, but it could also be abis-over-IP with a mixture of TCP and RTP/UDP.
+
+To further complicate the fact, multiple BTS can share one such pysical
+link. On a single E1 line, we can easily accomodate up to three BTS with
+two TRX each.
+
+Thus, it is best for OpenBSC to have some kind of abstraction layer. The BSC's
+view of a BTS connected to it. We call this 'bts_link'. A bts_link can be
+* all the TCP and UDP streams of a Abis-over-IP BTS
+* a set of E1 timeslots for OML, RSL and TRAU connections on a E1 link
+* a serial line exclusively used for OML messages (T-Link)
+
+A bts_link can be registered with the OpenBSC core at runtime.
+
+struct trx_link {
+ struct gsm_bts_trx *trx;
+};
+
+struct bts_link {
+ struct gsm_bts *bts;
+ struct trx_link trx_links[NUM_TRX];
+};
+
+Interface from stack to input core:
+======================================================================
+int abis_rsl_sendmsg(struct msgb *msg);
+ send a message through a RSL link to the TRX specified by the caller in
+ msg->trx.
+
+int abis_rsl_rcvmsg(struct msgb *msg);
+ receive a message from a RSL link from the TRX specified by the
+ caller in msg->trx.
+
+int abis_nm_sendmsg(struct msgb *msg);
+ send a message through a OML link to the BTS specified by the caller in
+ msg->trx->bts. The caller can just use bts->c0 to get the first TRX
+ in a BTS. (OML messages are not really sent to a TRX but to the BTS)
+
+int abis_nm_rcvmsg(struct msgb *msg);
+ receive a message from a OML link from the BTS specified by the caller
+ in msg->trx->bts. The caller can just use bts->c0 to get the first
+ TRX in a BTS.
+
+int abis_link_event(int event, void *data);
+ signal some event (such as layer 1 connect/disconnect) from the
+ input core to the stack.
+
+int subch_demux_in(mx, const u_int8_t *data, int len);
+ receive 'len' bytes from a given E1 timeslot (TRAU frames)
+
+int subchan_mux_out(mx, u_int8_t *data, int len);
+ obtain 'len' bytes of output data to be sent on E1 timeslot
+
+Intrface by Input Core for Input Plugins
+======================================================================
+
+int btslink_register_plugin();
+
+
+Configuration for the E1 input module
+======================================================================
+
+BTS
+ BTS number
+ number of TRX
+ OML link
+ E1 line number
+ timeslot number
+ [subslot number]
+ SAPI
+ TEI
+ for each TRX
+ RSL link
+ E1 line number
+ timeslot number
+ [subslot number]
+ SAPI
+ TEI
+ for each TS
+ E1 line number
+ timeslot number
+ subslot number
+
+
+E1 input module data model
+======================================================================
+
+
+enum e1inp_sign_type {
+ E1INP_SIGN_NONE,
+ E1INP_SIGN_OML,
+ E1INP_SIGN_RSL,
+};
+
+struct e1inp_sign_link {
+ /* list of signalling links */
+ struct llist_head list;
+
+ enum e1inp_sign_type type;
+
+ /* trx for msg->trx of received msgs */
+ struct gsm_bts_trx *trx;
+
+ /* msgb queue of to-be-transmitted msgs */
+ struct llist_head tx_list;
+
+ /* SAPI and TEI on the E1 TS */
+ u_int8_t sapi;
+ u_int8_t tei;
+}
+
+enum e1inp_ts_type {
+ E1INP_TS_TYPE_NONE,
+ E1INP_TS_TYPE_SIGN,
+ E1INP_TS_TYPE_TRAU,
+};
+
+/* A timeslot in the E1 interface */
+struct e1inp_ts {
+ enum e1inp_ts_type type;
+ struct e1inp_line *line;
+ union {
+ struct {
+ struct llist_head sign_links;
+ } sign;
+ struct {
+ /* subchannel demuxer for frames from E1 */
+ struct subch_demux demux;
+ /* subchannel muxer for frames to E1 */
+ struct subch_mux mux;
+ } trau;
+ };
+ union {
+ struct {
+ /* mISDN driver has one fd for each ts */
+ struct bsc_fd;
+ } misdn;
+ } driver;
+};
+
+struct e1inp_line {
+ unsigned int num;
+ char *name;
+
+ struct e1inp_ts ts[NR_E1_TS];
+
+ char *e1inp_driver;
+ void *driver_data;
+};
+
+/* Call from the Stack: configuration of this TS has changed */
+int e1inp_update_ts(struct e1inp_ts *ts);
+
+/* Receive a packet from the E1 driver */
+int e1inp_rx_ts(struct e1inp_ts *ts, struct msgb *msg,
+ u_int8_t tei, u_int8_t sapi);
+
+/* Send a packet, callback function in the driver */
+int e1driver_tx_ts(struct e1inp_ts *ts, struct msgb *msg)
+
+
+struct e1inp_driver {
+ const char *name;
+ int (*want_write)(struct e1inp_ts *ts);
+};