= OsmocomBB layer23 architecture =

layer23 is an (incomplete) MS-side implementation of the L2 and L3 GSM
protocols as described in GSM TS 04.06, 04.08 and others.

== Interfaces ==

L1 (on the phone) uses the L1CTL protocol to talk with layer23 (on the PC).

L2 (inside layer23) uses the RSLms protocol to talk with the L3 (inside layer23)


=== RSLms ===

RSLms is modeled after the GSM TS 08.58 Radio Subsystem Link protocol.  Despite
being designed for the network side, RSL seems a good match for the L2/L3
interface inside a MS, too.

At least the RLL (Radio Link Layer) part of RSL is 100% as applicable to the MS
side as it is for the ntwork side.

==== Lower interface (L2 to RSLms) ====

Layer2 calls rslms_sendmsg() with a msgb that has the msgb->l2h pointing to a
RSL header (struct abis_rsl_common_hdr).

==== Upper interface (L3 to RSLms) ====

Layer3 calls rslms_recvmsg() with a msgb that has the msgb->l2h pointing to a
RSL header (struct abis_rsl_common_hdr).

There are utility functions like rslms_tx_rll_req() and rslms_tx_rsll_req_l3()
for creating msgb's with the apropriate RSL/RLL headers.


=== LAPDm ===

LAPDm is the GSM TS 04.06 protocol

The lower interface (to L1) is using L1CTL

The upper interface (to L3) is using RSLms