== User-Plane Traffic via RTP

RTP (Realtime Transfer Protocol) is a protocol for streaming audio
and video data.  It is specified by IETF RFC 1889.

OsmoBTS A-bis/IP implements RTP as transport medium for circuit-switched
user-plane traffic, contrary to the E1 sub-slot based transport
specified in 3GPP TS 08.60.

The RTP transport endpoint parameters are configured using the RSL User
Plane Transport Management procedures described in <<user_plane_txp_mgmt>>.

RTCP is implemented in addition to RTP, on a UDP port number of the RTP
port incremented by one.

=== RTP Payload Formats

The RTP payload format depends on the voice codec used on the radio
channel.  The OsmoBTS is simply passing the GSM speech frames between
the Um radio interface channels and the RTP payload (and vice-versa).

No transcoding function is implemented in the BTS!

.RTP Payload formats
[options="header",width="60%",cols="15%,15%,70%"]
|===
| TCH | Codec | RTP payload format specification
| TCH/F | FR | IETF RFC 3551 Section 4.5.8
| TCH/F | EFR | IETF RFC 3551 Section 4.5.9
| TCH/F | AMR | IETF RFC 4867
| TCH/H | HR | IETF RFC 5993
| TCH/H | AMR | IETF RFC 4867
|===
