Trying to understand Ericsson Abis-IP + OML

Harald Welte

2010-11-29

I've recently been able to look at packet traces of an Ericsson BTS (which they call RBS) and have been working on understanding their proprietary Abis-over-IP protocol stacking and OML layer.

By now, all equipment makers have migrated their BTS products from classic TDM (E1/T1) lines to some form of IP-based back-haul. However, the GSM specs as published by ETSI and 3GPP still only specify the E1/T1 transport layer, and every vendor seems to invent their own protocol for back-haul.

What we already know (and support) is the ip.access style Abis-over-IP, where they have their IPA multiplex layer inside TCP. Inside that they then have pretty straight-forward 08.58 (RSL) and 12.21 (OML) messages.

With Ericsson, they use a stacking like this: Ethernet, IPv4, L2TP, custom-HDLC, RSL/custom-OML.

The custom HDLC layer (I have called it Ericsson HDLC, or short EHDLC) seems to work quite different from all other forms of HDLC that I've seen:

1 Byte Address
1 Byte Length (including the header!)
1-2 Bytes Control
n Bytes Data

The Control octets are just like in any HDLC, i.e. you have U/I/UI/S Frames and see commands like SABME, UA, RR, ... It mostly uses a two-octet control word that has both N(R) and N(S) for acknowledgements. At the beginning they actually do a XID exchange that seems compliant with ISO 8885 (if you cannot find that document, try reading the AX.25 spec instead, its XID is compatible with ISO 8885).

I have not fully understood the Address octet yet. I see lots of changes in the upper three bits, and it seems there is a SAPI or TEI that is the lower 5 bits of the octet.

Having a length field in an HDLC header instead of any flag bytes is indeed very uncommon to see.

The OML layer (called OM2000) is completely proprietary and shares nothing with the GSM specs 08.59/12.21 apart from the three byte header. However, I have managed to build a pretty complete dissector which you can find together with the EHDLC code in this rbs2409 patch which applies on top of the generic wireshark abis_oml.patch

It is my hope that this information (and particularly the dissector) will prove a valuable resource once we add Ericsson BTS support to OpenBSC. If there is anyone who can provide us real BTS (RBS) hardware, please let me know :)