E1 interface, layer 2

Hi,

i am staring at the sources right now, figuring out how openbsc uses which part of misdn to send/receive frames over the e1 link. I think it's doing something like this (please correct me if i am wrong:

- use timeslot 1 in hdlc mode for the signalling link - use misdns layer 2 to handle the TEI assignment, etc

What i am wondering about is: - which ETSI standard describs layer 2 for GSM abis?

Thanks a lot in advance for your feedback. :-)

Mit freundlichen Grüßen