What protocol is used at air interface (Um interface) of GSM?

Hi there,
The GSM air interface is described in the ETSI specification TS 145 005
You can find it on the ETSI website http://www.etsi.org/ under "Services and Products" (you'll have to register).
Mosieur_Oiso

Hi

It seems can not download the specification from the ETSI website . Right ?

Hi,

Sure, you can download the spec from the 3GPP web site in the specification area:
go here: http://www.3gpp.org/specs/numbering.htm.
Hope this helps.

RFCMOS