 Examples This is the method used to support multiple terminals on an ISDN user site that all belong to the same ISDN network address. The GSM application of SAPI and TEI in LAPD and LAPDm (see Fig. 3.21) is very different to ISDN since the connectivity structures to support are different: LAPDm is a variant of LAPD used in the GSM radio network. LAPDm uses the SAPI parameter for defining priorities between different types of signals, which is not in line with the intended use of SAPI. The TEI is (most likely) not used at The T-L table defines the Li-AAli relations. The LAPD agent must deliver the Li to the actor. It uses its TEI parameter for that, which is also not in line with the intended use of TEI. The model in Fig. 3.21 shows that LAPD must perform discrimination between data to be relayed in this network and data to be terminated in the BTS. Accord- ing to AMLn, this requires an interface type identifier (ITi) in the LAPD proto- col. LAPD uses the SAPI parameter for that, where SAPI=0 defines data to be relayed and all other SAPI values define data to be terminated. Using SAPI as an ITi is not in line with the intended use of SAPI. There is a lesson to be learned from this: the author initially learnt about LAPD from the LAPD standard (and therefore, basically, its use in the ISDN context). When later encounting GSM, he naturally expected that TEI and SAPI were used as described in the LAPD standard. It took him many frustrating hours to under- stand the LAPD applications in GSM. After finally understanding the actual needs in the BTS, and translating those needs to AMLn defined tables, it became clear how and why SAPI and TEI were used in GSM the way they are. If AMLn had been available when GSM was standardized, other more relevant names for SAPI and TEI would have been used in describing LAPDm (probably also another name would have been chosen for the protocol). It had saved a lot of time for all engi- neers who over the years have tried to understand what is going on in the BTS. 3.4.3 Switched Routes 