9 Special protocol operation on SAPI=0 and SAPI=3

3GPP44.006Data Link (DL) layer specificationMobile Station - Base Station System (MS - BSS) interfaceRelease 16TS

This sub-clause summarizes the specialities of the LAPDm protocol regarding the acknowledged mode operation on SAPI=0 and SAPI=3.

The purpose is to adapt the multiple frame operation of LAPDm to the needs and the characteristics of the GSM system.

The adaptations and simplifications specified in this sub-clause are mandatory for both entities, Mobile Station and Network for SAPI=0 and SAPI=3.

As indicated in sub-clause 8, the protocol operates on SAPI=0 in the following way:

‑ Data link establishment for SAPI=0 is always initiated by the Mobile Station.

‑ Data link establishment according to the contention resolution procedure (see sub-clause 8.4.1.4) is supported when initiated by the MS on the main DCCH immediately after "immediate assignment" of that radio channel. In other cases of link establishment contention resolution is not used.

‑ When changing the radio channel during assignment or handover procedures (layer 3 procedures), in acknowledged mode on SAPI=0, the MS supports continuous transmission of layer 3 messages without loss by offering the SUSPEND, RESUME and RECONNECT primitives, as specified in sub-clause 8.4.3. However, duplication of at most one layer 3 data unit (in the MS to network direction) is possible.

For protocol operation on SAPI=0 as well as on SAPI=3 the following holds:

‑ For multiple frame operation, the window size k = 1 shall be used.

‑ The data link layer entity is not allowed to transmit an RNR frame and therefore shall never enter the own‑receiver busy state. The same applies to the peer‑receiver‑busy condition. This is applicable, because buffer capacities necessary for avoiding such exception conditions are limited.

‑ When receiving, the data link layer entity may ignore RNR frames without notification.

‑ The procedure of enquiring the peer entity status by spontaneous transmission of a RR or REJ command frame, as indicated in sub-clauses 6.8.5 and 6.8.7, need not be supported. However, reaction on receiving such frames shall be as specified throughout the present document.