The detailed physical configuration of the Gb interface is subject to negociation between operators and equipment providers and is out of the scope of the present document.
Point-to-point physical lines or an intermediate Frame Relay network may be used. In the latter case, the two ends of the Gb interface may use different types of physical interfaces.
Each of the physical layer of the Gb interface shall conform to one of the following FRF 1.2 clauses. This does not mean that each BSS and SGSN equipment has to support all of these physical interfaces, it means that the supported physical interfaces shall be compliant with the corresponding clause of FRF 1.2.
clause 2.1.1: ANSI T1.403.
clause 2.1.2: V.35, physical circuit and DTE/DCE interface clauses.
clause 2.1.3: G.703.
clause 2.1.4: G.704.
clause 2.1.5: X.21.
clause 2.1.6: TIA/EIA-530-A.
clause 2.1.7: HSSI.
The Gb interface may be multiplexed with the A interface on the same E1 (2048 kbit/s), or T1 (1544 kbit/s) digital path. In case of E1 interface, ITU-T Recommendation G.704 shall be applied according to FRF 1.2 and TS 48.004
as appropriate, and in case of T1 interface ANSI T1.403 shall be applied according to FRF 1.1 and TS 48.004
In the case where multiple 64 kbit/s channels are used on an E1 (2048 kbit/s), digital path on the Gb interface, it is recommended to aggregate them into one nx64 kbit/s channel, see ITU-T Recommendation G.704, clause 5 and included clauses. In case where multiple 64 kbit/s channels are used on a T1 (1544 kbit/s) digital path on the Gb interface, it is recommended to aggregate them into nx64kbit/s (where 2 ≤ n ≤ 24) channel, see Bellcore TR NWT 1203. This approach optimises the use of the available bandwidth by taking advantage of the statistical multiplexing at the upper layer. However, this approach requires that no slipping occurs between individual 64 kbit/s channels e.g. when passing through intermediate equipment between BSS and SGSN.
The error rate experienced at the physical layer between the BSS and the SGSN shall be compatible with the operation of the upper layers.
The physical channels on the Gb interface shall be permanently reserved by means of administrative procedures.