The present document is an introduction to the 48.5xx - 48.6xx series of 3GPP Technical Specifications and deals with the definition of the Base Station Controller (BSC) to Base Transceiver Station (BTS) interface defined inside the Base Station System (BSS) for the GSM system. These Technical Specifications define the basic interface with some identified options requiring further elaboration. The BSC-BTS interface is mandatory within GSM only if the BSC and the BTS are not colocated.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

For the purposes of the present document, the abbreviations given in TR 21.905 apply.

The BSC-BTS interface shall be capable of supporting all the services offered to the GSM users and subscribers. In addition it shall also allow control of the radio equipment and radio frequency allocation in the BTS.

The BSC to BTS interface specifications should allow the following:

1. Connection of various manufacturers BTS/TRX to the same BSC, according to the location of the transcoder.
2. The use of several manufacturers BSC to the same type of BTS/TRX, according to the location of the transcoder.
3. The use of the same BTS/TRX in any PLMN, according to the location of the transcoder.
4. The use of the same BSC in any PLMN.
5. Separate evolution of BSC and BTS/TRX technology.
6. Separate evolution of O & M facilities.
7. Sub-multiplexing of speech channels on a 64 kbit circuit.
8. Evolution towards lower speech coding rates.
9. Location of transcoders either in BSC or in BTS.
10. Support of all services defined in the 02 series of 3GPP TS Technical Specifications.
11. A stepwise expansion of capacity in a BTS.
12. Different physical solution of the various equipment in the BTS.
13. Support of a single TRX forming a BTS.
14. Support of a set of TRX'S forming a BTS.
15. Support of a BTS as one entity.

The interface is defined to be at the terrestrial link of a remote BTS connected to the BSC. The BSC to BTS interface is specified by a set of characteristics, including:

1. physical and electrical parameters;
2. channel structures;
3. signalling transfer procedures;
4. configuration and control procedures;
5. operation and maintenance information support.

The definition of the BSC to BTS/TRX interface follows a layered approach similar to the ISDN. Layer 3 is for the most part based on Technical Specification TS 48.008 with additional procedures for control of radio resources. Layer 2 is based on the LAPD protocol. Layer 1 is either digital (at a rate 2048 kbit/s with a frame structure of 32 x 64 kbit/s time slots or at a rate of 64 kbit/s) or analogue with the data being passed by the use of modems (this latter case is a national option). In the case that the transcoder is positioned outside the BTS, the overall one way propagation delay between the Point of Interconnection to PSTN/ISDN and the MS is limited to 1,5 ms (approximately 300 km). With the transcoder in the BTS, the limit is 6,5 ms (approximately 1 300 km). These limits may be subject to increase resulting from savings made in the overall network. See also Technical Specification TS 43.050.

The full structure of the Technical Specifications specifying the BSC to BTS interface are as follows: