The present document defines the structure of the physical layer (layer 1) of the BSS-MSC interface for supporting traffic channels. Use of the physical layer for supporting Signalling System No.7 signalling links is covered in Technical Specification TS 48.006. The physical layer is the lowest layer in the OSI Reference Model and it supports all functions required for transmission of bit streams on the physical medium. For the present document only digital transmission will be considered, the use of analogue transmission is a national concern.

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 gieven in TR 21.905 apply.

All ITU-T recommendations referred to are Blue Book. Layer 1 shall utilise digital transmission:

• at a rate of 2048 kbit/s with a frame structure of 32 x 64kbit/s time slots, as specified in ITU-T Recommendation G.705 clause 3 for E1 interface; or
• at a rate of 1544 kbit/s with a frame structure of 24 x 64 kbit/s time slots, as specified in T1.102 specification for T1 interface.

Therefore the physical/electrical characteristics are defined in ITU-T Recommendation G.703 for E1 interface or ANSI T1.403 specification T1 interface. The functional characteristics are defined in ITU-T Recommendation G.732 clauses 2 and 3 for E1 interface or ANSI T1.107 specification for T1 interface. Fault conditions should be treated in accordance with ITU-T Recommendation G.732 clause 4 for E1 interface or ANSI T1.403 specification for T1 interface. Speech encoding shall be the A-law or Mu-law (for North America) as defined in ITU-T Recommendation G.711. The idle pattern must be transmitted on every timeslot that is not assigned to a channel, and to every timeslot of a channel that is not allocated to a call. The idle pattern shall be 01010100 in ITU-T based systems and 01111111 in ANSI based systems. Synchronisation at the BSS for the transmitted 2048/1544 kbit/s bit stream shall be derived from the received 2048/1544 kbit/s bit stream. Data encoding is covered in Technical Specification TS 48.020.