08.063GPPRelease 1999Signalling transport mechanism Specification for the Base Station System - Mobile Services Switching Centre (BSS - MSC) interfaceTS
a) This Technical Specification is applicable to the signalling between radio subsystems (BSS) and mobile switching centres (MSCs) in GSM PLMNs. It provides a minimum set of MTP requirements that may be implemented at a BSS or MSC, whilst maintaining compatibility with the implementation of a full specification of the MTP.
b) The Technical Specification defines the interface at the 64 kbits/s boundary to the BSS or MSC and applies primarily for digital access arrangements, the use of analogue arrangements isan option for PLMN operator.
Also, the Technical Specification defines the interface at the 56 kbits/s boundary to the BSS or MSC for T1 links.
c) The security of signalling links is a PLMN operator concern , however it is recommended that in the case where more than one multiplex system is required and security reasons dictate the use of a multiple link linkset, then each signalling link should be assigned in a different multiplex system. It is however noted that this is of little benefit if diversity of routing of the multiplexes is not used.
d) For initial implementations only the associated mode of signalling may be applicable at the BSS. Future evolution or economic reasons applicable to the interface may however make the use of STP working at the BSS attractive, in which case some of the simplifications in this paper will not apply.
e) A variety of information types may be supported by the signalling system, e.g. relating to circuit switched call control and packet communication. These are fully defined in the service series of Technical Specifications (the 3GPP TS 02.xx series and in IS-104  for PCS 1900).
f) For E1 link usage, the CCITT recommendations concerning the MTP shall be taken as being requirements unless covered by a statement in this Technical Specification.
g) For T1 link usage, the ANSI recommendations concerning the MTP  shall be taken as being requirements unless covered by a statement in this Technical Specification.