TDM-based GSM architecture with TRAUs¶

In the traditional GSM architecture prior to introduction of IP-based transport, all of the numerous different network elements were interconnected with TDM links, based on E1 circuits in Europe or T1 in North America. These TDM links carried everything: signaling traffic on CCS timeslots, voice call traffic and ISDN-style circuit-switched data calls on dedicated timeslots or sub-timeslots, and in later years GPRS packet data in Frame Relay transport.

GSM was philosophically envisioned as wireless ISDN, and the first generation of GSM MSCs were ISDN switches (see Siemens EWSD) with modified software. However, ISDN switches operate on 64 kbit/s B-channels, while GSM traffic channels, even at full rate, provide significantly lower payload bit rates. Speech calls over GSM are compressed with one of several voice-oriented lossy codecs (aka a vocoder), whereas in data calls between GSM and ISDN the rate adaption functions of V110 are split between the MS and the TRAU.

Whether each given call carries speech or data, the E1 or T1 circuit coming out of the BTS devotes only a 16 kbit/s or 8 kbit/s sub-timeslot to each active call (each GSM timeslot or half-timeslot, for TCH/F or TCH/H), not a full 64 kbit/s timeslot! Thus a network element is needed somewhere along the path between the BTS and the MSC (can be before or after the BSC) that expands each per-call 16 kbit/s or 8 kbit/s voice or data channel to an ISDN-compatible (and PSTN-compatible) standard 64 kbit/s channel. This network element is called TRAU, which stands for Transcoding and Rate Adaption Unit. For speech calls the TRAU implements the voice codec (transcoding); for data calls it completes the job of V.110 rate adaption that was started in the MS.

Each major vendor of GSM network infrastructure equipment made their own MSCs, BSCs and TRAUs. Per GSM architecture specs, the TRAU is logically a part of the BSS (Base Station System, a term that encompasses BSCs and BTSs), but its concrete implementation and physical location are up to the BSS vendor. From an operating cost point of view, colocating the TRAU with the MSC makes the most economic sense: the number of E1 (or T1) circuits needed on the MSC side of the TRAU is 4x or 8x the number of same-type circuits needed on the BSC/BTS side, hence colocation of the TRAU with the MSC greatly reduces leased line costs. Both Nokia and Siemens devised BSS architectures in which the TRAU is logically a part of the BSC, but can be physically located anywhere between the BSC and the MSC, including a rack in the same exchange room with the MSC.

The one physical implementation of GSM TRAU we are currently experimenting with is Nokia TCSM2.