GSM's data-over-cellular facilities are part of the Phase 2 portion of the GSM MoU. The key to delivering these enhanced services is something called SS 7 (Signalling System Number 7), a robust set of techniques or protocols designed to provide fast, efficient, reliable transfer and delivery of signalling information across the GSM network and to support both switched voice and non-voice applications. SS 7 is essentially the multimedia specification of GSM, proving data (data/fax capabilities), sound (voice mail) and images (fax mail) to the user.