aling system featured flexible message formatting for high-speed data transmission, designed to support ISDN and the transfer rates of SDH. SS7 also defined a separate network for transporting signalling messages, for far more than one trunk circuit. In the early phases of digital telephony, signalling and voice used the same path but could not use it at the same time. SS7 offers a signaling gateway into other network types, such as the mobile cellular operator's core (or fixed) network. Telecommunications infrastructure includes not just interlinked national public switched networks, be they circuit or packet switched, but also private networks - leased lines for data and voices, sometimes with private INFRASTRUCTURE TO 2030: TELECOM, LAND TRANSPORT, WATER AND ELECTRICITY - ISBN 92-64-02398-4 - © OECD 2006 2. TELECOMS INFRASTRUCTURE TO 2030 switches, for use by companies and public sector organisations. The interconnection of these two infrastructures has been the subject of much regulation. Private virtual networks (PVNs) have also been built on top of the public network. Future of the fixed network Under the pressures of mobile infrastructure competition, the arrival of Internet voice (VoIP) and regulation to open up the local loop, the fixed infrastructure - local loop and trunk network - is being divided up as unbundling is taking place while the profitable voice margins disappear. In response, incumbent operators of fixed networks are considering "converged" architectures for "co-opetition" with mobile and VoIP. An example is BT's 21st Century Network (21CN) architecture designed to provide both mobile connection and fixed line to and within the home ("bluephone") and office with a trunk network designed to be the interconnect and backhaul transport mobile traffic for other carriers on a wholesale basis. Fixed-to-mobile "convergence" is changing the fixed infrastructure architecture for all large fixed line operators. Fixed wireless access by radio - narrowband and broadband - has been around for decades, a