 MGW. Edge Nodes The SG encapsulates SS7 signalling (e.g. an ISUP message) within an IP packet. Conversely, it extracts SS7 signalling (e.g. an ISUP message) from an IP packet and transports it using MTP. Figure 11.11 Changing the transport protocol of ISUP in a signalling gateway 11.4.2 SECURITY FUNCTIONS The MGW security resources embrace various types of tunnelling, such as L2TP and GTP-U, IPsec and encryption. The MGC level controls the security measures taken in the MGW. 11.4.3 MANAGEMENT, BILLING CDR (call detail record) generation is one of the functions. 11.4.4 ACCESS SERVERS In general, present-day fixed access servers can be separated into two categories: narrowband access servers (NAS); and broadband access servers (BAS). Narrowband access servers, are used for Internet dial-up applications and for providing Internet access to businesses via E1, primary rate access (PRA) or fractional E1 access. As dial-up traffic increases, local exchanges are subject to longer holding times (typically 20 instead of 3 minutes per call). When used for Internet traffic, the capacity of the circuit mode transport part of the network is not being used in a cost-effective way - a 64 kbit/s channel is used per modem, whereas on average only a fraction of this capacity is utilized because of a bursty traffic pattern. Many Internet service providers have also expressed interest in outsourcing access server capabilities (Internet offload) to a telecom operator, and the narrowband access server migrates towards the local exchange or even towards remote subscriber switches (See for example Figure 13.5). Broadband access servers, which are part of a DSL solution, are used to consolidate permanent virtual circuits (PVC) and to provide frame relay, ATM or IP connectivity over the backbone. They are also used for selecting ISPs and corporate networks. Also the broadband access server can be situated close to the local exchange. Moreover, since Understanding Changing Telecommunications - Building a Successful Telecom Business many fu