one sender/receiver provides tones to be sent to and received from end users. DTMF tones can be sent to the far end of the connection as requested by a mobile station. DTMF tones can also be used with interactive messaging applications. Interactive messaging. The interactive messaging application provides subscribers with information messages on special conditions in the network or conditions that pertain to the service in use. To reduce the footprint, the signalling gateway application, which provides signalling interworking between IP, ATM and TDM networks, can be co-located with the media gateway node. See Figure 11.11. 11.4.1 QoS FUNCTIONS Quality of service (QoS) is supported by the media gateway through a combination of ATM traffic management, multi-protocol label switching (MPLS) and differentiated ser- vices (Diffserv) for IP. The selection of the AAL framing component is also QoS related as well as the protocols RTP, RTCP and RTSP. The MGC level controls the QoS handling in the 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).