M cells of ATM cells PLMN, IP, FR, X.25 Access node Access network Backbone network Access network (here ATM) AAL only at the gateway Figure 11.2 Illustration of a possible network structure: various access types, a gateway and a uniform ATM backbone. Layer 3 below AAL in the gateway is reserved for possible layer 3 signalling and addressing Edge Nodes SS7 ISUP ISUP/IP SIGTRAN Softswitch PSTN/ISDN switch network H.248 E1 speech Voice channels gateway RTP/UDP/IP Figure 11.3 A softswitch configuration Access Transport Service network Access transport = Optical rings Internet used when applicable server Other operator's server Large/medium enterprises network SGSN GGSN server control function Access Access IP packet backbone router transport Frame relay transport router Circuit mode, Ethernet (MGW) router (MGW) ATM, IP Access IP/VPN (MGW) transport (MGW) Access Access transport Access Access transport DSLAM transport GSM/EDGE transport Homes & SMEs WLAN hot spot Resource sharing Copper xDSL Fibre Ethernet DSL WCDMA Figure 11.4 Edge nodes constitute the interface between access and core network of edge nodes. See Figure 11.4. When MGW and edge router are collocated the router is often called a site router. 11.2 ACCESS AND BACKBONE NETWORKS 11.2.1 ACCESS CONNECTIVITY During all processes of change, the expensive access network has been a conservative power. Let us mention the digitization, which by definition is not yet complete for PSTN in the access part. The convergence of the existing networks towards a packet network Understanding Changing Telecommunications - Building a Successful Telecom Business with broadband capability must be profitable with strict control of investments. The access network investments are by far the most critical in order to reach that goal, since, for tradi- tional operators, the access networks represent more than 50 % of the overall investments. Fortunately all the fixed transmission media, whether originally designed for narrowband or higher bandwidths, have proven surprisingly feasibl