 APIs. For interconnected networks see Chapter 15. The control layer is related to MGW/edge node control (See Figure 14.1). There isn't normally a 1:1 relation between edge nodes/MGW:s and media gateway control nodes/control functions, unless the nodes are built in one physical unit. Figure 11.7 gives better perception. However, GGSN and releases of SGSN and other nodes are typically implemented in one physical unit. In homogeneous environments there is no need for a gateway, see for example Figure 14.5. Figure 14.2 shows a number of control parts or domains in the control layer, the common home subscriber server (HSS) function and a signalling gateway server function. The signalling traffic gateway is, strictly speaking, part of the connectivity layer. The description of control in this chapter covers control of the following domains of networks: Access transport Access transport = Optical rings Control network used when applicable Service layer (+ possibly IN service logic) Large/Medium Enterprises Large/Medium Other operator's Enterprises server server Internet network server. "LAN" server SGSN/GGSN server "LAN" Access function Ethernet transport Access router Packet backbone router transport (MGW) router (MGW) Access Access transport transport (MGW) (MGW) WLAN hot spot Access Access transport transport Access To be added: transport Cable modem GSM/EDGE Homes & SMEs Copper xDSL Resource sharing Fibre Ethernet DSL WCDMA PSTN/ISDN Figure 14.2 Three control parts or domains in the control layer Control Network Mobile and fixed voice networks ('circuit-switched domain'). ISUP/BICC signalling. Data uses the connectivity layer Mobile data networks ('packet-switched domain') as introduced in GPRS Mobile and fixed IMS multimedia networks. Primarily SIP signalling. Carries media individually and as multimedia Signalling over IP solutions. Gateways SIP-ISUP. Sigtran. 'Softswitch' with SIP/H.323 signalling. See Figure 14.3, central part. Voice trunking with H.323 gatekeepers accord- ing to Chapter 8, Section 8.8.3 Figure