this is known as 'break-out'. This point is called the break-out point. The new IMS control part is equipped with a break-out call function. Access Network Access node Server Client Layer 2 aggregation/ Edge node switching/multiplexing e.g. router Backbone network Transparent transfer Transparent transfer ATM, FR, Ethernet 01011000100 01011000100 Transmission section 1 Figure 13.16 A layered example of an access network There is always a need for layer 2 functions, such as framing (numbering and error control) in all types of packet-mode networks. The error control needs are large at the access part, since the quality of the line is lower there (radio, copper) than in the core network (fibre). A lower quality means more bit errors. X.25 layer 2 (LAPD) is an example of a strong layer 2 protocol, permitting transport over low quality lines. Figure 13.16 provides an example of an end-to-end layered view with access focus. Framing can be done by layer 2 in for example frame relay, ATM, Ethernet, PPP/SLIP. Layer 2 might change many times over a long connection, mainly depending on the type of traffic carried and its requirements on error rate, QoS differentiation needs and quality of the transmission line. Layer 1 can be PSTN base band, ISDN base band, xDSL above the frequency band of PSTN or PSTN/ISDN, SHDSL, PDH base band, STM-1, STM-X, SONET to take a few. Layer 1 normally changes many times over a long connection, mainly depending on transmission media and capacity need. Below layer 1 is the transmission medium, possibly split into wavelengths using dense wavelength division multiplex (DWDM). The physical layer and the link layer terminate at the edge node (almost by definition). The edge node/media gateway takes care of conversions to core technology in layers 1 and 2 and possibly even higher. The network layer should preferably be end to end, since it carries the necessary number to the called subscriber. In addition it often carries a security protocol (IPsecurity), for which end-to-end continuity is preferred.