f 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 feasible in the new telecom world. An example is the possibility to install VDSL (very high bit rate digital subscriber line) on copper pairs with a bit rate of about 50 Mbit/s. This requires of course a transmission technique that is tailored to the transmission medium. For mobile systems the investments can be based on upgrades utilizing existing fre- quency bands, with GPRS and EDGE as examples. This alternative is the cheapest but might not offer sufficient bandwidth for new services. The UMTS system employs a new frequency band but also makes better use of the existing band around 1800 MHz. Thus, the conservative access impact is not always true. The ongoing investments in mobile 2.5 G and in particular 3 G access systems, may in fact turn the access side more modern than the backbone. And other new access types such as Bluetooth and WLAN strengthen such a development. Existing media will successively be upgraded to a broader bandwidth. In any case, the number of access alternatives will increase rather than converge to a few standard alternatives. Mobile or fixed wireless, Cu pair, fibre and cable-TV will all remain as access media alternatives. The transmission media will for a long time carry different network services (PSTN, GSM, IP, ATM, frame relay, GPRS, UMTS). 11.2.2 CORE CONNECTIVITY For the network core the situation is different. The bit transport cost in this part is almost negligible compared with the access part. The reason is that this traffic is highly aggre- gated, compared with the mostly thin accesses. Therefore we can afford to build a fairly homogeneous backbone for the traffic transfer, opting for a standard type of packet trans- fer mode throu