ode in Fixed TDM The packet traffic (X.25, frame relay, ATM) often used/uses a certain leased transmission bandwidth between the packet switching points. A pure vertical solution uses dedicated transmission systems and cables as well. The SONET/SDH Phase. Managed TDM. Managed Cross-Connection The introduction of SONET/SDH removed the inflexibility of the manual cross-connection by introducing a new managed network level with 'cross-connect switches' operated by commands. The concept includes programmed automatic protection switching in case of faults. Stratification within the Packet Domain Both the frame relay and ATM transfer modes gained a strong foothold in the backbone. Now one could see the beginning of stratification, where frame relay traffic is carried in ATM cells, that are carried in SONET/SDH paths with capacities from the basic PCM level (T1 or E1) and upwards. The IP Layer. IP over ATM over SONET/SDH. IP over FR over SONET/SDH The IP protocol 'carries everything over anything'. 'Anything' could for example be frame relay or ATM. The stratification increases when IP packets are distributed to ATM cells Packet Backbone utilizing the AAL5 adaptation layer. ATM provides QoS to IP or other traffic, such as IP over AAL5 and voice over AAL1 or AAL2 adaptation layers. IP over ATM becomes a common stratification. Optical Point-to-Point Lines Develop into Optical Networks. One Window gives a 'Lambda' Capacity. Optical Cross-connections for Windows and Fibres The optical fibre capacity is split into wavelengths. The technique is called wavelength division multiplex (WDM). A recent development manages 160 wavelengths in one fibre. Figure 12.5 shows even greater capacity. Since such capacity is not often required, cheaper technologies can be chosen especially in metropolitan areas. An example is CWDM (coarse WDM) with 16 wavelengths. Figure 12.6 shows the development towards opti- cal networking. The forecasts around the millennium shift indicated a heavy increase in overall back- bone traffic. For fixed broadba