hen using a functional horizontal split of the network, each area represents a unique role or a logical network in the overall network. This role extends over the whole network. Networks and Technologies Conceptually, when applying a layered functional approach, the network is dissolved into horizontal slices, one for each functional area. Note that these slices also comprise functionality in customer premises equipment/networks! Layering opens up for stratification. Stratification means that different technologies with almost the same function in the network can be layered on top of each other. Stratification is commonly used in networks employing ATM in the packet backbone, for transport of TDM, X.25, IP and frame relay network services. In fact one network makes up the infrastructure for one or more other networks. See Figure 3.32 and Figure 3.33. Services Telephony (IP) VPN (IP) MM Switching and routing transfer mode(s) Carries everything over anything Frame relay Transmission PDH/SDH Fibre Figure 3.32 Example of stratification with up to three layered switching technologies. LL = leased lines Process Process Process layer Host-to-host layer IP network Internet layer Network interface hardware Ethernet layer Bearer network(s) Physical Communication Logical Communication Figure 3.33 A typical layered network Understanding Changing Telecommunications - Building a Successful Telecom Business As shown in Figure 3.33 the expression 'Network interface and hardware layer' can be used for underlying connectivity layers in IP stratification. A similar idea is used for tunnelling, a layering and stratification technique for pro- tection of data integrity and confidentiality in IP networks. Tunnels require 'extra' layers to encapsulate the often sensitive end-to-end user traffic. Tunnels are described in the Chapter 6. 3.10 THE CONTROL LAYER As seen in Figure 3.17 the control network will initially be more heterogeneous than other core layers. The present target control design is the IP multimedia subsystem, IMS, accord