L1 Voice compressed AAL2 Data AAL5 Data IP Voice over IP Data FR Voice over FR Figure 3.31 Various transfer modes networks into one network will take time'. See the CS (circuit switched or circuit mode) part in Figure 3.18. In any case protection of investments already made is likely to call for a kind of migration path. Examples of paths are included in the book. See Chapters 8, 13 and Appendix 3. The uncompressed IP header (especially IPv6) for a voice packet is in reality larger than the payload (IPv6/UDP/RTP 60 bytes, voice payload 32 bytes). For ATM there is no need for a tail since the length is always the same (53 bytes). With an increasing number of services and fewer networks the adaptation and enabling area becomes a key part from a business point of view. 3.9.1 STRATIFICATION When 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)