 IP AND TELECOM FOR BROADCAST ENGINEERING AND OPERATIONS Application Presentation ASCII Binary Virtual Session Terminal Transport TCP & UDP Network Data Link Virtual Tributary Physical Section Media FIGURE 5-5 Example of Three Layering Models The OSI model has been separated between layer 2 and layer 3. This is the division between hardware on the lower level and software on the higher level. The Internet model is left intact and placed above the SONET model. The VT layer has been added to the SONET model. Layers 2, 3, and 4 have been shaded with four different levels or shades of gray, reflecting the dividing layer between hardware and software in the OSI model. There is a similar division between the IP, and lowest layer in the Internet model and the VT/STS, or highest layer, in the SONET/SDH model. The data link layer is a point of commonality across all three. That is, voice and data networks use exactly the same physical layer network interfaces and protocols as the Internet does. For example, an unchannelized E1/T1, E3/DS3, or OC3/STM1 facility can be used to support all types of traffic defined in the site location architecture. Most depictions of SONET/SDH layering don't include the virtual tributary (VT) layer. If this layer is included on top of the SONET/ SDH model, it becomes a direct fit, or interface to the Internet model for plesiochronous (PDH) point-to-point links or leased/private line facilities. Another point worth mentioning is the fact that many purveyors of network equipment and services are offering direct 5 NETWORK ARCHITECTURE, FACILITIES, AND SERVICES interface between IP and SONET/SDH synchronous transport streams. In and of itself, it's an incremental step. However, if this physical layer capability is combined with differentiated services in the network, traffic aggregation, and type of service (TOS) capabil- ities in new and emerging network equipment, the result is a poten- tially dramatic and profound impact on ISDN/PSTN and plesiochronous (PDH) network facilities and services. Co