uipment is currently manufactured. We are in the early stages of deploying new muxes that operate at OC-768 and support 40Gbps. As of mid-2006, use of OC-768 connections outside research or testing networks was very rare, largely due to the fact that they are very expensive. How- ever, given that it is the next logical speed step in the SDH/SONET hierarchy, it is predicted that there will be greater adoption of this standard going forward. On the other hand, some people feel that electronic muxes really are not suitable for the higher data rates being made possible by advances in optical technologies and that we should concentrate on moving to all-optical muxes and switches instead. How do the high optical carrier levels relate to all the lower-level signals out there-such as those from a 1.5Mbps T-1 or a 2Mbps E-1? There are mechanisms that enable us to map signal levels below DS-3 (i.e., below 45Mbps) into what SDH calls virtual containers or what SONET calls virtual tributaries. A virtual container or tributary basically defines the data structure for the transport and switching of sub-51Mbps network services such as DS-1, E-1, DS-2, and E-3. Table 4.4 shows the various line rates that are supported and what existing standard each refers to. For most people, this type of detail won't make or break success in the industry, but it's important to know that a virtual tributary or virtual container can provide a highway for lower-rate data signals to coexist in high-speed optical pipes. In contrast to PDH, SDH/SONET is a synchronous infrastructure. This means that each of the network elements draws its clocking pulse from one clocking source-so everybody is marching to the beat of the same drummer. Instead of using special framing bits to delineate channels, SDH/SONET uses a special pointer bit in front of each conversation that essentially says "start of a new con- versation." When it's time to drop that channel off at a customer's premises, it is possible to identify it by its pointer bit and extract it without h