controllable. In fact, the network operator or service provider should be in firm and complete control of any and all aspects of their network except things and effects they can't control such as the weather, errant backhoes, unpredictable motorists, and similar forces known to occasionally inflict themselves on power and telephone poles. In addition to the cloud, other symbols are used to denote different types of facil- ities. Accordingly, a set of symbols should be adopted to define generic types of equipment and facilities to make block diagrams and top- ology maps. Table 3-2 shows some simple examples, most of which are used in this book. 3 CONTENT TRANSPORT NETWORK DEFINED TABLE 3-2 The famous cloud that typically summarizes important details inside the network. Here, it will be filled with one of four shades of gray. The darkest layer corresponds to Layer 1. Layers 2 and 3 represent continuous and discontinuous bandwidth with Services Layer in white at the top. Layer 1 Point-to-point wired or wireless transmission facilities. Truly the "media layer" including electrical, radio or optical transmitters, receivers, repeaters, and add-drop multiplex. Arrows indicate full duplex path or route. Some call this level the "bit level" because it is in a form of continuous clocked bits Layer 1 cross-connect facility. Used to groom Layer 1 bandwidth into and out of Layer 2/3-switch and router equipment, and provision point-to-point private line facilities. Not to be confused with Layer 2 Circuit Switch, or other Layer 2 functions. In PDH networks, this layer is still considered the bit level, however framing makes it's way into the picture in the form of channelization built on individual 64Kbs timeslots. Layer 2 Circuit Switch represents switching required to support POTS and ISDN service. When other framing and protocols replace POTS or ISDN, a different type switch is required. For example, Ethernet or ATM. Layer 2 Cell Switch - structured, continuous bandwidth Layer 3 Cell Switch - unstructured, discontinuous band