ssing traffic. That traffic may be flow only one way or both ways, and if the equipment or people at the site are busy or engaged with one site, it may not be possible to connect to a third site simultaneously. From a slightly different perspective, if two sites are busy, then the likelihood of either of them becoming engaged with a third site is limited. Therefore, use of the other possible links is significantly less likely, SO why spend money on all that capacity. However, without a fully meshed network, ways to commu- nicate between all the sites must be established, and the answer is switching. The carrier might provide the switching function, or it can be done with premises equipment. One last point is that meshing might occur at any one of the commu- nications layers, all the way from the physical layer through and including layer 4 of the OSI stack. Virtual paths at layer 1 are created in the electrical domain, across disparate physical facilities. Linking of virtual paths involves a com- bination of several factors such as physical aspects of the connector, a match between transmit and receive pins or pairs, signal polarity, and clocking. For example, digital cross connect switching systems are used to create a virtual path through the transmission network when provisioning private line services. POTS/ISDN switches and their signaling systems create virtual paths through the same transmission network to support voice grade connections enabling telephone calls, facsimile, and modem transmission. The mechanics of the connection 5 NETWORK ARCHITECTURE, FACILITIES, AND SERVICES involve connecting a transmit/receive pair on one side of the switch- ing system to a receive/transmit pair on another facility. ATM network architecture includes a virtual path layer, inside which virtual channels or circuits are created and placed. Routing and switch- ing are performed according to information in the virtual path identi- fier and virtual circuit identifier sections of the ATM cell header. The linking term is also ap