gateways, and interoffice transmission equipment. Transport services include switched services, nonswitched services, and virtual private networks (VPNs). Switched Services There are two main types of switched services: public and private. Switched public services include local calling, long-distance calling, toll-free calling, international calling, directory assistance, operator assistance, and emergency services. Switched private services can be switchable either because they are deployed within the CPE or because they are deployed on a carrier basis. With CPE-based services, you can add capabilities to the telephone systems onsite in the PBXs-a feature called electronic tandem networking. For example, you can use electronic tandem networking to gain some flexibility in routing around congestion points: If the preferred leased line from switch A to switch B is occupied or not available, the switch can decide how to reroute that traffic to still reach switch B, but through a different series of leased lines. However, because leased lines (also referred to as tie trunks) are mileage sensitive and dedicated to individual customers, they are very expensive; thus, not much private voice networking is done over tie trunks because there are several more attractive solutions, such as VPNs, which are discussed shortly. With carrier-based switched private services, a centrex customer could parti- tion and implement extensions across multiple local exchanges and thereby be able to switch traffic between those locations. Nonswitched Services Nonswitched services include leased lines, foreign exchange (FX) lines, and off- premises extensions (OPXs). With leased lines, two locations or two devices are always on, using the same transmission path. FX lines enable a toll call to appear to be a local call. For example, you might have a dedicated leased line that runs from your customer premises to a local exchange in a distant area where you call large numbers of customers. When any- one behind your PBX dials a number associate