Toll bypass allows customers to bypass the PSTN. The PSTN consists of the tandem time-division multiplexing (TDM)based switches used for long-distance (or toll) voice calls. Enterprise customers who typically depend on the PSTN for their interoffice voice traffic avoid toll charges by using an IP network with the Cisco routers that serve as the edge voice gateways. Toll bypass allows some Internet service providers (ISPs) to offer residential customers free, or very low-cost, long-distance voice calls by routing the calls over an IP network.
As an example, in Figure B-3, traffic from the enterprise PBX enters the Cisco routers that serve as edge voice gateways. The edge voice gateways, in turn, route the call over the IP network using the H.323 protocol. As shown, the enterprise customers avoid the TDM-based toll switches for their interoffice voice traffic and rely on an IP network.
Figure B-3. Toll Bypass