ing which path to send packets on, must assume that information regarding the status of the incoming direction is useful in deciding what to do when packets need to go back in that direc- tion on the opposite side of the full-duplex channel. This assumption is not always war- ranted. Networks can be clogged at a certain point, but only in one direction, just highway can have a traffic jam in only one direction. 19.2 Voice and Data Sharing Physical Transport The simplest case of mixing voice and data would not be seen as a single network at all by some, but it still has some of the economic benefits. If a pipe is laid with multiple channels, some of those channels can be used for voice and others for data. In the pub- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Hybrid Voice-Data Networks Hybrid Voice-Data Networks lic switched telephone network (PSTN), the larger trunks, DS-3 and higher, carry many DS-1 circuits via time division multiplexing (TDM). The channels are independent. As a result, it does not matter if the DS-1 link carries 24 voice calls or one 1.536-Mbps data signal. At each end, the voice calls are routed to a voice switch, and the data calls TDM equipment designed for data. We find a similar approach used with fiberoptic transport. Companies that own fiber lease capacity in two forms. Dark fiber is a set of glass tubes with a certain carrying ca- pacity from one point to another. The owner of the fiber leases it, and the lessee adds equipment and uses it for any purpose. Or the owner can set up a service, such as ATM or Synchronous Optical Network (SONET), and lease active capacity on a fiber back- bone. Sometimes three companies are involved. One company that owns the physical transport leases dark fiber to a local exchange carrier, long-distance carrier, or data network services company. This company lights up the fiber a