this problem, but neither of them is ideal. The first method is to provide excess capacity for peak demand on the voice service and let the data network deal with delay due to low bandwidth. This may be ac- ceptable because data systems have a higher tolerance for delay than do voice net- works. This approach is commonly taken by telephony carriers who want to use some 1There is a partial exception to this rule. Some carriers can carry voice and also carry low-speed data, such as modem or low-speed cellular data transmission, and allocate dynamically on de- mand. However, these lines are not best for data service; they are merely sufficient. 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 Key Data Concepts of their excess capacity to serve data customers. However, it still leaves a large part of the capacity assigned to voice use unused except at peak hours. The other approach is to have an alternate route for voice calls. Of the total trans- port capacity available, enough is given to data services to satisfy peak demand. Excess transport capacity is given to a limited number of voice channels. When these voice channels are used up, the switches at each end adopt an alternate route for any new telephone calls that come in. This solution is used commonly by private companies. The point-to-point links, which are leased for wide area network (WAN) data uses, have ex- cess capacity beyond the current demand for data, usually because they were leased with future capacity in mind. The excess capacity is given to the internal private branch exchange (PBX) telephone switches. It is the preferred route because it offers long-distance service within the office without a per-minute charge. Whenever the in- ternal network reaches capacity, the next request for an office-to-office call that comes to the PBX is sent to the chos