 one for the forward direction and one for the reverse direction, so that the voice path is completed in both directions. The 24 time-slot pairs that result when used to connect switching systems are still referred to as trunks. A DS-1 facility can be used for purposes other than carrying trunks, so a more general term for a time-slot pair is a channel. 2 Chap- ter 13 describes in greater detail the hierarchy of digital transmission facilities. There are two standards for interpreting the analog value of the voice signal and cre- ating an 8-bit digital representation. These two standards are commonly referred to as u-law and A-law. u-Law is used in North America and Japan, and A-law is used in Eu- rope. Many switching systems are capable of handling both, as well as converting be- tween them for international traffic. This subject is treated further in Sec. 18.1.2. 11.1.3 The private branch exchange (PBX) The private branch exchange (PBX) was developed originally to be a small switch with many of the functions of a local exchange office but located within the customer's offices and owned by the customer. Modern PBXs can be used in this manner for a few 2DS-1s can be set up without predefined channels. The subdivision of a DS-1 into 24 time slots is commonly referred to as D4 channelization. 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. The PSTN and Telephone Switching Key Telephony Concepts telephones up to tens of thousands of telephones and can be equipped not only with the features of a modern local exchange office but also with many advanced features required for the business customer, such as call centers and computer-telephony integration. Some PBXs such as those manufactured by Avaya, Inc., actually have been used as small local exchange offices and for enhanced services. Originally, businesses would purchase separate telephone numb