 Terms of Use as given at the website. First Things First Chapter 1 signals weaken faster than lower frequencies and, therefore, don't travel as far without amplification and regeneration. Bandwidth is the last characteristic that we will discuss here, and the quest for more of it is one of the great challenges of telecommunications. Bandwidth is a measure of the number of bits that can be transmitted down a facility in any one-second period. In most cases it is a fixed char- acteristic of the facility and is the characteristic that most customers pay for. The measure of bandwidth is bits-per-second, although today the measure is more typically thousands (kilobits), millions (megabits), or billions (gigabits) per second. Facilities are often called channels because physical facilities are often used to carry multiple streams of user data through a process called mul- tiplexing. Multiplexing is the process of allowing multiple users to share access to a transport facility, either by taking turns or using separate fre- quencies within the channel. If the users take turns, as shown in Figure 1-10, the multiplexing process is known as time-division multiplexing (TDM), because time is the variable that determines when each user gets to transmit through the channel. If the users share the channel by occu- pying different frequencies, as shown in Figure 1-11, the process is called frequency-division multiplexing (FDM), because frequency is the vari- able that determines who can use the channel. It is often said that in TDM, users of the facility are given all of the frequency some of the time, because they are the only one using the channel during their timeslot. In FDM, users are given some of the frequency all of the time, because they are the only one using their particular frequency "band" at any point. Figure 1-10 Time-division multiplexing (TDM) Inbound Estably Multiplexer Inbound Traffic Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill 