TABLE OF CONTENTS In telecommunications, information can be transmitted between two locations using a signal that can be either analog or digital in nature. In the telecommunications networks today, digital transmission is used almost exclusively, in which analog traffic, such as voice calls, is converted to digital signals (a process referred to as sampling) to facilitate long distance transmission and switching.
A high-pitched voice contains mostly high frequencies, while low-pitched voice contains low frequencies. A loud voice contains a high-amplitude signal, while soft voice contains a low-amplitude signal. Analog signals can be combined (i.e., multiplexed) by combining them with a carrier frequency. When there is more than one channel, this is called frequency-division multiplexing (FDM). FDM was used extensively in the past but now has generally been replaced with the digital equivalent, called time-division multiplexing (TDM). The most popular TDM system is known as the Tier 1 (T1) system, in which an analog voice channel is sampled 8,000 times per second, and each sample is encoded into a 7-bit byte. Twenty-four such channels are mixed on two copper pairs and transmitted at a bit rate of 1.544 megabits per second (Mb/sec). T1 in North America (E1 in the rest of the world) remains an important method of transmitting voice and data in the public switched telephone network (PSTN).1
A talking path (i.e., a switched circuit) in the PSTN can be either analog or digital or a combination thereof. In fact, a digital signal can be...
