Data Communications Networking Devices - 4th Ed.pdf

1.3 TYPES OF SERVICES AND TRANSMISSION DEVICES ______________________________ 17Figure 1.14Bipolar AMI) RTZ 50 percent duty cycleOn modern wide area network digital transmission facilities a modi®ed form ofbipolar return to zero signaling is employed. That modi®cation involves theplacement of pulses in their bit interval so that they occupy 50% of the interval,with the pulse centered at the center of the interval. This positioning eliminateshigh frequency components of the signal that can interfere with other transmissionsand results in a bipolar pulse known as a 50% duty cycle alternate markinversion AMI). An example of this pulse is illustrated in Figure 1.14.Now that we have a general appreciation for the type of digital signaling used ondigital transmission facilities and the rationale for the use of that type of signaling,we will discuss some of the types of digital transmission facilities available for use.In doing so we will ®rst consider the manner by which the digitization of voiceconversations was performed,as the voice digitization effort had a signi®cant effectupon the evolution of digital service offerings.Evolution of service offeringsThe evolution of digital service offerings can be traced to the manner by whichtelephone company equipment was developed to digitize voice and the initialdevelopment of digital multiplexing equipment to combine multiple voiceconversations for transmission between telephone company of®ces. The digitizationof voice was based upon the use of a technique referred to as Pulse CodeModulation PCM) which requires an analog voice conversation to be sampled8000 times per second. Each sample is encoded using an 8-bit byte,resulting in adigital data stream of 64 kbps to carry one digitized voice conversation.The ®rst high speed digital transmission circuit used in North America wasdesigned to transport 24 digitized voice conversations. This circuit,which isreferred to as a T1 line,transports a Digital Signal Level 1 DS1) signal. Thatsignal represents 24 digitized voice samples plus one framing bit repeated 8000times per second. Thus,the operating rate of a T1 circuit becomes 24 8 ‡ 1)bits/sample 8000 samples/second,or 1.544 Mbps.Each individual time slot within a DS1 signal is referred to as DS0 or DigitalSignal Level 0 and represents a single digitized voice conversation transported at64 kbps. Figure 1.15 illustrates the relationship between DS0s and a DS1. Inexamining Figure 1.15 note that the DS1 frame is repeated 8000 times per second,