Data Communications Networking Devices - 4th Ed.pdf

1.3 TYPES OF SERVICES AND TRANSMISSION DEVICES ______________________________ 21ting,and timing. Most DSUs are designed to operate at one of four speedsÐ2.4,4.8,9.6,and 56 kbpsÐwhile some DSUs also support 19.2 kbps operations. Thetransmitting portion of the DSU processes the customer's signal into bipolarpulses suitable for transmission over the digital facility. The receiving portion ofthe DSU is used both to extract timing information and to regenerate mark andspace information from the received bipolar signal. The second interfacearrangement originally developed for AT&T's Dataphone Digital Service is calleda channel service unit CSU) and was provided by the communication carrier tothose customers who wish to perform the signal processing to and from the bipolarline,as well as to retime and regenerate the incoming line signals through theutilization of their own equipment. Originally marketed as separate devices,almostall DSUs and CSUs designed for use on AT&T Dataphone Digital Service andequivalent facilities from other carriers are now manufactured as one integrateddevice which is commonly referred to as a DSU or a DSU/CSU. Since mostterminal devices connected to T1 lines contain a built-in data service unit,aseparate channel service unit is required for transmission on that type of digitaltransmission facility.As data is transmitted over digital facilities,the signal is regenerated by thecommunications carrier numerous times prior to its arrival at its destination. Ingeneral,digital service gives data communications users improved performanceand reliability when compared to analog service,owing to the nature of digitaltransmission and the design of digital networks. This improved performance andreliability is due to the fact that digital signals are regenerated whereas,whenanalog signals are ampli®ed,any distortion to the analog signal is also beingampli®ed.Although a digital service is offered in many locations,for those locationsoutside the serving area of a digital facility the user will have to employ analogdevices as an extension in order to interface to the digital facility. The utilization ofdigital service via an analog extension is illustrated in Figure 1.16. As depicted inFigure 1.16,if the closest city to the terminal located in city 2 that offers digitalFigure 1.16 Analog extension to digital service. Although data is transmitted in digital formfrom the computer to city 1, it must be modulated by the modern at that location fortransmission over the analog extension