Data Communications Networking Devices - 4th Ed.pdf

1.16 INTEGRATED SERVICES DIGITAL NETWORK ___________________________________ 163NT2The NT2 network termination 2) functional group includes devices that performswitching and data concentration functions equivalent to the ®rst three layers ofthe OSI Reference Model. Typical NT2 equipment can include PABXs,terminalcontrollers,concentrators,and multiplexers.InterfacesAs previously explained,the R interface is the point of connection between non-ISDN equipment and a terminal adapter. Although the R interface can consist ofany common DTE interface,most terminal adapters support RS-232 and V.35.The S interface is the standard interface between the TE and NT1 or betweenthe TA and NT1. This is the interface to a 192 kbps,2B ‡ D,four-wire circuit.The S/T interface can operate up to distances of 1000 m using a pseudo-ternarycoding technique. In this coding technique,a binary one is encoded by thetransmission of an electrical zero,while binary zeros are encoded by transmittingalternating positive and negative pulses. Since there are three signal states ‡ ,0,voltage) used to encode two symbols,the coding method is called pseudo-ternary.An example of this coding technique is illustrated in Figure 1.93.Figure 1.93Pseudo-ternary coding exampleThe T interface is the customer end of an NT1 onto which you connect an NT2.For basic access,the T interface is a 192 kbps four-wire,2B ‡ D interface. Forprimary access,the T interface is a 1.544 Mbps,23B ‡ D,four-wire circuit or a2.048 Mbps,30B ‡ D,four-wire circuit.The U interface is the ISDN reference point that occurs between the NT1 andthe network and is the ®rst reference point at the customer premises. Thecoding scheme for information on the U interface is known as 2B1Q which is anacronym for `two binary,one quaternary'. Under this coding scheme every twobits are encoded into one of four distinct states that are known as quats. The topportion of Figure 1.94 illustrates an example of 2B1Q encoding,while the lowerportion of that illustration indicates the relationship between each dibit value andits quats code.From the U interface,transmission occurs at 160 kbps to the telephone companycentral of®ce. Due to the use of 2B1Q coding a maximum transmission distance of18 000 feet is supported at a data rate of 160 kbps. This data rate represents144 kbps used for the 2B ‡ D channels and 16 kbps used for synchronization. In