|  | 2.5 | WAN Link Options
| 2.5.8 | Describe ISDN

Telephone companies developed ISDN with the intention of creating a totally digital network. ISDN devices include the following: Terminal Equipment 1 (TE1) -- Designates a device that is compatible with the ISDN network. A TE1 connects to an NT of either Type 1 or Type 2. Terminal Equipment 2 (TE2) -- Designates a device that is not compatible with ISDN and requires a TA. TA-Converts standard electrical signals into the form used by ISDN so that non-ISDN devices can connect to the ISDN network. NT Type 1 (NT1) -- Connects four-wire ISDN subscriber wiring to the conventional two-wire local loop facility. NT Type 2 (NT2) -- Directs traffic to and from different subscriber devices and the NT1. The NT2 is an intelligent device that performs switching and concentrating. As shown in Figure , ISDN interface reference points include the following: The S/T interface defines the interface between a TE1 and an NT. The S/T also is used to define the TA-to-NT interface. The R interface defines the interface between a TE2 and the TA. The U interface defines the two-wire interface between the NT and the ISDN cloud. There are two ISDN services: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). ISDN BRI operates mostly over the copper twisted-pair telephone wiring in place today. ISDN BRI delivers a total bandwidth of a 144 kbps line into three separate channels. Two of the channels, called B (bearer) channels, operate at 64 kbps and are used to carry voice or data traffic. The third channel, the D (delta) channel, is a 16-kbps signaling channel used to carry instructions that tell the telephone network how to handle each of the B channels. ISDN BRI often is referred to as 2B+D. ISDN provides great flexibility to the network designer because of its capability to use each of the B channels for separate voice or data applications. For example, one ISDN 64-kbps B channel can download a long document from the corporate network while the other B channel browses a Web page. When you're designing a WAN, you should be careful to select equipment that has the right feature to take advantage of ISDN's flexibility.