|  | 5.1 | ISDN
| 5.1.3 | ISDN reference points

Because Customer Premise Equipment (CPE) covers a wide variety of capabilities and requires a variety of services and interfaces, the standards refer to interconnects by reference points rather than specific hardware requirements. Reference points are a series of specifications that define the connection between specific devices, depending on their function in the end-to-end connection. It is important to know about these interface types because a CPE device, such as a router, may support different reference types; the reference points supported will determine what specific equipment is required for purchase. The table in Figure provides a summary of the reference points that affect the customer side of the ISDN connection. A sample ISDN configuration is shown in Figure , where three devices are attached to an ISDN switch at the Central Office (CO). Two of these devices are ISDN compatible, so they can be attached through an S reference point to NT2 devices. The third device (a standard, non-ISDN telephone) attaches through the R reference point to a TA. Although they are not shown, similar user stations are attached to the far-right ISDN switch.
| Lab Activity
This exercise will serve as a study guide to help reinforce your understanding of basic ISDN terms and devices to which they relate.

| Threaded Case Study
| Washington Project: ISDN Equipment and Media You need to identify what additional equipment and media will be necessary in order to implement an ISDN link in the Washington School District WAN design.

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| ISDN Reference Points