{"id":133,"date":"2014-02-25T22:12:32","date_gmt":"2014-02-25T22:12:32","guid":{"rendered":"https:\/\/learncisco.net\/index.php\/isdn-bri-and-isdn-pri-function-groups-and-reference-points\/"},"modified":"2023-01-19T17:28:52","modified_gmt":"2023-01-19T10:28:52","slug":"isdn-bri-and-isdn-pri-function-groups-and-reference-points","status":"publish","type":"page","link":"https:\/\/www.learncisco.net\/courses\/legacy-topics\/part-10-isdn-and-dial-on-demand-routing\/isdn-bri-and-isdn-pri-function-groups-and-reference-points.html","title":{"rendered":"ISDN BRI & PRI Function Group & Reference Point"},"content":{"rendered":"
The ISDN specifications identify the various functions that must be performed to support customer premises equipment (CPE). ISDN uses the term function group to refer to a set of functions that a piece of hardware or software must perform. Because the ITU wanted several options for the customer, it defined several different function groups. Because the function groups might be implemented by separate products, possibly even from different vendors, the ITU needed to explicitly define the interfaces between the devices that perform each function. Therefore, ISDN uses the term reference point to refer to this interface between two function groups.<\/p>\n
Most people understand concepts better if they can visualize or actually implement a network. A cabling diagram is helpful for examining the reference points and function groups. The cabling diagram for several most used examples is here:<\/p>\n
<\/p>\n
Router A is ordered with an ISDN BRI U interface; the U implies that it uses the U reference point, referring to the I.430 reference point for the interface between the customer premises and the telco in North America. No other device needs to be installed; the line supplied by the telco is simply plugged into the router’s BRI interface.<\/p>\n
Router B uses a BRI card with an S\/T interface, implying that it must be cabled to a function group NT1 device in North America. An NT1 function group device must be connected to the telco line through a U reference point in North America. When using a router BRI card with an S\/T reference point, the router must be cabled to an external NT1, which in turn is plugged into the line from the telco (the U interface).<\/p>\n
A router can connect to an ISDN service with a simple serial interface, as shown with Router C on the picture. Router C must implement an ISDN function group called TE2 (Terminal Equipment 2) and connect directly to a device called a terminal adapter using the R reference point.<\/p>\n
The following tables summarize the types shown in the picture above.<\/p>\n
Function Group<\/th>\n | Acronym<\/th>\n | Description<\/th>\n<\/tr>\n | ||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
TE1<\/td>\n | Terminal Equipment 1<\/td>\n | ISDN-capable four-wire cable. Understands signaling and 2B+D. Uses an S reference point.<\/td>\n<\/tr>\n | ||||||||||||
TE2<\/td>\n | Terminal Equipment 2<\/td>\n | Equipment that does not understand ISDN protocols and specifications (no ISDN awareness). Uses an R reference point, typically an RS-232 or V.35 cable, to connect to a TA.<\/td>\n<\/tr>\n | ||||||||||||
TA<\/td>\n | Terminal adapter<\/td>\n | Equipment that uses R and S reference points. Can be thought of as the TE1 function group on behalf of a TE2.<\/td>\n<\/tr>\n | ||||||||||||
NT1<\/td>\n | Network Termination Type 1<\/td>\n | CPE equipment in North America. Connects with a U reference point (two-wire) to the telco. Connects with T or S reference points to other CPE.<\/td>\n<\/tr>\n | ||||||||||||
NT2<\/td>\n | Network Termination Type 2<\/td>\n | Equipment that uses a T reference point to the telco outside North America or to an NT1 inside North America. Uses an S reference point to connect to other CPE.<\/td>\n<\/tr>\n | ||||||||||||
NT1\/NT2<\/td>\n | –<\/td>\n | A combined NT1 and NT2 in the same device. This is relatively common in North America.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n And here are the reference points:<\/p>\n
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