ISDN Reference Points

"This is my router, this is my switch, one is for routing, but I don't know which......"

Actual photo of eBay NOC under Denial Of
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All right people, sit down and shut up! As you know ISDN can be comprised of several individual components that can be wired together to form a small ISDN communications system. ISDN is a digital communications method cooked up by phone companies so that digital communications could be transported over an old, worn out infrastructure meant to carry analog signals.

In these here United States the phone company supplies ISDN on an RJ 11 jack and the signal uses only 2 wires. This is called a U interface. I fought in 'Nam so they could have an ISDN U type wall jack but as you know, we lost that one and Viet Nam uses the S/T interface. So does the rest of the world. But here in the good 'ol U. S. of A we enjoy getting our ISDN on a U interface. Actually the entire local loop back to the phone company's switch is referenced by the letter U.

ISDN can carry voice or data. Some phones are ISDN capable and they are known as TE1 devices. TE1 devices are considered "special". Phones (or computers) that are not compatible with ISDN are known as TE2 Devices. TE2 devices are considered "regular". Here in America because the phone company's local loop is a U reference point, we have to provide the power to run the circuit. When you are sitting at home about to make a crank call, and you hear the dial tone, that means that for analog calls the Telco is providing the power. But ISDN requires more power so the phone companies make us provide our own power. Many say that in America the ISDN local loop is referenced as a U point because it is "un powered". The device that provides the power is called an NT1.

Before we continue with our fascinating lecture on ISDN reference points, Sgt. Hartman wants you guys to get down and crawl under some barbed wire and then double time it to the main gate and back. I'll wait here for ya...

There is also a device called a NT2. Is used for sending signals to an ISDN capable PBX. Nobody living or dead has ever seen an ISDN PBX so the NT2 is a real money maker for what ever company supplies them to Cisco. Regular, non ISDN equipment requires a TA or Terminal Adapter to convert their analog signals to digital. Here is how the reference points look when you see them in your web browser:

Because NT2s provide signals to devices that never existed most vendors combined them with the NT1 which is used in America for line power. So the ISDN port on a Cisco router will either say S/T or U. If you are a WAN engineer in Dallas Texas and you just bought $1,000,000 worth of ISDN routers from Cisco with S/T interfaces, you've got problems.

The S/T interface uses a RJ 45 connector and 4 wires are used, 2 for power from the Telco and 2 for data. Here in the USA we use the U or un powered interface so we only use 2 wires.

Remember, TE1s are "Special" so they have a S reference. TE2s are "Regular" so they have a R reference. The United States starts with a U, so the Telco supplies us with an Un-powered U reference. Spain and Tonga are countries far from the United States so Spain and Tonga use S/T interfaces to connect to their Telco's.