![t1 loopback pinout t1 loopback pinout](http://www.sfcommunications.com/UploadedFiles/Images/wiring3.jpg)
The span of copper between HTUs when using HDSL is single pair. HTU stands for host terminal unit and there is usually a HTUC (HTU Central Office) and a HTUR (HTU Remote Location). The basics of it is that the only single pair part of the connection is between HTU’s. The concept is, for the most part, fairly simple but it took me a very long time to fully grasp the entire picture. Now, you might be wondering why the telco is selling you a DSL circuit and magically converting it into a T1 at demarc. HDSL stands for high bit rate digital subscriber line. How do they do this? In most cases they use a version of HDSL. However, with the emergence of better standards and technologies the telco is now, in most cases, able to deliver the T1 to demarc over a single pair. The interesting part of this discussion is that regardless of whether it’s a single or dual pair circuit you’ll ALWAYS have dual pair coming out of demarc and going into your WIC card. The fact of the matter is that both 2 wire (single pair) and 4 wire (dual pair) T1s do exist. They have a point if it’s 2 wire, it certainly sounds like a DSL circuit. They wonder why they would pay T1 prices for what they equate to a ADSL circuit. I can’t tell you how many people I have heard complaining about having a ‘2 wire’ T1. I’m going to start by explaining some of the basics of T1 wiring, discuss some relevant terminology, and pepper in some of my own experience throughout that might help your troubleshooting. Almost 99% of the time if there is an issue with a T1, its because of the wiring. Moreover, I think the number of turn ups that I have been a part of that went completely as planned are fewer than 5%. I’m not sure how many circuit turn ups I have been a part of that just fell through because the on site technician couldn’t get a CD light on the WIC card.