Since I'm the type of person that tends to (eventually) dot all the i's and cross all the t's... Once upon a time, in a digital land far away...no, that's not it. In a previous post, I covered the different types of loopbacks that I had discovered in my toolbox. At that time, I didn't know why the T1 loopback was different from the 56K loopback.
This site: Cisco Modular Access Router Cable Specifications
has all the pinouts you could ever want. It also has pictures of the devices and what's going over that individual wire (ground, tip, ring, receive tip, etc...). At the top of some of the tables, you will see pictures of loopback plugs and even the pinout of a null modem.
Table 14 shows you CT1/PRI-CSU pinouts. You can easily see why the loopback goes from 1-4 and 2-5. It connects Receive Ring to Ring and Receive Tip to Tip.
Table 16 shows you 56/64K CSU/DSU connector signals and pinouts. Again, you can see why the loopback goes from 1-8 and 2-7. It connects Transmit Ring to Receive Ring and Transmit Tip to Receive Tip.
Apparently all the devices that are connecting are looking for the signal on these pins and that's why the loopback is different for the different speeds. Actually, it's different because you're connecting to different devices rather than the same device at different speeds.
Now it all makes a lot more sense. I hope we all can sleep better now with that puzzle solved.
Posted by BlueWolf on October 18, 2005 05:29 PM