I used trial and error to get transmit and receive properly crossed between the two T1 cards. I'd like to know if there's any reason to be concerned about polarity within a single signal pair. ie 1-4 vs
If polarity does not matter in this application, does it ever matter in telecom applications?
Now-a-days, tip/ring polarity probably doesn't really matter as far as making services work. But at all my buildings (16 of them with several hundred DS-0 & DS-1s) I stress that it's important to keep tips and rings straight (that means tip conductors always connect to tip conductors and ring conductors always connect to ring conductors). In fact, I get my nose more than a little bit out of joint if I find things wired otherwise. Finding some MixMaster mess of wiring tells me the tech didn't understand what was being connected - he (or she) just started messing around with the interface until things started working. This reminds me of what an old line Bell System tech told me once: "Two wires - hoook 'em up; four wires - fxxx 'em up!" All that applies to tips and rings.
Transmit and receive pairs can sometimes be rolled, however. It depends upon the application, of course. If you're extending services from an outside carrier, for example, transmits should be connected to transmits and receives to receives (that is 1-2 to 1-2 and 4-5 to 4-5). But if you're connecting a "DCE" to another "DCE" - as in the case you cite - then you'll probably have to connect a transmit to a receive, that is 1-2 to 4-5). Does that make any sense? (Dang! I didn't think it would!)
Maybe this will help: Extending a carrier's demark to a DCE type CPE: 1 to 1, 2 to 2, 4 to 4 and
5 to 5 (a "straight through" connection) Connecting two local interfaces together: 1 to 5, 2 to 4, 4 to 2 and 5 to 1 (a "rolled" connection) (Notice above that the tips always connect to tips and rings always connect to rings).
I've found that if you draw a picture of it on a piece of paper it's easier to make it work once you get the wires and tools in your hands! Less frustrating, too! AND... Your boss will be pleased with your "documentation"!
Al, Thanks for the thoughtful reply. I understood everything you said -- at least I think I do.
But it's what you didn't say that I'm still curious about: Why is it a big deal to you that tips stay connected to tips if services will work either way? I guess this would come under the category of maintaining absolute signal polarity although I don't know why that might be useful.
Well, I guess maybe I over stated the insensitivity to polarity! DS-1s (generally) work with tips/rings wired straight or reversed. But Nortel Network digital telephones, for example, don't. Those sets require correct polarity. It's likey that other digital telephone sets require correct polarity as well. And there is probably quite a pile of other equipment that is polarity sensitive as well. So there are reasons for getting things right. I stand corrected! Thanks for pulling me back on course!
Another reason (to my way of thinking) is craftsmanship. I feel a technician that wires things to "the book" has put a little more effort into doing a good job. That shows up in some subtile ways: a job with a little better craftsmanship will likely last longer, work better and be easier to fix if it does break. At least that's what I preach to both my technicians and to my management! And they both believe it!
On T1, a tip/ring reversal will not impact a T1 signal although, as you found out, you need to get the transmit/receive pairs connected correctly.
The reason why is that on a T1, ones "1" are sent on the line are either a positive or a negative electrical pulse. A zero "0" is sent as no pulse. If you swap Tip and Ring, the pulses are inverted, but it doesn't matter. Even with Simplex Current (A scheme where DC power is supplied to the T1 interface), a Tip/Ring reversal does not matter.
NComm, Inc - We supply software for implementing the physical layer interface of a product including T1, E1, T3, E3, and SONET/SDH.