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November 18, 2004

Loopback Testing

I have a number of loopback plugs in my little toolkit and I thought it might be useful to cover/discover the use of each...

Loopback testing is used to isolate a problem to a certain section/device in a network. The wire for the signal going out is looped back to the wire for the signal coming in so that it creates a loop. This sort of simulates "everything else" and can help to prove that a certain section of the network or device is/isn't working properly.

There are software loopbacks (which loops the functionality) and hardware loopbacks (which are intrusive, but more effective in isolating problems).

Cisco has an excellent article on Loopback Tests for T1/56K Lines which covers hardware and software loopbacks.

My intent with this post is to cover four loopback plugs:
99-7912-042-1 RJ-48C T-1 Loopback Connector
99-7912-043-1 RJ-48S 4W 56K Loopback Connector
99-7912-044-1 RJ-11 2W 56K Loopback Connector
AC07-003376 RJ-45 Ethernet Loopback Connector

From another part of the Cisco site:

What is the difference between RJ-48 and RJ-45?
* RJ-48 has a keyed connector and a notched jack.
* RJ-45 does not have a key or a notch.
* Both have 8 pins

Although an RJ-45 connector will fit an RJ-48 jack, the key on an RJ-48 connector will prevent it from plugging into an RJ-45 jack.

To be honest, I'm sitting here looking at both and they look pretty similar. And the (supposed) RJ-48 loopback plug that I have easily fits into the RJ-45 jack of my NIC card. Oh well... It's really the pinout that's important. [And in case you're curious - the "C" identifies a surface or flushmounted jack, and the "S" identifies a single-line jack.]

99-7912-042-1 RJ-48C T-1 Loopback Connector
This connector has two wires. One wire loops from pin 1 to pin 4 and the other loops from pin 2 to pin 5. This is used to test a T1 connection. You can use it to replace either end of the cable that goes from the smart jack to the WIC card or CSU/DSU. This is to test a T-1 connection.

T1 Loopback Plug

99-7912-043-1 RJ-48S 4W 56K Loopback Connector
This connector also has two wires. One wire loops from pin 1 to pin 7 and the other loops from pin 2 to pin 8. This is used to test a 56K connection.

I wish I could say that I knew the difference in usage between the T1 and 56K loopbacks. I did a quick search and couldn't find any information regarding the reason that one uses 1-4/2-5 and the other uses 1-7/2-8. Speed itself shouldn't determine the wire pinout... My best guess is that one is digital and the other is analog. But, I will have to leave that for another post...

99-7912-044-1 RJ-11 2W 56K Loopback Connector
Since this is the only RJ-11 plug in the bunch, it should be pretty obvious as to when this should be used. The RJ-11 plug is the same type of plug that is used for your telephone line. Once again, I couldn't find any information on its usage with a quick search. My best guess here is that it's used to test both ends of a modem connection (POTS). This will also have to be left for a future post. I can't even find a good pinout diagram or any mention of the proper pinout for this loopback plug. From visual inspection, it uses one wire to loop from pin 3 to pin 4.

AC07-003376 RJ-45 Ethernet Loopback Connector
This loopback plug uses the familiar RJ-45 jack. This loopback also uses two wires. One wire connects pin 1 to pin 3 and the other wire connects pin 2 to pin 6. [These four pins should be familiar - they are the four 'slots' involved in a cross-connect cable.] This loopback connector is used to test Ethernet ports. Once again, I find that information on this topic is not readily found and will have to research this further.

This post may not have been as informative as I would have liked, but at least it distinguishes the different loopbacks by pinout. It always seems that every time I'm able to answer one question, it leads to three other new questions... Perhaps that's what I like so much about this field. No matter how much you learn, there's still more to be learned... (And once you learn most of it, they come out with something new which makes the old technology obsolete.) This is definitely a field where you will experience the perpetual joy of new discovery...

Posted by BlueWolf on November 18, 2004 10:01 PM