atcomsystems.ca/forum Forums Wiring - Telecom Carriers - Trunks & Services T1's, CSU's and DSU's T1 line coding

I've been working on a site that has 7 T1s going through a Canoga Perkins 3240S MUX.

The line coding we have our equipment set up for is B8ZS. One of the MUXs went down along with the T1s. They were down for s few days before the MUX was replaced. When we hooked up a test set to the demarc where the 7 T1s come in, the tester read AMI. I was told that since the site on the distant end was down and there were no users trying to transmit data, that B8ZS would not be present and the tester would see all 0's (AMI). When we were on the phone with the provider and they were doing testing with us our testers (Easy Tester II) B8ZS LED would flicker. Is that normal? Is B8ZS dependent on data on the circuit? So if there was no data it would be all 0's or AMI. Also we noticed that when we had a loop at the far end we would see a solid light on the testers B8ZS LED. Is that because the tester was sending a constant signal for B8ZS coding?

Thanks

Any test set I've ever used only lit the B8ZS light when the zero's threshold was reached, and yes your test set would be lit solid if sending a test signal to verify B8ZS. All zero's is not AMI it's an idle signal on a B8ZS circuit when no live signal is present, at least that's always been my understanding. Mike will correct me if I'm wrong.

You're correct, Bill. will667!
In a B8ZS coded circuit, the presence of "all-zeros" on your test equipment does not represent Alternate Mark Inversion (AMI) line coding. The absence of any "1s" simply means that no voltages are currently being applied to the circuit by any devices.

If it's OK, I'd like to provide my personal "keep-it-simple" explanation of "Bi-8-Zero-Sub" line coding: When the devices at both ends of a circuit are configured for B8ZS, the circuit becomes monitored for anytime that eight consecutive zeros are transmitted in either direction. When eight consecutive zeros are sent, B8ZS immediately inserts two successive "1s" of the same voltage (also called a bipolar violation) into the signal. The receiving device interprets and reacts to the bipolar violation as a timing mark, which keeps the transmitting and receiving devices synchronized.

Bill/Mike,

So I would see all 0's if the far end site had no users active on the network? And if so the test set would read it as AMI but that doesn't mean in this case that the end equipment is configured with it.

Thanks alot for your help guys. I'm so glad I found this message board.

Are you receiving any alarm indications on your tester while receiving the "all zeros"?
Have you confirmed the correct T1 framing for the devices at each end of the circuit?

Even though an idle AMI coded circuit will also show "all zeros", your far-end loop test implies that both end devices are configured for B8ZS line coding.

Mike,

If I recall correctly the test set did have an alarm on it. I didn't use the test set myself but i was present when the test was done.

The framing was confirmed on both ends.

So a soft loop on the far end CSU to the test set confirms B8ZS coding by indicating a solid green on the B8ZS LED?

We were seeing AMI on the test set at first before the T1s were plugged into the MUX. When the provider was testing with us we saw the intermittent B8ZS and AMI. After that we plugged the circuits back into the MUX and everything has been running clean for almost two days. I'm just trying to understand why the test set was showing AMI before I talked to the provider and then AMI and B8ZS it while I was on the phone with them. I thought at first it had to do with the circuits being down for a few days and the providers equipment went into an alarm state or it was looped away from our location to clear their alarms.

With the softloop up at the far end, a solid green B8ZS LED at your tester indicates that the far end device's line coding is configured for B8ZS.

As far as trying to identify if the line coding was updated from AMI to B8ZS while you were on the phone with the provider, I couldn't guarantee that. Based on my experience (at least with 5ESS), updating line coding or framing on a DS1 circuit requires that the circuit first be removed from service, the coding and/or framing configs updated, then restoring the circuit to service. If your provider had done this, you definately would have received errors at your tester during the change.

What happened here is that the carrier discovered they were misoptioned, and rather than admit it, they fixed it on the spot and didn't say anything about it, because they didn't figure you'd ever figure that out.

Typically a MUX failure, or a T1 open at the input to a fiber MUX will yield an Alarm Indication Signal (AIS) consisting of an Unframed All One's signal pattern.

To verify what I say, configure a test set to T1 no framing (T1 framed), all One's test signal. If you achive pattern synch with an All Ones's test pattern, then this is an indication of an AIS.

An All Zero's option that some fiber MUX equipment have, Telco Systems 828 FOX HUB for example, is used to propagate a loss of signal (LOS) condition that is required for selected vintage SLC systems to switch to a protection T1 span.

SLC-96, for example, will not switch to protection with the presence of AIS. The SLC-96 only recognizesa a LOS. Hence whythis option is there on some MUX equipment.

If optioned for B8ZS, the presence of eight or more logical zeros will trigger the B8ZS encoding.

A 1-8 test pattern sent unframed will not cause B8ZS to kick in, but if sent T1 framed, B8ZS will be detected is the line is so optioned.

One tip.

All T1 NIU's since 1998, and all HDSL units since 1999 support a feature called AIC-CI.

AIS-CI is a variant to the standard AIS alarm pattern. AIS-CI is sent into the network by NIU's & HDSL remote units upon detection of a loss of signal (LOS) from the attached CPE, or if the customer equipment is sending AIS. This tells the LEC technicians that the AIS "like" signal is not a network fault, but a fault that is originating from the CPE side of the DEMARC. ANSI T1.403 & ANSI T1.231 go into considerable detail with AIS & AIS-CI.

How AIS-CI can help a CPE tech, as well as a CO tech. If a far end CPE device has failed due to loss of local power or was disconnected, or for any reason there is a T1 loss of signal (LOS) into the far end T1 NIU, the presence of AIS-CI can be detected.

To distinguish between AIS, and AIS-CI, configure a T1 test stet to T1 (Unframed), and an All One's test pattern. If the test set measures T1 Pulses, and Pattern Synch (You will not have Frame synch in this test configuration), the signal is plain old AIS.

However if you achieve pattern synch, but with a burst of about 113 to 115 bit errors in about a one second in time intervalt, that is AIS-CI.

The embedded AIS-CI payload signature designed to be detected by specialized equipment disrupts the All Ones signal pattern, and yields the bursty erors indicated above.

CO tech's I showed this to liked this undocumented trick. It can't hurt trouble shooting T1's from the CPE side as well.

Regards,

Field OPS