kurt@hi.UUCP (Kurt Zeilenga) (09/22/87)
Well, now I need info on good DSU/CSU units for T1. Any information on good vendors would be greatly appreciated.
heker@JVNCA.CSC.ORG (Sergio Heker) (09/22/87)
Kurt, We have been using the AVANTI Tpac DSU/CSU multiplexer for over a year with good results. If you need more info contact me directly. -- Sergio ----------------------------------------------------------------------------- Sergio Heker tel: (609) 520-2000 Internet: "heker@jvnca.csc.org" Bitnet: "heker@jvnc" JOHN VON NEUMANN NATIONAL SUPERCOMPUTER CENTER, JVNCnet Network Manager -----------------------------------------------------------------------------
OPRBBDM@TCSVM.BITNET (Bryan McWilliams) (09/22/87)
Kurt, In response to request for CSU/DSU Go with Verilink. you can order them through DCA direct at 404-442-4244. They're 2395.00 each, but if you can get the educational discount you'll save another 30%. Bryan McWilliams Manager, Network Services Tulane University OPRBBDM@TCSVM 504-865-5631
enger%gburg.DECnet@BLUTO.SCC.COM ("GBURG::ENGER") (09/22/87)
We have a couple of T1 circuits, but the equipment on the ends functions
as its own "DSU", so all that is required are outboard CSU units.
We use Digital Link Corp. Model 551C.
I think we have had three blow out so far. And a fourth leaked span power
to its chasis! It turns out they had problems with the physical construction
of the units. A bolt came very close to the circuit board and often shorted
or arcked during electrical storms, etc. The factory supposedly installed
an engineering modification (cutting the excess off of the bolts probably)
to lower the likelyhood of lightning damage. When one of the units came back
from getting "the fix", its Percent Error Free Seconds display simply counted
down constantly.
The Error Free seconds display is a nice feature. It keeps an "objective"
eye on the status of the line. Makes it real easy to point the finger at the
phone company. Other than span power outages due to lightning blowing the
CO fuses, the only problem I have ever had with the T1s was a bad punch at
the undergroud, and the CSU EFS display showed it as an excessive error rate,
and indicated which direction the problem was in. The local operating
company fixed it promptly, no questions asked. They took one look at the
display on the csu, and went to work on the span.
If you wish to contact them, Digital Link is in Sunnyvale, Ca
Phone number 408-745-6200
Good luck,
bob
From: GBURG::ENGER "Robert M. Enger {CONTEL Fed. Sys} (301)840-4906" 22-SEP-1987 12:42
To: GBURG::MAILER,ENGER
Subj: RE: [TCP/IP Mail From: <weidlich@ludwig.scc.com>] ludwig non-functional when net is down
I don't "think" the ypbind has anything to do with the iebark reset.
I'll check into it.
bob
(I think rebooting the machine is what cleared your problem??)
bob
------eshop@saturn.ucsc.edu (Jim Warner) (09/23/87)
In article <8709221539.AA06394@ucbvax.Berkeley.EDU> OPRBBDM@TCSVM.BITNET (Bryan McWilliams) writes: >Kurt, In response to request for CSU/DSU Go with Verilink. you can >order them through DCA direct at 404-442-4244. They're 2395.00 each, but >if you can get the educational discount you'll save another 30%. > I wish I could be so positive about Verilink. It looks like they are nothing but trouble from my perspective. Ours get into funny states which they can't recover from without pulling the plug. I mean that literally. They have no on/off switch or reset button. The factory recommendation to unjam one of these things is to pop the modules out with the power on. That grates. There's not even a button to force it out of loopback mode (again - just pull the plug). I wish I could be more specific about what "funny states" means. What we think happens is that something happens to a phone circuit that causes a lot of errors. After several hours of whatever mistreatment the phone co has to offer, the Verilink finally gets confused, looses sync and locks up. After the phone line has gotten good again, the Verilink doesn't recover by itself. Besides that, they break a lot. We're working on our third unit this year. Another thing that's been a headache is their 8ZBS encoding. While the encoding itself isn't a problem, the equipment that our telco gives its people sees a circuit that is working perfectly as having thousands of errors per second. In fairness to Verilink, this last point is mostly a telco problem, but the bottom line is that our phone co can't passively monitor our line to tell us what it looks like from their perspective. That hurts.
OPRBBDM@TCSVM.BITNET.UUCP (09/23/87)
I'm sorry to hear about your Verilink problems. Were I suffering the same difficulty I surely would not have recommended the box. One question, are you span powered? South Central Bell (in Louisiana) provides T-1 via fiber only, so my boxes are powered by standalone 48 v. power supplies from Tellabs. You may be taking some hits on the span if that's where your D.C is coming from. We test our line with firebird sets, and they show zero errors. The gear the phone company uses is not nearly as sophisticated (at laest in our region). I've had 2 verilink 551V models running absolutely flawlessly for 10 months now. From my perspective it's an excellent unit.
CERF@A.ISI.EDU (09/23/87)
Sorry to seem dense, but I thought the idea behind 8ZBS was to force some ones in the stream to maintain sync - but that this was done in a way which the telco did NOT see anything funny. The telco equipment usually requires that there be some 1s in the data stream every so often to assure clock synchronization. If a data stream threatens to send a long sequence of zeros (as a crypto stream might), one puts in a gadget that will convert 8 zeroes into a different bit pattern, and on the receiving side, turn them back into zeroes. Why would the telco see that as lots of line errors? Vint Cerf
jas@MONK.PROTEON.COM.UUCP (09/24/87)
(This may be a little rusty, since I'm not near my PUB's, but I think I've got the spirit, if not the letter, right.) T1 coding is trilevel, with levels I will call -1, 0, and +1. A bit of "zero" is always represented as 0. A bit "one" of will will be represented as -1 if the last "one" was +1, and as +1 if the last "one" was -1. T1 uses regenerative reclocking repeaters along the line. They require flux transitions to ge the clock out of. If they don't get enough, they lose it. To get transitions, you've got to send some "one"'s. This is why the rule of no more than 7 (?) "zero"'s in a row. B8ZS avoids this by putting a code violation in the bit cell for the 8th "zero". Thus, if the last "one" was -1, the 8th "zero" will be -1, and if the last "one" was +1, the 8th "zero" will be +1. This offends Central Offices, who constantly check for code violations. This one way they decide the line is sick. The nation may not be fully B8ZS safe for another decade. Many CSU/DSU's offer to run without B8ZS. They use different schemes to put your raw sync data on the T1 line without needing B8ZS to provide data transparency. Their price is usually correlated with how many bits they waste doing this, and how high a clock rate you get at the sync (RS-449) interface.
CERF@A.ISI.EDU (09/24/87)
In the early days of digital voice, when T1 was invented, I recall that every 8th bit of each 24 channel sequence (192 bits total) was a 1 for signalling/sync purposes. That coding gave you only 56 kb/s of digital information, but avoided any problems with regeneration. Over time, the need for such signalling went down (after all, 1/8 of each channel was 8 kb/s which is a lot just for signalling). It sounds as if the coding trinary coding scheme didn't account for arbitrary data passing through repeaters. Sigh. Vint
jas@MONK.PROTEON.COM (John A. Shriver) (09/24/87)
Now that I'm at my office, I see I goofed a little on B8ZS. There are two complex patterns that replace the 8 "zero"'s, depending on the polarity of the last "one". If the last "one" was +1, then you send 0,0,0,+1,-1,0,-1,+1. If the last "one" was -1, you send 0,0,0,-1,+1,0,+1,-1. The fourth and seventh positions are bipolar violations. This is better than what I described before, as it gives you a lot more flux changes (4 vs. 1). I beleive the eigth bit in voice use is used for signalling (on-hook/off-hook), but not to provide one's density. It appears that the one's density is provided by having the PCM voice not have any voltage that comes out as seven "zero"s. (Or at least making this very infrequent.) However, the 56KB of DDS service definitely comes from stuffing that eighth bit as a "one". This is also what the cheaper DSU/CSU's do. For further reference, see AT&T PUB 62411.
hedrick@TOPAZ.RUTGERS.EDU (Charles Hedrick) (09/25/87)
8ZBS generates bipolar violations of a specific kind. Some telco equipment allows it and some does not. For single-channel T1 (i.e. two IP gateways that want the whole bandwidth of a T1 connecting them), we have used a gadget from Digilink that I call a T1-izer. It takes a vanilla synchronous signal, and supplies everything that is needed to pass it through T1. There are a number of switches that control the encoding. You can select 8ZBS, if your telco can hack that, or a more convservative encoding if they can't. (There is of course a slight benefit in terms of line speed if you use 8ZBS.) THis also has a DSU/CSU builtin. (However we more typically use Avanti equipment, because we need to multiplex several channels on one T1 line.)
eshop@saturn.UUCP (09/27/87)
After I wrote about my Verilink experiences, the machine room at the far end of our T1 line had an air conditioning failure. By the time the temperature had reached 85 F, our packet loss rate had climbed to something on the order of 70 percent. The guy at the other end of the wire opened the doors to the machine room (it was evening and fairly cool outside) and pulled the temperature down. The packet losses dropped back down to near zero. There are three Verilinks in the same cabinet in the aforementioned room (this is at Stanford). Only our line suffered when the temp climbed. My conclusion is that there is definitely something wrong with that Verilink. NOTHING should be that temperature sensitive, as the other two Verilinks in the cabinet testify. The environmental spec's for the 551VCC/U call for it to operate at up to 50 C (122 F) so it's clear we have a lemon.
mckee@MITRE.ARPA (H. Craig McKee) (09/28/87)
At the risk of discussing the obvious: You noted that one of three Verilinks behaved badly when the temperature went up. It may be that the distribution of cooling air within the cabinet was such that yours went out-of-spec. Regards - Craig