mbr@aoa.UUCP (Mark Rosenthal) (09/16/88)
I would like to rent a Time Domain Reflectometer to check out our backbone
cable, and I have a few questions.
1. Are TDR's which are advertised for diagnosing ethernet problems (e.g. the
one made by Cabletron) different in any way from TDR's which are not
specific to ethernet?
2. Do you know of any company (preferably in the Boston area) which will rent
an appropriate TDR?
--
Mark of the Valley of Roses
...!{harvard,ima}!bbn!aoa!mbrron@topaz.rutgers.edu (Ron Natalie) (09/17/88)
The Ethernet TDR's are generally cheaper. The other "wire" ones ought to work. The phone guys were going to loan us one they used, and were convinced that it was more than I needed, but the problem went away before I took them up on the offer. Of course if it's an OTDR, it's a different picture. I actually have a Cabletron unit. We've had it out a couple of times, but we haven't actually found a problem that it would have solved. Usually just serves to point out that the physical cable is NOT what is causing the failure. -Ron
eshop@saturn.ucsc.edu (Jim Warner) (09/18/88)
In article <425@aoa.UUCP> mbr@aoa.UUCP (Mark Rosenthal) writes: >I would like to rent a Time Domain Reflectometer to check out our backbone >cable, and I have a few questions.... What is special about the Cabletron TDR is (1) it uses negative going pulses to probe the cable and (2) it can superimpose a steady negative voltage onto the cable to force all the stations to defer. The rational for using negative going pulses is that's what transceivers normally put on the cable so you minimize the possibility of electrically stressing your installed transceivers. I'll buy that. Cabletron brought one of their TDRs out here for a sales demo. I found that it didn't do any more than the setup I was using which cost a whole lot less money. I use a conventional pulse generator and an oscilloscope. The requirements for the pulse generator is that it be able to generate negative going pulses ~50 nS long and that it have a separate output to send to the trigger input of your oscilloscope. One pulse generator that is suitable is B&K model 3300, which sells for about $400. Put the pulse generator close to the scope and the end of the E-net cable. Run a BNC cable from the generator to a 'T' connector on the 'scope input. Another BNC cable goes from the o'scope 'T' to a BNC to N type adapter on the end of the E-net cable. Set the pulse generator to produce 0.5V negative going pulses before you attach it to the E-net cable. A good repetition rate is anywhere between 1 and 10 KHz. A third BNC cable goes from a spare output on your pulse generator to external trigger input on your oscilloscope. You'll be measuring the distance to reflections in time units. Convert to distance units by multiplying by 0.77 ft per ns. You should divide your observed times by two to convert round trip time to one-way time. The reason for a triggering the oscilloscope independent of the signal on the E-net cable is that it then won't trigger on attempts of hosts to transmit. This readily serves the same function as Cabletron's custom negative bias generator. I would prefer a packaged portable TDR unit if I had to do TDR in the field where there was no electricity -- say in a man hole in the middle of the street. But I don't climb around in man holes... I am especially fast to get out my TDR setup whenever (1) we are dealing with a new contractor and the office wants to know if it's OK to pay the bill (2) someone wants to convert their network cable to RG58 with a type N to BNC adapter (3) a network expansion is being done with cable from a different vendor than the original install. Of course, if your records are less than perfect, TDR also provides the authoritative method for determining the length of a cable segment. jim
sung@mcnc.org (Wayne Sung) (09/19/88)
The Ethernet specific tdr's are capable of testing the cable without turning off all stations. Other tdr's such as the ones used in rf cable work do not have this capability.
kwe@bu-cs.BU.EDU (kwe@bu-it.bu.edu (Kent W. England)) (09/19/88)
In article <Sep.16.20.37.28.1988.20510@topaz.rutgers.edu> ron@topaz.rutgers.edu (Ron Natalie) writes: >The Ethernet TDR's are generally cheaper. [...] > >I actually have a Cabletron unit. We've had it out a couple >of times, but we haven't actually found a problem that it >would have solved. Usually just serves to point out that >the physical cable is NOT what is causing the failure. > >-Ron Just a follow-up to this: You should have a cable TDR'ed before installation when it is still spooled and TDR'ed after installation. The first test is the "cable acceptance" test and the second is the "installation acceptance" test. You can have the installer do both TDRs as part of his contract. Any contractor with Ethernet experience can do that. You usually won't need to TDR a cable again, unless you let just anybody install stuff on it. :-) You might be able to do without one, unless you have miles and miles of cable. However, another trick we use is strategically placed barrel connectors. We usually have floor cables run into a closet to access a riser. There are barrel connectors placed in the closets that let us physically partition a segment for diagnostics. It's actually easier and less disruptive than a TDR, for to TDR a cable you have to disrupt traffic on it. Ideally you want the cable completely shutdown to traffic, which may be impractical. We do all the transceiver installs ourselves and rarely find the need for a TDR beyond acceptance testing, but we do have one because they are relatively cheap. If you don't have an Ethernet analyzer, buy that first, then buy a LAN-MD (or other), buy the TDR last. Hope this helps. Upon further reflection [pardon that pun], a TDR would be nice when some other contractor damages your cable, but again we usually find it faster by inspection and by asking questions. The bright yellow color we use saves a lot of damage. I guess contractors will waste anything black or aluminum/steel colored (like broadband coax) before they'll touch that funny yellow stuff. (Anybody know a vendor that sells orange broadband coax?)
ian@lassen.SGI.COM (Ian Clements) (09/20/88)
Electro-Rent rents the Tektronix 1502 and 1503. The 1502 comes with a strip chart recorder and is good for about 500 meters of cable. The 1503 comes with a strip chart recorder and can be ordered with distance calibration option, P7 phosphor and a metric version of the recorder. The 1502 runs about 450 a month. I don't know about the 1503. Cheers, Ian
ron@topaz.rutgers.edu (Ron Natalie) (09/22/88)
Our best line we got from a contractor was "We thought that was an extension cord." We snuck into a building under construction and ran the Ethernet cable before they put the cieling grid in, tying it up very nicely out of the way. The ceiling people pulled it down so that it would be below the finished ceiling. -Ron