Re: [Discuss-gnuradio] USRP2 and long cables

[Matt Ettus]

Newman, Timothy wrote:
> Has anyone had any experience using a USRP2 with long Ethernet cables, 
> possibly up to the 100m max?  I don’t forsee any problems but I plan on 
> having several USRP2s distributed throughout a building and having all 
> the host PC’s in central location will simplify things.  Just wanted to 
> check if anyone has had any good/bad experiences with this. 


I don't have it handy, but I believe the spec on BER for 1000Base-T is 1 
in 10^10 at the max length of 100m.  So if you are running at full rate, 
you could see errors about once every 10 seconds.  The error rate drops 
dramatically as you shorten the cable, to the point where you never see 
errors at 25 meters.

1 every 10 seconds would be fine for normal internet communications, 
since TCP would retransmit and you'd never notice it.  However, for us, 
we are forced to drop packets with errors, so you'd see glitches.

Another issue is that in my experience, you may have trouble even 
getting any ethernet devices to auto-negotiate at that distance.  I 
tried with a 100m cable and couldn't get two Marvell ethernet cards to 
talk to each other.

My suggestions --

- try to limit it to 50m or so
- use high quality factory-made cable.  No hand crimping.
- don't join more than one cable

If you need to go farther than 75m, you could put a gigE hub in the 
middle, or you could use twisted-pair to fiber converters.  The fiber 
standards have 100x better error rates which are defined at much longer 
distances.


Matt