Newsgroups: comp.dcom.cabling
Subject: Re: Terminators? Why aren't they required as they are with a coaxial system?
References: <[email protected]>  <[email protected]> <[email protected]>
Robert Redelmeier  writes:

> Shrubby  wrote in part:
> > Thanks for the reply, if I unplug my RJ45 from the Telecoms
> > outlet socket on the wall, I am therefore removing the
> > NIC from the network and therefore the terminating effect,
> > why dont I have to plug a terminator in its place?
> 
> In theory, you should.  And one into the NIC.  And you would have
> to if there were other stations on the segment.  But each link
> is isolated buy the hub/switch, so it doesn't much matter if the
> drivers see opens, or drive voltage to Vcc.  No-one is listening.
> I don't even think this worsens EMI, because there's less current.

In twisted pair ethernet system there are separate wire pairs 
used for transmitting and receving (applies 10Base-T and 100base-TX). 
If there is no NIC or termination on the other end of cable, 
signal gets there, gets reflected back etc... the signa waveform 
on the cable itself is useless, but no problem nobody is listening. 
Sending data to pairs that do not have NIC connected does not 
cause network problems, the signal sent there does not get anywhere. 
The signal coming back from NIC to hub/switch is transported on the 
different wire pair...

The EMI issue is real. For this reason Ethernet hub/switch hardware is 
designed in siwch way that it sends data to only those ports that 
have an active NIC connected to them. In 10Base-T network, the 
hubswitch knows that NIC is connected to it when it receives either 
link pulses or real data from the NIC. In 10Base-TX system there 
is first link nagotiation (sending link pulses every now and them) 
and then when link is established a constant flow of bits 
(keeps the link up and clocks syncronized). 



-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/