Framing errors with TCP-IP

"Robert Wessel" <robertwessel2@yahoo.com> wrote in message
news:bea2590e.0308132254.560c9938@posting.google.com...
> Detlef.Bosau@tesionmail.de (Detlef Bosau) wrote in message
news:<8rmQCZ1jSJB@Detlef.Bosau>...
> > robertwessel2@yahoo.com meinte am 12.08.03
> > zum Thema "Re: Framing errors with TCP-IP":
> > > BTW, Manchester encoding does not eliminate DC current flow,
> > > although differential Manchester does.
> >
> > Could you please explain this a little bit more detailed?
> > I think, this is interesting.
>
>
> I had a bit of a brain fade when I wrote that: neither straight
> Manchester or Differential Manchester have any DC current flow.
>
> With Manchester, you always have a transition in the middle of each
> bit, thus half of each bit is transmitted at +Vx, and half at -Vx
> (presumably driving the same load, thus current is the same in both
> cases), thus netting zero net current flow.  In straight Manchester, a
> "one" is distinguished by a -Vx at the start of the bit, in
> Differential Manchester a "one" bit is distinguished by an additional
> transition at the beginning of a bit.
>
> Anyway, you want to avoid DC current flow because the entire
> transmission system, including the transmitter electronics, the wire,
> and the receiver electronics, has numerous components that can act as
> capacitors, and store energy.  If there's DC current flow, things can
> get charged up, which by itself can lead to saturation on transistors,
> which can lead to reduced sensitivity or responsiveness at the
> receiver, or just present a higher net impedance for any additional
> signal with the same DC polarity.  And once charged up, it'll
> discharge when the DC polarity reverses, which will almost certainly
> result in a large burst of noise on the line.  If your signaling
> scheme has DC current flow, you generally need to provide some method
> for draining the current away before any of the bad things happen.
> It's typically easier just to avoid the problem in the first place.

Well, for coaxial ethernet there IS a DC component.  It uses a current
source which is either on or off.  The average voltage level is used for
carrier detect and collision detect, which requires fairly tight tolerances
on the impedance and detection circuits.  Transformer isolation is used on
the AUI cable, power is supplied to the transceiver which uses a DC to DC
converter to isolate the AUI from the coax cable.

This would also be true for differential manchester, though.

-- glen