From: Michael Spencer (labmich2002@xxxxxxxxx)
Date: Sat Aug 24 2002 - 05:28:32 GMT-3
Hi guys,
Enough has been said about synchronous vs asynchronous. The logic that ethernet
uses Manchester encoding leads to believe that it is synchonous. Anyway if it
is asynchronous then where is the start bit and stop bit between each character
. Secondly does RFCs say anything about this?
Guys think about it and come to a conclusion.
Wade Edwards wrote:You got me on the T2 standard but the other ones yes.
-----Original Message-----
From: Przemyslaw Karwasiecki [mailto:karwas@bellsouth.net]
Sent: Friday, August 23, 2002 3:54 PM
To: bk-lists@kesslerconsulting.com
Cc: Wade Edwards; 'Asim Khan'; 'Michael Snyder'; 'Michael Spencer';
ccielab@groupstudy.com
Subject: RE: ethernet
I believe that again, answer is not simple. :-)
It depends if we are talking about 100BaseTX or 100BaseT4 or 100BaseT2
I think that TX is using 2 pairs and 4b/5b encoding, but still needs
a lot of spectrum from channel, so works only on CAT5 cables.
T4 can run on CAT3 (less bandwidth available) but uses 4 pairs.
Finally T2 uses some very sophisticated modulation technique,
but can run on 2 pairs only, and low grade (bandwidth) CAT3 cable
If I am mistaken, please correct me,
Przemek
On Fri, 2002-08-23 at 16:20, R. Benjamin Kessler wrote:
> For what it's worth, I believe FastEthernet utilizes the same 4b/5b
> encoding method that FDDI does
>
> -----Original Message-----
> From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf
Of
> Przemyslaw Karwasiecki
> Sent: Friday, August 23, 2002 2:34 PM
> To: Wade Edwards
> Cc: Asim Khan; Michael Snyder; Michael Spencer; ccielab@groupstudy.com
> Subject: RE: ethernet
>
> I don't know if Ethernet is Synchronous or not. But:
>
> Synchronization of clocks in receiver and transmitter is achieved
> by manchester encoding, not frame preamble.
> Frame preamble is used to delineate frames.
>
> Difference between RS232 and Ethernet is, that after start bit,
> clock of the receiver is free running in RS232, hence you cannot
> reliable send more then couple of bits. (6,7,8). On the other hand,
> in Ethernet, receiver needs to synchronize its clock to the signal
> from the wire. It is possible, because of manchester encoding,
> and PLL circuity, keeping receiver clock in synch.
>
> BTW -- this system has horrible overhead. It doesn't utilize
> available bandwidth of transmission channel in efficient way.
> There are many techniques addressing this issue.
> For instance FDDI was using some kind of encoding with only 25%
> overhead, and some extra signalization incorporated into encoding.
>
> As far as comparison between access methods to Ethernet media
> and other medias, you are basically right. But does it implies
> automatically that Ethernet is Asynchronous?
>
> I guess, we are trying to attach synchronous/asynchronous label
> to some broad concepts, like Ethernet or ATM, which are basically
> a sets of different, simpler components. Those components possibly
> can be qualified as either synchronous or asynchronous.
>
> But I am not an expert in semantics. :-)
>
> Take care,
>
> Przemek
>
> On Fri, 2002-08-23 at 13:38, Wade Edwards wrote:
> > But with that logic, modems (specifically the connection from DTE to
> > DCE) use synchronous communication because the start and stop bits
> > synchronize each character of data. The receiver sees the start bit
> and
> > synchronizes to the transmitter clock. The clocking is transferred
in
> > the encoded data.
> >
> > I just don't think of it that way.
> >
> > By the nature of Ethernet, as you start adding more and more devices
> to
> > that medium total throughput goes down because each device is trying
> to
> > access the medium in an asynchronous manner. If there is never any
> data
> > to transmit on an Ethernet network (with keepalives turned off)
there
> is
> > no clock.
> >
> > On an ATM network, if there is never any data to transmit there is
> still
> > clocking and order.
> >
> > To me this mean asynchronous for Ethernet and synchronous for ATM.
> >
> > L8r
> >
> > -----Original Message-----
> > From: Asim Khan [mailto:asimmegawatt@yahoo.com]
> > Sent: Friday, August 23, 2002 2:43 AM
> > To: Przemyslaw Karwasiecki; Michael Snyder
> > Cc: 'Michael Spencer'; ccielab@groupstudy.com
> > Subject: RE: ethernet
> >
> > I think ethernet is synchronous. The reason is in any
> > synchronous transmission, the receiver uses a clock
> > which is synchronized to the transmitter clock. The
> > clock may be transferred by either:
> >
> > 1)A seperate interface circuite.
> > 2)Encoded in the data (like Manchester encoding,AMI
> > encoding).
> >
> > Now in ethernet an encoded clock is used.
> >
> > Regards.
> >
> > Asim Khan
> >
> > --- Przemyslaw Karwasiecki
> > wrote:
> > > Ethernet is using something called Manchester
> > > encoding.
> > > It basically means, that in order to provide clock
> > > synchronization
> > > between frame transmitter and receiver, each zero is
> > > represented
> > > by sequence of 01 and each one is represented by 10.
> > > By doing so, it makes it possible to maintain clock
> > > synchronization
> > > even in case frame contains long sequence of zeroes
> > > or ones.
> > > And, yes, before each frame, there is a short
> > > sequence send
> > > called preamble (but I believe this is layer 1 not
> > > 2),
> > > which makes it possible to delineate beginning of
> > > the frame.
> > >
> > > Is it synchronous -- IMHO yes, but it depends on
> > > definition
> > > of the term synchronous.
> > >
> > > Przemek
> > >
> > >
> > > On Thu, 2002-08-22 at 17:17, Michael Snyder wrote:
> > > > Where's the clock?
> > > >
> > > > Believe every Ethernet transmission starts with a
> > > series of one's and
> > > > zero's sent before the packet header.
> > > >
> > > > This layer two header provides the clock. So it
> > > it's async before the
> > > > packet is transmited, and synced as the packet is
> > > transmited.
> > > >
> > > > Does this help?
> > > >
> > > > I have a better question for you, is ATM sync or
> > > async. Really? You
> > > > don't think there's a sync'ed clock signal on the
> > > fiber cables. About
> > > > as clear as mud huh?
> > > >
> > > > -----Original Message-----
> > > > From: nobody@groupstudy.com
> > > [mailto:nobody@groupstudy.com] On Behalf Of
> > > > Michael Spencer
> > > > Sent: Thursday, August 22, 2002 1:04 PM
> > > > To: ccielab@groupstudy.com
> > > > Subject: ethernet
> > > >
> > > > Is ethernet synchronous or asynchronous?
> > > >
> > > >
> > > > ---------------------------------
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