From: Przemyslaw Karwasiecki (karwas@xxxxxxxxxxxxx)
Date: Fri Aug 23 2002 - 17:54:25 GMT-3
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 <karwas@bellsouth.net>
> > 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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