From: Wade Edwards (wade.edwards@xxxxxxxxxxxxxxxxxxx)
Date: Fri Aug 23 2002 - 17:59:38 GMT-3
Got it. So within an Ethernet frame (largest that is) there are 24000
states with 12000 pieces of information (or bits). It is like AMI or
B8ZS in the T1 world. Need to keep the ones density or the timing goes
and so does the circuit. The only difference is that T1's are bipolar
and Ethernet is unipolar (I think).
One key thing you said was that timing is always synchronized as long as
there is a transmission occurring. I assume it is correct to say that if
there is no data, then there is no synchronization. Synchronization
occurs at the beginning of each transmission (or Ethernet frame) and
continues until the end of that transmission (again a frame). Just like
the way asynchronous devices communicate with the start and stop bits.
I still see Ethernet as asynchronous from the standpoint of there not
being clocking (and synchronization) if there is not any data to send,
just like other asynchronous communication methods.
Still learning.
-----Original Message-----
From: Przemyslaw Karwasiecki [mailto:karwas@bellsouth.net]
Sent: Friday, August 23, 2002 3:47 PM
To: Wade Edwards
Cc: Asim Khan; Michael Snyder; Michael Spencer;
ccielab@groupstudy.com
Subject: RE: ethernet
On Fri, 2002-08-23 at 15:52, Wade Edwards wrote:
(snip)
>
> This is what I found about Manchester encoding: A method of
transmitting
> bits which enables the receiver to easily synchronize with the sender.
> To me this means that Ethernet starts sending the preamble of a frame
> and the receivers synchronize with the Manchester encoded data stream.
> This gives them time to synchronize before actual data (i.e. network
> traffic) is sent.
>
In manchester encoding, receiver clock is _continuously_ synchronizing,
while receiving data. Otherwise, in 1500 octets == 12000 bits
Ethernet frame, receiver clock would have to be nearly perfectly
accurate to avoid slips at the end of the frame.
Manchester waste bandwidth of communication channel,
but provides clock all the time, during transmission.
Each one is transmitted as 10 and each zero as 01,
so regardless of pattern of bits in frame (can be all zeros,
can be all ones, can be whatever) there will be ALWAYS transition
of a signal in each and every bit. Those transitions are used to
synchronize receiver clock.
Przemek
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