From: Wes Stevens (wesley@stevens.name)
Date: Tue Jan 13 2004 - 10:12:48 GMT-3
Priscilla,
I thought the main purpose of the preamble was collision
detection? You don't need all the packets in the preamble to
sync clocks. If all you need to do was sync clocks you could
make it a lot shorter. The collision detection is not need
these days as I have not seen too many hubs still out there
and do not remember ever using a 100 meg hub.
---- Original message ----
>Date: Mon, 12 Jan 2004 21:06:15 GMT
>From: "Priscilla Oppenheimer" <nobody@groupstudy.com>
>Subject: RE: Preamble at Speeds Above 10 Mbps [7:82377]
>To: cisco@groupstudy.com
>
>Ken Chipps wrote:
>>
>> I recently read in a Cisco Press book that the preamble is
not
>> required at
>> speeds above 10 Mbps. Why is that? Is this due to changes
in
>> the basic
>> standards or due to better hardware? Anyone have a
reference on
>> this so I
>> can find out more?
>>
>> Ken Chipps
>
>The preamble is still used in all varieties of Ethernet. As
far as a
>reference, you can download the IEEE standards for yourself.
They no longer
>charge for them. Go to http://standards.ieee.org/getieee802/
>
>Also, check Charles Spurgeon's Ethernet pages here:
>http://www.ethermanage.com/ethernet/ethernet.html.
>
>Maybe you better tell us more of the quote from the Cisco
Press book. Taken
>out of context maybe it just sounds like nonsense?? (I'll
give them the
>benefit of the doubt.)
>
>The purpose of the preamble is for the recipient to synch up
to the
>clocking. Without synchronization, the recipient can't know
when bits start
>and end. There's no separtate clocking signal on Ethernet,
as you know. It's
>not like V.35 and other serial synchronous technologies
where there's a
>separate clocking signal. Instead, the sender sends out with
its CPU clock
>and the recipient receives with its CPU clock.
>
>Various Ethernet technologies use different methods for
encoding ones and
>zeros. All of them insist that there must be changes from
high to low or low
>to high voltage in the bit periods to enable
synchronization. Ethernet uses
>Manchester encoding. 100Base-T uses MLT encoding which is
multilevel and
>actually goes between high, low, and medium along with 4B/5B
encoding.
>1000-BaseT uses PAM5, etc.
>
>They all use a preamble.
>
>Priscilla
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