Path: utzoo!attcan!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!think.com!snorkelwacker.mit.edu!bloom-beacon!deccrl!news.crl.dec.com!shlump.nac.dec.com!koning.enet.dec.com
From: koning@koning.enet.dec.com (Paul Koning)
Newsgroups: comp.dcom.lans
Subject: Re: Ethernet Vs. IEEE 802.3 ??????????
Message-ID: <18269@shlump.nac.dec.com>
Date: 17 Dec 90 21:11:14 GMT
References: <3443@lectroid.sw.stratus.com> <1990Dec5.145439.9133@cs.utk.edu> <13213@uudell.dell.com>
Sender: newsdaemon@shlump.nac.dec.com
Reply-To: koning@koning.enet.dec.com
Organization: Digital Equipment Co., distributed systems architecture
Lines: 56


|>
|>They can coexist on the same wire but unless your physical interface is
|>sophisticated enough to recongize both types at least some of the data
|>will not get through.  Which is better is probably a religous question. ;-)

The sophistication needed is trivial.

|>They are not the same, but they are somewhat similar.  
|>
|>The Ethernet frame format looks like this:
|>	8 octet preamble
|>	6 octet destination address
|>	6 octet source address
|>	2 octet type
|>	46-1500 octets of data
|>	4 octet CRC
|>
|>The 802.3 frame format looks like this:
|>	7 octet preamble
|>	1 octet start frame delimiter

Same as Ethernet (Ethernet doesn't have separate names for the two
parts, but the definition is IDENTICAL.

|>	2 or 6 octet destination address
|>	2 or 6 octet source address

No, only 6 octet addresses are allowed.  See section 4.4.2.1.

|>	2 octet length
|>	46-1500 octets data and pad (in 10BASE5 definition)

Not just 10Base5; it applies to all 10 MHz versions.

|>	4 octet CRC
|>
|>The preamble and SFD of 802.3 are basically the same as the preamble of
                                    ^^^^^^^^^ 
They ARE the same; only the words used to describe them differ.

|>ethernet.  The 10BASE5 version of 802.3 specifies use of 6 octet
                 ^^^^^^^ Actually, then 10 MHz version in general
(including 10Base2, a.k.a. "ThinWire", a.k.a. "cheapernet".

|>addresses so these are the same as ethernet.  The type field of ethernet
|>is, I've been told, always less than some number and the length field of
|>802.3 is always greater than that number.  This *might* be a way to tell
|>the two frame types apart, but don't count on it.  Both formats require

It's actually the other way around: 802.3 Length is in the range 0-1500,
while Ethernet ProtocolType has the larger values.  And why be so tentative
about this?  You certainly CAN count on it; it works; it IS the way to
tell the two apart.

	paul