Re: IEEE 802.2, 802.3 & Ethernet questions


Ron D. Thornton (panda!teddy!rdt@husc6.harvard.edu)
12 Aug 88 16:39:13 GMT


As noted, the differences between V1 ethernet, V2 ethernet, and 802.2/802.3
are both electrical and header format. On the coax, all 3 versions are fairly
compatible. I don't have the specifications in front of me, but IEEE probably
changed some edge specifications just to be different.

On the tranceiver to controller electrical side:

Version 1 used DC coupling on the tranceiver cable interface which make it
totally incompatible with Version 2 or 802.3 tranceivers (some tranceivers/
controllers can be strapped for either DC or AC coupling).

Version 2 changed to AC coupling on the tranceiver cable interface. V2 also
specified that collision detect from the tranceiver should be forced on when
a transmission is ended to provide a heartbeat or verification from the
tranceiver.

803.2 took Version 2, played with some differential levels and driver/receiver
specifications but remains reasonable compatible with the Version 2 tranceiver
interface. It also added an optional control signal from the controller
to the tranceiver and added an optional 3rd state to the collision presence
signal (oscillate at 5Mhz instead of 10Mhz).

On the packet header side:

All 3 versions define the first 6 octets as destination address, and next 6
as source address.

Version 1 and 2 ethernet use the next 2 octets as type field to define the
next higher protocol level.

802.3 uses those 2 octets as a length field. Short packets may need to
be padded and this field tells where data ends and padding starts. The
ethernet specs leave it to the next level in the protocol to determine
what is data and what is pad. 802.3 relys on the next level up, 802.2 to
define more octets that are then used to determine the next level of protocol.

There is an escape in the 802.3 header specification that allow 802.3 and
ethernet packets to co-exist. The maximum packet length is 1500 and any
802.3 packet with a length field greater than maximum can be used in a user
defined fashion. By only using ethernet type codes greater than 1500 you
know at a glance if a packet is 802.3 or ethernet. All internet type codes
have values greater than 1500.

        -Ron-
        rdt@genrad.COM



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